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Steel Boats: A Strong Alternative

By Geoff Payne
Updated: August 7, 2002

Adventure cruising down Chile’s exciting southern waterway, we chose to make a side trip up one of the many fiords. Like most, this one was uncharted. “Must be as deep as the hills are high around us,” Margaret and I agreed. The crew of an approaching local fishing boat waved enthusiastically as we tacked from shore to shore against a fine breeze.

Those fishermen were really waving their hands in the air at us. “Guess they’ve never seen a sailboat with such good windward ability,” I thought as we left them rapidly astern. If we hadn’t dusted them so completely, perhaps we would have seen their hands go down onto their heads and then over their ears. Full sail and at some seven knots of boat speed, 13 tons of Skookum plowed onto a pile of sharp glacial boulders lurking below the surface.

Was the boat holed due to this colossal blunder? Was the keel parted from the hull? Was the rudder torn off? Was that the end of our cruise? Well, there was a loud bang, we felt the cockpit rapidly rise then suddenly fall, but on we sailed, red-faced and with sails luffing to slow us down. Skookum’s full keel tapers down to a 2 1/2-inch-diameter solid-steel bar. That and the heavier keel plating probably made more impression on the rocks than the rocks did on us. A boat of other material could have sustained trip-terminating damage. Once again, my decision to build in steel had paid off.

Joshua, Williwaw, Damien II: These famous steel cruisers bring to mind high-latitude epic trips, often among ice. But steel is not just for extremist cruisers. As a matter of fact, the finest steel hulls are passing you by, indeed overtaking you under sail. Only you don’t realize they’re made of steel.

On Skookum’s cabin table is a small offcut from the hull plating. It’s often passed around among visitors aboard. “Sure is strong,” folks say, “but isn’t it heavy?” Skookum displaces 28,000 pounds, which is on the heavy side for a 40-footer. But there are respected fiberglass and wood designs of similar size that are heavier still.

They say lighter is faster; it’s true. When it comes to extended cruising, though, the rules are not that simple. A cruising family accumulates literally tons of weight aboard. A serious cruising boat easily could contain over 4,000 pounds of fluids, spares, tools, literature, outboards, and other provisions. That kind of weight stresses and hampers a lightweight craft. The modern steel-hulled cruising boat will be of generous displacement, proportions that will accept a large payload without loss of performance.

Your cruising boat must have good performance. It should tack smartly, carve along to windward at six or seven knots and surge before the trade winds, leaving a straight white wake. Any properly designed, medium- to heavy-displacement sailboat with a big spread of well-cut sails ought to have sparkling performance. (The 1970s saw some successful steel racers.)

Margaret and I have short-tacked slalom courses up narrow buoyed channels, eased sheets to race afternoon sailors, and logged some 180-mile days at sea in Skookum; all this with a displacement/length ratio of 450. That bit of weight in our steel construction equals a lot of comfort on the ocean — especially in rough weather.

No one doubts steel’s strength. It takes over 30,000 psi of force to deform it. Steel is an “orthotropic” material; that means it’s equally strong in all directions. Try to bend or rip a circle of steel any which way you like, it won’t give in any easier. A piece of wood will split down the grain. Wood is thus an “isotropic” material — stronger in one direction. Isotropic materials (including fiberglass, which has extra thickness roving here and there) are very efficient for boatbuilding because their strengths can be aligned to counteract predictable forces of water and wind. This results in a far lighter structure. Even though a piece of deck steel need only support your dainty weight, the minimum practical plating would still take thousands of pounds to pierce.

The end result in steel is an enormously strong structure. Skookum’s mast and rigging loads are so well resisted at deck level that our lee shrouds barely slacken, even under full sail and hard on the wind.

Of major concern to the designer of an all-steel sailboat is the weight of the steel superstructure. Don’t expect to find apartmentlike accommodations inside a steel-decked offshore cruiser, for that would raise the center of gravity unacceptably. The thickness of any framing also represents lost volume inside a steel hull. Used to good effect, it becomes valuable insulation, covered with paneling. Because structural bulkheads are seldom required in steel hulls, cabin layout can be very flexible. Free of constraints inside Skookum, I created a wide-open, bright and light-filled interior. Without fail, newcomers aboard remark upon how roomy the boat feels belowdeck.

Steel sailboat design has come a long way in recent years. Two aspects of the preceding paragraph have come under rigorous review as designers and manufacturers constantly come up with better steel craft. First, the superstructure need not be made of steel. Secondly, in some cases, the framing can be done away with.

Once upon a time a steel hull might have looked pretty much like a wood one under construction: ribs galore. To support large expanses of steel of minimum thickness and to keep it beautifully smooth and fair, light framing (transverse and longitudinal) is definitely required. Building a fair, curve-plated, round-bilged metal hull is a skill and an art — a task for the professional boatyard. However, if the curved cross section of a sailboat is approximated in straight lines, then the plating of the whole hull is considerably simplified. The fore-and-aft joint lines between plates are called “chines.” The smooth-looking sphere that is a beach ball is actually made up of once-flat tapered strips; each seam is a chine. Done right, a multi-chine steel hull is both easy to build and puts a sweet curve or two along the topsides of that vessel. But it’s a challenge on the drawing board. Done poorly, the chines appear sudden and awkward and make for a boxy looking sailboat.

Chines also introduce lines of strength into the hull (a bit like the way a floppy sheet of paper folds into a sound little aircraft). This has led designers to say, “Aha. Maybe we don’t need the frames!” Indeed it can be done, and there are plenty of such designs available. Chined, frameless hulls do require heavier plating, so there is no great weight savings. “Frameless” construction is a hotly debated topic among metal-boat designers and builders. In fact, Skookum’s chines are strengthened by longitudinal stringers, and floors in the keel provide transverse support. To completely forego all framing yet still adhere to responsible engineering principles would render a small boat heavy indeed.

Having mastered the multi-chine concept, designers and builders saw the opportunity to go one better: eliminate at least those chines visible above the waterline by introducing a “radius chine,” a narrow curved piece of steel that disguises any sudden turns in the plating. So long as that piece of steel can be cut from either a cylinder or a cone, the task is not too hard. In fact, the whole hull can be plated in “conically developed” shapes (frames required though). Steel boats like these are the ones sailing right by you looking like molded fiberglass.

Steel can be worked into nearly any shape imaginable. Clipper bows, canoe sterns, deep fin keels, tumblehome topsides, bowsprits or reverse transoms can all be achieved at commensurate cost. The skeg supporting the steel cruising boat’s rudder can be made so strong that the arrangement could hardly be considered vulnerable to damage by floating objects. Skookum’s stern even incorporates a welded tab and stopper arrangement to support the rudder in the event of the hove-to boat being thrown backward by a big sea.

Welding allows the creation of fabulous custom work on deck. Stainless steel bollards, chain plates, towing eyes, lifting lugs, vents and fillers can all be elegantly incorporated into the deck in an utterly waterproof manner.

Even the thinnest practicable steel plating (about 7/64-inch, or 12-gauge) is too heavy a material for a sailboat much less than 30 feet in length, hence you’ll find few really small steel cruisers. Any thinner plating creates problems with welding, maintaining a fair shape and corrosion tolerance.

Stock plans in steel for popular-size (35-foot to 45-foot) cruising boats generally show a medium- to medium-heavy displacement craft with average internal accommodation. These plans cost from $500 to $1,500, reflecting a wide variation in the amount of information given. Full-size templates for plating are even available with some designs. Very serious consideration should be given to the selection of the design: The one to two percent of the finished value of your project that you invest in plans could be 100 percent responsible for ultimate success…or disappointment. And you won’t find out until the first day’s sail. Designers’ work is best not to be messed with — generally it’s not on the page if it’s not important. A custom design in steel could run to 10 percent of the boat’s value.

Chined construction, a method that greatly simplifies hull plating, is an attractive option for amateur builders. With little more than a welding machine and good cutting and handling equipment, a steel hull can be backyard built. Once I had learned how to handle the long pieces of steel properly, I found the hull construction to be most rewarding. Sparks flew, there was smoke and grit, but in essence it was a bit like sewing: I made Masonite patterns for each strake, traced around them and cut the material to shape, tacked it onto the upside down temporary frame, then finally seamed it all together. Welding is so immensely and immediately strong that I was as convinced then of the boat’s colossal strength as I am now, 50,000 miles later. Full-strength welding meant I could carry out 100 percent corrections of occasional cutting errors.

What About Corrosion?** Talk of steel and the word “rust” comes up straightaway. Rust is a chemical reaction and salt water speeds it up, but not as much as you would think. Ice scraped the paint off Skookum’s waterline about a meter back from the bow. Although I didn’t get around to touching it up until nearly a year later, no major harm was done to the plating. What worried me more was rust inside the hull, in the hidden corners of the bilges. Only after four years of hard sailing did we remove the cabin sole (wisely, I made it all demountable) and after a thorough scrub, we found areas of scratched paintwork. Nothing serious, nor structural — just awkward to sand and touch up.

The steel deck, unlike the hull, is not only continually doused in salt water, but also trafficked and abraded. Anchors, chain, winch handles, harbormasters’ boots — they inevitably knock off paint. Very soon, out weeps a trickle of brown. But at least you can see it! Unlike rot or ultraviolet deterioration or osmosis, rust gives itself away practically the day it starts. It’s not difficult to remedy, just tedious.

On Skookum we have some nuisance rust spots that repeatedly need rubbing back and touching up. Repainting means a full four or five coats of touch up, so the process is a protracted one. In every case, these bits of rust around hatches, coamings, stanchions and winches could have been avoided had I done things differently in the first place. Companies well experienced in steel boat production have developed excellent detailing on deck.

Given that recurring rust problems occur on deck, and that a boat doesn’t sail upside down, why then not construct the deck of something else? It’s called composite construction and it’s commonplace. Strong plywood decks and cabins can be built over steel framing. Epoxy and fiberglass take care of the sealing and finish. Aluminum decks can be married to steel hulls. Composite construction has other merits, such as less weight and less magnetic interference. (Tons and tons of steel certainly have an effect on a compass. Our classic five-inch-diameter steering compass stands on its own binnacle and was some 20 degrees off upon installation. Standard correctors inside the unit reduced this to a known five degrees on east and west headings. Electronic compasses can have sensors placed inside the mast or on a radar post and thus removed from steel’s magnetic clutches.)

Corrosion comes in another and more wicked form: electrolysis. Put nearly any other common metal underwater near steel and a battery current flows. More often than not it is steel that loses the electrons. Little volcanoes of corrosion erupt on unprotected steel, and these inflict damage much faster than rust. Electrolysis is a threat to any kind of boat, but especially to metal-hulled ones.

All steel craft sport little zinc pads on the keel, rudder and propeller shaft. These “sacrificial anodes” corrode instead of the hull, so must be maintained. Corrosion vigilance is the price one must pay for the reassuring strength of steel.

The corrosion specter heavily devalues older steel boats, especially if a bit of the brown stuff is visible. Boats that have not had the protection of modern paint systems might be picked up, for a “steel.” If you’re planning to recondition an older steel craft, first establish if you can gain access to all the steel surfaces. Even then the cost of dismantling, preparation and recoating will be considerable.

Coatings Offer Excellent Protection** Rust and electrolysis can only get a grip on bare steel. Coatings have advanced in recent decades and offer excellent protection. One system coats the sandblasted steel with coal tar combined with epoxy. Another paint is substantially zinc. Or, the whole boat can be “flame sprayed” with aluminum or zinc — the ultimate treatment (see the “Save The Steel” sidebar, following). Most seagoing steel is protected mainly by epoxy paint. As many as nine coats go on — primer, high builds, hard and gloss coats (polyurethane). It’s a significant investment in paint, but very effective and attractive.

That little piece of plating that we keep by the cabin table to show visitors was cut out of the finished transom. The thickness of paint buildup seen in the cross section is impressive. In fact, we’ve taken to saying we’re sailing around the world in an epoxy boat lined with steel. These paints ought to last a very long time. As long as the steel remains coated, our boat is going to be around longer than we are.

One other coating proven on steel hulls is sprayed polyurethane foam insulation. About 1 1/2 to two inches of this closed-cell substance, sprayed inside from the turn of the bilge up and over the deckhead, transforms a clammy, tinny chamber into a quiet refuge, cozy or cool as required. It’s superb insulation that retains or repels heat, eliminates condensation, dampens deck noises and sticks tenaciously to (lightly painted) steel, keeping air and water from ever initiating interior rust.

There’s an image of steel boats being dank and clammy belowdeck. Perhaps those that are uninsulated are that way. In fact, the coziest and sweetest smelling cabins I’ve experienced have been aboard steel craft. The Mexican “lancha” drivers used to think we were “locos” to live inside a black steel hull…until they came below and found it to be airy and cool. Spray foam has kept Skookum comfortable to live in at all latitudes.

The Security Issue** Steel sailboats are over-engineered — for wind and water forces, that is. Can the vessel to which you entrust your family’s life be too strong? What if on a calm and sunny day you tied up, went into town, and returned to find the local ferry had T-boned your boat into a concrete wharf? It happened. That metal sailboat completed its circumnavigation — a bit dented, that’s all.

In or out of the water an all-steel sailboat with polycarbonate hatches also will be a formidable barrier to burglars, even bullets. If seacocks connect to metal standpipes extending above the waterline and shafts have metal stuffing boxes, then a steel craft might survive an internal explosion or fire. Charred, but still floating.

Our boat’s nearly invincible strength had to become our insurance policy in far southern latitudes — no underwriter would cover us. So why are we shopping around for coverage now that we’re back in busier, foggier waters? Afraid of being run down? On the contrary — with 13 tons of momentum, our pointy-ended boat could sink something 10 times its size. We need liability insurance.

With such strength and so solid a feel, by providing so smooth a ride, the well-appointed steel sailboat is a Mercedes Benz of ocean cruisers. Cost is not in the upper luxury level, especially if you are home-building, for which steel is well suited. Extra expenses for the rig and proper sail area to drive such a sturdy craft is why a performance, steel sailboat is not going to be the cheapest option. In value, appearance and performance steel cruisers are right in there with equivalent-size boats made of other materials. And if it comes to the c-r-u-n-c-h, they’re incomparably stronger.

Before next you stroll the marinas, put a fridge magnet in your pocket. Slide it on to some really pretty boats — you might just get a surprise. Marine steel craft have come a long way in recent years.

———————————————————————— After taking a couple of years off from cruising to build a house, to research a biography of yacht designer (and uncle) Alan Payne and to fill the cruising kitty, Australians Geoff Payne and Margaret Hough are planning to take Skookum next summer for a tour of the Canadian Maritimes and beyond.

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9m Steel Catamaran; thoughts?

Discussion in ' Boat Design ' started by rustybarge , May 20, 2016 .

rustybarge Cheetah 25' Powercat.

Hi All, Can steel make a good materials choice for a small 30' Cat for home building, , and what plus/minus points does 3mm plating pose? http://www.bodenboatplans.com/popup_image.php?pID=23&image=0 http://www.bodenboatplans.com/popup_image.php?pID=23&image=1 BB260 "Steel Catamaran 9 Metre" $595.00 Length 9.0 m Displacement 6500-7500 kg Material Steel Hull Draft 0.85 m LWL 7.9 m Hull Weight 4500 kg steel Beam 3.8 m Max Speed 12 kn Motor capacity 2 x 60 hp Fuel Type Diesel Fuel Capacity 700 litres 100mm x 50 x 3mm cross beams folded plate Fabricated frames 3mm flanged plate. 30 x 3 mm stringers. 2.5 mm superstructure . . A 9 metre steel power catamaran for long distance economical cruising. The alternative way of spacious live a board cruising under power. Also available in aluminium construction and a 4 stroke outboard version in steel or aluminium. An exceptional design. There is nothing else like this seagoing cat on the internet Steel, or steel extended version , also aluminium construction with extended cabin and flybridge Single chine catamaran Hull tunnel to waterline is 300mm loaded and 370mm light Propeller dia 400mm. Fresh water 200 litres. Sewage tank 60 litres. Intended for living aboard for extended periods in the Med. where provisions are available on a weekly basis. This craft was designed so that all nominated fitting out items including tanks etc. were available from the Vertus catalogue, which is readily available world wide. Accommodation is two double berths forward, galley and dinette in wheelhouse saloon. The cockpit is relatively large and is ideal for relaxing outdoors. The shower/toilet space is unusual in that the toiler is located at a lower level on one side and the shower on the starboard side. There is internal entry to each of the machinery spaces with larger maintenance hatched in the cockpit. Sub division with bulkheads make this a very safe boat. The engines shown in the plan are 60hp Dutz as shown in the Vertus catalogue At first we thought that a 9 metre steel catamaran would not be practical. A preliminary design established that providing it was built in 3mm plate,with fabricated frames and tunnel beams,a satisfactory design was possible. The hulls are deep to carry the additional weight and the craft is designed to run at displacement speed with initial estimates indicating a top speed of 14 knots and a very economical cruising speed of 10 knots. Particular attention has been given to the machinery installation with the hull aft designed to accommodate the propellers, reduce draught and enable the catamaran to sit on the bottom. The original craft was designed in 2002 for use cruising the Greek islands. The advantage of steel is that the craft can be built in the open and shot blasted and epoxy painted on completion of the steel fabrication. Click to expand...

Stumble Senior Member

Let's see.... The design displacement is roughly four times what a 30' cat should weigh, and the speed is pretty marginal. Setting aside the difficulty in welding steel this thin, you would be far better buying a 30' fiberglass sailing catamaran and adding bigger engines. It would be pretty inefficient compared to a designed power cat, but still far better than this.... Thing.

Stumble said: ↑ Let's see.... The design displacement is roughly four times what a 30' cat should weigh, and the speed is pretty marginal. Setting aside the difficulty in welding steel this thin, you would be far better buying a 30' fiberglass sailing catamaran and adding bigger engines. It would be pretty inefficient compared to a designed power cat, but still far better than this.... Thing. Click to expand...

fredrosse USACE Steam

Actually no difficulty welding this steel thickness with a wire feed machine. Stick welding would be difficult for me, but I am no professional welder. With a quality MIG machine even I could weld very well for a thin steel hull. Cutting the steel with an abrasive wheel is also easy, with virtually no distortion. Steel weight for 3mm thickness is about the same as 35mm plywood, whereas in plywood you could use 20mm thickness. So the hull plating in steel is considerably heavier than with plywood, but most of us know that already. Steel hulls, with a good coating system, can hold up very well, and if that is your preference, then proceed to get what you want.

fredrosse said: ↑ Actually no difficulty welding this steel thickness with a wire feed machine. Stick welding would be difficult for me, but I am no professional welder. With a quality MIG machine even I could weld very well for a thin steel hull. Cutting the steel with an abrasive wheel is also easy, with virtually no distortion. Steel weight for 3mm thickness is about the same as 35mm plywood, whereas in plywood you could use 20mm thickness. So the hull plating in steel is considerably heavier than with plywood, but most of us know that already. Steel hulls, with a good coating system, can hold up very well, and if that is your preference, then proceed to get what you want. Click to expand...

Richard Woods Woods Designs

I doubt if you will get 14 knots with twin 60hp on a 7.5T catamaran. Has someone achieved that speed with one? I recall you have posted before about the powercat you were building. So I am pretty sure you know my Skoota 28, which is, as Stumble says, nearly a quarter the weight of the steel boat. The hulls are 6mm, the decks 9 and 12mm. Yet has proven to be a successful live aboard cruiser. My Skoota 32 is currently being built and will be under 3T in the water If you think that a steel boat will be cheaper remember to add in the cost of bigger engines and the running costs. I suspect the bridgedeck will slam as heavy boats need higher clearance The 9m Catalac doesn't weigh 7T! Richard Woods of Woods Designs www.sailingcatamarans.com

Richard Woods said: ↑ I doubt if you will get 14 knots with twin 60hp on a 7.5T catamaran. Has someone achieved that speed with one? I recall you have posted before about the powercat you were building. So I am pretty sure you know my Skoota 28, which is, as Stumble says, nearly a quarter the weight of the steel boat. The hulls are 6mm, the decks 9 and 12mm. Yet has proven to be a successful live aboard cruiser. My Skoota 32 is currently being built and will be under 3T in the water If you think that a steel boat will be cheaper remember to add in the cost of bigger engines and the running costs. I suspect the bridgedeck will slam as heavy boats need higher clearance The 9m Catalac doesn't weigh 7T! Richard Woods of Woods Designs www.sailingcatamarans.com Click to expand...

Here's a commercial steel cat, just slightly bigger ...lol. http://nbcommercialboatsales.com.au...ommercial-vessels/24-m-steel-catamaran/133490

waikikin Senior Member

rustybarge said: ↑ My project is to home build a small coastal cruising cat on a small budget. The advantage of steel is easy welding; alloy needs an experienced expert . The materials for a 30' plywood cat come to about $20k using good quality marine ply. Here in Europe steel is at an all time low of £300/ton because of Chinese dumping and oversupply from European mills. I estimate £2k would cover the material costs in steel. Click to expand...

SamSam Senior Member

9m Steel Catamaran; thoughts? Click to expand...

waikikin said: ↑ Rustybarge, Steel wins as a low cost start to fabricate, the other materials soon catch up when you factor in abrasive blasting and paint system....welding consumables/gas/ abrasive discs are a minor add on but add up, then you need to build the interior which generally means lining the habitable areas which in effect is building a thin ply/laminate boat inside much of the steel one. It would be one tough little cat, one was for give away incomplete in Wollongong a few years back. At the other end of the project you would have a tough, comfy, slow piece of paradise..... on resale which you may not care about could be very low. Cecil E was a respected designer of commercial craft and some pleasure craft. If he was designing today in the market available that boat might be very different than presented. I'd never say don't build it but keep your eyes wide open at the big pic. Al the best from Jeff. Click to expand...

SamSam said: ↑ It's in your moniker rustybarge...rust. In thin steel, a corrosive pit is halfway through the skin. The worst is from the inside out, from condensation. I imagine creating and maintaining a protective coating in all the nooks and crannies inside those narrow, deep hulls would be a chore. Click to expand...

rustybarge said: ↑ This problem had occurred to me, especially if there are parts of the frames with doublers . how do you stop corrosion once its got into two thin steel plates spot welded together? I suppose after good layer of two pack paint, some sort of wax treatment like waxoil could be injected into overlapping sections of the steel work. But is plywood any better if it gets wet and starts rotting, and how would you dry it out in narrow hard to get at sections? Click to expand...

whitepointer23 Previous Member

what about alloy glued instead of welding. i read somewhere that alloy boats can be glued together just like plywood. i think the bond is also a lot stronger than glued ply bonds. even a combination of epoxy and rivets like an aircraft.

waikikin said: ↑ Well you need to seal weld the perimeter, I'm assuming you're indicating the gussets? lapping the framing. The "secret" with steel or timber for that matter is ventilation and limbering(drainage holes), an effective paint scheme is very important to steel, access to or elimination of void areas as well. The 3K structurally complete cat is fantasy if including coatings even in Pound$. A plywood or composite cabin may be ok if the landing/upstand/coaming is in staino, the fastenings required generally compromise coatings to steel & eventual results blow up with oxygen mixing with steel Jeff. Click to expand...

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Sailboat Hulls: Steel Vs Fiberglass

Last Updated by

Jacob Collier

August 30, 2022

For decades, sailors and boat owners have been having hotly contested debates about the merits of steel hulls vs fiberglass hulls in sailboats.

The major benefits of boats with steel hulls are that they are very strong, durable, and can be repaired easily. On the other hand, a fiberglass hull offers your boat a smooth and sleek look that is very pleasing. They are also lighter, faster, and require less maintenance than steel boats.

Whether you are building your own sailboat or thinking of buying one, getting the right material for the hull is of paramount importance. Your choice of material should depend on consideration of multiple factors, including its durability, stability, maintenance, repairs, weight, comfort, safety, and cost.

We have a team of sailing experts who have spent decades on the water and have set sail on boats built of all types of materials available. So who better to walk you through the pros and cons of steel and fiberglass hulls?

Table of contents

‍ Steel Hull vs Fiberglass Hull: Top 10 Factors to Consider

Let us take a look at some of the major factors that can help you determine whether a boat with a steel hull or fiberglass hull will be a better choice for you.

Sailboats with steel hulls are much more durable and stronger than those with fiberglass hulls. Steel sailboats have a more rigid structure and are quite robust so they can better understand grazes, rubs, and bumps when out in the open water.

In case of impact, a steel hull will bend and may become dented; however, a fiberglass hull has a higher possibility of breaking. That’s because steel is more ductile and can withstand strong blows without losing its toughness.

Fiberglass is a lighter material than steel, making fiberglass boats lighter. Many people prefer this quality since it means that the boat will travel faster on water and will require less power and wind energy to move than a boat with a steel hull. This means lower fuel consumption and more savings. However, a fiberglass boat will be more prone to be buffeted by the winds since it is lighter.

Anti-Corrosion Properties

The sailboat manufacturing industry now uses state-of-the-art technology and makes use of the best quality materials to make the hull. Steel corrodes when exposed to the atmosphere. However, if the right alloy is used for making the hull, it will resist saltwater corrosion, without even needing special paint.

Steel boats also experience electrolytic or galvanic corrosion, but they can be avoided with the use of insulated electrical connections and sacrificial anodes.

Fiberglass does not corrode. However, it can still suffer from osmosis if the fiberglass had air bubbles at the time of lamination. This can cause water to collect in the void, forming blisters that can weaken the hull. Fiberglass may also become damaged from ultraviolet radiation.

Since steel boats are heavier than fiberglass boats, it means they are more stable on the sea, particularly if you experience choppy waters. A fiberglass boat, on the other hand, is lighter, and hence sailors may experience a rougher journey on choppy waters.

In addition, due to its extra weight, steel boats drift slower and more predictably, which is particularly useful for anglers.

Maintenance

Many steel boats require greater maintenance since they are more prone to corrosion. Galvanic corrosion can occur when two different metals are placed together. Hence, it is important that you ensure that high-quality materials, joints, and screws are used on the hull. It is important to rinse the hull with fresh water once it is out of the sea.

Fiberglass boat hulls do not have welds and rivets and you do not need to worry about the hull rusting. However, it can experience osmosis issues, which can cause serious problems if they are not treated in time.

Both fiberglass and steel boats require antifouling application to prevent barnacles, algae, and other sea organisms from sticking to the hull. However, antifouling can be more expensive for steel boats.

It is easy to repair small dents in steel boats. However, if the damage is extensive, it can be more complicated and costly to repair or replace large sections of steel hulls. Welding a boat hull is a specialized job that requires trained professionals.

It is easier to repair a broken fiberglass hull, but it may never have the same strength and durability as the original hull since the structural tension will not be equal at all points.

Fiberglass boats are made of petroleum-based products that are flammable. Hence, in case of a fire, they will burn easily and quickly. A steel boat is much safer since it cannot burn. In addition, a significant impact from an unidentified floating object can result in a breach in a fiberglass hull easily and open up a waterway into the boat that can cause it to sink. Steel, on the other hand, can withstand larger impacts without compromising the integrity of the boat.

Steel boats operate much louder than fiberglass boats, especially in turbulent seas at high speed. Steel is also a good conductor of heat and if it is not well-insulated during construction, it can become uncomfortably warm in the summer and cold in the winter. On the other hand, boats with fiberglass hulls do not transmit heat well and are more comfortable.

When it comes to aesthetic appeal, fiberglass hulls have a sleeker, shinier, and more polished look. Steel hulls often have marks of reinforcements on their hulls and they need to get a nice paint to look good. In most cases, steel hulls are covered with putty to hide any construction defects. This putty should be polished so that the boat has a nice finish and is done in a controlled environment to keep out dust.

As you can imagine, this process is complex, costly, and drives up the price of the boat.

It is easier to manufacture fiberglass hulls and mold them into more complex shapes. This can lead to faster production and lower construction costs. Sailboats with steel hulls are more expensive, as we mentioned before because they require welding, heavy-duty grinding , and specialized cutting tools and are more labor-intensive.

When Should You Choose a Steel Boat?

Steel hulls are stronger, durable, and more impact-resistant than their fiberglass counterparts. Dents in steel hulls can be repaired easily and although steel is prone to corrosion, this can be managed by special paints, insulation, and some regular maintenance.

If you are deciding on a circumnavigation or want to go out on a long spree in the water, you need a solid and dependable boat that you can rely on when you venture into new territories.

A well-maintained sailboat gives you the confidence to enter into unfamiliar rocky coasts and reduce your worries about hitting UFOs. However, keep in mind that steel boats may be slower than fiberglass boats, particularly if they are smaller vessels.

When Should You Choose a Fiberglass Boat?

Fiberglass boats are generally prettier than steel boats since they have a smooth and polished hull. They also do not require protective paint on their hull since they are corrosion-free and hence quite low maintenance. In addition, they are lighter and faster than their steel counterparts and do not cost as much.

However, one big concern of a fiberglass hull is that it is not as strong as a steel hull. If the boat hits a hard object, the fiberglass may break, which can be dangerous on the open seas, particularly in choppy waters.

Still, fiberglass boats are an excellent option for racing and even long-distance cruising in areas that do not have sharp rocks.

The type of sailboat you choose depends on your sailing style and your needs. So make sure you consider all the factors before you invest in a steel or fiberglass boat.

Types of Sailboat Hulls

What Is a Sailboat Hull?

Born into a family of sailing enthusiasts, words like “ballast” and “jibing” were often a part of dinner conversations. These days Jacob sails a Hallberg-Rassy 44, having covered almost 6000 NM. While he’s made several voyages, his favorite one is the trip from California to Hawaii as it was his first fully independent voyage.

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Catamaran Design Formulas

Post author By Rick
Post date June 29, 2010
10 Comments on Catamaran Design Formulas

Part 2: W ith permission from Terho Halme – Naval Architect

While Part 1 showcased design comments from Richard Woods , this second webpage on catamaran design is from a paper on “How to dimension a sailing catamaran”, written by the Finnish boat designer, Terho Halme. I found his paper easy to follow and all the Catamaran hull design equations were in one place. Terho was kind enough to grant permission to reproduce his work here.

Below are basic equations and parameters of catamaran design, courtesy of Terho Halme. There are also a few references from ISO boat standards. The first step of catamaran design is to decide the length of the boat and her purpose. Then we’ll try to optimize other dimensions, to give her decent performance. All dimensions on this page are metric, linear dimensions are in meters (m), areas are in square meters (m2), displacement volumes in cubic meters (m3), masses (displacement, weight) are in kilograms (kg), forces in Newton’s (N), powers in kilowatts (kW) and speeds in knots.

Please see our catamarans for sale by owner page if you are looking for great deals on affordable catamarans sold directly by their owners.

Length, Draft and Beam

There are two major dimensions of a boat hull: The length of the hull L H and length of waterline L WL . The following consist of arbitrary values to illustrate a calculated example.

L H = 12.20 L WL = 12.00

After deciding how big a boat we want we next enter the length/beam ratio of each hull, L BR . Heavy boats have low value and light racers high value. L BR below “8” leads to increased wave making and this should be avoided. Lower values increase loading capacity. Normal L BR for a cruiser is somewhere between 9 and 12. L BR has a definitive effect on boat displacement estimate.

Tags Buying Advice , Catamaran Designers

Owner of a Catalac 8M and Catamaransite webmaster.

10 replies on “Catamaran Design Formulas”

Im working though these formuals to help in the conversion of a cat from diesel to electric. Range, Speed, effect of extra weight on the boat….. Im having a bit of trouble with the B_TR. First off what is it? You don’t call it out as to what it is anywhere that i could find. Second its listed as B TR = B WL / T c but then directly after that you have T c = B WL / B TR. these two equasion are circular….

Yes, I noted the same thing. I guess that TR means resistance.

I am new here and very intetested to continue the discussion! I believe that TR had to be looked at as in Btr (small letter = underscore). B = beam, t= draft and r (I believe) = ratio! As in Lbr, here it is Btr = Beam to draft ratio! This goes along with the further elaboration on the subject! Let me know if I am wrong! Regards PETER

I posted the author’s contact info. You have to contact him as he’s not going to answer here. – Rick

Thank you these formulas as I am planning a catamaran hull/ house boat. The planned length will be about thirty six ft. In length. This will help me in this new venture.

You have to ask the author. His link was above. https://www.facebook.com/terho.halme

I understood everything, accept nothing makes sense from Cm=Am/Tc*Bwl. Almost all equations from here on after is basically the answer to the dividend being divided into itself, which gives a constant answer of “1”. What am I missing? I contacted the original author on Facebook, but due to Facebook regulations, he’s bound never to receive it.

Hi Brian, B WL is the maximum hull breadth at the waterline and Tc is the maximum draft.

The equation B TW = B WL/Tc can be rearranged by multiplying both sides of the equation by Tc:

B TW * Tc = Tc * B WL / Tc

On the right hand side the Tc on the top is divided by the Tc on the bottom so the equal 1 and can both be crossed out.

Then divide both sides by B TW:

Cross out that B TW when it is on the top and the bottom and you get the new equation:

Tc = B WL/ B TW

Thank you all for this very useful article

Parfait j aimerais participer à une formation en ligne (perfect I would like to participate in an online training)

Stainless Steel

I am occasionally asked, "What about building a boat in Stainless?"

A structure built in stainless will weigh approximately the same as one built in mild steel, although on occasion one may be able to make use of somewhat lighter scantlings due to the somewhat higher strength of stainless. There are several major drawbacks to the use of stainless, not the least of which is cost. Stainless of the proper alloy will cost nearly six times the price of mild steel!

Even if it were not so costly, stainless has numerous other problems:

Stainless is quite difficult to cut, except by plasma arc.
Stainless work hardens when being formed and can become locally tempered such as when being drilled.
Stainless deforms rather extremely when heated either for cutting or for welding, meaning distortion will be very difficult to control.
Stainless, even in the low carbon types, is subject to carbide precipitation in the heat affected zone adjacent to the weld, creating an area that is much more susceptible to corrosion as well as to cracking.
Stainless is subject to crevice corrosion when starved of oxygen. This can be prevented only by sandblasting and painting the surfaces wherever an object is to be mounted onto the stainless surface. The same applies to the back side of any stainless fittings which are applied to hull surfaces.

If the above issues with stainless can be properly accounted for in the design and building of the vessel, then stainless can be a viable hull construction media.

Type 316-L stainless is generally the preferred alloy. Type 316-L is a low carbon alloy, and is used in welded structures to help prevent carbide precipitation in the heat affected zone. When available, the use of type 321 or 347 stainless will be of considerable benefit in preventing carbide precipitation, since there are other alloying elements (tantalum, columbium, or titanium) which help keep the carbides in solution during welding.

In my view, as a builder the main battle one will face is the rather extreme distortion levels when fabricating with stainless. Stainless conducts heat very slowly and has a high expansion rate. Both of these characteristics conspire against maintaining fairness during weld-up. Short arc MIG welding will be an imperative. In fact Pulsed MIG will probably be desired in order to sustain the right arc characteristics while lowering the overall heat input.

Copper Nickel

Another material which should be considered along with steel, stainless, and aluminum is Copper Nickel. One can ignore paint altogether with CuNi, inside, outside, top and bottom. Copper Nickel acts as its own natural antifouling. In fact, bare Copper Nickel plate performs better than antifouling paint..! Being a mirror-smooth surface, any minor fouling is very easily removed.

Besides not having to paint CuNi and its natural resistance to fouling, CuNi is also easy to cut and weld, it has relatively high heat conductivity, it is extremely ductile, and it is therefore very favorable with regard to distortion while welding.

There are two alloys of Copper Nickel which are the most common: 70/30 CuNi, and 90/10 CuNi. The numbers represent the relative amounts of Copper and Nickel in the alloy. Having a greater amount of Nickel, 70/30 CuNi is the stronger of the two and also the more expensive of the two.

In the US as of February 2007, 90/10 CuNi was priced around USD $8.50 per pound, and 70/30 CuNi around USD $13.00 per pound, both based on a minimum order of greater than 15,000 pounds. In other words, roughly ten to fifteen times the cost of the same structure in steel. I have not investigated current (2015) prices for CuNi, but we can be certain they are higher (i.e. the value of the dollar less) thus the ratio of costs vs. steel much higher.

The issues with CuNi are not only those of cost, but also of strength. For example, the ultimate strength of 90/10 Cu Ni is about one third less than that of mild steel, and the yield strength about half that of mild steel. In practice, this means that a hull built of Cu Ni will have to use heavier scantlings. CuNi, being slightly heavier than steel per cubic foot, the CuNi hull structure will end up being slightly heavier than an equivalent steel hull structure.

In most materials, we usually "design to yield." This means that the ultimate failure strength of a material is more or less ignored, and the yield strength is instead used as the guide for determining scantlings. For example, if we were to desire a 90/10 CuNi structure having the same yield strength as there would be with a similar steel structure, we would be tempted to actually double the scantlings. Naturally this would result in quite a huge weight penalty, BUT....

In practice, a CuNi structure need not be taken to this extreme. Using the ABS rules to calculate the scantlings, an all 90/10 Cu Ni structure will have around 25% more weight than a similar structure in steel. It is best to use the same plate thickness as with steel, and compensate for the lower yield strength by spacing the longitudinals more closely.

It is unlikely that one would choose CuNi for the internal framing, primarily because of its cost, its relatively low strength, and the relatively much larger scantlings and weight that would result. In other words, there is no reason not to make use of CuNi for the hull skin in order to take full advantage of its benefits, but it is possible to use a stronger and less expensive material for all the internal framing.

What is the best choice for the internal framing...? Probably type 316-L Stainless . As long as the various attributes of stainless are kept in mind, this is a combination having considerable merit. Here is why...

Stainless can be readily welded.
One can easily make a weld between stainless and Cu Ni.
Scantlings of stainless internal framing would not need to be increased, in fact they would be less than those required for mild steel.
The weight of stainless internal framing would therefore be roughly 10% less than with mild steel, or approximately equal to the weight of a Corten steel internal structure.
316-L Stainless costs (February 2007) around USD $4.50 per pound based on a minimum order of 10,000 pounds. Therefore the cost of stainless is roughly half that of 90/10 Cu Ni, and about one third the cost of 70/30 Cu Ni... Combined with there being much lighter scantlings, the overall cost factor would be reduced considerably.

With this strategy the weight can be kept to roughly the same as an equivalent mild steel structure.

And to further reduce costs, NC plasma cutting or water jet cutting can be used for all plates and internal structure.

Are there still more options to reduce costs...?

Fiberglass...! Compared to the weight and cost of an all CuNi / Stainless structure, both cost and weight can be reduced by using fiberglass for the deck and house structures, or possibly just for the house structures. A cold moulded wooden deck and / or superstructure is also a possibility.

Even with GRP or composite wood for the house structures, it probably would be most advantageous to plate the deck with Cu Ni. In so doing, one could then use CuNi for all the various deck fittings: stanchions, cleats, bitts, etc. Pipe fittings are readily available in either alloy of CuNi, so this would be a natural. The resulting integral strength and lack of maintenance would be an outstanding plus.

While the expense of Copper Nickel may seem completely crazy to some, given a bit of extra room in the budget and the will to be completely free from ALL requirements for painting, this is the bee's knees....! The savings realized by not having to paint the entire vessel inside and out - EVER - will go quite a long way toward easing the cost differential.

Per existing research on a number of commercial vessels, their operators have shown a very favorable economic benefit over the life of a Copper Nickel vessel. This is due to there being a much longer vessel life; far less cost for dry docking; zero painting costs; no maintenance; no corrosion; few if any repairs; etc.

Per the Copper Alliance, and organization that has studied the economic benefits of CuNi for boat hulls, the cost saved on a commercial vessel's maintenance routine pays for the added cost of the CuNi structure within 5 to 7 years. And... if the resale value of a CuNi boat is considered, the ROI is further enhanced.

Monel 400 is an alloy of around 65% Nickel, around 30% Copper, plus small percentages of Manganese, Iron and Silicon. Monel is extremely ductile, and therefore will take considerable punishment without failure. Monel is easily welded, and Monel has extraordinary resistance to corrosion, even at elevated temperatures.

Monel is much stronger than mild steel, stronger than Corten, and stronger than the usual varieties of stainless. As a result of this greater strength, Monel could be used for the entire structure. As compared to a similar steel structure, Monel will therefore permit lighter scantlings and would allow one to create a lighter overall structure than with steel. Alternately one could use the same scantlings in order to achieve a vessel having greater strength .

To reduce costs even more, one could use the same strategy as with CuNi, i.e. use Monel just for the plating, and then use 316-L Stainless for the internal framing. This is probably the sweet spot, offering light scantlings and extraordinary freedom from on-going maintenance costs.

If cost is not an important factor, an all Monel structure may well be the ultimate boatbuilding material of all time.

Titanium has been used in the aircraft and aerospace industries for quite a long time. As well, several Russian submarines have been built using Titanium. With very high strength alloys available, extreme nobility on the galvanic scale, virtual immunity to corrosion in sea water and in the atmosphere, and about half the weight of steel, there are only a few considerations that stand in the way of Titanium being the "perfect" hull material, not least of which is cost .

Cost : Due to the higher cost of titanium as compared to, say stainless or aluminum, the choice in favor of using titanium for a fabricated structure such as a boat must be made on the basis of the resulting structure having lower operating costs, longer life, or reduced maintenance in order to justify its use. In other words, titanium will only be chosen if it is perceived to have a lower total life cycle cost.

Plastic Range: Among the Commercially Pure (CP) grades of Titanium, and with most Titanium Alloys there is little spread between the yield point (the point at which a material is deformed so far that it will not return to its original shape when released) and the ultimate failure point. Thus most grades and alloys of titanium have a very limited plastic range.

Elongation : The percentage of elongation before failure is on par with mild steel, and is roughly twice that of aluminum. Thus most grades of CP Titanium and most alloys are readily formable, and have a fatigue resistance on par with steel.

Stiffness: Another characteristic is "stiffness" which is expressed by the modulus of elasticity. For steel, it is 29 million psi. For aluminum, it is 10 million psi. For Titanium, it is 15 million psi. This indicates behavior that is somewhat closer to aluminum in terms of material rigidity. In other words, Titanium will flex about twice as much as steel, but about 50% less than aluminum. Interestingly, Ti has about the same modulus of elasticity (stiffness) as Silicon Bronze, but Ti has less stiffness than copper nickel, which has an elastic modulus of 22 million psi.

Welding: Yet another consideration is the welding of Titanium, which is somewhat of a mixed bag due to several of the material's properties.

The melting point of Titanium (3,042 deg F) is well above that of steel (2,500 deg F) and about three times that of aluminum (1,135 deg F). Titanium forms a very tough oxide immediately on exposure to the air, and is highly reactive with nitrogen, therefore welding must be done only after thorough cleaning of the weld zone, and the welding process must assure a complete inert gas shroud of the weld zone both on the side being welded and on the opposite side. The weld zone must then continue to be shielded until the metal cools below 800 degrees.

These factors may provide considerable difficulty, but they are surmountable by thorough attention to detail, good technique, and aggressive measures to assure post-weld shielding. These factors however dramatically increase fabrication costs over that of other metals.

Among the other material properties that contribute to ease of fabrication of any metal are its heat conductivity, and its thermal expansion rate. Aluminum expands twice as much as steel per degree of temperature change, and is three times as conductive thermally. The thermal conductivity of aluminum is a big help, but the expansion makes trouble in terms of distortion. As a benefit though, an equivalent aluminum structure will have greater thickness and thus locally greater yield strength, so the score is more or less even between steel and aluminum, with aluminum having a slightly greater tendency toward distortion while welding.

With Titanium, this latter consideration will be the overriding factor in determining the minimum practical thickness for plating. Thermal conductivity is given as 4.5 BTU / Sq Ft / Hr/ Deg F / Ft for Titanium. For steel, it is 31, for aluminum it is 90. Thermal expansion is given as .0000039 in / in / deg F for Titanium, about 50% the expansion of steel and about 30% that of aluminum. These figures seem to indicate that the material would be fairly stable while welding, but that welds would take a much longer time to cool as compared to steel and vastly longer compared to aluminum. In other words, the heat would not dissipate - it would remain concentrated in the weld zone.

Industry consensus is that Titanium is slightly more prone to distortion due to welding as compared to steel. Considering these factors along with its much higher strength, as a very rough guess a thickness of around 3/32" may possibly be the minimum practical thickness for a welded structure in Titanium, with 1/8 inch thickness being a more likely lower practical hull thickness limit. As a comparison, the minimum thickness for other materials (mainly due to welding ease and distortion issues) is 10 gauge for mild steel (.1345"), and 5/32" for aluminum, although 3/16 inch thickness is a more practical lower limit for aluminum boat structures.

Corrosion: Titanium is extremely corrosion resistant due to the immediate formation of a tenacious Titanium Dioxide on exposure to air or oxygenated water. This means it is practically immune to corrosion in sea water, but there is one catch... Like aluminum, Ti depends on free access to oxygen, therefore it can be susceptible to crevice corrosion wherever it is deprived of free access to oxygen and cannot form a protective oxide. Crevice corrosion can be prevented in the same way as is done with aluminum, and some grades of Ti are more resistant to crevice corrosion than others.

Titanium Grades: Titanium Grade 2 is the most commonly available Commercially Pure (CP) grade, having 40k psi yield, 50k psi ultimate strength and a 20% elongation before failure. It is highly formable and weldable, and is available in most shapes, i.e. plate, bar, pipe, etc. These are highly favorable properties for hull construction.

Titanium Grade 12 includes Mo and Ni for a higher strength alloy having superior resistance to crevice corrosion, with 50k psi yield, 70k psi ultimate strength and an 18% elongation before failure. The 20k psi spread between yield and failure is a highly favorable property. It is a highly formable grade, readily weldable and is available in a variety of plate sizes, pipes and bar shapes. All of these are highly favorable properties for hull construction, making Grade 12 one of the best choices to be favored for boat structure.

Titanium Alloys : An interesting Titanium alloy is the experimental Alloy 5111 (5% Al; 1% SN; 1% Zr; 1% V; 0.8% MO) with 110k psi yield, 125k psi ultimate strength and a 15% elongation before failure. Described as "a near alpha alloy having excellent weldability, seawater stress corrosion cracking resistance and high dynamic toughness." It has a high elongation before failure, a "medium" overall strength of about twice that of mild steel, and has a slightly greater spread between its yield point and failure point than the "high" strength Titanium alloys. It is favored for submarines, but its high strength is not especially necessary for boats or large yachts.

Another Titanium alloy is the proprietary ATI Alloy 425 being made by Allegheny Technologies Inc. (ATI) who are targeting this alloy at ship structures. With 132k psi yield, 152k psi ultimate strength and a 13% elongation before failure, its use is likely to be relegated to applications requiring very high strength. Its low elongation before failure is an indication that it could be prone to cracking, and it is unlikely to be a candidate for typical boat structures (i.e. non-military usages).

Light weight, high strength, immunity from corrosion in sea water... sounds ideal. Although it is obvious that Titanium would be an outstanding hull material, it requires extreme care during construction, thus labor costs would be high. If those factors can be mitigated or if cost is not an issue, then Titanium may possibly be the "ultimate" boat hull material...!

Despite its immunity from corrosion in sea water, a titanium hull will still require paint below the WL in order to prevent fouling.

Relative Cost

If we ignore the cost of the hull materials themselves for a moment and consider what may impact costs in other ways, we can observe the following... Vessel construction costs will vary more or less directly with displacement, assuming a given material, and a given level of finish and complexity in the design. Since displacement varies as the cube of the dimensions, we can see that the costs for a vessel will increase exponentially with size.

With regard to the complexity of a vessel the same can be said. Complexity in whatever form affects cost perhaps to the fourth power...! Assuming a given budget, a simpler boat can just plain afford to be made larger!

Estimating actual construction costs is relatively straightforward but it does require a detailed look at every aspect of the process. A reliable construction cost estimate must consider the hull material, degree of finish, complexity of structure, building method, whether the structure is computer cut, the complexity of systems specified and the degree of high finish for the joinery. This is only possible with a well articulated vessel specification, a complete equipment list, and a detailed set of drawings that show the layout and the structure.

Assuming we are considering vessels of equal size and complexity, when all is said and done, and if painted to the same standard on the exterior, an aluminum vessel may possibly be around 10% more expensive to build than the same vessel in steel. If the aluminum vessel is left unpainted on the exterior except where necessary, many yards can build for less in aluminum than in steel, or might quote the two materials at parity. This has been verified by several yards via actual construction estimates for boats of my design.

As compared to a steel boat, maintenance will be less costly on an aluminum boat and resale value will be higher. Taken as a whole, any increased hull construction costs for an aluminum hull will shrink into insignificance in the context of the entire life of the boat.

Of course a Copper Nickel, Monel, or Titanium vessel will be considerably more costly than one built in steel or aluminum, however in terms of longevity a boat built with any of those metals will provide the ultimate as a family heirloom...

For more information, please review our comprehensive web article on Boat Building Costs .

The materials of construction need not dictate the aesthetics of a vessel. Much can be done to make a metal boat friendly to the eye. On the interior for example with the use of a full ceiling and well done interior woodwork, there will generally be no hint that you're even aboard a metal boat.

On the exterior, if metal decks are preferred for their incredible strength and complete water tightness, one can make the various areas more inviting by devious means. An example would be the use of removable wood gratings in way of the cockpit. Fitted boat cushions made of a closed cell foam work equally well to cover the metal deck in the cockpit area, and some will prefer to laminate a cork or teak deck over a painted and protected metal deck.

Many metal boats we encounter seem "industrial" in their appearance. In my view, classic and traditional lines, if attended to faithfully, will completely eliminate that industrial look. With a bit of classic gracefulness introduced by the designer, a metal boat will be every bit as beautiful as a boat of any other material.

My design work often tends to be drawn toward fairly traditional aesthetics, which some may regard as being somewhat old fashioned. What I have done in these designs however, is to take maximum advantage of up to date materials and current knowledge of hydrodynamics, while retaining the look and feel of a classic boat. In so doing, my overall preference is to provide a boat that is very simple, functional, and rugged, while carrying forth a bit of traditional elegance.

Everyone's needs are different of course. When considering a new design, nearly anything is possible. The eventual form given to any vessel will always be the result of the wishes of the owner, the accommodations the boat must contain, the purpose for which it is intended, and the budget that is available for its creation.

Regarding Hull Form

Efficiency and performance are high on the list amongst the myriad considerations that go into shaping a hull. With metal hulls, there is always a question of whether a vessel should be rounded or "chine" shaped.

Assuming two vessels are of equally good design, whether the hull is rounded or single chine will not have much impact on their performance, i.e. they will be more or less equivalent. Here are a few considerations that may be of some benefit when considering the choice between rounded or single chine hull shapes...

If one were to take a single chine hull form and simply introduce a fairly large radius instead of the chine, the newly rounded vessel's wetted surface would be less; displacement would be less; and initial stability would be less, and the comparison somewhat skewed.

In terms of interior hull space, a chine hull form will often be slightly less wide at sole level and slightly wider at the waterline level, so possibly a bit less room to walk around but larger seats and berths. The single chine hull form will have slightly greater initial stability (greater shape stability), and will therefore have slightly greater sail carrying ability at typical heel angles under sail. The single chine hull form will have greater roll dampening (faster roll decay). The rounded hull form will have a slightly more gentle rolling motion. The chine hull form will have slightly greater wetted surface. This implies that the rounded hull form will have slightly less resistance at slow speeds where wetted surface dominates the total resistance. The chine hull can be designed to equalize or reverse that resistance equation at higher speeds due to wake differences resulting from the chine hull being able to have a slightly flatter run.

Aside from these generalities, relative performance would be difficult to pre-judge. We can however observe the following:

Given the same sail area, when sailing at slow speeds in light airs, one might see the rounded hull form show a slight advantage due to having slightly less wetted surface.
When sailing fast , a chine hull form will be more likely to exhibit greater dynamic lift, especially when surfing.
Especially in heavier air, one might even see a slight advantage to windward with the chine hull.

Given that those observations do not reveal any special deficiency with regard to a single chine hull we can additionally observe the following:

When creating a new design, wetted surface is one of the determining factors of sail area.
Having slightly greater wetted surface, a single chine hull should therefore be given slightly more sail area, so its slightly greater wetted surface will become a non-issue .
If the chine hull is given slightly more sail area, it will therefore be subject to a slightly greater heeling force.
However the single chine hull form will have inherently greater "shape stability" in order to resist that heeling force.
One can therefore expect the sail carrying ability to be essentially equalized .
Therefore with good design, there is no performance hit at low speeds, and there is ordinarily a performance gain at high sailing speeds.

Among the above considerations, the one factor that seems to favor the rounded hull form most definitively is that of having a slightly more gentle rolling motion. In other words, a slower "deceleration" at the end of each roll. On the other hand, rolling motions will decay more quickly with a single chine hull form. Even these factors can be more or less equivocated via correct hull design.

Rounded Metal Hulls

As we have seen, one cannot claim that a rounded hull form is inherently better in terms of performance without heavily qualifying that claim. The primary trade-offs between a rounded hull and a chine type of hull form for metal boats therefore turn out to be purely a matter of cost and personal preference.

I have designed several rounded hulls for construction in metal. These are true round bottom boats designed with the greatest ease of plating in mind. Some are double ended, some have a transom stern, others have a fantail stern, and still others have a canoe stern where stem nicely balances the shape of the stern.

Having an easily plated shape, any of these rounded hull forms can be economically built. These rounded shapes require plate rolling only in a few places and are elsewhere designed to receive flat sheets without fuss. These are not "radius chine" boats. They are simply easily plated rounded hulls.

With any of these types, the keel is attached as an appendage, there being no need when using metal to create a large rounded garboard area for the sake of strength, as would be the case with a glass or a wooden hull. This achieves both a more economically built structure, as well as a better defined keel for windward performance under sail and better tracking under sail or power.

Plating on these rounded hull types is arranged in strips having a limited width running lengthwise along the hull. Usually the topsides can be one sheet wide, the rounded bilge one sheet, and the bottom one larger sheet width.

Examples of these rounded hull types among my designs are Jasmine , Lucille 42 , Lucille 50 , Benrogin , Greybeard , Fantom and among my prototypes such as Josephine and Caribe . While these might be imagined to have a "radius" chine shape, they are in fact true rounded hull forms. In other words, the turn of the bilge is not a radius but is instead a free form curve between bottom and topsides. Both bottom and topsides have gently rounded sectional contours that blend nicely into the curve at the turn of the bilge. With the exception of the turn of the bilge, all of the plating on these designs is developable and will readily bend into place making these vessels just as easily constructed as any radius chine shape. In other words, 85% to 90% of the vessel is able to be plated using flat metal sheets without any pre-forming.

What's the difference between this and a radius chine...?

In my view the visual difference between radius chine and rounded hull forms is very apparent, strongly favoring the rounded shape, yet the labor required and the consequent cost is the same. Due to the gentle transverse curvature given to the surfaces above and below the turn of the bilge, the appearance is a vast improvement over the relatively crude radius chine shape.

Radius Chine Metal Hulls

Looking around at typically available metal boat designs we quickly observe that the "radius chine" construction method has become fairly common. Here, a simple radius is used to intersect the "flat" side and bottom plates. Although the radius chine shape takes fairly good advantage of flat plate for most of the hull surface, it is not a more economical construction method than the easily plated rounded hull shapes described above - nor is it nearly as attractive.

One reason for the popularity of the radius chine is that nearly any single chine boat can be converted to a radius chine. This is often done without any re-design of the hull by simply choosing an appropriate radius, and using rolled plate for that part of the hull. Radius chine construction does add quite a few extra hours to the hull fabrication as compared to single chine hull forms.

In my experience there is no benefit whatever to employ a radius chine shape over that of an easily plated rounded hull form. The radius chine hull will always be easily recognized for what it is... a radius chine shape rather than a true rounded hull. By contrast a gently rounded hull form will be vastly more appealing visually.

Chine Hull Forms

A single chine can be quite appealing, especially when used with a more classic / traditional style. A few single chine examples among my sailing designs are the 36' Grace , the 42' Zephyr , the 44' Redpath , the 56' Shiraz , along with a number of others such as the prototype designs for a 51' Skipjack , or the 55' Wylde Pathaway .

As supplied, metal plate is always flat . When building a boat using flat sheet material, it makes the most sense to think in terms of sheet material and how one may optimize a hull design to suit the materials, without incurring extra labor. I am attracted to the single chine shape for metal boats. In my view the single chine shape represents the most "honest" use of the material.

In this regard I feel traditional styling has much to offer, keeping in mind of course the goals of seakindliness, safety, and of excellent performance. As with many traditional types, there is certainly no aesthetic penalty for using a single chine, as is evidenced by reviewing any of the above mentioned sailing craft.

Assuming that by design each type has been optimized with regard to sail area and hull form, it becomes obvious that the typically pandered differences between the performance of a rounded hull form versus that of a single chine, unless heavily qualified, are simply unsubstantiated.

In fact, since costs are significantly less using single chine construction, one can make an excellent case in terms of better performance via the use of a simpler hull form....!

How is this possible...?

With metal boats, labor is by far the largest factor in hull construction, and as we have observed greater complexity pushes the hours and the cost of labor up exponentially. Therefore dollar for dollar, a single chine vessel can be made longer within the same budget .

This means that in terms of the vessel's "performance per dollar" the single chine vessel can actually offer better performance (i.e. greater speed) than a similar rounded hull form...!

By comparison, a multiple chine hull form offers practically no advantage whatever. A multiple chine hull will require nearly as much labor as a radius chine hull. The only savings will be eliminating the cost of rolling the plates for the actual radius. In my view, multiple-chine shapes are very problematic visually, and they are much more difficult to "line off" nicely. There will be just as much welding as with a radius chine shape, and in general a multiple-chine hull will be considerably less easy to keep fair during construction.

If you look at the designs on this web site, you'll soon discover that there are no examples of multiple-chine vessels among my designs, whether power or sail....

Basically, multiple chine shapes cost more to build, and in my view multiple chine shapes are not as visually appealing. As a result the preference has always been to consider the available budget and to make a graceful single chine boat longer for the same cost, and realize some real speed, comfort and accommodation benefits...!

In the end what ultimately defines a good boat is not whether she is one type or another, but whether the boat satisfies the wishes of the owner.

Keel Configuration

The keel of any vessel, sail or power, will be asked to serve many functions. The keel creates a structural backbone for the hull, it provides a platform for grounding, and it will contain the ballast.

In a metal boat, the keel is not just "along for the ride." In a metal vessel the keel can contain much of the tankage including a meaningful sea water sump, and the keel can serve as the coolant tank for the engine essentially acting as the "radiator." It is usually convenient to allow at least one generous tank in the keel as a holding tank.

A metal hull can take advantage of twin or bilge keels without any trouble. It is an easy matter to provide the required structural support within the framing. Often, bilge keels can be integrated with the tanks, allowing excellent structural support.

An added advantage with both sail and power boats is that the bilge keels can be used as ballast compartments. Having spread the ballast laterally becomes a big advantage in terms of the vessel's roll radius, providing an inertial dampening to the vessel's roll behavior.

Bilge keels can also be designed to permit a good degree of sailing performance to a power vessel which has been set up with a "get-home" sailing rig. Aboard a power vessel, when faced with the choices involved with having an extra diesel engine as a "get-home" device in the event of failure of the main engine, I would very seriously consider the combination of bilge keels and a modest sailing rig.

Bilge keels will usually make use of a NACA foil section optimized for high lift / low drag / low stall. With metal, this is easily accomplished.

Integral Tanks

Integral fuel and water tanks are always to be preferred on a metal boat. Integral tanks provide a much more efficient use of space. Integral tanks provide added reinforcement for the hull and ease of access to the inside of the hull. Integral tanks are very simple to arrange for during the design of the vessel. If the tank covers are planned correctly there will be excellent access during construction as well as in the future for maintenance.

The one exception to this generality is that polyethylene tanks may be preferred for black or grey water storage, since they can be readily cleaned. This is especially so in aluminum vessels, due mainly to the extremely corrosive nature of sewage. In steel vessels, when properly painted there will always be an adequate barrier, and integral black and grey water tanks again become viable. For aluminum construction, if integral holding tanks are desired the tanks must be protected on the inside as though they were made of mild steel... and the coatings must not be breached...!

Please see my article on Integral Tanks for more on this question...

Scantling Calcs

Hull size, materials of construction, and the location of the specific region of the structure in question will each have a bearing on the results of the scantling calcs. The method of calculating the hull structural scantlings is usually processed as follows, assuming first that the vessel data is already given (hull length, beam, depth, freeboard, weight, etc.).

Select plate material according to owner preference, available budget, and desired strength or other material properties Select preferred plate thickness according to availability, suited to vessel size and displacement Calculate local longitudinal spacing to adequately support the plate Select frame spacing to satisfy the locations of interior bulkheads or other layout considerations Calculate scantlings required for longitudinal stringers to satisfy their spacing and the span between frames Calculate scantlings required for transverse frames according to the depth of long'l stringers and the local span of the frames.

Per item 3, when considering an alternate material it is possible that due to a difference in plate yield strength as compared to the original design material (say steel), that the long'ls will be placed slightly more closely (say for the same thickness of plate, but a plate of lesser strength).

Generally, since the long's support the plate, they are the primary variable when plate thickness, or strength, or location is changed. It is no big deal to the structure, to the overall weight, or to ease of the building of the vessel (as compared to say steel) to have a tighter long'l spacing. This is the proper strategy to accommodate plate of different strength or thickness.

Once the plate is adequately supported, then scantlings of items 5 and 6 can be calculated according to their spans and the material strengths for the chosen framing materials.

It becomes obvious from the above that it is an advantage (in terms of weight) to select a relatively lesser thickness of plating, and a relatively more frequent interval for internal framing. On the other hand, it is usually an advantage in terms of building labor to select plate of a slightly greater thickness and a less frequent framing interval (so simpler internal structure).

Please see my article on Using the ABS Rule for a more detailed look at how scantlings are determined.

Frameless Construction...?

There is quite a lot of misleading and incorrect information associated with the implied promise of "frameless" metal boats, a notion that is pandered by several offbeat designers and builders. The concept of "frameless" metal boats is attractive, but flawed.

If one applies well proven engineering principles to the problem of hull design as detailed above, one quickly discovers that for the sake of stiffness and lightness, frames are simply a requirement. For example, in order to achieve the required strength in a metal vessel without using transverse framing will require an enormous increase in plate thickness. Even with light weight materials such as aluminum alloy this would automatically result in a substantial weight penalty..

With light weight materials such as aluminum, one can certainly gain some advantage by the use of greater plate thickness, primarily in terms of maintaining fairness during fabrication, and in terms of ruggedness in use. Still, as strong as metal is, even with light weight materials there is definitely a need to support the plating and to reinforce and stiffen the structure as a whole using frames and stringers.

In general, the most suitable arrangement for internal structure is a combination of transverse frames and longitudinal stiffeners. Framing may sometimes be provided in the form of devious strategies... For example framing may be in the form of bulkheads or other interior and exterior structural features, placed in order to achieve the required plate reinforcement. Many so-called "frameless" boats do indeed make extensive use of longitudinals in combination with bulkheads or other internal structure to reduce the span of the longitudinal stiffeners.

While it is true that many metal boats are successfully plated , and their plating then welded up without the aid of metal internal framing during weld-up, in order to provide adequate strength in the finished vessel, frames must then be added before the hull can be considered finished. Even on a hull that will eventually have substantial internal framing this construction sequence can provide a big advantage when trying to maintain fairness during weld-up.

Experienced metal boat builders and designers have often come to recognize the potential benefits of building a metal boat over molds which do not hold the boat so rigidly as to make trouble during the weld-up. However, the competent among them also know that to leave the boat without internal framing is quite an irresponsible act.

Please see my articles on Framing and Frames First for more on this subject.

Framing Systems

Framing systems are several, but can roughly be categorized into

Transverse Frames Only Transverse Frames with Longitudinal Stringers Web Frames with Longitudinal Stringers.

Among those, the Transverse Frames Only system is fairly common in Europe. In the US, the most commonly system used is the second system, where transverses are used in combination with longitudinal stringers.

In terms of scantlings, typically, long'ls will be half the depth, but approximately the same thickness as the transverse frames. It is an ABS requirement that transverse frames be twice the depth of the cut-out for the long'l.

Among some light weight racing yachts, a system of Webs with fairly beefy Long'l Stringers is the preferred approach, or alternately Webs with smaller Intermediate Transverse Frames, in combination with Long'l Stringers..

A somewhat generalized walkthrough of the usual design sequence is as follows:

For any given vessel size, plating will need to be a certain minimum thickness suited to that vessel size. For that given minimum plating thickness (for that particular boat) the long'l stringers will need to be a certain distance apart in order to adequately support the plate. The dimensions of the Long'l Stringers are determined by the vessel size, the spacing of the long's and the span of the long's between transverse frames. Finally, the dimensions of the Transverse Frames are determined according to the vessel size, the frame spacing, the span of the frames between supports, and by the requirement that the frames be no less in height than twice the height of the long's.

In other words, by this engineering approach the transverse frames are considered to be the primary support system for the long'l stringers, and the long'l stringers are considered to be the primary support system for the plating.

When a long'l member becomes the "dominant" member of the structure (usually locally only), it ceases to be referred to as a long'l stringer, and becomes instead a long'l "girder" (an engine girder for example).

If long'l stringers are not used, then the frames are the only means of support for the plating. They must therefore be more closely spaced in order to satisfy the needs of the plating for adequate support. In general though, long'l stringers are to be considered highly desirable, primarily because they contribute considerably to the global longitudinal strength of the yacht.

When calculating the strength of any beam, there is a benefit when the beam gains depth (height). Beams of greater height have a higher section modulus. Just as with beams of greater height, when calculating a vessel's global longitudinal strength it is the height of the vessel that makes the greatest contribution. Small and medium sized power and sailing yachts usually have very adequate height , so long'l strength calculations are less critical. For larger yachts or for yachts which have a low height to beam ratio, there it is necessary to consider long'l strength very closely. Witness the catastrophic failures of several recent America's Cup vessels....!

As a general guide to the boundary of acceptability, the ABS rules consider that a vessel must be no more than twice as wide as it is high (deck edge to rabbet line), and no greater than 15 times its height in overall length. Beyond these limits, a strictly engineering "proof" must be employed rather than the prescriptive ABS Section Modulus and Moment of Inertia requirements for calculating the strength of the global hull "girder."

The ABS Motor Pleasure Yachts Rule, 2000, is a very suitable scantling rule for boats of any material. Originally created for "self propelled vessels up to 200 feet, the scope of the Motor Pleasure Yachts Rule has been subsequently restricted to vessels between 79 and 200 feet. In that size range, the ABS Rules for Steel Vessels Under 200 Feet, and the ABS Rules for Aluminum Vessels may also be applied, in particular to commercially used vessels. For sailing craft of all materials, the ABS Rules for Offshore Racing Yachts is applicable to sailing vessels up to 79 feet.

The most appropriate means of assessing the adequacy of structure is to assure that a vessel's scantlings comply with the applicable ABS rule, or alternately the applicable rule published by Lloyd's Register (England), German Lloyds (Germany), Det Norske Veritas (Norway), Bureau Veritas (France), etc.

As we can see from the above, framing is highly desirable for any metal yacht. Without framing, plate thickness would become extreme, and consequently so would the weight of the structure...

Computer Cutting

The labor involved in fabricating a metal hull can be reduced by a substantial amount via NC cutting. What is NC...? It simply means "Numerically Controlled." Builders who are sufficiently experienced with building NC cut hull structures estimate that they can save between 35% and 55% on the hull fabrication labor via computer cutting.

As an example, a fairly simple vessel of around 45 feet may take around 2,500 hours to fabricate by hand, complete with tanks, engine beds, deck fittings, etc. ready for painting. If one can save, say 40% of those hours, or some 1000, then at typical shop rates the savings can be dramatic. By comparison, the number of design hours one must spend at the computer to detail the NC cut files for such a vessel may amount to some three to four man-weeks, or perhaps some 160 hours.

With this kind of savings, the labor expended to develop the NC cut files will be paid for many times over. In fact, the savings are sufficient that NC cutting has the potential to "earn back" a fair portion of the cost of having developed a custom boat design...! Where there may be any doubt, please review our web article on how we use CAD effectively to develop our designs for NC Cutting .

Anymore, it is inconceivable to build a commercial vessel of any size without taking advantage of NC cutting. While this technology has been slow to penetrate among yacht builders, these days it is plain that builders and designers who ignore the benefits offered by computer modeling and NC cut hull structures simply have their heads in the sand. A possibly entertaining editorial on this is subject is Are We Still in the Dark Ages ...?

Paint Systems

Small metal boats are not designed with an appreciable corrosion allowance. They must therefore be prepared and painted in the best way possible in order to assure a long life.

Current technology for protecting steel and aluminum boats is plain and simple: Epoxy paint .

When painting metal, a thorough degreasing is always the first step, to clean off the oils from the milling process, as well as any other contaminants, like the smut from welding, which have been introduced while fabricating.

The next important step is a very thorough abrasive grit blasting on a steel boat, or a somewhat less aggressive "brush blast" on an aluminum boat. The process of sand blasting a metal boat is expensive and can in no way be looked at with pleasure, except in the sense of satisfaction and well being provided by a job well done.

While there is no substitute for grit blasting, there are ways to limit the cost of the operation. When ordering steel, it is very much to a builder's advantage to have it "wheel abraded" and primed. Wheel abrading is a process of throwing very small shot at the surface at high speed to remove the mill scale and clean the surface. Primer is then applied. Having been wheeled and primed, the surfaces will be much easier to blast when the time comes.

In terms of the paint system, aluminum boats are dealt with more easily than steel boats. Aluminum must be painted any place a crevice might be formed where things are mounted, and should also be painted below the waterline, if left in the water year-round. The marine aluminum alloys do not otherwise require painting at all.

On an aluminum boat, any areas which will be painted should receive the same aggressive preparation regimen used on steel: thorough cleaning, sand blasting, and epoxy paint. Aluminum is less hard than steel, so sand blasting aluminum is relatively fast compared to steel. The blast nozzle must be held at a greater distance and the blast covers the area more quickly.

On a Copper Nickel or Monel vessel, there would simply be no need for paint anywhere.

Many schemes are used to insulate metal boats. Insulation is mentioned here in the context of corrosion prevention mainly to point out that regardless of the type used, insulation is NOT to be considered an effective protection against corrosion. As with anywhere else on a metal boat, epoxy paint is the best barrier against corrosion.

Sprayed-on foam is not to be recommended. While popular, sprayed-on foam has many drawbacks that are often overlooked:

Urethane foam is not a completely closed cell type of foam. With time, urethane foam will absorb odors which become difficult or impossible to get rid of. This is especially a problem when there are smokers aboard.
Nearly all urethane foam will burn fiercely, and the fumes are extremely toxic. Blown in foam should therefore be of a fire retarding formulation, and should additionally be coated with a flame retarding intumescent paint.
Sprayed-on foam makes a total mess, requiring extensive clean-up. The clean-up process actually further compromises the foam due to breaking the foam's surface skin.
Sprayed-on foam requires that an intumescent paint be applied, both for the sake of fire suppression, and in order to re-introduce the seal broken by the clean-up of the spray job.

A much better insulation system is to use a Mastic type of condensation / vapor barrier such as MASCOAT, which adheres well to painted steel surfaces, as well as unpainted aluminum surfaces. It creates a barrier to water penetration, and an effective condensation prevention system. Applied to recommended thicknesses of around 60 mils, it is effective as insulation. Further, it is quite good at sound deadening, is fire proof, and will not absorb odors. Mascoat DTM is used for insulation, and Mascoat MSC for sound attenuation, very effective on engine room surfaces and above the propeller. Both are effective whether on a steel or an aluminum boat.

These mastic coatings can be painted if desired. In more severe climates the mastic coatings can be augmented by using a good quality flexible closed cell cut-sheet foam to fit between the framing. The best choices among these flexible cut-sheet foams are Ensolite and Neoprene. There are several different varieties of each. The choice of insulation foam should be made on the basis of it being fireproof, mildew proof, easily glued, easy to work with, resilient, and if exposed, friendly to look at. Ensolite satisfies all these criteria. Ensolite is better than Neoprene in most respects, but is slightly more expensive. One brand offering good quality flexible foam solutions for boats is ARMAFLEX.

Styrofoam or any other styrene type of foam should be strictly avoided. Go get a piece at your local lumber yard and throw it onto a camp fire.... You will be immediately convinced. The same applies to any of the typical rigid or sprayed-on urethane foams. They are an extreme fire hazard and cannot be recommended.

Zincs are essential on any metal hull for galvanic protection of the underwater metals (protection against galvanic attack of a less noble metal by a more noble metal), as well as for protection against stray current corrosion.

In the best of all possible worlds, there would be no stray currents in our harbors, but that is not a reality. Regardless of the bottom paint used or the degree of protection conferred by high build epoxy paint, zincs must be used to control stray current corrosion, to which we can become victim with a metal boat, even without an electrical system, due to the possible presence of an electric field in the water having a sufficiently different potential at one end of your boat, vs the other end...!

The quantity of zinc and the surface area must be determined by trial and error by observing real-world conditions over time. However as a place to start, a few recommendations can be made. As an example, on a metal hull of around 35 feet the best scheme to start with would be to place two zincs forward, two aft, and one on each side of the rudder. With a larger metal boat of say 45' an additional pair of zincs amidships would be appropriate. As a vessel gets larger the zincs will become more numerous and / or larger in surface area.

Zincs will be effective for a distance of only around 12 to 15 feet, so it is not adequate to just use one single large zinc anode. Zincs will ideally be located near the rudder fittings, and near the propeller. The zincs forward are a requirement, even though there may be no nearby hull fitting, in order to prevent the possibility of stray current corrosion, should the paint system be breached.

Using the above scheme, after the first few months the zincs should be inspected. If the zincs appear to be active, but there is plenty left, they are doing their job correctly. If they are seriously wasted, the area of zinc should be increased (rather than the weight of zinc). During each season, and to adjust for different marinas, the sizes of the zincs should be adjusted as needed.

Good electrical connection between the zinc and the hull must be assured.

Bonding is the practice of tying all of the underwater metals together with wires or bonding strips. It is done in order to 'theoretically' bring all of the underwater metals to the same potential, and aim that collective potential at a single large zinc. It is also done in order that no single metal object will have a different potential than surrounding metal objects for the sake of shock prevention.

However for maximum corrosion protection, metal boats will ideally NOT be bonded. This of course is contrary to the advice of the ABYC. Keep in mind though that the ABYC rules represent the consensus of the US Marine Manufacturers Association, and are therefore primarily aimed at satisfying the requirements aboard GRP vessels, about which the MMA is most familiar. Naturally, aboard a GRP boat the boat's structure is electrically inert and not subject to degradation by corrosion, therefore aboard a GRP boat there is no reason to recommend against bonding - except perhaps the fact that bonding all underwater metals using a copper conductor invites the possibility of stray current corrosion of those underwater metals due to the possible potential differential in the water from one end of the boat to the other.

Little by little though, the ABYC is learning more about the requirements aboard metal and wooden vessels, and recommendations for aluminum and steel boats have begun to appear in the ABYC guidelines. Even so, the corrosion vs shock hazard conundrum aboard metal boats is not 'solved' since the solutions are not as simple as they might at first seem. For an introduction to some of the issues with regard to bonding, please see our " Corrosion, Zincs & Bonding " booklet.

Electrical System Considerations

Aboard a metal vessel, purely for the sake of preventing corrosion the ideal will be to make use of a completely floating ground system. In other words, the negative side of the DC power will not permitted to be in contact with the hull nor any hull fittings, anywhere. With a floating ground system, a special type of alternator is used which does not make use of its case as the ground, but instead has a dedicated negative terminal.

This is contrary to the way nearly all engines are wired. Typically, engines make use of the engine block as a mutual ground for all engine wiring. Also, the starter will typically be grounded to the engine, as will the alternator. And typically the engine is in some way grounded to the hull, possibly via the coolant water, or possibly via a water lubed shaft tube, or the engine mounts, or even a direct bonding wire, etc.

Needless to say, for the sake of preventing corrosion, there should not be a direct connection between the AC shore power and the hull. This includes that insidious little green grounding wire. This whole issue is avoided if a proper marine grade Isolation Transformer is installed, which has as its duty to totally isolate all direct connections between shore power and the onboard wiring. This is done by 'inducing' a current in the onboard circuits, thus the electrical energy generated has been created entirely within the secondary coils, and is therefore entirely separate from the shore side power.

The purpose of the green grounding wire is to return any leakage current back to ground onshore, rather than to leak away through the hull and its underwater metals into the water, seeking an alternate path to ground. If a leakage current of greater than 10 milliamps exists onboard (not at all uncommon), it presents an EXTREME hazard to swimmers nearby. This is especially dangerous in fresh water where a swimmer's body provides much less electrical resistance than the surrounding water, and the swimmer thereby becomes the preferred path for any stray currents in the water. With a leakage current above 20 milliamps, death can (and has) become the result. Above 100 milliamps, and the heart stops. Serious business.

The shore side green grounding wire must be brought aboard and connected to the primary side of the Isolation Transformer. It creates a 'fail safe' return path for the AC current seeking ground. But on the secondary side of the Isolation Transformer it serves no purpose onboard because the secondary side will have created an entirely independent electrical system, generated onboard , and not tied to shore power.

Separately, there should ideally be a green grounding wire in the onboard electrical system, however it should not be tied to the shore side green grounding wire. Recommendations differ here, and the Isolation Transformer should be chosen on the basis of providing COMPLETE isolation of the onboard electrical system from the shore power system... What this means is that if a particular Isolation Transformer's wiring diagram recommends connecting the shore side green grounding wire to the onboard green grounding wire (effectively defeating its very purpose) that Isolation Transformer should be rejected as a candidate for placement onboard.

Other "black box" devices should be avoided, including "zinc savers" or impressed current systems, etc. On a military vessel, commercial vessel, or large crewed yacht where these systems can be continuously monitored, such "active" protection schemes may have some merit. However on a small yacht, which may spend long periods with no-one aboard but which may still be plugged into shore power, an "active" system will not be attended to with any regularity, and could easily fail and develop a fault that could potentially cause rapid corrosion, resulting in considerable damage.

The ideal electrical system onboard will be entirely 12v or 24v DC, energized via a large battery bank. All installations should have an Isolation Transformer on the shore power connection. Onboard, the secondary side of the transformer can then be connected to marine quality battery chargers. Some battery chargers are available that have a built-in isolation transformer, but they should be screened on the basis described above. Then onboard if the only thing the Isolation Transformer connects to onboard is a large battery charger, then there is no real connection between the onboard DC system and the shore side AC system.

Using such a system, it is possible to have onboard AC power provided by inverters, directly energized by the large battery bank. This provides yet another barrier between the onboard AC electrical system and the shore power system. It also provides other considerable advantages.... For one, some types of isolation transformer can be switched in order to accept either 110v AC or 220v AC, and to output either voltage , depending on what the onboard equipment requires (essentially just the battery charger in this case). Since the isolation transformer and the battery chargers are also frequency agnostic, if all onboard AC is generated by inverters, you then have a truly shore power agnostic system. All onboard equipment will either be DC, or will be AC generated onboard by the inverters at the requisite frequency and voltage required by the onboard equipment.

Where this scheme gets defeated rather quickly is where there must be an air conditioning system, and / or a washer / dryer, all of which are very power hungry. But we can still keep from bringing shore power onboard to directly serve those items by using the above described system (i.e. shore power > isolation transformer > battery charger > battery bank > inverter > onboard AC system) in combination with an onboard AC generator. In this way, all AC current onboard will be generated onboard, either via the inverters for low current draw items, or by the generator when high current draw items are used, and frequency / voltage suddenly become a non-issue...

The whole point is to keep shore power OFF the boat by limiting its excursion only to the Isolation Transformer, where it stops completely. With all onboard power being created entirely onboard, there is no hazard to swimmers posed by stray currents attempting to seek ground onshore, because the onboard "ground" is, in fact, onboard...

I know there are those who will disagree with the above statements about electrical systems. Whether you agree or disagree, please don't come all unglued over these matters and instead, for much more complete information on these topics, please see the resources mentioned below...

We can see that metal can make considerable sense as a hull building material. On the basis of strength, ruggedness, ease of construction, first cost, and ease of maintenance, there is plenty of justification for building a metal hull, whether steel, aluminum, Copper Nickel, or Monel.

Steel wins the ruggedness contest. Aluminum wins the lightness contest. Copper Nickel and Monel win the longevity and freedom from maintenance contest.

Part of the equation for any vessel is also resale. In this realm, aluminum does very well, albeit in this country not as well as composite construction. This is mainly a matter of market faith here in the US where we are relatively less educated about metal vessels. As for resale, a vessel built of Copper Nickel will fare extremely well. After all, the Copper Nickel or Monel vessel will have essentially been built out of money...!

Metal is an excellent structural material, being both strong and easily fabricated using readily available technology. In terms of impact, metal can be shown via basic engineering principles and real world evidence to be better than any form of composite. If designed well, a metal boat will be beautiful, will perform well, will be very comfortable, and will provide the peace of mind achieved only via the knowledge that you are aboard the safest, strongest, most rugged type of vessel possible.

It is said among dedicated blue water cruisers in the South Pacific that, "50% of the boats are metal; the rest of them are from the United States....!" Although this statement may seem so at times, it is fortunately not 100% true!!

It is my hope that the above essay will be of some value when considering the choice of hull materials. If you are intending to make use of metal as a hull material you may wish to review the article " Aluminum for Boats " that first appeared in Cruising World magazine, and the article " Aluminum vs. Steel " comparing the relative merits of both materials. Also, in defense of steel as a very practical boat building medium check out the article on " Steel Yachts ."

In addition, there are two excellent booklets available on our Articles and Other Links page. The first of them, the " Marine Metals Reference " is a brief guide to the appropriate metals for marine use, where they will be most appropriately used. It also contains welding information and a complete list of the physical properties of marine metals. The second booklet, " Corrosion, Zincs & Bonding " offers a complete discussion of electrical systems, corrosion, zincs, and bonding.

The 8 Main Characteristics of a Catamaran Hull

Posted on May 28, 2022

Ever wonder why your catamaran ride is so much smoother than other boats, including yachts? Why don’t I get as oozy as when I’m on other vessels, even when the water is choppy?

It’s all about the hull, my fellow water enthusiasts. Not just one but two hulls, positioned in such a way as to give us the most perfect ride we can get on the ocean, without us having to grow fins.

Catamarans have a wide beam, instead of a ballasted keel like a monohull vessel, which provides its steadiness. It has a more shallow draft and smaller displacement than a monohull with a similar length. Its hull offers stability, more space, privacy, no heeling, lower hydrodynamic resistance, and more.

Now that you’ve gotten the gist of it, let’s look in-depth at the hull characteristics.

main characteristics of a catamaran hull

1. Lower Hydrodynamic Resistance

Hydrodynamic resistance of the combined two hulls is usually lower than equivalent monohulls, needing less power from the sails or the engines. A catamaran’s broader posture on the water can decrease the feeling.

It also lessens the motions caused by waves, which can also produce smaller wakes than on a single-hulled vessel.

A cat’s hulls have a less wetted surface area, which means they burn less fuel. The boat may be propelled by one engine in mild winds.

2. Reduced Heeling

The term “heeling” refers to the tendency of a sailboat to lean to one side due to the force of the wind on its sails. The boat is oriented so that the wind hits the sails at an angle and pushes them to one side of the boat, propelling the boat.

Catamarans are the safest way to navigate the oceans because they have no heel angle. Cruising catamarans are safer than monohulls for their crew because they offer better protection and a no-heel environment.

As a result, the crew will be less exposed, make fewer mistakes due to exhaustion, and arrive at their destination more rested.

3. Less Displacement

Boats that employ buoyancy to support their weight are referred to as displacement hulls. To provide its name, it is partially immersed and moves by causing water to be dislodged.

Its weight is equal to the amount of water it dispenses. In stormy water, it maintains its sturdiness. As a result, cruisers and sailboats alike frequently make use of this design.

4. Stability

In place to evade capsizing as well as heeling, the catamaran depends on hull stability, while hull stability depends on buoyancy and beam. About half of a typical cruising catamaran’s length is its beam.

Say, for example, that the boat was 50 feet in length, the beam might be about 25 feet broad so that you could maintain the balance between heel and righting moment.

On a cat, passengers who are prone to motion sickness will be far less affected by the impacts of motion than they would be on a monohull. The cook’s task is made much easier while traveling and at anchor because of the cat’s extra stability. When compared to monohulls, catamarans have less rock and roll.

Once you’ve decided to sail, you won’t have to worry about scrambling to stow things or securing them with bungee cords. Relatively rough seas have little effect on most of the ship’s equipment.

5. A Bridgedeck Connects Them

You can’t ignore this reality! Bridgedeck clearance, or the gap between the water and the bridge deck, is an important factor in a catamaran’s safety and reliability. Ocean waves have room to flow between the hulls thanks to the bridge deck clearance.

Our catamaran’s hulls create waves that converge underneath the bridge deck, requiring a larger buffer for the bridge.

If you don’t have enough clearance, your catamaran will be pounded by the waves.

In addition to slowing your catamaran down by 3 to 4 knots, rough seas can also put your guests’ health and safety at risk, as well as inflict significant damage to your boat and rigging.

6. Can Maneuver Shallow Water

A boat’s draft is the distance from the water’s edge to the hull’s center of gravity. It is important to know the depth of a boat’s draft since it determines the quantity of water that can be displaced for safe passage. As a result of their parallel hulls’ buoyancy, catamaran vessels can have fewer drafts without affecting their stability or their ability to maneuver.

When it comes to hull stability and handling, monohull vessels are built with a deeper draft to protect against capsizing as well as heeling. Because of this, the catamaran was deemed suitable for use in shallow waters.

Boating activities such as swimming and fishing aren’t the only ones that benefit from being able to dock in shallow water. A boat with a shallow draft will be more maneuverable in areas where you may have to sail through a variety of shallow areas, some more than others.

Monohull boats with deeper drafts may be unable to access shores, intertidal zones, coral reefs, or even sandbars because of their shallower depths. Also, shallow-water swimming and snorkeling are two of the most popular pastimes for sailors plus their families. To ensure a smooth experience, you’ll need secure access to shallow waters.

There are several types of fishing that require a catamaran’s dependability and readiness, and these include trout, oystering, and clamming. These activities become much easier when you can navigate shallow waters without worrying about running aground or striking underwater objects.

7. Allows Cat to Have More Space

It certainly does. Eating and preparing food can be done side by side. One hull is used as a big cabin in the “owner versions,” which are fantastic for parties.

Most comparable-priced monohulls lack the amount of interior room seen on catamarans, especially in the primary salon, galley, plus cockpit. As a result, their cabins tend to be larger, and now even the tiniest cat in the group has a stand-up headroom in each one.

8. Offer More Privacy

The distance between the two hulls allows for greater privacy than on monohull yachts, which tend to be closer together.

Because of the configuration, a cat offers more solitude than monohulls, and the increased separation between the main living area and the cabins makes it simpler for children to go off to sleep at a normal hour when aboard.

What Are the Differences Between Catamaran and Monohull Sailing?

Most characteristics of sailing a catamaran resemble those of sailing a monohull. In most Catamaran Sailing Boats, the abilities you acquire on a monohull, you can transfer to a cat. A few minor distinctions must be made though, including these.

Keep a steady speed during the tack to avoid “winning,” which can occur if you relax the mainsheet too much. Winning occurs when a catamaran’s larger mainsail tries to direct the boat towards the direction of the wind.

Gybing on the monohull necessitates much more caution, therefore you have to slow down your gybe considerably. Travelers on catamaran boats have an advantage since they can sustain a pace while gybing to depower the main.

You can tell when you’ve got too much sail on a monohull by the way the boat is heeling, which tells you it’s time to reef. Since catamarans don’t heel, we have to be super cautious when reefing the enormous mainsail because it’s so heavy.

In most cases, the first reef will be thrown in between wind speeds of 18 and 20 knots (determined by the size of our yacht) and the second reef will be thrown in as the wind speeds reach up to 25 knots.

Many elements of sailing a catamaran are fairly similar to those of a monohull, so making the switch should not be too difficult.

Are Catamarans Safer Than Monohulls?

Exactly what I was looking for! People are less likely to drown when sailing on a catamaran than on a monohull, thanks to the boat’s greater stability. They are bigger, more steady vessels. In most cases, this makes cats “safer” than a monohull of the same size.

Having two engines makes catamarans “safer” in the event of an engine breakdown. Sailing is the sole alternative on a monohull boat if the motor is out of commission. In the event of an emergency, the second engine on a catamaran is always ready to assist!

Is Sailing A Catamaran Easier Than Sailing A Monohull?

It is more difficult to sail a monohull because of heeling and the constrained space it has to offer. In greater gusts, monohulls heel, making it harder to accomplish most activities.

Sailing aboard a heeling vessel is more difficult for a variety of reasons, including heading forward to a reef, having to haul in a sail, or just moving around the boat.

The greater stability and space provided by catamarans, on the other hand, make moving around the boat easier than on other types of boats. The fact that catamarans are typically referred to as “easier” to sail is due to this.

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