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How Sailboats Measure Up

• By By Jeremy McGeary
• Updated: October 17, 2012

Sailboats by the Numbers

Boat reviewers rely on numbers to describe some of the key attributes of their subjects, such as length, beam, draft, and displacement. And while judgments on interior layouts and decor are subjective, these figures describing dimensions are not. There are, however, other numbers commonly cited in spec boxes that can prove more elusive, since they attempt to put a numerical value on how a sailboat might be expected to perform while under way. The commonly used ratios are sail area to displacement (SA/D), displacement to length (D/L), and ballast to displacement (B/D). And though they’re so commonly used that a certain amount of dogma has accrued around them, these figures can, in fact, be misleading, or at least misunderstood. And the result is that a boat can be assigned attributes based on numerical values that don’t take into account how sailboat design has changed over the past several decades.

Here, then, is a look at those ratios, what they attempt to describe, and how they should be interpreted when you go off exploring new and used models. (Click to page 2 for a more in-depth explanation.)

Sail Area/Displacement (SA/D)** An automobile buff seeking a high-performance ride looks for a high power-to-weight ratio and compares the horsepower/curb-weight ratios of different cars. For a sailboat, the SA/D provides the same metric. The horsepower comes from the wind on the sails and is proportional to the sail area; a boat’s weight is its displacement (in pounds, kilograms, or tons).

Initially, the SA/D only really gives a measure of potential acceleration rates (in case any physicists are reading this), but since displacement is a key factor in the resistance a boat encounters when moving through the water, SA/D also has a bearing on potential maximum speed.

The traditional calculation for SA/D compares sail area in square feet to displacement in cubic feet. In the formula, displacement in pounds is divided by 64 (the density of seawater) to obtain cubic feet, which are in turn converted to square feet to make the ratio unit-free.

On a spreadsheet, the formula would be S/(D/64) (2/3).

Nominally, the higher the SA/D, the more lively the boat’s sailing performance. The vessel will accelerate more quickly and have the potential for higher speed. But to be able to compare boats with any degree of precision (or fairness), we have to use similar numbers. The displacement must be in the same condition, either light ship (nothing on board) or fully loaded, and the sail-area measurement must reflect the normal working sail plan. Racing boats have measurement certificates from which these numbers can be reliably extracted. The specifications provided in cruising-boat brochures might not be consistent between builders, but we have to assume they are.

Boats measured in the 1970s and the 1980s for racing under the International Offshore Rule for the most part had SA/Ds between 16 and 17, based on the sum of the mainsail triangle (M = P E/2) and 100-percent foretriangle area (100%FT = I J/2). The measurement system favored small mainsails and large headsails, and since designers of cruising boats stuck close to the IOR sail plan, the IOR value for SA/D became the yardstick. An SA/D above 17 said “fast boat,” and anything below 16 said “slow boat.”

After the IOR fell out of favor, cruising-boat design drifted away from raceboat design, and sail plans began to change. Today, many boats are designed with large mainsails and small jibs, and most builders publish a “total sail area” number that includes the standard jib (often as small as 105 percent) and the roach in the mainsail (which is significantly greater on modern boats with full-battened mainsails than on IOR boats).

These builder-supplied numbers are more readily comparable against competing models, but using them in the SA/D formula makes the boats look “faster” than older models. This is a false comparison, because the sail area used for the older boats doesn’t include the extra area in, say, a 150-percent genoa.

The table “Sailboats by the Numbers” (see page 79) illustrates this. It shows SA/Ds calculated for a selection of modern boats and boats from past eras, all about the same length, using different numbers for sail area. For each model, it shows five SA/Ds. SA/D 1 is calculated using the sail area provided by the builder. SA/D 2 is calculated using M (P E/2) and 100% FT (I J/2). SA/D 3 is calculated using M + 105% jib. SA/D 4 is calculated using M + 135% jib. SA/D 5 is calculated using M + 150% jib. The only SA/D that includes mainsail roach is SA/D 1.

Let’s look at some examples. The 1997 Beneteau Oceanis 411 has a published sail area of 697 square feet on a displacement of 17,196 pounds. That gives an SA/D 1 of 16.7 (the same as SA/D 2), which for decades was considered very respectable for a cruising boat.

In 2012, the current Beneteau Oceanis 41 has a published sail area of 902 square feet (453 mainsail + 449 jib) and a published displacement of 18,624 pounds, to give an SA/D 1 of 20.5. Wow! Super-high performance! But this is for the standard sail area, with the 449-square-foot jib (just about 100% FT and typical of the trend today toward smaller jibs that tack easily). Plug in the calculation using I, J, P, and E and SA/D 2 drops to 18.9 because it doesn’t include mainsail roach, which is about 16 percent of the total published mainsail area.

Go back to the 1997 model, tack on a standard-for-the-day 135-percent genoa, and the SA/D 4 becomes 20.7. (If we added in mainsail roach, typically about 11 percent of base mainsail area before full-battened sails, we’d have 21.4.) The 1997 boat has essentially the same horsepower as the 2012 model.

Looking at current models from other builders, the SA/Ds based on published numbers hover around 20, suggesting that designers agree on the horsepower a cruising sailboat needs to generate adequate performance to windward without frightening anyone.

The two boats in our chart that don’t at first appear to fit this model are the Hunter 39 and the Catalina 385, but they’re not really so far apart.

The Hunter’s SA/D 2 is 16.1. Its standard jib is 110 percent (327 square feet), and the rest of the published sail area is in the mainsail—664 square feet, of which 37 percent is roach!

Catalina is a little more traditional in its thinking. If you add the standard 135-percent genoa, the SA/D becomes 21.2—right in the ballpark. (It’s still there at 19.7 with a 120-percent genoa.)

The table shows that, for boats targeted at the “performance cruising” market, the SA/D numbers using actual sail area lie consistently around the 20 mark. To go above that number, you have to be able to fly that sail area without reefing as soon as the wind ripples the surface. To do that, you have to elevate stability—with broad beam, lightweight (i.e., expensive) construction, deep bulb keels, and fewer creature comforts.

Displacement/Length (D/L)** While sailboat builders and buyers are interested in displacement in terms of weight, naval architects view it as volume; they’re creating three-dimensional shapes. When working in feet, to get a displacement in pounds, they multiply cubic feet by 64, the density in pounds per cubic foot of seawater. (Freshwater boats displace more volume because the density of fresh water is only 62.4.) The D/L ratio is therefore a measure of immersed volume per unit of length—how tubby the hull is below the waterline.

According to conventional wisdom and empirical studies, the lower the D/L, the higher the performance potential. This is mainly due to wavemaking resistance being lower for slender hulls than for tubby hulls.

In the D/L formula, displacement in pounds is divided by 2,240 to convert it to tons to bring the values to manageable numbers, so D/L is displacement in tons divided by .01LWL (in feet) cubed.

In a spreadsheet, the formula would be D/(2240*(.01L)3), where D is the displacement in pounds and L is LWL in feet.

In the early days of fiberglass boats, the Cruising Club of America rule was the principal dictator of boat shapes. Because it was a waterline rule, designers kept waterlines short to keep ratings low and relied on long stern overhangs immersing to add “sailing length” when the boats heeled. Carbon fiber was available only to NASA, and boats had full interiors, so “light displacement” wasn’t really in the cards. A D/L of 300 was considered dashing, even risky. Many still-popular designs from the 1970s and 1980s have D/Ls as high as 400; see the Bounty II.

Fast-forward 40 years. Boats now have plumb bows and plumb sterns and waterlines almost as long as their LOAs—there are no rating penalties on a cruising boat. The boats’ weights haven’t changed much because, although builders try to save weight to save cost, the boats are so much bigger. The hull and deck surface areas are greater, and all that extra internal volume can be filled with furniture. The effect on D/L ratios has been drastic—just look at the table. A D/L ratio above 200 today describes a heffalump.

But do these lower D/Ls actually buy you any more speed? Yes and no.

Yes : Because speed is proportional to the square root of the waterline length. Today’s 40-footer has a much longer waterline than yesterday’s and ought to sail as fast as yesterday’s 50-footer. It might also benefit from reduced resistance due to a smaller cross-sectional area, but it also might have greater wetted-surface drag due to the longer immersed length. When sailing downwind in waves, though, the lower-D/L boat will surf more readily.

No : Because, as we saw above, the power-to-weight ratios (SA/D) of modern boats aren’t effectively any higher, and certainly aren’t in the realm that would allow our cruising sailboats to climb out of the displacement zone and plane. In most conditions, the lower-D/L boat is still trapped in its wave.

In the days of the IOR, a D/L of 250 was still pretty racy; see the 1978 Catalina 38. Today, even a D/L as low as 150 doesn’t make a boat a speedster if it can’t carry the sail area to make it so. To compete at a level with a Volvo 70, look for a D/L of about 40 and an SA/D of 65.

Ballast/Displacement (B/D)** The ballast/displacement ratio is simply the ballast weight divided by the boat’s total displacement. Since ballast is there to give the boat stability, it’s easy to jump to the conclusion that the higher the B/D, the stiffer the boat.

However, B/D doesn’t take into account the location of the ballast.

Take a boat that has a total displacement of 20,000 pounds and put its 8,000 pounds of ballast in the bilge. Now take the same boat and put the 8,000 pounds of ballast 4 feet deeper in a bulb at the bottom of a deep fin keel. Same ballast ratio (0.4), but very different stability.

When looking at B/D, therefore, we must ask about the configuration of the keel: How low is the ballast?

Stability analysis is complex and involves beam, hull cross-section, and length, among other factors, of which B/D is just one.

Since the late 1990s, builders of sailboats intended for sale in the European Union have been required to provide stability data, including a curve of righting arm at angles of heel from 0 to 180 degrees—far more information than anyone can divine from a B/D number and a much more useful measure of a boat’s inclination to stay upside down in the unlikely event (the way most people use their boats) that it exceeds its limit of positive stability.

CW contributing editor Jeremy McGeary is a seasoned yacht designer who’s worked in the naval-architecture offices of David Pedrick, Rodger Martin, and Yves-Marie Tanton and as a staff designer for Camper & Nicholson.

To read the related article, How To: Measure Sail Area, click here.

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The Definitive Guide to Sailboat Hull Types

If you’ve ever been on a sailboat or any kind of boat, one of the first parts of the boat you saw was its hull and you might not have even known it.

Simply put, the hull is the bottom part of a boat that rides in and on top of the water. When a sailboat is underwater, it’s accompanied by the keel and the rudder.

Just like knowing the different types of sails , knowing the hull type on your sailboat means you’ll have a better understanding of how your boat operates while it’s out on the water.

All in all, the hull of any boat is meant to keep the boat afloat and to ensure minimum resistance against the water while being propelled forward. Now let’s dive into the different sailboat hull types and even some other types of hulls in boats in general!

Main Sailboat Hull Types

There are two main hull types that we’ll be looking at that encompass the many other types of hulls that vary from these two main types.

Depending on the type of boat you have, you’ll be floating around with one or the other. We’ll take a look at what you can expect if your boat has either of these hull types.

Displacement Hulls

The most common sailboat hull type you’ll find out there is the displacement hull, which is very effective at pushing the water aside and powering through it during forward propulsion.

A displacement hull is often found not only on sailboats, but also fishing, freight, cruise, and other larger boats.

All boats that have a displacement hull will be limited in their speed based on the waterline length of the hull. Regardless of how much power you use, whether it’s from the wind or motor, the maximum speed can’t be increased.

This is why you’ll see people mention the waterline length of a boat’s hull when putting them on the market to sell.

The big advantage of having a displacement hull is that they require far less power to get moving across the water compared to the other main hull type; the planing hull.

What this means is that your boat will be able to cruise for a long time with the same amount of energy, which also allows you to carry more items on board.

Planing Hulls

It’s almost guaranteed that your sailboat won’t have a planing hull since they’re most commonly found on powerboats and personal watercrafts (PWCs), like jet skis.

Planing hulls allow the boat to lift itself out of the water, reducing drag and increasing the speed of the boat.

Almost any boat that’s equipped with a planing hull will be able to attain a speed much greater than a boat with a displacement boat.

The main reason for this is the lift that’s produced when traveling at high speeds which reduces drag on the water.

The maximum speed of a boat with a planing hull is dependent on the horsepower of the engine and how much of the hull can be removed from the water while still cruising.

The biggest advantage of having a planing hull is that your boat will be able to pick up speed quickly and reach a greater maximum speed.

This allows for shorter journey times. However, there needs to be a source of all that energy, which comes directly from a combustion engine. The faster a boat with a planing hull goes, the larger the cost of fuel will be.

How Planing Works

The way planing works is actually pretty interesting, so I thought I’d dive into it a bit. Even though a sailboat is virtually guaranteed not to have one, it’s always nice to know how other boats operate while out on the water.

1. Displacement

Before a boat with a planing hull actually planes, it starts out acting like a displacement hull.

As a matter of fact, a boat with a planing hull needs to reach a certain speed before it starts to produce lift. Before that happens, it’s essentially a displacement hull.

While a boat with a planing hull is picking up speed and lifting itself out of the water, it’s in a plowing mode.

You’ll know when a boat is in plowing mode when the bow of the boat is elevated and the boat is throwing a relatively large wake. The goal, however, is to move from plowing mode to planing mode, which requires further acceleration.

Once the boat with a planing hull reaches a certain speed, it’ll leave plowing mode and enter planing mode.

As I already described, planing is when the hull is gliding across the water with a smaller amount of the hull dragging in the water when compared to the previous modes. Different boats will start planing when reaching different speeds.

Common Sailboat Hull Styles

Now that we’ve gone over the two main types of hulls you’ll find in sailboats and other types of boats, we have a good foundation for the hull styles you’ll commonly see when out on the water.

There are three main hull styles that you’ll see quite often, so let’s take a look at those.

By far the most common hull style you’ll see on sailboats is the monohull, which is simply a single hull.

Traditionally, a sailboat will have a monohull and they can be found all over the place. It’s probably the style of hull that comes to most peoples’ mind when imagining a sailboat.

Monohulls on sailboats are virtually all displacement hulls. As we went over previously, this allows your sailboat to cruise for long stretches and has a greater efficiency compared to planing hulls.

However, most boats that exist on planet earth are monohulls, including powerboats, which can also be of the planing hull type.

When it comes to a monohull on a sailboat, the only way it can keep its stability is to have a proper keel attached to it.

A keel is a wing-like object that sticks out of the bottom of the hull in the water and provides a sailboat with ballast for stability. It’s important to understand how a keel works when operating a sailboat with a monohull since it’s one of the main reasons a sailboat can move forward without tipping.

There are certainly a lot of monohull sailboats out there, but there’s no doubt that you’ll also see your fair share of catamarans.

Catamarans are sailboats with two hulls and operate quite differently than their monohull cousin. Catamarans are known to be fast and are likely to outrun most monohull sailboats.

Unlike monohull sailboats, catamarans can be fitted with displacement hulls as well as planing hulls. However, even if they have a planing hull they can still produce a relatively good amount of cruising time and do so rather efficiently.

Catamarans are a bit different than monohulls in the sense that they can reach greater speeds. There are several reasons for this. For one, a catamaran doesn’t need a ballast for stability since the broad stance between the two hulls provides enough stability.

This means there’s no need for a large, heavy keel. Second, they’re often built out of lightweight materials that allow the boat to reach a higher maximum speed compared to heavier sailboats.

Also, if a catamaran has a planing hull, it’ll have the ability to produce lift resulting in reduced drag on the water and even greater speeds.

Unfortunately, catamarans do have the disadvantage of being more likely to capsize in unwanted high-wind situations.

Also, it’s very difficult for a catamaran to recover from capsizing as opposed to a monohull sailboat that has a good ballast from its keel.

You might have already guessed from the name, but I’ll state the obvious anyway. A trimaran is exactly like a catamaran but with three hulls instead of two.

Often times you’ll see a trimaran look like a monohull sailboat with a pair of hulls attached to its side.

Similar to a catamaran, trimarans can hit speeds much greater than your average monohull sailboat. As a matter of fact, they’re known to be “unsinkable” under the situation that the hulls on the port and starboard side of the central hull are completely filled up with water.

One of the coolest aspects of having a trimaran is that when it has a planing hull and/or a hydrofoil, the trimaran’s central hull will lift completely out of the water.

This gives it the effect that it’s floating across the air, which is the result of lift produced from the planing hull or a hydrofoil. It’s very cool to see this!

Sailboat Hull Bottoms

Apart from the main boat hull styles, like the monohull, catamaran, and trimaran, there are hull bottoms that pop up in the world of boating that can differ in style and function.

These hull bottoms are more of a deeper look at the hulls of a monohull, catamaran, or trimaran, so you can think of them more as a feature of any of the previously mentioned styles of hull.

Flat Bottom

A very common hull bottom for boats that are derived from the planing hull type is a flat bottom hull.

The flat bottom hull is considered to be one of the less stable styles of hulls, especially when confronted with rough waters.

However, you’ll often find them on boats that don’t necessarily ride in these situations, including fishing or taxi areas.

• Good for small lakes and rivers due to having a shallow draft.
• Able to hit relatively high speeds once entering planing mode.

Disadvantages

• Not good at handling choppy waters resulting in a rough ride.

Round Bottom

When it comes to sailboats, you’re most likely going to run into monohull sailboats that have a displacement style hull with a round bottom.

While these are the most common hull bottom for sailboats, they can also be found on smaller boats that are used for fishing, canoeing, and other similar kinds of boats.

• Easily moves through the water due to being a displacement hull type.
• When accompanied by a keel, it produces a great amount of stability from the ballast.
• Without a keel, it can roll when entering and exiting the boat as well as when waves are present.
• Less maneuverable compared to other hull styles.

Deep ‘V’ Bottom

If you’re operating a powerboat, then in all likeliness your boat has a planing hull with a deep ‘V’ bottom.

Since deep ‘V’ bottoms are found on planing hulls, these types of boats will be able to pick up speed quickly and at high maximums. This is the most common setup for powerboats out on the water.

This is the most common type of powerboat hull. This hull type allows boats to move through rough water at higher speeds and they provide a smoother ride than other hull types.

• Provides a smooth ride compared to its flat bottom rival.
• Good at handling rough water.
• Requires more power to plane compared to its flat bottom rival.
• Cannot handle sharp turns very well resulting in potential rolling or banking.

Multi-Chine Bottom

We took a good look at multi-hull styles like the catamaran and the trimaran earlier, which are the exact style of hulls that have a multi-chine bottom.

A multi-chine bottom is a great example of a displacement hull on either a catamaran or trimaran as it’s the most common bottom you’ll find.

• In a multi-hull boat, it has a great amount of stability due to its wide beam.
• In a multi-hull boat, it needs a large area when either tacking or jibing.

Main Parts of a Sailboat Hull

There’s some terminology I threw around while describing the many types of hulls a sailboat and other types of boats have.

As is the case with a lot of activities, learning the terminology is just something you have to do.

Thankfully, the terminology will eventually sink in overtime and eventually you’ll be able to ring off any hull terminology that comes up.

The bow is simply the most forward part of a sailboat and, thus, the very front of the hull.

The stern, conversely to the bow, is the most backward part of a sailboat and, thus, the very end of the hull.

The port side of a hull is the left side. I always remember this with the phrase “I left my port on the table”, with the port being wine.

This just so happens to also be the side where boats will have a red light turned on at night, which is the color of port wine.

The starboard side of a hull is on the right side.

Opposite the port side, in the evening boats will have a green light turned on and will be located on the starboard side of the boat.

Fore is a sailor’s way of saying “forward”.

Aft is a sailor’s way of saying “back”.

A transom is the aft-most (see what I did there?) section of the boat that connects the port and starboard sections of the boat.

The flare of a hull is where the hull starts to form a large angle the closer the hull gets to the deck.

The waterline is the line around the hull where the water touches when under a normal load.

Waterline Length

The waterline length, once referred to as the Load Waterline Length (LWL), is the length of the hull where the waterline is located.

This is not the entire length of the boat.

Length Overall (LOA)

The length overall (LOA) is, you guessed it, the overall length of the boat. This is measured from the tip of the bow to the end of the stern.

The freeboard is the space on the hull of a boat above the waterline and below the deck.

The draft is the length from the bottom-most part of a boat (the tip of the keel on a sailboat) and the waterline.

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• Displacement Hull

A Heavy or a Light Displacement Hull for Offshore Cruising?

In the early days of sailboat cruising, heavy displacement hull forms were the only choice for long-distance offshore cruising, solely because light materials for the hulls, spars, rigging and all the other fittings and equipment that go to make up a sailboat just weren't available.

Displacement Length Ratios:

Under 90 -  Ultralight;

90 to 180  -  Light;

180 to 270  -  Moderate;

270 to 360  -  Heavy;

360 and over  -  Ultraheavy;

These days, with the advent of exotic composite laminates and lightweight fittings, super-strong and ultra-light displacement hull forms are regularly seen crossing the world's oceans.

But there are plenty of sailors around that still favour the easy motion of heavy displacement hulls for offshore cruising, cheerfully conceding the extra days spent at sea for the benefit of increased comfort in rough seas.

Elsewhere on this website we've seen how displacement has a consequential influence on stability, performance and comfort. Now let's turn it around and look at the strengths and weaknesses that each one - from ultra-heavy to ultra-light displacement hull types - has to offer.

Heavy and Ultra-Heavy Displacement Hulls

With  Displacement/Length ratios  of 360 plus, ultra-heavy displacement hull styles have fewer devotees these days, though for passionate cruising traditionalists it's de rigueur. Heavy displacement sailboats of this type will have a full (or long) keel, which will bring with it some benefits - and some significant limitations.

In light winds, a boat of this type will sail slowly - if at all - due to the hull drag caused by its high wetted area and the power required to shift its massive weight.

It will only just be getting into its stride when other more moderate types are taking in reefs.

Apart from its contribution towards a sailboat's stability the next most important function of a keel is to resist leeway. Unfortunately, long, low aspect ratio keels aren't very good at this so they must make up in area what they lack in efficiency.

To counteract the hull drag caused by the surface area more sail area is required, so to enable the boat to stand up more ballast is needed, which is why long-keelers need to be heavy and why they are often underpowered.

My first 'proper' cruising boat (a Nicholson 32 Mk10 , 'Jalingo II' ) with a length/displacement ratio of 394 was a craft of this type. Jalingo's long keel kept her tracking as if on rails, but it was important to keep plenty of way on to get her through the wind when a tack was called for. Her motion underway was sedate, and as comfortable as you could get for a boat of this size. Her vee'd forward sections gave a soft ride but she did show a tendency to bury her bows, making her a little wet at times.

But my enthusiasm for her excellent directional stability at sea largely evaporated during close-quarters manoeuvring. She was a nightmare in a marina where going astern with any degree of directional certainty was well-nigh impossible. 

When heeled, the general symmetry of the immersed hull section will mean they should remain well balanced at high heel angles, but the barn door proportions of their unbalanced rudders and the fact that they are often raked off the vertical make them heavy on the helm at all times.

With their relatively shallow draft, protected propeller and rudder these boats will take the ground well, and should breeze over floating ropes and nets without problem.

The high load carrying capacity of heavy displacement hulls will be greatly appreciated by live-aboard sailors, which together with their other attributes will probably make them best suited for those sailors with ambitions to spend much of their time offshore in remote areas of the world.

But for those of us who are more inclined to spend our time island hopping in the Caribbean and Mediterranean, and cruising offshore in Europe and the USA their sluggish performance will make them less attractive.

A Moderate Displacement Hull

Moderate displacement sailboats are a natural development of the heavy displacement hull types, with a moderate length fin keel and a separate rudder which is either transom hung or supported on a skeg.

On GRP boats, the fin keel may be part of the hull moulding and have its ballast encapsulated within. This avoids the need for keel bolts, and the corrosion and security issues often associated with them.

Although still on the heavy side by modern standards, with a Displacement/Length Ratio of around 300, this type remains a firm favourite with many long-distance cruisers.

Performance, whilst not of the 'ocean greyhound' nature, should be adequate in most conditions and owing to the separation of keel and rudder, manoeuvrability under both power and sail will be much improved.

For a given Sail Area/Displacement ratio her sail area will be less than for the heavy displacement type, making her easy to handle for a small crew. Directional stability and balance will be dependent on the quality of the design, and there's no reason why both shouldn't be excellent.

Light Displacement Hull

Driven partially by the need for economy in a competitive market - lighter means less material - and an increasing demand for better performance, more and more yachts are falling into this category.

Typically with a Displacement/Length Ratio of around 200, a modern light displacement production boat - often dubbed a 'cruiser/racer' - will sport a medium aspect ratio fin keel'. The rudder will be either transom hung, or be supported by a short skeg, or be a cantilevered spade type. The underwater shape will be dinghy-like, with minimal overhangs at bow and stern to maximise waterline length.

Artwork by Andrew Simpson

A lot of ballast is clearly not an option for a light displacement boat so much of its stability is gained through increased beam.

This means that when excessively heeled the asymmetry of the immersed hull sections coupled with the broad beam carried well aft can make them hard on the helm.

Much is to be gained by reefing these boats early and sailing them fairly flat. Performance will be brisk in nearly all conditions, especially off the wind, when hull speed may well be exceeded with a light displacement hull of this type .

Sailing hard on the wind in vigorous conditions will be less comfortable than in a heavier displacement craft. The flatter forward sections can tend to pound, and the ride is likely to be on the lively side.

Apart from beating to windward in heavy weather they are a delight to sail, pointing high and tacking through the wind with ease - and passage times shouldn't be disappointing.

Handling under power, both ahead and astern, will be good. Except, that is, when at low speed in a crosswind. This was brought home to me during our early days with Alacazam (our custom yellow and white wood-epoxy sailboat shamelessly featured in the logo of this website) , when motoring astern out of a marina berth in Leixões in Portugal. The wind was blowing from the direction I wanted to go, but as soon as I cleared the berth and put the helm over the wind blew the bow off so I was pointing at the next berth down.

Someone once said that the height of stupidity is doing the same thing over and over again, and expecting a different result. I thought of this as I found myself zig-zagging sideways down a cul-de-sac, greeting a series of worried looking crews on the way.

Fortunately,  Alacazam steers astern almost as well as ahead so the obvious solution - which fortunately occurred to me before we reached the seawall at the end - was to let the bow blow around and motor out astern past my visibly relieved audience until there was enough room to turn. Another lesson learned.

The load-carrying capacity of smaller light-displacement boats can be a concern. Clearly if you load, say, 1,500lb of stores and equipment on a 25ft boat with a Displacement/Length Ratio of 200 it will have a greater effect than if you loaded the same amount onto a forty footer of the same Displacement/Length Ratio. The 25 footer's Displacement/Length Ratio would increase to 242 and the forty footer's to 210 - obviously a more performance-sapping penalty for the smaller boat.

Ultra-Light Displacement Hull

These ultra-light displacement boats (ULDBs) are probably at least one step too far for the vast majority of offshore cruising sailors. Sharing many of the characteristics of the previous category but more so, these will be beamier, lighter and deeper drafted. Keels will be high-aspect ratio and of such depth to prevent anchoring anywhere near the beach. Performance in the right conditions though will be awesome.

These types will readily unstick themselves from the limitations of hull speed and plane like dinghies, and it should come as no surprise that Ted Brewer's comfort ratio isn't high on the list of design considerations.

To build such a light displacement hull whilst making her sufficiently strong calls for exotic materials and hi-tech building techniques, both of which come with a high price. So much so that cruising versions are generally owned by people with Lamborghinis, and backyards the size of Regents Park.

Optimum performance, handling and comfort can't all be found at the same place on the sliding scale of displacement.

Displacement, or more accurately the Displacement/Length Ratio has a greater influence on the way in which a boat behaves in a given set of conditions than any other parameter, and should be a crucial consideration for a prospective buyer.

Whilst boats at the heavy end will have a more comfortable motion, passage times will be slower and handling more cumbersome. At the other end, the blistering performance of a ULDB will shake your fillings loose.

Somewhere though, between these two extremes, lays your ideal heavy/light displacement hull compromise.

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Semi Displacement Hulls – A Detailed Guide For Owners

Understanding some of the basics is fundamental to understanding all the latest trends and most exciting technologies in boat design. 

For all their variations and quirks, boat hulls come in only three forms. Here’s a look at the various types, along with a closer look at the most misunderstood—the semi displacement hull.

Table of Contents

Displacement hull, planing hulls, displacement hull vs planing hull, semi displacement hull, identifying boat hull type, boating always involves compromise and balance, hull shape faqs.

Three Types of Boat Hulls

Even though there seem to be thousands of different types of boats, there are actually only three hull forms in the world. Sure, designers are constantly tinkering with little things to make them faster or more efficient. But boats will always have either a displacement, planing, or semi displacement hull.

The first type of boat is the oldest – the trusty old full displacement hull. Displacement hulls have been around forever – dugout canoes, cargo ships, cruise liners, and sailboats are all displacement hulls. 

What common things tie these seemingly different boats and ships together? For one, their design stability and the buoyancy that keeps them afloat comes from the water that their hulls displace. Hence the name – displacement hull.

A full displacement boat has a top speed limited by its waterline length. The longer the boat, the faster its theoretical hull speed is.

What is Water Displacement?

When discussing boat design, it is impossible not to talk about the elephant in the room—Archimedes. Ok, he wasn’t an elephant. He was a Greek mathematician and physicist—but you get the idea.

Archimedes was one of the first to apply math to the physical world, and he came up with the law of buoyancy. Archimedes Principle states that the upward force on an object immersed in a fluid (buoyancy) is equal to the weight of the fluid that the object displaces. 

For boats, this simply means that so long as the boat displaces an amount of water that weighs more than the boat, the boat will float. 

So what is a displacement hull? It is one that derives its buoyancy from the weight of the water it displaces. If weight is added to the boat, it sinks lower and displaces more water. 

The key here is that this is the only force that keeps the boat afloat. 

On the other end of the spectrum, we have the modern planing hull boat. If a boat goes fast, it must have a planing hull. Envision speed boats, power racing boats, most fishing boats like bass boats or center consoles, and things like jet skis or skiffs.

When these boats are not moving, they act as displacement hulls. Their shape displaces enough water to keep them afloat. But once they start building up speed, the water passing over the bottom of their hulls creates lift. 

That is the critical difference. At some point, the hull of a planing boat gets on plane—it lifts out of the water. It is no longer displacing enough water to keep it afloat. Instead, a dynamic lifting force is being created by the water flowing under the hull. Once that force diminishes (as the boat slows down), the boat will start displacing water again and will once more become a displacement hull boat. 

There are a lot of differences between these two hull designs. They look different, and they function differently. 

Planing hulls are built for speed, and many compromises are made in the design to accommodate this need. It takes a lot of power to get any boat on plane. Engines must be big and powerful, which means they’re also thirsty. That means they’ll need large fuel tanks. 

In all other aspects of the boat, weight must be limited. A heavier boat will require more power to lift onto plane, so overloading a planing hull means it might not be able to get up to speed. Planing designs favor lighter materials and thinner construction to keep weight down—all benefiting performance.

On the other end of the spectrum, displacement hulls are all about everything other than speed. By committing to a displacement hull, the builder says, “speed is not the priority here.” So what is the advantage, if any, to an old-school displacement hull?

First and foremost, displacement hulls are efficient. It takes very little power to get a displacement hull to speed, which means less fuel and smaller engines need to be carried. 

There are also fewer limitations on how much weight a displacement hull can carry. A displacement hull will use about the same amount of power, whether lightly loaded or heavily loaded. As a result, displacement hulls are the types of design used for things like cargo ships and supertankers. 

Even if you aren’t hauling cargo, all of the room that a wide displacement hull provides makes for excellent living accommodations. On a cruising boat, this type of design usually gives you the most space for staterooms, galleys, heads, and salons – in other words, they are comfortable to live on.

The contrast is the planing hull, which often sacrifices creature comforts due to the hull design’s shape and depth constraints. Without adding enormous superstructures topside, there just isn’t much room inside a planing hull for living accommodations.

Finally, displacement hulls can be designed to be highly stable and self-righting. Sailboats, for example, with their deep keels and heavy ballast, are designed to be stable in all conditions. For this reason, displacement hulls are considered to be the most seaworthy design.

Ride Quality and Comfort Compared

When it comes to cruising boats, displacement hulls often get a bad rap for their ride quality. While they are safe and stable, the ride tends to be rollier than any other type of vessel. Their rounded hulls and low center of gravity allow them to slip through the waves, but they feel each and everyone nonetheless. 

Most cruising boats add some form of stability to reduce roll in a seaway. Sailboats use their sails, the pressure on which keeps the boat heeled over and resists the motion of the waves. Slow trawlers tended to have small stability sails, which did the same thing. Modern boats sometimes include stabilizers that keep them level and resist the motion of the seaway.

Rolling is less of a factor on planing boats since the boat rides above the waves. But the speed at which the boat moves creates another factor – a pounding as the hull goes from wave to wave. Depending on the vessel, this can be more tiresome and exhausting for the crew than the ride on a displacement vessel.

Of course, there’s also a limit to how much the boat can take. The stability and safety of a planing boat require the operator to pick and choose how and when they want to tackle each wave. This means the skipper’s job is never-ending in rough seas. 

At the helm of a displacement boat, the skipper might set the sailboat autopilot to a heading and take their lumps. But at the helm of a boat on plane, the helmsperson is more likely to be actively making adjustments to the boat’s course to find the smoothest ride and prevent jaw-clenching bounces.

So, what is the third type of boat then? A semi displacement hull could also be called a semi planing hull. This type of boat gets its buoyancy and stability from a combination of the two factors listed above – buoyancy and dynamic lifting forces on the hull.

Like all boats, the semi displacement hull design starts out as a displacement hull when sitting in the water. Its hull design makes a dynamic lifting force as it accelerates because of the water flowing around it – much like a planing hull does. The difference is that a semi displacement hull will never get entirely on plane. Its stern will always sit low, and a portion of the force floating the hull will always come from displaced water.

There are fewer examples of semi displacement hulls because they have limited commercial use. However, the hull design is well known from the Maine lobster boat, which handles offshore seas and conditions well while traveling quickly out of the harbor. A variant of this design used on cruising boats is the Downeast. 

Semi displacement designs are also common on modern “fast trawlers,” as well as on offshore deep-sea fishing boats like those made by Hatteras and Viking. Several brands of well-known motor yachts also use semi displacement designs to keep their speeds up while offering excellent living space.

Semi Displacement Boat Performance

That means that semi displacement hulls are not as fast as planing hulls but faster than displacement-only hulls. For a cruising boat, this is perfect because it provides a skipper with options—the option to go slow and safe fuel or the option to put the pedal to the metal and get there a little faster.

Like their speed, they find a compromise in many aspects of their use and form. 

For example, their hulls are generally shaped to resemble a slightly sleeker displacement hull. That’s good news for a cruising or liveaboard boat because it means lots of space for storage and living accommodations.

Offshore Ride Quality in a Semi Displacement Hull Boat

The design also has enormous implications for ride quality because the semi displacement boat does not inherit either of the nasty tendencies of the other two. It is stable and steadfast like the displacement hull, but it has enough speed to power through the chop and roll that might make the slower boat wallow. 

Likewise, it does not have the same lift on the hull as the planing hull does, so it does not come to the same jarring halt as planing hulls do in big chop. Instead, the boat plows the water out of its way, pushing the waves ahead and forging its own path. 

The result is a smooth ride in many different conditions. A semi displacement design is less likely to be rolled but a seaway, and it’s less likely to experience a stop-and-start jerky ride than a planing hull might. This type of boat is great in rough weather and experiences the least wave motion of any design, so long as conditions allow for the speed to be kept up. At displacement speeds, it is less stable than a full displacement boat.

Have you ever wondered how you would know what sort of hull a boat has? There are a few ways to tell.

If you’re lucky enough to see a boat in a boatyard, you analyze the shape of its underwater features.

• Displacement hulls often have round sides and round bottoms. Their transoms are more likely to be curved or rounded, as well. Displacement hulls tend to be very deep.
• Planing hulls have flat or deep v shaped bottoms with hard chines on the sides to create lift. They often also have strakes–vertical surfaces running fore and aft–that help deflect waterflow and make lift.
• Semi displacement hulls tend to look like displacement hulls in the front, with tall, deep bows, and planing hulls in the back, with flat transoms and a flat aft hull underwater.

If you’re on the water, a great way to tell is by looking at the speed a boat moves and the wake it makes.

• Displacement hull boats and ships move a slow speeds and make very little wake. There is seldom much whitewater in the wake and only a small bow wave that pushes water out of the way.
• A planing hull also makes very little wake, but it is white water pushed out of the way by the hull and powerful propeller.
• A semi displacement hull makes an enormous wake when at cruising speed.

Consider this as you evaluate the various boat hull designs – there is no perfect boat. 

There isn’t even a perfect boat for a particular boater. All boats represent some form of compromise. Perhaps even more surprising is how we grow and change as boaters. The more years that go by, the more you may find yourself appreciating boat designs that once might have turned you off. 

As much as we romanticize them, boats are equipment built for a purpose. As your purpose and use of boats change, so will the sorts of boats that interest you. 

What is a semi displacement hull?

A semi displacement hull can go faster than a displacement hull, but not as fast as a planning hull. In more technical terms, its buoyance and stability while cruising come from both buoyancy provided by the hull and dynamic lifting forces created by the hull moving through the water. 

What type of hull is best for rough water?

Much depends on the boat in question and the preferences of the skipper. The safest boat will likely be the slow displacement hull boat, designed to plow through seas and chop comfortably. A well-designed and well-ballasted displacement hull is exceptionally stable and is self-righting. The limitation of these boats is that they are slow – they are limited to their hull speed. 

Skippers looking for a faster ride but still smooth and comfortable may opt for a semi displacement hull. These boats are not efficient performers in the miles per gallon category, but good ones provide an exceptionally smooth ride in rough chop. The extra speed can also help a skipper time the seas for the best ride and get you out of the soup faster.

What is the most efficient hull shape?

In terms of fuel efficiency, the most efficient hull shape is the displacement hull. This type of boat is limited to its theoretical hull speed. They sit low in the water and push the water out of their way. Speeds less than their hull speed require very little power—and, therefore, very little fuel.

Matt has been boating around Florida for over 25 years in everything from small powerboats to large cruising catamarans. He currently lives aboard a 38-foot Cabo Rico sailboat with his wife Lucy and adventure dog Chelsea. Together, they cruise between winters in The Bahamas and summers in the Chesapeake Bay.

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Email This Page to a Friend Preview: What About a Sailboat’s Displacement? Doug Hylan Discusses Light and Heavy

April 5, 2013

A mong the dockside pundits, the discussion of light vs. heavy displacement usually revolves around the ability of a cruising sailboat to carry the necessary provisions and gear for extended cruising. I would like to consider the question from another angle: appearance and cost.

L ight displacement boats have some real advantages. Up to a certain point, lighter displacement saves money, both in initial cost and continuing expenses. Cy Hamlin pioneered this idea with his Controversy yachts produced at Mt. Desert Yacht Yard in the 1960s. Many people are surprised to learn that boats, like meat, tend to cost by the pound, not the foot. Compared to a heavy displacement 40-foot sailboat, a lighter boat of the same length will require smaller sails, lighter rigging, smaller ground tackle, a smaller engine, and less ballast.

U nder many conditions, lighter boats can also be faster. For one thing, they can cheat the devil of hull speed, even plane if they are very light and have the proper hull shape. This was one reason that MARY ANN, the first Barnegat Bay A Cat, was able to sweep aside the older heavier competition on the bay, even though they sported vastly bigger sail plans.

N at Herreshoff, the Wizard of Bristol, took advantage of both of these factors. One of the many facets of his wizardliness was his intuitive feel for light, stiff and strong construction. His boats generally had lighter structure than the competition, making them both cheaper to build and faster under sail. Even when a rating rule demanded that boats be of a certain displacement, lighter construction meant that more of that weight could be put into ballast. Ballast is not only one of the cheapest components per pound in a boat, but a greater proportion of ballast increases stability, meaning that the boat can stand up to more wind before reefing.

A esthetics, however, is one area where lighter boats have trouble competing. While they may not be able to enumerate the reasons, most people will admit that older boats tend to look more graceful and appealing than their modern sisters. Although several factors are involved, a major one is freeboard, or the amount of hull that shows above the waterline. Older boats generally have less, and just as with cars, lower and sleeker usually looks better.

I n the world of boat design numbers (called hydrostatics), the relative “lightness” or “heaviness” of a boat is defined by its displacement/length ratio, usually abbreviated as D/L. I’ll spare you the formula, but keep in mind that this is a dimensionless number – in other words, it makes no difference if the boat is a dinghy or an ocean liner. If her D/L is 400 she is considered to be in the heavy displacement realm. If the D/L is 150, she is pretty light.

W hy do lighter boats tend to be higher sided? Archimedes who purportedly jumped out of his tub and ran naked through the streets of ancient Syracuse shouting “Eureka!” is credited with the answer. The heavier a boat is, the more there is of it under water. The more there is under water, the less of it there needs to be above water to get the same amount of headroom.

C onsider the two boats shown below, one an old style heavy cruiser (D/L 400), the other a modern light displacement sister (D/L 150). Both boats have an overall length of 33′, a waterline length of 28 feet and a draft of 5 1/2 feet. They both have the same 6-foot headroom, but the cabin sole of the lighter boat must be much higher, so either the topsides or the cabin (usually both) has to be moved up to get the same headroom.

S o, tall people who are unwilling to bump their heads (or bend over to avoid it) can be the ruination of good looks in small, light displacement boats. Daysailers, where headroom is not considered essential, are generally immune to this problem, and once a yacht gets to 60 or so feet in length, there is plenty of height in either type for homo erects.

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Offshore sailboat choices

Editor’s note: There are many types of hull design, rig configuration and material choices for offshore sailboats. We talked to four voyaging couples and asked them about the pros and cons of the choice they made when they decided on a voyaging boat.

Rich and Cat Ian-Frese Tayana 37 Anna Rich and Cat Ian-Frese left Seattle after an 11-year refit of their Tayana 37 cutter, Anna . They started out heading in the general direction of South America, where they eventually took a right turn and crossed the South Pacific. They have refit, cruised and lived aboard Anna since 2000.

Rich has a background in research engineering and spent many years working on research grants in emerging laser technologies. He has worked as a project director on R&D projects for the U.S. Department of Energy, the National Science Foundation and NASA. Cat spent over 20 years teaching elementary school, including special education.

Ocean Navigator: Why did you decide to voyage in a heavy-displacement boat? Rich and Cat Ian-Frese: My wife and I decided to voyage in a full-keel (with cutaway forefront) heavy-displacement monohull, a Tayana 37 cutter rig, because it has a comfortable and easy motion in the ocean. It feels solid and stable and safe in big waves, even in big confused waves — although the comfort factor is noticeably reduced in big confused waves, as it would be on any vessel in rough seas where the waves are big, short-spaced and coming from multiple directions. This will cause bashing into a headwind and heavy, lumpy rolling dead downwind. In our opinion, a heavier displacement vessel will absorb getting knocked around better than a light-displacement monohull.

ON: What are the advantages and disadvantages of this type of offshore boat? R&CI-F: The advantages of a heavy-displacement monohull, like the Tayana 37 cutter, are a very stable and comfortable ride in normal ocean conditions. The Tayana 37 has modest initial stability for a moderately heavy displacement boat, and it will comfortably heel over initially to dig in and pick up speed and add to its waterline length. And then its secondary stability kicks in, which is effective in keeping you sailing at a comfortable heeling angle. We’ve never felt the heeling angle to be unsafe, even in big rolling seas.

We don’t feel that there is any significant disadvantage to a heavy-displacement boat that is used for ocean voyaging, unless speed is your priority. We’ve never been in a situation where not being able to outrun bad weather resulted in a bad outcome. We think that it would be advantageous at times to attain a knot of speed for every knot of wind, like some fast monohulls and catamarans we know of, but we’re happy to slip along at 4 to 7 knots. It seems less stressful on the boat and rig, and less stressful on the crew as well, especially when conditions deteriorate. Besides, we like being on the ocean and don’t place a lot of emphasis on getting from point A to point B in record-setting time. But that’s just us; we can certainly see how hull speeds of 10 to 20 knots can be appealing when you actually have a chance to get ahead of a forecast bad weather system or have simply just had enough rolling on a 3,000-mile passage.

Many heavy-displacement vessels are full keeled. And while a full-keeled monohull like the Tayana 37 has advantages on the ocean — like righting itself quickly if rolled in heavy seas, and having an easy motion in normal seas, an excellent balance of primary and secondary stability, and a modest turn of speed — there is perhaps a disadvantage in close-quarters work.

In marinas where getting into a Travelift dock or a fuel dock involves backing up in a straight line in windy conditions or in a strong current, maneuverability can be a challenge. We could never back up our Tayana 37 down a narrow fairway and expect to end up exactly where we want to go. It’s not in the nature of the Tayana 37. Bow thrusters are, of course, an option, but we didn’t have thrusters on our boat. Many other boats with long keels, other than the Tayana 37, are simply bad when the transmission is thrown into reverse. Occasionally, we are lucky and the wind and current help us along. But generally the less distance we need to go in reverse, the better. Normal docking and turning in narrow fairways is fine. Just don’t expect consistently perfect results in reverse gear in a heavier breeze or fast-running current with some full-keel boats.

We had a Pearson 30 a long time ago. It had a light displacement and a spade rudder. It could turn on a dime and back down a long narrow fairway in almost any conditions. But there are tradeoffs with any boat. We would never think of taking that vessel on a long ocean voyage with its unprotected rudder. When most of your time is spent on longer passages in unpredictable conditions, a good stable boat that you have complete confidence in when the weather goes south is most important.

ON: For voyagers considering a heavy-displacement sailboat, what advice would you give? R&CI-F: We wouldn’t hesitate to sail a heavy-displacement boat. Personally, we’d feel more secure in conditions that were other than ideal. A heavy-displacement monohull doesn’t guarantee comfort in heavy weather, but it will buffer the ride and offer a degree of stability and security that you may feel is lacking in a light-displacement boat. If you want speed and comfort running downwind in the South Pacific, then a catamaran might be right for you. There are light-displacement catamarans and also moderately heavy displacement oceangoing catamarans about 48 to 50 feet in length that could take you just about anywhere that a shorter, stout, heavy-displacement monohull could. We choose the Tayana 37 cutter for its ruggedness in the ocean. It’s comfortable for a short-handed crew to operate, it’s economical at 37 feet and it has never let us down when in heavier ocean conditions over the past 20 years. It’s utilitarian. You might call it the Jeep of sailboats.

Phil and Lynda Christieson Kauri ketch Windora Phil and Lynda Christieson from New Zealand have owned and cruised extensively on Windora , a 43-foot Kauri ketch, for the last 26 years. They circumnavigated with their two sons in the ‘90s and are now completing a six-year cruise in the higher latitudes.

Ocean Navigator: Why did you decide to voyage in a wooden-hulled boat? Phil and Lynda Christieson: When we went looking for a cruising yacht, we were extremely lucky to find Windora , a proven offshore vessel, within our budget. Coming from New Zealand, where wooden boats are built from far superior materials, there was never any question that a wooden boat was inferior, and we learned very quickly that we had an exceptional boat. Being of wood construction, the interior is a piece of artwork, leaving you in no doubt that you are on a real boat, not a floating caravan.

ON: What are the advantages and disadvantages of this type of offshore boat? P&LC: Windora is a composite wooden boat, strip-planked with a heavy layer of epoxy glass fiber. This creates a high-strength, totally dry hull. It provides the most comfortable environment to live in, both in tropical and high-latitude climates. Repairs and maintenance can be done in the most remote places with easily available materials. There are large areas of the planet where you are treated differently because you sail a wooden boat. You are not just another white plastic boat; you stand out in the crowd. A traditionally planked wooden boat cannot be left on the hardstand for extended periods.

ON: For voyagers considering a wooden-hulled sailboat, what advice would you give? P&LC: First choice would be wood-composite construction, which lets you use two-part urethane paint systems, allows for minimum maintenance, and gives you the advantage of being able to store the boat ashore in the most extreme climates. You need to ensure the surveyor has a good understanding of wooden boats, as they are the most complex in construction of all the materials. A poorly built wooden boat can be hugely expensive to put right. Never touch any wooden vessel with iron fastenings in it.

Dave and Sherry McCampbell St. Francis 44 catamaran Soggy Paws Dave and Sherry McCampbell left Florida in May 2007 and headed west across the Pacific via the Panama Canal. They spent eight years getting across in their 1980 CSY 44 monohull. By 2015, they were ready for a faster boat and switched to a 2005 St. Francis 44 MK II catamaran. After four years exploring much of eastern Southeast Asia and a number of significant modifications, they now have the perfect cruising home.

Ocean Navigator: Why did you decide to voyage in a multihull? Dave and Sherry McCampbell: We are full-time international cruisers. There were many reasons we switched from our 1980 CSY 44 monohull to our 2005 St. Francis 44 catamaran. But the bottom line is that it was better suited to our increasing age and desire for more safety, more comfort and less maintenance. Below are the most important reasons to us that we switched. These mirror some of the most important advantages and disadvantages of a catamaran versus a monohull.

ON: What are the advantages and disadvantages of this type of offshore boat? D&SM: Here are what we see as the main advantages.

Level sailing: Cats sail relatively flat, so there is far less fatigue on a passage. Because of this, we can finally read and do computer work most of the time underway. That was rarely possible on the CSY monohull while rolling along at a 10- to 15-degree heel. This is really important for full-time cruisers and not well understood by the monohull cruising community. See the Navy study from a few years ago on page 24 of my presentation link below.

Layout: There is typically about 40 percent more room on a modern cat than on a monohull of similar length. A cat layout is much more cruising-friendly, with daily living, navigation and watch-standing areas up, and bunks, storage, heads and mechanical spaces down. The main saloon and cockpit are on the same level, so there’s no need for a ladder transit between them. It is also easier to access multiple storage lockers along the sides of two hulls instead of one, or searching for things under bunks and in the bilges.

Two engines: Modern cats generally have better speed and fuel economy while motoring. Motoring at 5 knots with one small engine properly loads up the engine and uses roughly half the diesel we used to use with the CSY. That means we can carry roughly half the fuel we used to carry for a 1,200-nm range. Two engines also means a full spare parts inventory is always on board, and there is no drama if one should develop a problem needing repair at sea.

Maintenance: The newer boat and more room mean generally easier maintenance for electrical and mechanical equipment. No teak on deck means no varnish work ever! A more modern rig makes rigging work and sail handling easier.

Unsinkability: Many modern cats won’t sink regardless of damage, due to a thick foam-cored hull, waterproof crash compartments and lack of lead keel. Our cat has a 1.25-inch foam-cored hull and deck, and is advertised as non-sinking unless really overloaded. That is a really comforting feeling while underway in deep water hundreds of miles from land. We think staying aboard is a better option than having to abandon ship into a life raft.

Stability: Cats have better stability at anchor, in a seaway or riding to a sea anchor. Little rolling means better sleep at night. It also means most things left on counters and tables will stay put underway in reasonable conditions. Availability of strong, wide bridle attachment points at the ends of the forward crossbeam reduce yawing and therefore ground tackle loads.

Speed: Most comparisons I have read indicate about a 20 percent speed increase on long passages. We rarely want to go more than about 8 knots, and we start reefing at about 7 knots. This compares to reefing at 6 knots on the CSY. We are comfortable at 7 to 8 knots on the cat. In the open sea, we consider anything more than about 9 knots uncomfortable due to increased boat motion and rig loads.

Sail handling: Wide, flat decks with little roll mean safer sail handling and reefing at sea. The jib is relatively small compared to the main, so it is easier to handle than on most monohulls. Also, no pole is required for downwind sails or a spinnaker.

Dinghy storage: Cats offer much safer and more convenient dinghy storage if lifted on high davits aft between the hulls. Typically, modern cats allow the dinghy (with motor on) to be taken out of the water easily and launched quickly. There is no need to remove the outboard and store the dinghy on the foredeck before making a passage.

Draft: The cat’s shallow draft gives many more anchoring options. This is especially important if looking for that mangrove-lined, unoccupied tropical cyclone hole. The ability to do a free haulout for repairs or a bottom paint touch-up on a beach is a huge advantage. It is easy to do on many beaches with just a few feet of tide.

Most of the monohull vs. catamaran comparisons, as well as catamaran features, are well covered in our PowerPoint presentation, “ Evaluating Modern Catamarans ,” available on our website.

ON: For voyagers considering a multihull, what advice would you give? D&SM: There is no perfect catamaran with all the features you may want, so be prepared to compromise somewhat. However, knowing what works and what does not for the cruising you plan to do is important. Be sure to research this carefully before starting to look for a cat.

With the number of catamarans being produced on the rise, there are many designs to choose from. However, not all are created equal. Although most cruisers spend 90 percent of the time in port, due consideration should also be given to features that enhance safety and comfort at sea. Most catamarans are optimized for tropics cruising and are probably not the best choices for high-latitude voyaging.

There is plenty here to consider before purchasing a cruising catamaran. Much more is on the Internet. Many modern cats are built for the lucrative charter trade and may have features that don’t work well for full-time bluewater cruising. Some of these can be corrected or improved, some cannot. Be suspicious of exaggerated dealer claims, ask for proof of anything that doesn’t seem right, and ask specific questions. Consider making a list of what to look for before going shopping.

As with almost all cat owners I’ve talked to now that we have made the switch to the “enlightened side,” we would never go back.

David Content & Roslyn Stewart Aluminum sloop Barefoot David Content and Roslyn Stewart have been sailing their boat, Barefoot , in the Pacific for eight years. Roslyn has previously sailed in Papua New Guinea and northern Europe. David has sailed extensively in the North and South Pacific. 

Ocean Navigator: Why did you decide to voyage in an aluminum boat? David Content & Roslyn Stewart: My present voyaging sailboat is Barefoot , a 43-foot aluminum boat designed by Angelo Lavranos and built by Dearden Marine. I chose an aluminum boat after having already sailed more than 50,000 ocean miles in an excellent 36-foot, IOR-design fiberglass boat. Most influential in the decision was wanting a solid, strong boat that was still reasonably lightweight and would withstand the stresses and wetness experienced for weeks at a time on offshore passages.

I learned from cruisers sailing metal boats that they did not develop leaky chain plates, slack stays and shrouds, water intrusion in the rudder, loose keel bolts or squeaky interior liners at the bulkheads, even after thousands of ocean miles. For me, an aluminum boat was a better choice than steel because steel boats require a watertight paint coating on all metal surfaces. I wanted to avoid the paint maintenance, and I desired the lighter weight of an aluminum fabrication.

ON: What are the advantages and disadvantages of this type of offshore boat? DC&RS: An aluminum boat is difficult to paint and doesn’t need it above the waterline. On Barefoot , the unpainted topsides are gray and protected by a naturally developing oxide coating. Some sailors perceive this as an advantage, while others prefer to paint the topsides for cosmetic purposes.

An obvious advantage of aluminum is that the robust cleats, chocks, handrails, stanchion sockets, chain plates, anchor rollers and genoa track bases are all permanently welded directly on the deck. In addition to being extremely strong, no potentially leaky deck penetrations exist. Inside the boat, aluminum frames and stringers serve as solid attachment bases for the interior handrails, autopilot ram, steering cable sheaves, and the generator and watermaker mounts.

Non-obvious advantages of an insulated aluminum boat are numerous. One example is that the insulated hull pulls the cabin and bilge temperature toward the water temperature rather than the air temperature. This means in the tropics we’re able to stow fruit and veggies in the cool bilge, and that no deck canopy is needed to keep the cabin cool, even in the tropical sun. Conversely, when sailing at high latitudes, the water temperature is warmer than the air and that keeps the boat naturally warmer.

Two disadvantages of aluminum boats are the cost of construction and the watchfulness required to avoid corrosion problems. Well-built aluminum boats are more expensive than production fiberglass sailboats. Aluminum fabrication costs are based on the material expense of proper aluminum alloys for hull plating, frames and stringers, and on labor costs for skilled aluminum welders. The construction process is more labor intensive than fiberglass boats.

Potential corrosion sources in aluminum boats are well understood by builders these days. Following best practices in electrical wiring and equipment isolation from the hull eliminates most corrosion risks. An aluminum boat owner can prevent corrosion by learning a few unique requirements. For example, an isolation transformer must be wired in the boat if connected to shore power; a charcoal filter must be used when filling an aluminum water tank from a city chlorinated water supply; never moor the aluminum boat next to a steel boat; never use chlorine cleaners for anything (use vinegar); and always have installed and regularly check an LED light indicating a fault from a connection between the hull and the positive or negative side of the battery.

ON: What is your advice for voyagers considering an aluminum vessel? DC&RS: They should follow the same process as with the considered purchase of any boat: Talk to sailors who have a similar boat, inspect as many aluminum boats as possible, and consider having a custom boat built by selecting a good naval architect and finding a small, experienced aluminum boatyard for the build.

By Ocean Navigator

What Is Sail Area Displacement Ratio?

Last Updated by

Gabriel Hannon

August 30, 2022

The sail area-displacement ratio is one of the simplest ways to measure the expected performance and speed of a sailboat.

While there is no single number that can encapsulate the performance of every different type of sailboat, the sail area-displacement ratio, or SA/D, provides a relatively robust way to discuss the relationship between sail power and weight that determines many aspects of a boat’s acceleration, maneuverability and performance capabilities.

This article will discuss the methods of calculating the sail area-displacement ratio, what it tells us about a boat’s performance, how it can be used to compare and categorize different boats, and some of its potential shortcomings. As boat building technology and modeling software advance, there are more significant and complex ways to measure performance characteristics. This is becoming especially prominent with the rise of foiling boats and other non-displacement hull plans. Still, the SA/D is a simple, back-of-the-envelope calculation that you can do to ballpark the performance of a boat.

Over years of racing, working with sailors and suppliers, I know how important it is to have a sense of a boat’s performance abilities. This can mean the difference between a single overnight trip and a long slog through the doldrums. Especially as you go to buy a new boat, having a sense of the performance range that you want and ensuring that the boat you purchase has these characteristics will make your decision much easier.

Table of contents

‍ Calculating the Sail Area-Displacement Ratio

For those of us fortunate (or perhaps unfortunate) enough to have had to memorize the concept of a ratio in the 5th grade, calculating the sail area-displacement ratio is a relatively straightforward endeavor. You need to take a measurement of the sail area, which is generally given in square feet, and a measurement of displacement and divide them. There are, however, light complications with both measurements. Here is the formula, and you can read on in this section if you want to understand the various nuances in the calculation and derivation of it, or skip to the next section if you just want to understand what it means for your boat.

When calculating the sail area, there are different points of view on how to accurately measure effective sail area. As the interactions between mainsail, overlapping or non-overlapping headsails, and off-the-breeze sails are not always the same depending on the point of sail. Because the different setups utilize these areas differently, relative sail plan measurements do not always represent how much sail area is directly being used to power the boat. Comparing the sail area-displacement ratio on a plan with a 150% overlapping genoa or a non-overlapping jib by simple square footage will favor the former, even if they have the same effective power. This doesn’t mean that you can’t use the SA/D to compare boats, but you just want to make sure that both boats are using the same method of measurement and that you account for such sail plan differences. Regardless, the sail area component, which will be the numerator in our ratio, is generally just equal to SailArea(ft2), indicating that it is measured in square feet.

The fundamental value of the denominator is a little bit more mathematically confusing, as it requires some conversions in order to return the final ratio. The displacement of a boat is simply equal to its weight, which can be given in pounds. We want our ratio, however, to be unitless, so we have to do some transformations. First, we realize that a displacement in pounds causes the boat to displace a certain volume of water, which we can measure in cubic feet. A single cubic foot of water weighs approximately 64 pounds, i.e. it has a density of 64 lbs/ft3. Therefore, if we want to determine the cubic footage that a boat of a certain weight displaces, we would have to divide its weight by that density. This gives us a converted value for displacement in cubic feet. Finally, we convert cubic feet to square feet by raising the measurement to the power of 2/3, which leaves us with a ratio of square feet over square feet. With this, we can finally perform our SA/D calculation.

Performance Implications and Categorization

The point of the SA/D is to give you an intuitive sense of how a boat will perform under sail. Sail area is one of the best sailing analogues for horsepower. Still, boats are much more dependent on the costs of these larger sail plans--longer, heavier, wider--and so sail area does not tell you as much about a boat as horepower does about a car. This is why we need to incorporate displacement as a shorthand for how big the boat is, and then use the ratio between the two to discuss performance.

Boat Categories

In general, sail area-displacement ratios float around the high teens. Heavy, casual cruising boats tend to be in the range of 15-16, while high performance racing boats can push over 20. A SA/D of 20 is generally accepted as a cutoff for modern performance cruising and racing hulls.

There are helpful niche-types within this larger range, however, so we will discuss the breakdown of what each ratio range should tell you about a boat in which you might be interested.

<15: Motorsailors and Auxiliary Sailboats

Boats in this range are never meant to operate purely on sail power, and cannot really be expected to make much headway under sail alone. With a good breeze, you can certainly save a little on gas, but you will not be cutting through the harbor with any particular excitement.

15-16: Average offshore cruiser

These boats, while a little bulky, need that bulk to handle the rigors and strains of offshore sailing. This is your first level of acceptable performance for making way under sail power alone.

16-17: Coastal cruiser

Sleeker and with more aggressive sail plans than their offshore counterparts, cruising hills  SA/D range starts pushing you into the sporty range. You can expect decent maneuverability and fresh speeds, but you won’t be breaking and sea-speed records, and you may well want to stay within sight of land.

17-19: Racing yachts

Once you get into the high teens, SA/D figures tend to point towards racing hulls with minimal capabilities for cruising and distance sails. These boats are engineered to go fast, shave off every bit of weight possible, and do everything they can to cut through the water. This means getting slimmer and lighter, both to reduce the displacement, in terms of weight, and the sheer amount of ‘wetted surface’ in the water, which introduces drag. It will be hard to say much about a boat’s relative performance within this range, however, as racing boats tend to have more factors effecting their performance at the slimmest of margins, but this range is, nonetheless, a good indicator that you should not expect to be taking such a boat for a long cruise.

20+: Performance racers, Dinghies, One-Design Sailboats

Once you get into the 20s, you may be presented with one of two options. Either you have made it into high-performance racing mono- and multi-hulls tuned for racing, or you have found your average sail around dinghy. The Laser, the Olympic Class singlehanded sailboat and also one of the most common sailboats in the world, has a SA/D of 47.3 by a quick calculation, but this certainly does not mean that it is the highest performance boat in the world: it just means that it is very light! A J-70, a widely competitive racing monohull has a more reasonable SA/D of 24.7, and is a good example of demonstrating a fast, light, and snappy boat that at least resembles the others on this list.

Shortcomings and Alternatives

As with any attempt to compactify an incredibly complex problem into one or two numbers, the SA/D has its share of shortcomings. In particular, it can tell you very little about how a boat will perform on the racing front and is unlikely to help you truly distinguish between the performance of boats within a small range. A boat with an SA/D of 17.2 may just be a fast cruiser, and the foiling boats used in the America’s Cup may well be so far off the charts that it is not even relevant to measure the SA/D for them (in fact, the AC72’s, by a quick calculation, have an SA/D of 81, which is not even on the charts).

Other measurements of performance have come to prominence in an effort to replace the SA/D. Yacht designers use velocity prediction programs to model the hydrodynamics of hull shapes, weights and sizes against sail plans across various windspeeds. Performance Handicap Racing Fleet (PHRF) ratings are tabulated based on years of empirical data from racing times to give an appropriate handicap for timed races, which does a good job of leveling the playing field across boats and giving you a sense of how fast or slow a boat is in a vacuum. Most boat outside the racing field, however, are not tabulated in such systems. Polars demonstrate the expected speeds for a boat at different wind angles and velocities, though they are dense and hard to read graphics.

So there is no single metric that can be used to determine a boat’s performance. Critics of the sail area-displacement ratio will claim that it is a number that does not tell you anything, especially as it can be manipulated by boatbuilders to sell their newest line even if it is not representative of a boat’s true performance. Still, even if it is not useful at the edges of the competitive racing field or can be subject to questionable sail area accounting, the rough categorization bands are useful for a quick summary of a boat’s fundamental characteristics. You would never buy a car just by reading its horsepower, but it is still good to know it in a pinch.

Happy sailing!

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I have been sailing since I was 7 years old. Since then I've been a US sailing certified instructor for over 8 years, raced at every level of one-design and college sailing in fleet, team, and match racing, and love sharing my knowledge of sailing with others!

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Hull speed is a phenomenon of displacement boats, and not of planing boats. Most sailing boats and all ships displace water—move it aside—as they plow through it. Planing craft, such as most motor boats, glide over the top like a surfboard. It takes more energy to push water aside than it does to slide over the top of it, and so displacement boats move at a more sedate pace than their lighter planing cousins. Some small sailing boats can be made to plane, but the general rule is that sailing boats are of the displacement type. Hull speed is usually an upper limit to the speed of displacement boats.* It is unsurprising that such a limit exists: we have seen how drag increases with speed, and so sooner or later drag will balance out the drive force and a sailboat will not be able to go faster. Yet there is a surprise in store for those of you who are not familiar with sailing: the hull speed of a given boat depends on its hull length at the waterline. It is not obvious from a simple consideration of drag why this should be so, but it is a well-attested fact, often quoted in the sailing literature, that the maximum natural speed of a displacement boat (in knots) is 4/3 the square root of waterline length in feet.

A key feature of the phenomenon, again well known to any sailor, is that hull speed has been reached when the bow wave of the boat lengthens to the waterline length. At lower speeds, there may be three or four complete waves seen to lap along the boat hull, but this number decreases as the boat picks up speed and reaches, pretty closely, one complete wave by the time the boat reaches her hull speed. It may be possible for her to go faster than hull speed, but this requires a disproportionate amount of effort. In other words, the hydrodynamic drag

* There is one trick by which a small displacement boat can exceed hull speed without expending enormous effort, and that is by surfing. Riding along the front of a wave is not the sole preserve of surfboards.

Figure 6.1. (a) Your hull-speed raft, viewed from above. Note the direction of motion. (b) When the bow wavelength is less than the distance between the long beams, drag is reduced compared to the case of (c). In (c) bow wavelength equals the distance between beams because the aft beam is more submerged. So hull speed is reached when hull length equals bow wavelength. Consequently, hull speed is limited by hull length.

force that is acting to hold back the boat increases rapidly once hull speed is reached. My goal in this section is to explain to you, in simple physics terms, why these phenomena occur.

Which is why I have press-ganged you into service onboard the undignified vessel illustrated in figure 6.1. She is a wooden raft with two long logs fore and aft that stretch way beyond her beam. These logs are not there to provide flotation, please note—we will suppose that the raft has enough buoyancy without them—but rather to illustrate hull speed. You set the primitive sail and drift off to the right. The forward log generates a bow wave which spreads out in the wake, as waves do. You notice something that you have seen many times before in other craft: the bow wave size (amplitude) increases as the vessel speed increases. This makes sense because the hull is pushing water aside, the displaced water has to go somewhere, and the faster you go, the more water is moved. So the wave size increases. Now you pick up speed, and so the wavelength of the wake, as observed alongside your hull, stretches out until exactly one wave lies between the two extended logs at bow and stern. The raft speed that gives rise to this condition is her top speed, you

Figure 6.2. Your hull-speed barge. Bow waves forward of the center of gravity, CG (open circle) exert a buoyancy force (vertical arrows) proportional to wave height that acts to rotate the barge hull counterclockwise. Similarly, waves aft of the CG act to rotate the hull clockwise. If we can assume that drag forces are proportional to counterclockwise torque (a dominant CCW torque means that the barge is climbing a hill created by its bow wave), we can show that hull speed occurs when bow wavelength equals hull length.

find. It is clear why: the aft log is now submerged, and so experiences more drag than it did earlier, when there was no wave crest at the hull stern (see fig. 6.1). So, drag force peaks when bow wavelength equals hull length, in this simple example.

Now we are able to see where the old formula for hull speed comes from. The speed of a bow wave, or of any other surface water wave,1 is c where c2 = gk/2p. Here l is the water wavelength, and g is the constant acceleration due to gravity. Now the raft speed, v, equals the water wave speed, c, so that v = VgL/2p (since hull length, L, equals water wavelength at hull speed, as we just saw). Substitute numbers and we arrive at the old formula.

The ungainly raft has served her purpose, and you can now abandon her. The lesson learned is intuitive, and yet it gives us a basis for understanding quantitatively what hull speed is about. Now I can do another calculation, this time a little more realistic. The math is more involved (you need not wade though it), but the basic idea is again quite intuitive. Figure 6.2 shows the profile of a steep-sided hull plowing through water and generating a bow wave, which oscillates along the line of the hull. This vessel is kept afloat by the buoyancy force, and we can see that the buoyancy force is going to be different at different points along the line of the hull because the wave height varies along the hull. Buoyancy that acts forward of the hull CG (shown in fig. 6.2) will create a counterclockwise torque that tends to twist the hull about the CG—trying to make it do a backflip. The buoyancy force aft of the CG produces a torque that acts in the clockwise sense. These two more or less cancel* but not quite. If the counterclockwise buoyancy torque is just a little bigger than the clockwise torque, the boat will tilt backwards, until her stern goes deep enough to generate a compensating torque. We would then be left with a boat that is going uphill, trying to reach the crest of her own bow wave.

Where am I going with all this? Roughly speaking, counterclockwise torque equates to uphill motion, and uphill motion leads to increased drag, for reasons that will soon be made clear. So, I am saying that increasing the unbalanced counterclockwise torque generated by a bow wave will increase drag. If this increase should suddenly take off at a certain speed, then we have found our hull speed. In fact, I can calculate the torque generated by the bow wave. You can see that as the bow wavelength changes, the torque will also change because the manner in which buoyancy force is distributed along the hull length changes with wavelength (fig. 6.2). The results of this calculation are plotted in figure 6.3. (For those interested, the math is provided in this endnote 2 in sufficient detail for you to reproduce the calculation.2) In figure 6.3 we see once again that drag force takes off for water wavelengths exceeding hull length, more or less.3

For simplicity, the hull of figure 6.2 was given vertical sides, but most boats don't have vertical sides, for a host of reasons. Recall that, in the Age of Sail, ships of the line were given a tumblehome cross section to deter boarders. Nowadays we are less likely to have to repel nefarious enemies swarming over our gunwales with cutlass in hand, casting a single bloodshot eye (the other being patched) in search of our gold doubloons. Hull sides are angled but the other way, with cross sections resembling a martini glass rather than a brandy glass. In plain language: more V-shaped. Here are some physics reasons for different hull cross sections.

——'Rounded hull bottoms are stronger than V-shaped hulls, but the latter will be deeper for the same displacement and so will better resist leeway.

*Just as well, because backflipping boats would be pretty uncomfortable.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 Water wavelength / L

Figure 6.3. Hull speed is limited by drag. In the simple model described in the text, the drag increases with water wavelength, l, as shown (L is hull waterline length). Here, drag force is set arbitrarily to 1 at zero speed. If the bow wave is assumed to have constant amplitude, independent of speed, then drag changes with speed as shown. For a more realistic model, with bow wave amplitude increasing with speed, the curve looks similar. In this simple model, hull speed occurs at l « 1.2L because for longer waves (higher boat speed) the drag force becomes too strong.

•—A large deck area is desirable, but large hydrodynamic drag is not. For a hull of a given displacement, the choice of hull shape is constrained by the trade-off between these two characteristics. •—'An angled hull—say one that is V-shaped—will have greater reserve buoyancy. That is, the righting moment will increase as the hull heels further and further. •—'During heeling, the waterline along an angled hull will not be symmetric about the longitudinal axis; the port side waterline length and shape will be different from that on the starboard side. This asymmetry can assist the boat to head up while heeling. Thus, even without aerodynamic assistance from her sails, a boat may automatically

point to windward when heeling solely because of hydrodynamic forces acting on the hull. •—'Different angled hull shapes beneath the waterline assist with planing. For certain boats, such as racers, this is important because planing requires less displacement, less wetted area, and so less drag—and hence increased speed.

The physics of angled hull shapes casts an interesting light on the capabilities of some ancient ships . Certain ancient ships were built with a lot of overhang at the bow and stern, but this practice is usually thought to have been of little value for the old square-riggers because these ships were supposed to be nippy only when running or on a broad reach. Today, such hull shapes are utilized to increase hull speed while heeling because the waterline length is increased when the hull is heeled over. This lengthened waterline increases boat speed on a beam reach, for example. It seems plausible to suppose that ancient vessels with overlapping bows and sterns may have been capable of traveling across the wind at speed. Indeed, such a hull design offers no other advantage for these square-rigged vessels. (An overhanging bow and stern increases deck area, but for merchantmen—and in ancient times most of the sailing ships were merchant vessels because warships were oar-powered—deck area was not such a big deal. Volume of the hold was what mattered.) For a downwind point of sail, extended hull length above the waterline will increase pitching motion when traveling downwind; this is bad, and yet the overhanging bow and stern must have conferred some advantage or these ancient ships would not have been built this way.

Continue reading here: Keel Appeal

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Readers' Questions

Is it possible for a displacement boat to exceed hull speed?

No, it is not possible for a displacement boat to exceed its hull speed. Hull speed is the theoretical maximum speed that a displacement boat can reach, and it is determined by the length of the waterline. When a boat exceeds its hull speed, it starts to climb up on its own bow wave and create excessive drag, making it difficult to go any faster.

What can you say about the speed of a boat that makes a bow wave?

The speed of a boat that makes a bow wave is usually quite fast, as the bow wave is usually associated with a boat moving at high speeds.

How to calculate hull speed?

Hull speed, also known as displacement speed, is the speed at which a boat hull moves through the water. It is calculated by taking the square root of the waterline length of the boat in feet and dividing it by 1.34. The formula is: Hull Speed = √LWL / 1.34 where LWL = waterline length in feet.

What is maximum hull speed for a boat?

The maximum hull speed for a boat is typically 1.34 times the square root of the waterline length of the boat in feet. For example, the maximum hull speed for a boat with a waterline length of 20 feet would be about 24 knots (1.34 x √20).

Why catamarans sail faster than hull speed?

Catamarans sail faster than hull speed because of their unique hull design. Their twin hulls provide greater stability and lift than a single hull, which results in less drag on the boat. This reduced resistance allows the boat to move more quickly through the water, resulting in higher speeds than what is normally achieved with a traditional hull design. Additionally, the width of the catamarans hulls also distributes the weight of the boat more evenly, which further reduces drag and increases speed.

Boat Hull Types, Shapes & Designs

10 Common Types of Boat Hulls

Displacement hulls, planing hulls.

• Flat Bottom
• Tri-Hull (Tunnel Hull)

Semi-Displacement Hulls

Multi-hulls.

Explore Different Boat Types

Large ships, some trawlers and traditional recreational sailboats have displacement hulls. They are slower moving but quite steady under way and are capable of carrying large loads with relatively small propulsion units. Displacement hulls are usually round on the bottom with ballast placed low in the center. At rest, round hulls tend to roll with the waves and swells.

Most powerboats and personal watercraft have planing hulls that ride on the water at higher speeds. They behave like displacement hulls at low speed but pop up onto a plane usually around 15-16 MPH depending on the design and load. Planing hulls come in a variety of shapes, each of which has it benefits and disadvantages.

Flat Bottom: Flat-bottomed boats are very stable and can carry a heavier load. They require only a small engine to get on plane but can ride rough and wet in chop or heavy weather. Small aluminum or fiberglass bay and fishing boats often benefit from flat hulls, which have a shallow draft and provide a good amount of deck space both of which are ideal for fishing on calm bodies of water such as small lakes and ponds or slow rivers.

V-Bottom: Deep V hulls cut through waves and ride smoothly in chop. They take a bit more power to push up onto a plane, tend to roll or bank in sharp turns and due to the angle of the hull, have less interior volume for stowage or accommodations. Fast, distance fishing boats like center consoles tend to have a V bottom so they can run fast on open water to get to the fishing grounds quickly.

Tri-Hull or Tunnel Hull: Popular with fisherman as well as with sport boat enthusiasts, tri-hulls, also called cathedral hulls, have a combination M-shaped bottom. They’re quite buoyant and stable and they get on plane quickly. They offer good volume below and significant deck space above. At speed, they tend to pound when they encounter choppy water so they’re ideal for lakes of calm bays.

Pontoon: Pontoon boats ride on (typically) aluminum tubes. Traditional pontoons have two tubes but newer designs have three and are called tritoons . Pontoon boats are all about deck space and make excellent boats for families and entertaining on the water. The newer tritoons can carry large outboards and so they’ve become planing boats capable of towing for water sports or reaching distant fishing spots.

Learn More: Tritoon vs. Pontoon

Semi-displacement hulls combine rounded sections for increased storage and tankage, and flatter hull sections to partially lift the forward part of the hull out of the water, thereby decreasing drag at high cruising speeds. They generate large bow and stern waves and may need high horsepower engines to get on plane. Larger, cruising motor yachts lean toward the semi-displacement design.

Boats with separate and distinct hulls are called multi-hulls and can be catamarans or trimarans. Multi-Hulls can be either power or sailboats and have displacement or planing hulls depending their shape and the size of their engines.

Catamarans: Catamarans have two hulls with a deck or trampoline in between. Their benefits include excellent stability and depending on size and type, significant living space aboard. Large cats (35 feet and over) have become popular in charter use because they offer more interior and deck space and an easier motion to induce less seasickness. With two engines, catamarans are very maneuverable but they do require more room to turn and berth. Small catamarans usually have just a trampoline in between the hulls and make fun daysailers.

Trimarans: Trimarans are often (but not always) sailboats. They have three hulls: a main hull and two amas (side hulls used for stability). On some smaller trimarans, the arms that hold the amas can fold inward, making the trimaran narrower and in some cases trailerable. Trimarans require smaller engines and they sail faster primarily due to the reduced wetted surface (the area in contact with the water), which cuts down on drag.

When choosing a boat type, consider your primary use for the boat and let that guide you to the optimal hull shape.

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Unveiling 13 Sailboat Hulls: Navigate Waves Like a Pro

Ever wondered why sailboats cut through waves like butter? Dive in and discover 13 hulls that make your sea adventures unforgettable!

A sailboat is only as good as the hull, and it ultimately determines how well you can navigate through the water. The hull of a boat plays a massive role in what type of water you can sail through and your overall speed. So, what are the types of sailboat hulls and how are they different?

The main types of sailboat hulls are planing hulls, displacement hulls, and semi-displacement hulls which offer the best of both worlds. Multi-hull boats such as pontoons and tritoons have even weight distribution and can handle rough waters. Flat-bottom sailboats are the most stable, but they don’t work well in deep waters.

Catamarans and trimarans feature space between each hull which adds stability and protects the deck from water. Choosing a sailboat with the ideal hull for you is essential in finding one that you will keep for years to come. Follow along as we explore the different types of sailboats and see what makes them unique.

Sailboat Hull Types

There are 13 types of sailboat hulls ranging from bilge keels and fin keels to displacement hulls. The ideal sailboat hull varies for you based on factors such as what type of water you’re in and weather conditions.  For example, some hulls, such as flat bottoms, are ideal for shallow and smooth water .

On the other hand, semi-displacement hulls are perfect for every application whether you’re in shallow or deep water. Let’s take a look at the different types of sailboat hulls and see how they differ.

1. Planing Hull

Sail Magazine

Planing hulls are the first of the three major categories of sailboat hulls. You can find planning hulls with 2 different shapes: v-shaped and flat-bottom hulls.  Planing hulls sit on top of the water and don’t sink deep like other types .

Many boaters and enthusiasts prefer this design because of how well a boat with planning hulls can move across the water.  Most fishing sailboats feature planning hulls because of how smoothly they can glide on the surface whether you’re on an ocean or lake . Boats with planing hulls can also move faster than other types of boats, and that is their main appeal.

2. Displacement Hull

Improve Sailing

Boats with displacement hulls are slower than boats with planing hulls, but that doesn’t mean that they’re bad . While they don’t move as fast, many boaters consider displacement sailboat hulls to be much smoother.  This comes in handy if you live in an area with rough waters and strong winds .

A displacement hull is rounded instead of flat at the bottom like a planning hull. The main downside to sailboats with displacement hulls is that you will likely use more fuel than you normally would. That is because the shape isn’t as aerodynamic and you’ll need the extra engine power to move through the water.

3. Semi-Displacement Hull

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As the name suggests, semi-displacement hulls combine the best of both worlds between planing and displacement hulls. A semi-displacement hull is both flat and rounded at certain parts providing both speed and stability.  They aren’t as fast as a flatter planing hull, but they’re faster than a standard displacement hull .

The unique shape of semi-displacement hulls helps reduce resistance. This alone can help take a load off of your engine and let it work optimally under most water conditions. You also get the benefit of extra storage in most cases because boats with semi-displacement hulls have storage-friendly floor plans.

4. Multi-Hull

Multi-hull boats, such as pontoons and tritoons, are smooth and easy to sail . There are separate hulls on each side of the boat that provide stability and let you power through rough waters. On a pontoon, each hull is a large tube filled with air known as a toon.

Multi-hull boats generally sit higher above the water than most boats because of their unique design. They are popular for fishing, cruising, and entertainment.  A key downside to multi-hull boats is that they typically operate loudly because the propeller may not be fully submerged in the water .

5. Monohull

The vast majority of sailboats that you will come across have a monohull . They are easy to sail, transport, and even dock at a marina because of their simple design. As the name suggests, they only feature one hull and are suitable for calm and rough water.

You can save money with a monohull sailboat compared to a multi-hull sailboat like a catamaran. A key advantage to monohull sailboats is that they are incredibly safe. You don’t have to worry about capsizing as much as you would with a multi-hull sailboat.

6. Flat-Bottom

Sailing Magazine

Flat-bottom hulls are essentially the simplest form of planning hulls. You can find flat-bottom hulls on the majority of sailing dinghies, and that’s what they are most suitable for.  They aren’t ideal for oceans or rough waters, but flat-bottom sailboats are perfect for rivers and lakes .

Rowboats also feature flat-bottom hulls, and they aren’t known for being particularly smooth. You get less precision with flat-bottom hulls, especially if you have to steer and turn unexpectedly. Otherwise, you won’t have trouble with a flat-bottom sailboat hull if you go out for a quick fishing trip in an area you’re familiar with that has smooth waters.

7. Catamarans

Catamarans feature a unique take on the traditional multi-hull design . They feature 2 hulls with space between them that usually features a deck. Sometimes, the space between each hull features a trampoline or even a small pool or tub.

They aren’t suitable liveaboard boats, but they are perfect for taking out for a day of cruising and fishing. Catamarans are as smooth as possible, but that sometimes comes at the cost of speed. However, they often feature multiple engines which can consume a lot of fuel but also put less strain on each engine.

8. Trimaran

Quiberon 24 Television / Youtube

Trimarans are essentially a step up from catamarans because they feature a third hull. Many people prefer the stability that trimarans offer over catamarans.  The extra stability also helps increase the speed that you can cruise at with a trimaran .

They are also safer than catamarans because the multi-hull design allows for perfect weight distribution. That’s not to say that catamarans are unsafe, but the extra hull that trimarans feature is more durable.  Most of the weight lies on the center hull and the rest is distributed between the 2 outer hulls .

BlueWater Yacht Sales

Deep v hulls are another type of planing hull, but they are less common than some of the other varieties. Granted, high-end modern powerboats often feature a deep v hull, but they come at a high price.  The v design allows the hull to cut into the water easily which lets you easily control the boat in any type of water condition .

Generally, the deadrise goes between 21 and 26 degrees for a deep v hull which is ideal for many boaters. However, boats with deep v hulls are primarily geared toward anglers and aren’t ideal for cruising at high speeds. Deep v hulls are usually made out of aluminum which means that they will be loud as they glide across the water.

10. Bilge Keel

Bilge keel hulls are specifically designed to reduce the risk of a boat rolling . The strange shape of a bilge keel hull lets it stand upright whether you’re on the shore or in shallow waters. This makes them much easier to maintain than many other types of boats.

The bottom of a bilge keel hull features multiple fins in a row that helps ensure a smooth ride. They never feature more than 2 keels which means that they have a shallow draft and you can easily beach them. The one downside to sailboats with a bilge keel hull is that they are difficult to transport to a port because of the bottom.

11. Bulb Keel

Bulb keel sailboats feature a teardrop-shaped ballast that increases the boat’s stability.  They are even faster than bilge keel sailboats because of how hydrodynamic they are . Unlike some types of hulls, a bulb keel works just as well on the sea as it does on lakes and rivers.

They feature incredible weight distribution because of the inclusion of an extra ballast. However, you have to be careful with bulb keel sailboats in shallow waters near the shore. They are more susceptible to damage at the bottom so they can be difficult to bring to shore and require precision.

12. Fin Keel

Jordan Yacht Brokerage

Unlike bulb keel and bilge keel hulls, fin keel sailboats are perfect for raising . Fin keel hulls improve the draft of a sailboat which comes in handy when you want to reach high speeds. Some sailors use fin keel sailboats to travel long distances across the water, especially if the weather is in their favor.

They are a perfect happy medium between flat and round-bottomed hulls offering the best of both worlds.  Fin keel sailboats are also quite comfortable because of the unique bottom shape that can easily handle choppy waters . With that said, they aren’t ideal for beginners because they can be difficult to steer compared to standard flat-bottom hulls if you are inexperienced.

13. Cathedral Hull

Jeff Clark / YouTube

Cathedral hulls get their name from their appearance which is similar to that of a classic cathedral. The unique appearance is one of the biggest benefits of cathedral hulls because the whole boat takes on that shape. They feature sharp bows and high sterns that are immediately recognizable.

With that said, cathedral-hull sailboats can be difficult to steer compared to flat-bottom or rounded hulls because of their bulky shape . You get plenty of storage with cathedral hulls which makes them perfect for long day trips with many people. They are incredibly stable because of the wide beams and wide berth, so there isn’t a serious risk of capsizing as long as you pack your cargo well.

What Type of Hull is Best For Rough Waters?

Any type of boat with a v-shaped hull is best for rough waters. Whether it’s a deep v or shallow v, this hull design makes it easy to cut through rough water without getting too much on your deck.  The last thing that you want is to go through rough waters and take on excess water weight onboard .

They are specifically designed to glide across the water without sinking low which is necessary for choppy waters. Boats with v-shaped hulls also often come with high-performance engines, so they offer the best of both worlds.  Generally, deep v hulls are the most precise and smoothest when it comes to rough waters .

They are perfect for keeping course which is essential if you’re in rough waters that can kick you off of your path. You can find many deep v hulls that are made out of fiberglass which is incredibly durable and withstand water exposure. Fiberglass v-shaped hulls are also easy to repair either by yourself or at a professional shop at a low cost.

What is The Most Stable Hull Design?

Flat-bottom hull sailboats have the most stable design for shallow water and multi-hull boats are the most stable in deep water. The inclusion of multiple hulls adds stability in deep water that prevents water from landing on the deck. This can save you expensive repairs and can also prevent your sailboat from capsizing.

Pontoons and tritoons are multi-hull boats and they are specifically popular because of their stability.  You can’t find a more stable design than a flat-bottom hull if you plan to cruise in shallow waters . Flat-bottomed hulls are also typically the fastest when you aren’t far from shore, especially if you are in smooth waters.

The box shape of flat-bottomed hulls is conducive to gliding across shallow water. However, they struggle to ride across waves and choppy waters because of the wide surface area. Multi-hull boats such as pontoons and tritoons are the best option if you plan to take your boat out to lakes, rivers, and oceans because they thrive in any scenario.

What is Better Flat Bottom or V-Hull?

Flat-bottom boats are better than v-hull boats for most uses, but v-hulls are better in choppy and deep waters. You can get by with a flat bottom in oceans and lakes alike, but they don’t always do well in deep water.  Conversely, v-hull boats can tear through rough and wavey water even at steep depths .

V-hull boats don’t do well in shallow waters and you are more likely to get stuck than you would be with a flat-bottom boat. Of course, you can always have someone push from behind when you depart, but that doesn’t help much when you return to shore.  V-hull boats are the better option for deep waters, however, even if you are in rough water .

Flat-bottom boats take on more water than v-hull boats unless you stay in calm waters. It’s always worth choosing a boat that won’t take on water that will weigh it down. However, if you’re looking for a reliable boat with a high capacity, then I would recommend looking into v-hull boats.

What Type of Hull Cuts Through Water?

Displacement hulls are the best at cutting through the water, especially when compared to planing hulls. They don’t rely on a powerful engine to cut through the water because of their design. Displacement hulls displace water once you lower them in from the shore.

This displacement isn’t ideal for speed, but it is perfect for rough waters and strong winds.  You can take sailboats with displacement hulls out on the ocean without having to worry about waves . They also work well in freshwater, but they are less necessary because of the lack of waves compared to the ocean.

Otherwise, you can get the best of both worlds with a semi-displacement hull . They aren’t quite as precise as displacement hulls, but they are better at navigating choppy waters than planing hulls. You sacrifice a little bit of speed, but the shape of a boat with a displacement hull lets you power through waves without veering off of your path.

How Long Do Sailboat Hulls Last?

Sailboat hulls last for an average of 15 years, but many of them can last for 20 years or longer. It ultimately depends on how well you maintain them and how often they are in the water.  For example, a sailboat that you always keep in the water and rarely store in a dry place may need hull repairs and replacement much sooner .

Sailboat hulls are susceptible to algae damage, and that is more likely if you always keep them in the water. Cleaning the hull of a boat is essential to protect them from algae and examine them for potential damage.  Fiberglass is the best material for a boat hull, even compared to aluminum which was the standard for years .

Fiberglass hulls can last for up to 50 years or more with regular cleaning and maintenance. A sailboat’s hull won’t last as long if it suffers damage neglecting maintenance or hits the shore too fast. The best way to increase the longevity of your sailboat is to take it out of the water every once in a while and scrub the hull to remove algae.

How Do You Inspect a Sailboat Hull?

The best way to inspect a sailboat hull is to take it out of the water and clean it . You can easily inspect a sailboat’s hull if it is clean and dry, or else you will mix cracks and dents. Cracks are the most important thing to look out for because it’s best to catch them early on.

You should be concerned if you come across cracks because they can eventually worse and threaten your boat’s structural integrity. This is especially true if you have a sailboat with an aluminum hull that you regularly take out onto saltwater.  Aluminum can eventually break down in saltwater so it’s important to inspect it regularly, especially after 10 years or more .

The hull is the first part of a boat that you should inspect because hull damage can cause a boat to sink. Always inspect your boat’s hull if you sail too fast in shallow water because that is when you risk the most trouble. Clean and dry your boat’s hull, then follow along it closely to look for spots that don’t glisten as much. This will indicate a weak point, scratch, tear, or dent.

Fastest Sailboat Hull Design

Multi-hull, trimarans, and flat-bottom boats feature the fastest sailboat hull designs . They are hydrodynamic which lets them glide through the water with minimal resistance. Trimarans move incredibly fast, especially in salt water, as long as they aren’t weighed down with too much cargo.

Careful packing reduces the necessity to haul less cargo because trimarans have incredible weight distribution. However, factors such as water conditions and what type of body of water you are on ultimately play a huge role.  Boats move up to 2% faster when in saltwater than in freshwater no matter which type of sailboat hull design you have .

Other factors such as your boat’s capacity and how much cargo you are carrying make a huge difference as well. Any type of boat with a planing hull is a safe bet if you want to move quickly through the water. Avoid sailboats with a displacement hull if you value speed because they often move the slowest.

So, What Are the Types of Sailboat Hulls?

The three main types of sailboat hulls are planing hulls, displacement hulls, and semi-displacement hulls.  Bilge keel, bulb keel, and fin keel hulls are similar but have different practical applications between freshwater and saltwater . Trimarans and catamarans feature sturdy hulls that are highly regarded for their even weight distribution and roomy storage.

Multi-hull boats such as pontoons and tritoons sit above the water and can withstand rough waters because of how high they sit. Displacement hulls are the best option if you need to cut through choppy waters and maintain your routing. Otherwise, consider a flat-bottomed hull if you primarily stay in shallow water because of how stable they are.

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Yachting Monthly

• Digital edition

Busting the hull speed myth

• julianwolfram
• December 13, 2021

Waterline length is not the defining factor in maximum boat speed that we all think it is. Julian Wolfram busts the hull speed myth

Modern hull forms, like this Jeanneau SO440, use chines to create volume forward while keeping a narrow entrance at the waterline

Every sailor is delighted when the breeze picks up and the boat really starts to get going with a bone in her teeth.

Julian Wolfram is a physicist, naval architect, former professor of ocean engineering at Heriot-Watt in Edinburgh and a Yachtmaster Offshore who has cruised and raced for 45 years

The crew will want to know how fast she will go and perhaps surreptitiously race her against any similar sized boat in the vicinity.

Speculation may start about what allows one boat to go faster than another – is it the hull shape or the sails?

It is easy to spot good, well-trimmed sails but what about the hull ?

The important part is not visible below the water surface. However there is one key indicator that is often very apparent – the waves generated by the sailing yacht.

When a yacht picks up speed the wave pattern around it grows and the greater the speed the bigger the waves .

The energy in these waves is proportional to the square of their height – double the height and the energy goes up by a factor of four.

This energy comes from the wind , via the sails and rig , making the hull push water out of the way.

If less of this wind energy was wasted in producing waves the yacht would go faster.

When a typical displacement monohull reaches a speed-to-length ratio of around 1.1 to 1.2 (speed in knots divided by the square root of the waterline in feet) up to half the wind energy driving it is usually wasted in generating waves.

The hull speed myth: Half angle of entrance

So how can we tell if a yacht will sail efficiently, or have high wave resistance and waste a lot of energy generating waves?

The answer starts back in the 19th century with the Australian J H Michell.

In 1898 he wrote one of the most important papers in the history of naval architecture in which he developed a formula for calculating wave resistance of ships.

Light displacement cruising boat: The bow of this Feeling 44 is finer than older cruising boats

This showed that wave resistance depended critically on the angle of the waterlines to the centreline of the ship – the half angle of entrance.

The smaller the angle the smaller the height of the waves generated and the lower the wave-making drag.

A knife blade can slice through water with minimal disturbance – drag the knife’s handle through and you generate waves.

The big hull speed myth

For a displacement hull the so-called ‘hull speed’ occurs when the waves it generates are the same length as the hull.

This occurs when the speed-length ratio is 1.34.

It is claimed that hulls cannot go significantly faster than this without planing. It is called ‘the displacement trap’ but is a myth.

Heavy displacement cruising boat: An older design has a bow that is several degrees wider

As an example, consider a 25ft (7.6m) boat that goes at 10 knots in flat water.

This is a speed-length ratio of two. That is the average speed over 2,000m for a single sculls rower in a world record time.

The reason for this high speed is a half angle of entrance of less than 5º. Hobie Cats, Darts and many other catamarans have similarly low angles of entrance and reach even higher speed-length ratios with their V-shaped displacement hulls.

These hulls also have almost equally fine sterns, which is also critically important to their low wave resistance.

The monohull problem

Now a monohull sailing yacht needs reasonable beam to achieve stability and, unless waterline length is particularly long, the half angle of entrance will inevitably be much larger than those on rowing skulls and multihulls .

In his 1966 Sailing Yacht Design Douglas Phillips-Birt suggests values of 15º to 30º for cruising yachts.

Many older cruising yachts with long overhangs and short waterline lengths, for their overall length, have values around the top of this range.

Busting the hull speed myth: A Thames barge is a similar length and beam to a J-Class, but its bluff bow, built for volume, makes it much slower. Credit: Alamy Stock Photo

Newer sailing yachts, with plumb bows, have somewhat smaller half angles and a modern 12m-long fast cruiser may have a value around 20º and a racing yacht 17º or 18º.

Size matters here as, to achieve stability, a little yacht is likely to have a bigger half angle than a large one, such as the German Frers-designed 42m (138ft), Rebecca which has a half angle of entrance of under 13º.

Rebecca also has a fine, elegant stern which helps minimise the stern wave – I’ll come back to sterns and stern waves.

Interestingly the half angle of entrance is not mentioned in the otherwise excellent 2014 Principles of Yacht Design by Larsson et al, although it is currently used as one of the parameters in the preliminary estimation of wave resistance for ships.

While it is still particularly applicable to very slender hulls, naval architects are not generally familiar with Michell’s work.

His formula for wave resistance involves quadruple integrals of complex functions.

German-Frers’ designed Rebecca has a half angle of entrance of just 18°. Credit: Cory Silken

These are not ‘meat and drink’ for your average naval architect, and only a few mathematically inclined academics have much interest in theoretical wave resistance.

Michell’s work is rarely, if ever, covered in naval architecture courses now.

Nowadays the emphasis is much more on numerical methods, high-speed computers and computational fluid mechanics (CFD) using the so called Navier-Stokes equations.

Examining these equations, which apply to any fluid situation, does not give any insights into wave resistance, albeit they can model wave resistance very well when used in the piecewise manner of CFD.

It is very easy to measure the half angle of entrance at the design waterline when a yacht is out of the water.

Take a photograph directly upwards from the ground under the centreline at the bow.

Busting the hull speed myth: Multihulls achieve high speeds due to fine hulls, light displacement and ample stability. Credit: Joe McCarthy/Yachting Monthly

Now blow this up on a computer screen, or print it off at a large scale, and measure the angle with a protractor.

Alternatively, if you have a properly scaled accommodation plan drawn for a level close to the design waterline this will yield a reasonable approximation of the half angle of entrance.

Unfortunately there is not a simple relationship between the fineness of the bow and the wave drag.

But, all other things being equal, the smaller the half angle the better.

It is easy to measure and is a useful parameter to know when comparing yachts.

Stern shape and hull speed

The half angle of entrance cannot be taken alone as a measure of wave drag, and the fairness of the hull and in particular the run aft is also critical.

Just as the half angle of entrance dictates the height of the bow wave, so the fineness of the stern is a key influence on the height of the stern wave.

Consider the water flowing around both sides of the hull and meeting at the stern.

Modern race boats, like Pip Hare ‘s IMOCA 60, combine a fine angle of entrance with wide, flat hulls for maximum form stability and planing ability. Credit: Richard Langdon

If these streams meet at a large angle the water will pile up into a high stern wave.

On the other hand if they meet at a shallow angle there will be less piling up. A fine stern can maintain a streamline flow of water.

However if the sides of the hull meet at the stern at a large angle then the streamline flow will tend to separate from the hull, leaving a wide wake full of drag-inducing eddies.

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In many modern designs the hull sides are not far off parallel at the stern and it is then the upward slope of the buttock lines that are critical and, again, the shallower the slope the better from a hull drag perspective.

The slope of the buttocks can easily be measured if the lines plan is available and a good indication can be obtained from a profile drawing or a photo taken beam on with the boat out of the water.

Drawing a chalk line parallel to the centreline and half a metre out from it will provide a buttock line that can be checked visually for fairness when the boat is viewed from abeam.

A rowing scull easily exceeds its theoretical max hull speed. Credit: Alamy Stock Photo

Again, the smaller the angle the better – provided the transom is clear of the water.

An angle of more than 17º will lead to separated flow and eddy making. This also happens if the transom is immersed.

The greater the immersion the greater the drag, so weight in stern lockers on modern boats can be critical.

Modern hull design

The modern wedge shape attempts to resolve the conflicting demands of a small angle of entrance, good stability and a fine stern.

The plumb bow extends the waterline forward and, with the maximum beam taken well aft, the hull forward can be relatively narrow, providing a low half angle of entrance.

The stern is wide, which helps achieve good stability, but at the same time the buttocks rise slowly at a shallow angle to the water surface.

This gives a smooth and gradual change in the hull’s cross section area ensuring the water flow remains attached to the hull and that the stern wave is kept low.

A modern cruising boat gains stability from a wide stern, but needs twin rudders

This wide, flat stern also helps surfing down waves and possibly planing.

Some designs have chines just above the design waterline which increases usable internal volume and gives a little more form stability when heeled.

However, as soon as the chine is immersed there will be separation along the chine edge as water will not flow smoothly around a sharp edge.

It is just not possible to get the chine perfectly aligned with the streamlines of the water flow in all sailing conditions and there will be some extra drag at times.

There are two downsides to the wedge- shaped hull.

Overloading aft will create a large increase in drag

First the boat has to be sailed at a small angle of heel to keep the rudder properly immersed and to avoid broaching. This can be offset to some extent by using twin rudders .

The second is that the weight must be kept relatively low.

This is because a relatively small increase in weight causes a big increase in wetted surface area at the stern and hence in the frictional drag which makes the boat slower, particularly in light airs.

This is the downside of slowing rising buttocks and the reason why dinghy sailors get their weight forward in a light breeze .

Displacement Length Ratios

Traditionally for sailing yachts the displacement-length ratio has been used as a measure of speed potential, partly because it is easy to calculate from the yacht particulars.

It is waterline length (in metres) divided by the cube root of displacement (in cubic metres or tonnes).

A heavy boat, such as the Heard 35, will have a value of about 4 to 4.8.

A more moderate displacement boat, such as the Hallberg Rassy 342 or Dufour 32 Classic, will have a value in the range 5 to about 5.5; whilst a racing boat may a value of up to, and even over, 7.

A heavy displacement cruising boat with a fair run aft is less affected by additional weight

However the displacement length ratio can be misleading as making a hull 20% deeper and 20% narrower will keep the displacement the same but will significantly reduce the half angle of entrance and the wave drag.

It is interesting to note a Thames barge in racing trim has the same length-displacement ratio as a J class yacht, but their speed potential is vastly different.

Finally I should mention the older ‘length-displacement’ ratio, which is quoted in imperial units.

This is calculated by dividing a boat’s displacement in tons (2,240 pounds) by one one-hundredth of the waterline length (in feet) cubed.

Credit: Maxine Heath

It is still used in the USA and should be treated with caution.

The myth that your boat’s speed is only restricted by it waterline length does a disservice to its designers, and does little to help you understand how to get the best from her when the wind picks up.

Have a look at how the boat is loaded, how you sail on the wind, your boat handling and how much canvas you ask her to carry and you may discover more speed than you expect.

The remarkable John Henry Mitchell

Pioneer of wave theory

It’s worth saying a little more about the remarkable John Henry Michell.

He produced a series of ground-breaking papers including one that proved a wave would break when its height reached a seventh of its length.

He was the son of Devon miner who had emigrated to the gold mining area near Melbourne.

He showed such promise that he got a scholarship to Cambridge.

He was later elected a fellow of the Royal Society at the age of 35 – not bad for the son of a Devonshire miner.

His brother George was no slouch either – he invented and patented the thrust bearing that is named after him.

The half angle of entrance became the traditional factor for assessing the fineness of hulls.

It is defined as the angle the designed waterline makes with the centreline at the bow.It varies from less than 5º for very fine hull forms up to 60º or more for a full-form barge.

At higher speeds, modest increases in the half angle can give rise to substantial increases in wave resistance.

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A Closer Look at the Rationale Behind the Ratios

Sailboat performance varies based on the eye of the beholder. Racers want light-air alacrity and a willingness to plane while cruisers want directional stability and reasonable speed with moderate sail area. A boats design dimensions and a few simple ratios give some hints about these attributes.

Most performance-oriented boat designs focus on around-the-buoy racing; these assume that there will be a large crew to keep sail area and trim optimized and to provide movable ballast and that the boat must cope with windward and leeward legs of a race course. Offshore, point-to-point race boat designs are greatly influenced by racings handicapping rules. In either case, these rules penalize performance, and design factors like ultra-wide beam-intended to leverage crew weight for righting moment-are of little value to shorthanded cruisers. Make sure that the boat youre looking for is fast in the context under which it will be sailed.

The sail area-to-displacement ratio (SA/D) compares energy and resistance-much like a horsepower-to-weight comparison in an automobile. As the SA/D ratio grows higher, so does the vessels potential speed under sail. However, too much sail area and too little righting moment means a very tender boat. Too little sail area and too much displacement means you can brag about carrying full sail in 20 knots, but your boat will move like a sea buoy in 7 or 8 knots of breeze.

The ballast-to-displacement ratio (B/D) of a boat tells you how much secondary righting moment to expect from the keel. The smaller and lighter the vessel, the more important it is for this number to be higher for stability as well as for performance reasons. Bulbs and other keel-tip shapes lower the vessels center of gravity (CG) and can lessen the need for a 40-percent B/D ratio. A deeper draft can also lower the CG and can improve on-the-wind performance.

A boats displacement-to-length ratio (D/L) has a lot to do with the resistance of a hull shape moving through the water, and since skin drag is the big enemy at lower speeds, the D/L ratio tells us a lot about a boats light-air performance. By increasing the boat length and keeping the displacement the same, decreasing displacement, or doing both, the D/L ratio decreases, and the boat will go faster in light air. Wave-making kicks in as the major resistance at higher speeds, and the implications of the D/L ratio lessen.

In a nutshell, when it comes to performance under sail, light displacement is fast; deep-keel boats point higher and sail more efficiently to weather; full, flat sections aft cause a boat to plane sooner; and more sail area delivers more power. When it comes to delivering the goods in an open-ocean context, seakeeping ability is an important factor in performance as is the amount of punishment the boat and crew can endure.

Ordered by sail-area-displacement (SA/D), this table illustrates a progression from a heavy-displacement cruiser (Westsail 32) to a fast and light racer (Farr 400). Note how the designers altered ballast, sail area, and displacement to reach their goals.

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