sailboat motor size calculator

How to Calculate Outboard Motor Size for Sailboats

It seems so complex to pick the right engine size for your sailboat. I was done with complex calculations and tried to make it easier here.

How to pick the right outboard motor size for your sailboat? To get the right amount of horsepower needed to efficiently propel a sailboat, divide the displacement of the boat (in lb) by 550. You need approximately 1 HP per 550 lb of displacement or 4 HP per 2200 lb. Most sailboats don't need a motor with more than 30 HP.

In this article, I'm talking about small outboard engines for sailboats. We're talking about displacement hulls here, so in other words: keel boats. They need more power than flat bottoms.

But they're not powerboats - so it's not our mission to go fast. It's our mission to get decent speed, good control over the boat, and the best possible fuel efficiency. Without breaking the bank of course.

On this page:

How to pick the right motor size, other factors that are important for size, why is the right motor size important, is there a max hp for sailboats, in conclusion, related questions.

Sailboats need way smaller engines than powerboats. That's great news (unless your ultimate goal is speed), because it's cheaper to buy, cheaper to drive, and cheaper to maintain.

The amount of power you need is related to the hull displacement of your boat.

I like to use the simple formula:

HP = displacement (lb) / 550

So 1 HP for every 550 lb displacement, and 4 hp per 2200 lb.

Here, HP is the amount of horsepower you need to reach the maximum hull speed. This is in optimal conditions. So you have smooth water, no windage, a clean and polished hull, and so on.

If you want to get it absolutely right, you also need to correct for propellor size. And of course, a lot of other factors come into play (more on that later). But generally, these engine sizes will work with the following weights:

Weight	HPs	Typical boat length
1,000 lb	1-2 HP	18'
2,000 lb	4 HP	20'
3,000 lb	6 HP	22'
4,000 lb	8 HP	24'
5,000 lb	9 HP	26'
6,000 lb	11 HP	26'
7,000 lb	13 HP	27'
8,000 lb	15 HP	28'
10,000 lb	18 HP	30'
12,000 lb	22 HP	32'
15,000 lb	28 HP	36'
18,000 lb	34 HP	40'

That sounds about right to me. But remember that these are all rough estimates: I just try to give you a ballpark figure. There is no one formula to get an exact number. The hull design, sailing conditions, and your personal preference are all very important.

If you're serious about getting a new engine, I definitely recommend to get advice from an expert . But you know, salespeople always recommend the Turbo version. Remember that you don't have to overpower a sailboat. Usually you don't need anything over 30 HP. So at least you now know what will work on average.

What is hull displacement?

• Hull displacement is the weight of the boat, or the amount of water the boat displaces.
• Maximum hull displacement is the weight of the boat when it's fully loaded, including crew.

The weight of the boat is the same as its displacement, because the weight of any object is exactly equal to the weight of the water it displaces (aka: pushes aside). This is called Archimedes Principle.

The weight slightly differs in saltwater from freshwater, because saltwater is heavier. In saltwater, the boat gets a bit lighter. So in theory you can use a smaller engine for a bluewater boat, but in practice this is offset by the stronger current and wind.

How to find the displacement of your boat?

Most manufacturers simply give you the displacement of your boat. If you can't find any data, because, for example, you own an old boat, you can weigh your boat on a truck scale. You can also haul it out and measure it (which is painstaking work).**

Tip: if you're gonna weigh your boat, simply drive it onto a truck scale, and retract the weight of the trailer from the total weight.

Of course, it's not so simple. This formula gives a rough estimate. But for me this was way clearer than all that black magic that I get when I ask people what size engine I should get.

Let's look at the things this formula doesn't take into account.

You need more HPs	You need less HPs
4-stroke engine	2-stroke engine
smaller propellor	larger propellor
gas (less torque)	diesel (more torque)
multihull (high windage)	monohull
long distances or against wind	just in and out marina
bluewater sailing	lakes and inland sailing
wooden boat	fiberglass boat

2-strokes are more powerful than 4-strokes. Two-stroke engines fire once every revolution and four-strokes fire once every other revolution. This makes the 2-stroke twice as powerful. They provide more torque at a higher RPM. But they also wear more quickly. The 4-stroke will last you a lot longer, and its also more fuel efficient.

The right propellor size is just as important as having enough horsepower. With a smaller prop diameter, it has to work harder to generate the same propulsion as a larger diameter. But you can't just go larger always. The prop affects the RPM of your engine, and you have to get in the right range (more on this later). You also have to check the maximum diameter that fits your boat.

Diesels have more torque, because the compression rate is higher than that of gasoline engines. So if you consider a diesel, you can do with less HPs.

High windage hulls (multihulls) need a bit more. A multihull (or larger hull in general) suffers from more friction because of the larger surface. So the engine needs to work a little harder.

If you sail longer distances under power , or against the wind it's a good idea to get a larger engine (but not too large). This helps you to save on fuel since you have lower RPM. Especially if you sail offshore or on open sea. The engine needs to work harder due to stronger wind and current.

If you're just sailing in and out of the marina under power, you may need less HP.

Smooth hull designs need less HPs than bulky hull designs, like the classic wooden clippers and crabbers for example.

It matters to get the right size outboard motor for a couple of reasons.

First of all: smaller engines are cheaper, so you save money on buying the engine.

Secondly: smaller engines use a lot less fuel, so you save money on using the engine.

Thirdly: smaller engines are cheaper to maintain: so you save money on maintenance.

So why not get the smallest engine and get the best fuel economy? There are a couple of advantages to getting a (slightly) bigger engine:

• More power means more control (easier to stop the boat, in case you need to)
• Finding the sweet spot might actually reduce fuel consumption

The sweet spot

To perform optimally, an engine should get up to speed. The problem with an overpowered boat is that the engine won't rev up to 80 - 90% of the RPM. This kills fuel efficiency and also the cooling system won't operate optimally.

• The optimal cruising RPM of the engine is about 85-95% of the maximal RPM
• You should reach cruising RPM at hull speed, so your engine should be at about 90% RPM

The propeller size is very important for the RPM. If your prop diameter is too wide, the engine can't get up to speed and struggles to build power. Bad for fuel economy, bad for the engine, and bad for performance.

On the other hand, if your prop is too small, you don't make use of the engine's full power.

If you struggle to get to high RPM, your prop is too large. If your engine is constantly in the red, you're underpropped.

So don't go too big on the prop, but also don't go too small. The easiest way to get it right is to check the engines manual and see what the manufacturer recommends.

You can definitely go too big on a sailboats engine. An overpowered yacht doesn't make any sense. True, it can look cool, but it can't feel cool. Every displacement hull has a maximum hull speed. That means that it cannot go any faster than the max speed. So if your engine can cruise at that speed, it's not getting any better.

The problem with displacement hulls is that they displace the water, or in other words: they push the water in front of them. They cannot move any faster than they can push away the water. And because the resistance increases as speed increases, there's an absolute, physical speed limit for each keelboat.

That's why powerboats have to get out of the water to reach top speed.

Fun fact: the longer your boat, the higher the hull speed. Want to know the maximum hull speed for your boat? You can find it in this article .

So, you can't go faster than your maximum hull speed, so a 50+HP engine is kind of ridiculous. Bear in mind that a large engine also has the following disadvantages:

First of all: larger engines are more expensive, so you spend more money when buying the engine.

Secondly: larger engines use a lot more fuel, so you spend more money when using the engine.

Thirdly: larger engines are more expensive to maintain: so you spend more money on maintenance.

Also, if your engine is too big, it doesn't reach the optimal cruising RPM, so your fuel economy also gets really bad FAST.

I suggest getting the smallest possible engine that gets you to maximum hull speed while it's at roughly 90% of the RPM. As long as it gives you enough control and good handling, it will get you there. If you give up on going fast, you can actually get really good fuel economy and your engine will last you probably 20 years.

If you want to go fast, a sailboat is not the right one for you. You should instead get a powerboat.

I'm just kidding. Read my 13 Reasons Why Sailing is Better Than Powerboating here .

Do sailboats have motors? Most sailboats are power assisted boats, which means they have a small auxiliary engine to cruise in light air. When a sailboat is sailing under engine power, it is considered a motorboat and it doesn't have right of way.

Thanks for answering my questions.

Taylor Bishop

Thanks for explaining how you can figure out what size you need for an outboard motor. You mentioned that you should find the displacement by weigh a boat on a truck scale. I’m interesting to learn if you need to regularly weigh it in case the hull displacement could change or if it will always be consistent.

Shawn Buckles

Hi MitI, you’re welcome, my pleasure.

Hi Taylor, my pleasure.

You don’t need to weigh your boat regularly, as the hull displacement will stay consistent. You could literally see the hull displacement as the amount of space your hull takes up in the water. So as long as you don’t make any major changes to the hull shape or ballast of your boat, you should see no differences in displacement.

Roger S Johnson

How do you measure for shaft size, most outboard motors are for flat bottom and say measure to the bottom of the boat, most sailboats tapper to the aft. Where do you measure for a tapered bottom sail boat?

Will a 5 horse Honda 4 stroke be ok for a 25 foot Pearson Commander sail boat. Thanks for your time Luke

I think it would be Luke.

Great post, thanks for the info. A naive question from a soon-to-be sailor: I’m considering buying a 28 ft sailboat, with 2500 kg (ca. 5500 lbs) displacement. The engine is in pretty good condition, but is old and the original one (from 1977!), so I am also thinking of an alternative scenario in which it fails. I know that in my area replacing an inboard engine will cost double the price I’m putting down for the boat, and since I’m on a budget, that simply won’t be an option and outboards seem to be cheaper. So the question is: is it possible to put an outboard engine on all boats? Is there some factor that would make it impossible to mount an outboard engine on the boat? Thanks!

Garth Powelson

What is minimum length that a sailboat can go without an outboard. Does a 29’ “require by law” to have engine?

Hello Mr. Buckles, Thanks for the informative article. I’m looking to get the smallest possible outboard for my 1.5 ton displacement fiberglass monohull Hood 23’ sloop. Can I get away with a 4HP?!? What size prop would I need?!? (I’m only going to use it when there is NO wind, and, if I can stay 4HP or below, I am not required to register my vessel—which is pretty cool, so here’s hoping!)

Thanks again, Ship

Hi, I’ve got a older Pearson 39’ . I’m looking to remove the old 40 ho westerbeke and go electric. Unsure of what hp is going to be needed?

emilio h javier

i am purchasing a catalina 22 ft. i have in mind a 4 HP motor. what would be the length of the shaft.

I am considering buying a 25 ft sailboat with a 7200 lb displacement. The boats top speed is listed at 7knots per hour but the diesel motor does not work. The owner has a 9.9hp outboard that can be purchased with the boat. Is 9.9hp enough to power the boat to at least 5 to 6 knots per hour? Thanks. Rick

What weight outboard would be too much for a 20’ Santana, displacement 1,350 lbs? I don’t want too much weight at the back. I want the boat to be seaworthy.

I have not seen this amount of BS in years :) I’m not a marine engineer, yet physicist & avation engineer. You even can’t tell the difference between mass of the vessel and diplacement :D Fcking genius.

Leave a comment

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Maximum Boat Horsepower Calculator

If you’re curious to know what the largest engine you can legally put on your boat is, try this calculator. It uses the federal regulations to compute the largest outboard engine you’re allowed to install.

The federal government regulates the horsepower of outboard motors that can be installed on boats. The maximum HP is determined by calculating the boat’s factor (the length multiplied by the width) and then looking up the allowable engine size in a table.

STEP ONE: Work Out Your Factor

Step two: convert factor to maximum horsepower, federal regulations about maximum horsepower, calculation method, manufacturer limits, the final word, horsepower calculator.

This step-by-step calculator will help you work out your maximum boat horsepower.

First, you need to workout your FACTOR, this is calculated using boat length and transom width. This calculator will help work out your FACTOR:

Using your FACTOR worked out above, you can find your maximum horsepower.

If the FACTOR is 52 or below, use this table:

Factor As Calculated Above	0-35	36-39	40-42	43-45	46-52
Horsepower Capacity (All Other Boats)	3	5	7.5	10	15
Horsepower Capacity (Flat Bottom/Hard Chine Boats)	n/a	3	5	7.5	10

If the FACTOR is 53 or more, use these calculations (round up to nearest 5):

If the boat has remote steering and at least 20″ transom height	(2 × Factor) −90
If the boat has no remote steering, or less than 20″ transom height	(0.5 × Factor)−15
If the boat has no remote steering, or less than 20″ transom height	(0.8 × Factor)−25

I’ve been playing around with the idea of putting a larger outboard on my boat , and decided to read the rules and regulations to see what’s allowed. The federal government has regulations for just about everything, so it should come as no surprise that they created one for the maximum horsepower capacity of your boat.

The Federal Regulations governing outboard motor horsepower capacity are listed in 33 CFR § 183.53 – Horsepower capacity . Feel free to read these regulations!

I did, and I discovered they are written in lawyerly language with a lot of exceptions and footnotes. Every time I found an outboard engine that interested me, I had to perform a tedious calculation and refer to the rules to see if it was legal or not. After a dozen or so times, I became very frustrated and decided to developed this tool. It makes the evaluation process extremely simple and eliminates some of the mistakes that I kept making.

Feel free to use this handy calculator to determine the largest outboard engine that is legal for your boat.

The federal government uses two methods for determining the largest engine allowable for your boat. One method is a calculation based on your boat’s dimensions and the other is a performance test. The calculator above uses the computational method. This approach calculates the “factor” for your boat that is spelled out in the rules. The factor is simply the length multiplied by the width of your boat. Based on this value, you lookup the maximum horsepower in a table. If you have a flat bottom boat, you’ll need to downsize the engine horsepower.

For large boats, there are a lot of special rules and things to consider. Each one of these factors results in an adjustment of the maximum allowed horsepower. Here are features your boat may have that will affect the allowable engine size:

• remote steering
• transom height
• boat bottom

Other Methods

Most boats have a capacity plate that gives you a lot of information about your boat’s design capacity, including the maximum engine horsepower. If your boat was built after 1972 then it almost certainly has a National Marine Manufacturers Association Certification Plate. This plate lists the maximum motor horsepower that the boat designers built the boat to handle.

The capacity plate also lists the maximum number of passengers as well as the maximum weight your boat can safely carry. You can find this plate mounted near the helm.

If your boat doesn’t have a capacity plate or you can’t find it, you can likely find the information in the owner’s manual. Most manuals clearly state the maximum horsepower that the manufacturer deems safe and appropriate for your boat.

Don’t have the manual? Just call your boat’s manufacturer and ask. I’ve found the manufacturers to be very helpful. Every time I have a question about my boat, I’ve been able to get answer.

If you’re thinking about installing a bigger engine on your boat, you need to know what the federal regulations say. This calculator is a great screening tool to play around with, but it’s not meant to be a substitute for common sense or an expert’s opinion. You need to talk with a competent professional before making the decision about whether you can safely increase your boat’s power and how much. Talk to your mechanic and then contact your boat’s manufacturer to see what they say. The last thing you want to do is spend a lot of money only to create a safety hazard for you and your family.

Related Articles

What is the Best Outboard Motor for Saltwater  How to Measure Your Outboard Motor Shaft Length Picking the Best Small Outboard Motor

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What Size Outboard Motor for What Size Boat? (Boat Horsepower Chart)

Once you buy a boat you want, it is time to determine what size outboard motor for what size boat. As you can guess, there is no magic formula for estimating it precisely, but you can get rough numbers when you check relevant information.

You should start with a boat type , a maximum passenger number, and the equipment weight you plan to carry. Then, you can determine the maximum horsepower recommended for your vessel. Finally, think about 2-strokes and 4-stroke motor options, right propeller size , and fuel type. Sounds complicated? Let’s make it easier!

Table of Contents

Motor Size Matters

Things to consider when selecting an outboard boat engine, ways of pair an outboard motor with your boat size.

Yes, an engine size matters , and it is crucial to pair it with an adequate boat size to let it run smoothly.

Bigger motors with higher horsepower offer numerous benefits for comfortable boating, such as:

• You can sail at high speed
• A more sizable engine provides better handling, particularly when sailing at midrange speeds
• It makes water sports cozier
• You can count on better time maneuvering
• Such an engine will give you better control and provide more secure sailing in harsh weather condition
• Surprisingly, a motor with a bigger fuel tank uses less fuel

Downsides of a small engine

A too-small engine typically has difficulty powering a bigger boat so it will spend more fuel than a bigger model. Another problem is sailing with more passengers and equipment since extra load requires a motor with more horsepower.

Therefore, you should check the max HP a boat can handle before purchasing. That rating will show you a top weight it can carry, and you can risk uncomfortable and unsafe sailing, plus possible Coast Guard fines when exceeding it.

18 feet (5.5 m)	1,000 pounds (454 kg)	1 to 2 HP
20 feet (6 m)	2,000 pounds (907 kg)	4 HP
22 feet (6.7 m)	3,000 pounds (1,360 kg)	6 HP
24 feet (7.3 m)	4,000 pounds (1,815 kg)	8 HP
25 feet (7.6 m)	5,000 pounds (2,278 kg)	9 HP
26 feet (7.9 m)	6,000 pounds (2,720 kg)	11 HP
27 feet (8.2 m)	7,000 pounds (3,175 kg)	13 HP
28 feet (8.5 m)	8,000 pounds (3,629 kg)	15 HP
30 feet (9 m)	10,000 pounds (4,536 kg)	18 HP
32 feet (9.75 m)	12,000 pounds (5,443 kg)	22 HP
36 feet (11 m)	15,000 pounds (6,804 kg)	28 HP
40 feet (12.2 m)	18,000 pounds (8,165 kg)	34 HP

The vessel horsepower ranges from 2.5 to 1,000+ HP for contemporary outboard motors . You should pick out the most powerful one your boat can handle to provide smooth and comfy sailing.

Horsepower is used to measure the engine power. On average, your vessel will need 5 HP per 2,200 pounds (1,000 kg) its weight. Small boats shouldn’t use more than 75% to 80% of the max HP because of the possibility of rough waters and headwinds while sailing.

Only that way, you can count on enough thrust to drive as desire. You should think about a few things when determining how much horsepower your boat needs, including:

Manufacturer limits

Manufacturers’ recommendation always shows a maximum passengers number and cargo weight your boat can carry. You can also find the absolute max boat HP in the manual.

If you have a boat built after 1972, you can find a capacity plate with this information inside the transom. Older vessels, home-built models, and those made overseas typically don’t have that plate.

However, you can contact the manufacturer or look for their manuals online to discover limitations and recommendations for your boat.

Horsepower-to-weight ratio

The only way to precisely determine your boat’s necessary horsepower is to consider its weight. You can calculate this ratio in horsepower per pound or vice versa.

HP/boat weight = required horsepower per 1 pound (0.45 kg)

boat weight/HP = number of pounds per horsepower

Let’s say you have a boat weighing 5,000 pounds (2,268 kg) with an engine of 300 HP. The calculation will look like:

• 5,000 pounds / 300 HP = 16.6 pounds (7.5 kg) per horsepower
• 300 HP / 5,000 pounds = 0.06 HP per 1 pound (0.45 kg)

You should do the math to match the desired outboard engine with your boat size correctly. The result will show how fast your boat will go since its speed will be higher when these numbers are lower.

Horsepower your boat needs will also depend on its use. Any additional equipment weight or pulling water skiers behind the vessel increase the power it needs.

Number of passengers

In this case, the rule is simple. Your boat engine will work properly only when it carries 25 to 40 pounds (11.5 – 18 kg) per horsepower.

Once you have this information, you can check the boat weight and add the total weight of the passengers you plan to host on it. Then, you can effortlessly determine engine HP that matches your boat.

Fuel efficiency

As you can guess, the motor HP impacts the boat’s fuel efficiency, but that doesn’t mean a higher-horsepower engine necessarily consumes more fuel.

For instance, your motor with lower HP will use more gas at full throttle than the one with higher HP running with less throttle.

Engine type

Both 2-stroke and 4-stroke engines are good options. You should choose one or another, depending on the feature crucial for you.

The rule of thumb is that 2-stroke motors are lighter and less expensive, while 4-stroke models with the same power are quieter. However, these differences are diminishing nowadays.

Engine weight

One of the vital things is to match the outboard engine weight and boat size . That way, you will ensure sufficient power for the desired speed without jeopardizing safety and maneuverability.

Most 2 to 10 HP outboard engines weigh 35 to 90 pounds (16 – 41 kg). On the other hand, you should count on 100 to 160 pounds (45.5 – 72.5 kg) for a 20 to 30 HP motor. In this case, you can have a problem carrying a too heavy engine around.

Required speed

The rule of thumb is that your boat needs more power to reach a higher speed. The equation says that 1 HP can move 40 pounds (18 kg) of boat weight at 20 mph (32.2 km/h). According to that, you can calculate that your boat weighing 2,000 pounds (907 kg) needs 50 HP to reach the speed of 20 mph (32.2 km/h).

Federal regulations

In the US, it is illegal to overpower your boat. The Code of Federal Regulations clearly defines vessel overpowering, so you should check it before getting into a situation to break federal law.

You should know that your boat’s horsepower directly affects its insurance , and models with higher HP typically has higher premium costs. Keep in mind that the insurance company won’t cover a boat with an upgraded engine that exceeds the max HP recommended by the manufacturer.

The best way to properly pair an engine with a boat of a particular size is to determine its horsepower.

2 to 3.6 HP engine  

These small, less than 40 pounds (18 kg) weight, 2-stroke outboard engines are an excellent option for portable boats, like:

• Inflatable boats
• Small pontoons
• Small sailboats

They can push small boats at 5 to 9.5 mph (8 – 15 km/h), depending on weather conditions, current, as well as boat weight and design.

5 to 6 HP engine

This horsepower range is ideal for boats weighing 8 to 11.5 pounds (18 – 25 kg), including:

• 12 feet (3.65 m) long inflatable boats
• Portable folding boats
• Large canoes
• 8 to 12 feet (2.5 – 3.65 m) long inflatable and dinghy boats

Such an engine can propel your vessel with one person at 10 to 17.5 mph (16 – 28 km/h).

8 to 9.8 HP engine

You should pick out this small but powerful 11.5 to 20.5 pounds (25 – 45 kg) heavy outboard engine for your:

• 10 to 14 feet (3 – 4.5 m) long inflatable and aluminum boats
• Large portable folding boat and sturdy canoe
• Up to 14 feet (4.5 m) long light fiberglass boats

An outboard motor in this range is an excellent option for propelling a boat carrying a light load and two people at 9.5 to 17.5 mph (15 – 28 km/h).

15 to 25 HP engine  

Although powerful, these 16.5 to 23.5 pounds (36 – 52 kg) heavy 4-stroke outboard engines are still in a small outboard HP class. They are an excellent choice for:

• 10 to 14 feet (3 – 4.5 m) long inflatable boat
• 12 to 16 feet (3.65 – 5 m) long Jon boat , fiberglass boat , and aluminum fishing boat

These smooth-running engines can propel most boats to 25 mph (40 km/h) when carrying several people and heavy loads.

Whatever boat type you have, you should buy the smallest possible outboard engine that can allow a max hull speed with 90% of the revolutions per minute. Unless you want a higher-speed boat , you can use your 20 HP engine for twenty years and get an excellent fuel economy.

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Look I’m getting confused. I’m looking at 20 to 21 foot fiberglass star craft. My weight is around 260 my girl is around180. The dealers are trying to sell me a 150 yamaha. A northern dealer is trying to sell me a 20 ft boat with a 70 hp yamaha. The price is nice for the 70 hp but I’m not shore that it’s good for that boat. Help.

The lower the “hp/lb” value, the faster the boat. For example, you have a 3,000-pound boat with a 200-horsepower outboard motor. A pound will correspond to 0.067 HP.

The weight boat is one of the variables to consider when finding the maximum horsepower. Calculating the ratio between horsepower and weight is straightforward. You only need your boat’s weight and its boat HP rating.

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• How Much Horsepower Do I Need for My Boat?

If you’ve been searching for a boat to purchase, or have reached the point at which you’d like to get a new engine, chances are you’ve asked the question, “How much horsepower do I need for my boat?” While the individual concerns may be different — wanting to make sure there’s enough horsepower or not too much horsepower — it’s a question many people who are interested in purchasing a new boat ask.

Sometimes, the follow-up question is, “Is it illegal to overpower a boat?” Whether you’ve been asking yourself one of these questions, or are just curious about how horsepower can affect the cost of fuel and insurance, we’ve gathered several pieces of information to educate you on horsepower. Through reading this information, you’ll be able to learn more about where it came from, how it’s calculated, how to determine the amount you need and the dangers that come with having too much.

What Is Boat Horsepower?

Horsepower is defined as a unit of power equal to 550 foot-pounds per second used to measure the power of an engine. Today, it applies to boat and auto engines, but its original use was to describe the power of a steam engine. In the late 1700s, a Scottish engineer named James Watt invented the first steam engine, which improved on a design  pioneered by Thomas Newcomen in 1712 . It was a big deal. This new steam engine could do the same amount of work as the former Newcomen engine, but used only one-quarter of the fuel.

Unfortunately, comparing the steam engine to the Newcomen engine wasn’t effective marketing, as most of the population was still using horses for mechanical work. To market his new product to this audience, Watt knew he had to come up with a way to compare the work of horses to the work of his invention. Through his experiments, he determined one horse could do about 33,000 foot-pounds of work in one minute — meaning a horse could lift a 33,000-pound weight one foot in one minute. Therefore, he defined one horsepower as 33,000 foot-pounds of work per minute — or 550 foot-pounds per second.

There were a few flaws with this new measurement — the biggest being the assumption that a horse could continue to work at that consistent rate instead of tiring out. However, Watt didn’t let that fact bother him, and it didn’t bother his customers, either. Comparing the power of a horse to the power of a steam engine showed Watt’s steam engine could do the work of five horses, and it went on to become an integral part of the Industrial Revolution.

How Is Boat Horsepower Calculated?

The relevance of horsepower, the measurement of 550 foot-pounds per second, didn’t stop with the Industrial Revolution. The measurement of horsepower was converted into other units of measurement. For example, other experiments determined one horsepower equals 746 watts of energy. In other words, if you put a one-horsepower horse on a treadmill, it would be able to operate a generator producing 746 watts. Engineers also did calculations to relate horsepower to torque, which is especially important for boat engines.

Torque is easiest to explain through an example. Imagine you have a large socket wrench with a two-foot handle. You apply 50 pounds of force to that handle — when you do that, you’re applying a torque or turning force of a total of 100 pound-feet to the bold. Keep in mind, with that calculation, you could get the same result — 100 pound-feet of torque — by applying one pound of force to a socket wrench with a 100-foot handle, or by applying 100 pounds of force to a socket wrench with a one-foot handle.

In an engine, torque produces power — so being able to relate it to horsepower is critical. A device called a dynamometer applies a load on the engine and then measures the amount of power it produces to determine torque. Marine dynamometers are available, too — taking into consideration boat operation by simulating on-the-water operation. Once the torque has been determined, you’re able to convert torque to horsepower by multiplying torque by revolutions per minute (rpm) and dividing that product by 5,252. The divisor, 5,252, comes from a series of calculations that convert rpm to radians per second.

Through using a dynamometer, you’ll be able to see the horsepower versus rpm values for the engine, which is especially important for calculating peak horsepower. Engines will have a point in rpm at which the power available from the engine has reached its maximum. This is known as peak horsepower — it’s often documented as “___ HP at ____ rpm.”

Determining How Much Horsepower You Need

Once you understand the history of horsepower and how that’s connected to the calculation we use today, it’s natural to wonder how much horsepower your boat needs. There are several factors to take into consideration when you’re determining how much horsepower you need — the manufacturer’s limits and recommendations, boat horsepower-to-weight ratio, fuel efficiency, use of the boat, number of people on the boat and an industry rule of thumb. Here are some things to consider in each of these areas to help you answer the question, “How much horsepower do I need for my boat?”

Manufacturer Limits

The easiest place to start is with the boat’s capacity plate — or owner’s manual. In addition to giving you a maximum for passengers and cargo, the manufacturer will also spell out the absolute maximum boat horsepower. If you have a boat that was built before 1972, came from overseas or was home-built, you may not have a capacity plate, but boat owner’s manuals are often available online. If not, you can always contact the boat’s manufacturer to inquire about their limits and recommendations for your particular boat model .

Boat Horsepower-to-Weight Ratio

When you’re trying to determine the amount of horsepower you need, it’s essential to consider the weight of the boat. The boat horsepower-to-weight ratio is simple to calculate and can be expressed in horsepower per pound or pounds per horsepower.

Let’s say, for example, your boat weighs 5,000 pounds, and it has a 300-horsepower engine. Taking 5,000 divided by 300 gives you a result of 16.6 pounds per horsepower. Doing the opposite calculation — taking 300 divided by 5,000 — gives you a result of 0.06 horsepower per pound. The lower the number, the faster your boat will go. Remember the boat horsepower-to-weight ratio once you’ve decided on horsepower and are matching outboard to boat size. While one or two outboards may give you the same horsepower result, keep in mind additional weight will accompany each additional outboard motor.

Fuel Efficiency

The amount of horsepower you choose will impact your fuel efficiency. However, just because you have a higher-horsepower engine does not necessarily mean you’re going to use more fuel. According to  Boating  magazine , running your gas engine between 3,000 and 3,500 rpm and your diesel engine at three-quarters throttle is the sweet spot for fuel efficiency. If you’re running a lower horsepower engine at full throttle all the time, it’s going to use more gas than a higher horsepower with less throttle. Keep this fact in mind as you consider what horsepower to choose.

For those who prefer exact calculations,  Boating  magazine  has provided some calculations you can use to run some of the numbers. You’ll be calculating the gallons of fuel that are burned per hour (GPH). To do this, you need to know gasoline weighs approximately 6.1 pounds per gallon, while diesel weighs about 7.2 pounds per gallon. You also need to know a well maintained four-stroke gasoline engine is estimated to burn about 0.5 pounds of fuel per horsepower per hour, while a similar diesel engine is estimated to burn 0.4 pounds of fuel per hour.

The equation is GPH = (specific fuel consumption x HP)/fuel specific weight. For example, if you want to determine the fuel consumption for a 300-horsepower gasoline engine, you would calculate (0.50 x 300)/ 6.1, giving you a result of 24.5 gallons per hour.

As with most decisions related to boating, the use of the boat is always a factor. Are you using the boat just to cruise with friends and family ? Or will you be pulling water skiers, wakeboarders and tubers ? The addition of pulling someone behind the boat — and the additional weight of storing watersport accessories — increase the need for power and are often a reason to consider adding horsepower to your boat.

Number of People

Once you’ve considered the use of your boat, the next question to ask is what the normal number of passengers for your boat will be. If it’s usually just you and a friend or a spouse, that weight is different than if you enjoy boating with several of your family members and friends.

Rule of Thumb

If you’re the type of person who doesn’t need precise calculations — or just doesn’t want to be bothered with them — the  Boat Trader  blog offers a rule of thumb to use when determining the amount of horsepower for your boat. The rule of thumb is based on weight alone, and says you should have between 40 and 25 pounds of weight for each horsepower.

For example, a 5,000-pound boat could have an engine with between 125 and 200 horsepower. Yes, the range is wide, but that’s because boats have a variety of different designs and handle differently. While this rule of thumb can be a helpful way to get a ballpark range, it still requires some guesswork when it comes to considering your boat handling.

Federal Regulations

Is it illegal to overpower a boat? According to the federal government, yes. There are a few different ways you can define overpowering a boat. The horsepower capacity section of the  Code of Federal Regulations  contains its definition of overpowering a boat.

There are two different ways the federal government uses to determine the maximum horsepower for any given boat — one is a computation, and the other is a performance test. The way that is used depends on the boat. For the majority, the computation method is best to use. You multiply your boat length by the transom width. Then you take that number, which is known as the “factor,” and  match it to a horsepower capacity according to the Code of Federal Regulations Table 183.53  — Outboard Boat Horsepower Capacity, shown below.

Don’t forget to adjust the result based on remote steering, transom height and boat bottom.

The performance test method is for boats that are 13 feet or less in length, have remote wheel steering, have a maximum capacity of no more than two persons and at least a 19-inch transom height — or at least a 19-inch motorwell height and at least a 15-inch transom height. Through this method, there are very specific instructions for boat preparation addressing everything from motor mounting to fuel tanks to ensure consistency. There are equally as specific instructions for the conditions in which you can perform this test.

The first part of the performance test is the quick-turn test. Setting the throttle at a low maneuvering speed and facing straight ahead, you then turn the wheel 180 degrees in half a second or less and hold it there. If you can complete the 90-degree turn without losing control of the boat or reducing the throttle, your boat has passed the test. Repeat, increasing the turn in speed until you can no longer pass the test, or you reach the maximum throttle. The maximum horsepower the boat can use while still completing this test is defined as the maximum horsepower capacity, unless it is more than 40 horsepower, in which case, the maximum horsepower capacity is capped at 40.

The good news is, these rules are in place for boat manufacturers, so assuming your boat’s manufacturer is following the federal regulations, you can consider the maximum horsepower capacity listed on your boat meets the federal regulation for its maximum horsepower.

Insurance Considerations

The amount of horsepower your boat has will influence your boat insurance, which is another fact to consider. There are three main areas of insurance the amount of horsepower you choose for your boat will affect — overall coverage, premium cost and type of policy.

Overall Coverage

There is also a chance your insurance company will not cover a vessel that exceeds the boat manufacturer’s max horsepower. This is an extremely important factor, as it could affect your ability to get insurance coverage for your boat. If your boat is currently insured and you’re considering a motor upgrade, make sure you know your insurance company’s rules for horsepower limits. If you upgrade without abiding by these rules and notifying your insurance company, there’s a good chance they won’t cover any claim you file.

Premium Cost

While abiding by your insurance company’s rules for boat horsepower, it’s important to remember that doesn’t mean a change in horsepower won’t bring a change in your premium cost. As a rule of thumb, boats with higher horsepower will be more expensive to cover.

Type of Policy

In addition to solely considering the horsepower of the boat, the overall size of the vessel, which takes horsepower into account, may determine the type of boat insurance policy you need to get. For example, your homeowner’s or renter’s insurance policy will usually cover smaller powerboats with less than 25 horsepower. Boats that are larger and have more than 25 miles per hour horsepower almost always require a separate boat insurance policy.

Dangers of Overpowering Your Boat

Bigger and faster is not always better. Putting more horsepower behind your boat may seem like an innocent way to add some excitement to your boating experience, but it could cost you a significant amount of money in fines, lawsuits and damage to your boat. Here are a few examples of how your need for speed can get you in trouble when it comes to boat maximum horsepower.

Breaking the Law

Is it illegal to overpower a boat? In some cases, yes. There are federal laws in place to ensure the appropriate horsepower limits are listed on all boats. State and local laws regarding overpowering your boat vary. Make sure you know the state and local laws for the areas in which you’ll be boating. Otherwise, your excess horsepower may be putting you at risk for fines and other consequences.

Accident Lawsuits

Even if your state and local laws don’t address overpowering your boat, you’re still putting yourself at risk in other ways. If you’re in an accident, the fact that your boat’s horsepower is above and beyond the manufacturer’s recommendations will be in the accident report. While you may not have fines because of breaking state and local laws, you are very susceptible to being found negligent and the victim of a lawsuit, especially in a case where there are damages.

Too Much Weight

In recent years, more horsepower hasn’t always meant more weight. However, it is still true in some cases, and that additional weight is another component of high horsepower that can be dangerous. For example, the additional weight can make a self-draining cockpit useless, leading to flooding problems.

Boat Damage

Even with additional horsepower that doesn’t add weight to the boat, the additional speed applies pressure that can cause significant damage to your boat. Every part of your boat, from the transom to the bow, was created to withstand a certain amount of pressure and stress. If you decide to ignore the boat’s maximum horsepower and overpower it, you’re exposing every part of your boat to pressure above and beyond what it was designed to endure, risking significant damage to the hull of your boat.

Finding a Boat With the Ideal Balance

As a boat manufacturer, at Formula Boats we know the balance of giving you the power and speed you want while making sure safety is a priority. If you’re considering purchasing a boat, our online  boat builder  gives you the opportunity to fully customize several different boat models with a few different horsepower options. You can be sure horsepower options for each boat model we provide are within the limits we believe maximize your performance while maintaining safety.

Discover a boat you like through our boat builder? We have dealers located throughout the country ready to help you find your boat. Get started by  searching for the dealer  location closest to you on our website.

Even though we narrow down the options, it can still be tough to choose the amount of horsepower that will give you performance based on boat weight and use, but also fuel efficiency. If you’re interested in one of our boat models, but are still wondering how much horsepower you need, we’re here to help — please don’t hesitate to  contact us .

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Prop calculator.

Category	3 bladed prop	2 bladed prop	4 bladed prop
Prop Size
Prop Pitch

Calculated values from inputs used to derive size and pitch

Power required numbers, prop pitch numbers.

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I am not a spokesperson for the US Coast Guard or ABYC. For an official interpretation of regulations or standards you must contact the US Coast Guard or other organization referenced..

HORSEPOWER FOR OUTBOARD BOATS

HORSEPOWER CALCULATIONS FOR OUTBOARD MOTORBOATS WITH ENGINES GREATER THAN 2HP

Reasons for Safe Horsepower ratings

Start-in-gear  protection requirements for outboards.

The Rules are in 33 CFR Subpart D 183.51 - 183.53

Outboard horsepower ratings are based on a variety of factors.

These include; Centerline length. Maximum Transom width. Transom Height. Type of steering: remote or tiller. Hard Chine Flat Bottom or Other.

There is a formula for computing Safe Horsepower based on various combinations of the above factors. See the table below.  This only applies to a monohull boat less than 20 feet in length (6.096 meters)

Canada uses similar formulas for computing outboard power in kilowatts. (1 HP = 0.745 KW)

The below formulas do not apply to: sailboats , canoes, kayaks , and inflatable boats , that are designed or intended to use one or more outboard motors for propulsion. It does not apply to boats that are true multihulls.  A multihull makes two or more separate footprints in the water. A pontoon boat is a multihull.

Taken from Title 33 Code of Federal Regulations Subpart 183.53

.

Example 1: Bow Rider - Length = 19' 6" Transom width = 84" remote steering and 20" transom height.

(Length x Transom Width X 2)-90 = HP = ( 19.5 X 7.0 X 2) - 90 = 273 - 90 = 183 (136 KW)

Horsepower = 185 (138 KW)

Example 2: Length 12' Transom width = 46" Tiller steering and 15" transom height.

( Length x transom Width) = 45.96    Horsepower from table = 10 (7.5 KW)

Horsepower is rounded to the next multiple of five.

: is to be measured at the widest point of the transom including permanently attached parts of the boat such as rub rails. Click on the image for the full size
: Length does not necessarily include such things as swim platforms. However if the is molded into the hull and contributes to the buoyancy of the boat, it is included. If you need some more information on this call the at  . Click on the image for the full size
means: The boat is flat bottom if you can lay a straight edge across it, and there is no vee or curvature. A hard chine has no curvature.

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Beware of steering changes :  If your boat is rated for remote steering, and the buyer wants you to reconfigure the boat from remote steering to tiller steering, the boat can no longer be rated for the higher horsepower allowed with remote steering.  You will have to change the labels to the lower horsepower rating.

Conversely: if the boat is rated for tiller steering and the buyer wants it to have remote, you will have to put on a  label reflecting the higher horsepower rating.

Special Rules : For boats that meet the following qualification there is an optional test course method for determining horsepower:

13 feet or less in length. Outboard Powered. 2 or less passengers. Remote steering and 20 inch transom height.

See Horsepower page 2 :

How did we get these rules for power on outboard boats? Is there a better way

Most of the horsepower (or kilowatts in some countries) ratings for monohull outboard boats, used today, were developed back in the 1950's by the BIA and the Yacht Safety Bureau.  They ran tests on various boats and came up with the formulas for different types of boats. Primarily the limits on power are due to two factors, one is simply handling of the boat. Putting too much power on the back of a boat can make it handle erratically, spin out, and difficult to steer. The other is weight.  Putting a much larger engine on the transom can make the boat sit stern down and make it easy for a following wave to roll over  the transom and swamp the boat. 

Also different types of hull shapes handle power in different ways.  Flat bottom boats with hard chines are much easier to skid sideways in a turn, or to catch a chine and trip the boat, possibly flipping it, than a boat with a vee or rounded bottom.  Boats with a shorter transom height sit lower and are easier to swamp than a boat with a full height transom (20 inch). Engines come in standard shaft lengths, generally 15 inch, 20 inch and 25 inch, i.e. short shaft, standard, and long shaft.

But, over the years both boats and engines have changed considerably, so shouldn't the rules have changed?  Well, probably, but with everyone required to use the same formulas, at least they present a level playing field. Everyone has to use the same rules. Tests conducted in 2003 indicated that many boats may actually be overpowered using these formulas. More testing needs to be done.

Is there a better way?  There may be. Way back when, ABYC developed a test course standard for boats that don't have to meet the US Federal regulations.  This course is basically the same one that was originally used to obtain the formulas and measures a boats ability to maneuver through a course with turns, without becoming unstable.  The Coast Guard used this test course when it developed a separate standard for boats 13 feet or less, with one or two passengers, a 20 inch transom height and less than 40 horsepower. These were popular in the 80's, and called thrill craft, before the advent of Personal Watercraft. 

In the 1980's the Coast Guard and Mercury Marine did a lot of testing on different types and sizes of boats instrumented with accelerometers. These instruments measured the acceleration sideways in a turn.  The basic theory was, the more power, the greater the sideways, or lateral accelerations. Unfortunately the data revealed that there was no correlation, between power and lateral accelerations in a turn. There was more of a correlation between acceleration and the boat hull type. 

The International Standards Organization (ISO) has developed it's own test standard, a collision avoidance test, or barrier test.  An imaginary barrier is set up on the water and the boat with rated power, run at full throttle, has to avoid "hitting" (actually crossing over) the barrier by turning at a specific distance which is calculated based on the length of the boat. The test is repeated with larger engines until it can't avoid the barrier. The highest power with which it could complete the turn is the rated power.

On the more technical and scientific side, some naval architects and engineers have done research into resistance on planing hulls and developed formulas based on weight, wetted surface at speed, resistance, angle of the vee and other factors.  These formulas are used frequently to determine power for larger inboard powered planing hull boats but aren't really applicable to small (under 20 feet) outboard powered boats.

So where does that leave us?  The Coast Guard has been doing some testing comparing the ISO standard to the US standard to see how a boat rates under each standard. Whether this will develop into a new standard is open to speculation.

Start-In-Gear Protection:  Another important safety consideration is to prevent engines from being started while the boats drive is in gear.  This matters because people get tossed out of the boat when the boat suddenly starts and jumps forward.  Statistics showed a significant amount of  accidents in which someone had gone aft to work on an outboard engine,  the engine started up in gear and the person was thrown over the transom of the boat.  So the USCG adopted a regulation requiring outboard motors with greater than 115 lb. of thrust (about 2 HP or about 1.5 KW) to have a device that prevents the engine from being started when in gear.  However, ABYC has a standard, P-14 Mechanical Propulsion Control Systems, which requires start-in gear protection on inboard and inboard/sterndrive boats as well.  So most power boats have start-in-gear protection. Not to be confused with Kill Switches.

Kill-Switches : Emergency engine stop switches that stop the engine if the device is tripped by the operator being knocked down or thrown out of the boat.  Usually this is a simple lanyard attached to the key, or a switch.  When the lanyard pulls the key out or trips the switch the engine stops. There are more sophisticated devices on the market.  This is now a USCG requirement.   ABYC does have a standard for it, A-33 Emergency Engine/Propulsion Cutoff Devices, and most boats produced today have them,  especially Personal Watercraft. The industry standard for PWCs requires them to have a Kill Switch.

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Boat Horsepower Calculator

• Use this Boat Horsepower Calculator to estimate the power required to efficiently move your skiff. See the article “ Do You Really Need That Much Power? “
• Use English Units (pounds, cubic feet), no commas, please!

Category	Value	Comments
Hull dry weight, in pounds:
Fuel tank capacity, in gallons:
Fuel Density (in pounds/gallon):		Gasoline is about 6.1 lbs/gallon
Approximate Motor Weight (in pounds):		Just a guess, to start. Adjust using real data after you get close.
Secondary Propulsion Weight (in pounds):		This could be a kicker motor or an electric trolling motor
Number of Batteries:
Avg. Battery Weight (in pounds):		A typical battery is 65 lbs.
Number of passengers:
Avg. Passenger Weight (in pounds):
Livewell capacity (in gallons):
Water density (in pounds/gallon):		Saltwater is approx. 8.5 lbs/gallon
Ground tackle weight (in pounds):		Includes anchors, chains, ropes, etc.
Fishing Tackle Weight (in pounds):
Electronics Weight (in pounds):
Miscellaneous Gear, Group #1 (in pounds):		Notes on #1:
Miscellaneous Gear, Group #2 (in pounds)		Notes on #2:
Miscellaneous Gear, Group #3 (in pounds)		Notes on #3:
Total weight (in pounds)
Hull deadrise (in degrees)
Desired Cruise Speed (in statute MPH)
Estimated Horsepower:

\n") msg.document.write("\n") makeChartBar(msg, boatName[i], boatName[j], "LOA", LOAMax, LOA[i], LOA[j]) makeChartBar(msg, boatName[i], boatName[j], "LWL", LWLMax, LWL[i], LWL[j]) makeChartBar(msg, boatName[i], boatName[j], "Beam", beamMax, beam[i], beam[j]) makeChartBar(msg, boatName[i], boatName[j], "Displacement", displacementMax, displacement[i], displacement[j]) makeChartBar(msg, boatName[i], boatName[j], "Sail Area", sailAreaMax, sailArea[i], sailArea[j]) makeChartBar(msg, boatName[i], boatName[j], "Capsize Ratio", capsizeRatioMax, Math.round(capsizeRatio[i]*100.)/100., Math.round(capsizeRatio[j]*100.)/100.) makeChartBar(msg, boatName[i], boatName[j], "Hull Speed", speedMax, Math.round(speed[i]*100.)/100., Math.round(speed[j]*100.)/100.) makeChartBar(msg, boatName[i], boatName[j], "Sail Area to Displacement", sailAreaToDisplacementMax, Math.round(sailAreaToDisplacement[i]*100.)/100., Math.round(sailAreaToDisplacement[j]*100.)/100.) makeChartBar(msg, boatName[i], boatName[j], "Displacement to LWL", displacementToLWLMax, Math.round(displacementToLWL[i]), Math.round(displacementToLWL[j])) makeChartBar(msg, boatName[i], boatName[j], "LWL to Beam", LWLToBeamMax, Math.round(LWLToBeam[i]*100.)/100., Math.round(LWLToBeam[j]*100.)/100.) makeChartBar(msg, boatName[i], boatName[j], "Motion Comfort", motionComfortMax, Math.round(motionComfort[i]*100.)/100., Math.round(motionComfort[j]*100.)/100.) makeChartBar(msg, boatName[i], boatName[j], "Pounds/Inch", PPIMax, Math.round(PPI[i]), Math.round(PPI[j])) msg.document.write("

Report any problems to . will continue to host Carl's Sail Calculator on his Web site; please direct correspondence to him.

Carl's Sail Calculator v3.55 . For multihulls, try this site

This page works with all standard browsers on Mac OSX, Windows 7 or later, and Linux. It does not render properly on Apple iPads and iPhones running iOS 10. This is an OS problem beyond my control.

Some data were moved and recalculated from earlier versions. If you find any basic measurements that you know to be incorrect for any of the boats please send the corrections to Tom .

: When you select a boat, its parameters appear below in .

") for(i=0;i ")} // -->

") for(i=0;i ")} // -->

Select one boat in each column above, and press

: Note that length overall, length of waterline, and beam are in feet, displacement in pounds, and sail area in square feet. Do not use or in your numbers, which should be in the form, for example, 1000.50. Note that this site uses the American standard, with a period instead of a comma as a decimal delineator. you follow number entered with the letter " " and then click on the page anywhere outside the entry box. Doing this will convert each of your entries to the native units (feet, square feet, and pounds0) used by the calculator. Thus if you enter 1000m for the displacement in kilograms, it will be converted to 2204.6 pounds.

								*

Press .

to e-mail the data on your boat to Tom:

: This area displays the parameters of the boat selected. Do not enter values here. Click on any of the Derived Quantities boxes for an explanation of the box.

: You can search for boats in the database you selected in Part 1 by their parameters. Select any number of conditions.

: You can find your 'ideal' boat by doing a weighted search. For example, you can search for the boat that has the highest combined normalized scores in 'Motion Comfort' and 'Sail Area to Displacement' giving one a 60% weight and the other 40%, or whatever! You can also do low searches, for example, you can search for the boat that has the highest normalized score in 'Motion Comfort' and the lowest normalized score in 'Capsize Ratio' giving one a 30% weight and the other 70%, or whatever. A 'high' search is done as a percentage of the highest boat in the parameter. So, if the boat with the highest Sail Area to Displacement has a value of 48, a boat with a Sail Area to Displacement of 24 would receive a value of .5. For a 'low' search it is the inverse. That is, if the boat with the lowest capsize ratio has 1.3, a boat with a capsize ratio of 3.9 would receive a value of 0.33. Only boats within the specified length range and in the database chosen in Part 1 will be searched. You can also eliminate any type or types of boat from those searched by entering their names separated by commas in the first field below. For example, entering 'Herreshoff,Bolger' would eliminate any boat with either name in its name. The results (the top three boats, their scores and the average score for boats searched) are reported in the text area below.

Output Field:
Minimum Length:
	Capsize Ratio	Hull Speed	SA/Disp	Disp/LWL	LWL/Beam	Motion Comfort	Pounds/Inch
Weights:
Search Direction:

: The material here is taken from an article by in (February 2001. pp. 81-84) entitled . To really understand the numbers calculated below you should consult this article or his book . A note on the Maximum Sailing Speed calculated below: This is also from Gerr's work. He has determined that the classic formula for Hull Speed ( 1.34 Sqrt(LWL) ) does not always apply, the 1.34 is not a constant, leading to, in some cases, much higher speeds. However, Gerr observes: "

To use this form, select a boat, enter a Horsepower and Prop Type.

\n") for(i=0;i ")} // -->

Press

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Boat Propeller Calculator

You can use this boat propeller calculator to determine one of five variables: boat speed, propeller slip, propeller pitch, engine gear ratio, or engine revs.

To use the calculator, input four of the five variables. The calculator will automatically compute the 5 th variable.

Boat Speed mph kts kph

Engine Revs RPM RPS

Gear Ratio : 1

Propeller Pitch inch cm mm

Propeller Slip %

Propeller slip provides an indication of the efficiency with which a boat is traveling through the water. However, it is not to be confused with propeller efficiency.

If there was an absence of slip as the propeller circulates through the water, the boat will, theoretically, move forward at a distance that matches the propeller pitch.

A propeller pitch can be understood by considering the action of a screw. The pitch indicates the distance that each rotation contributes to the extent to which the boat moves forward on each rotation. For instance, if the propeller moves forward 15 inches every time it completes a full turn, the nominal propeller pitch is 15 inches.

The propeller revolution rate is determined by dividing the engine rpm by the gear ratio. The propeller rpm multiplied by the propeller pitch indicates the distance forward a boat will move forward every minute.

However, as a result of propeller slip, there is a difference between actual and theoretical speed. Specifically, the actual speed is typically 10-20% slower than the propeller than the theoretical speed when the boat is traveling at its top speed.

The slip can be much higher at lower speeds, often above 50%. Typically, the slip reduces as the speed increases, generally diminishing to under 10%.

'Gear ratio' defines the number of drive shaft revolutions per propeller revolution. As such, if an engine has a gear ratio of 2:1, the engine drive shaft will turn twice for every propeller revolution. It is common for 200-HP engines to have a ratio of 1.86:1. Engines that have a smaller horsepower will have a higher ratio in the region of 2.33:1.

Calculator Equations

The following equations are used within this calculator:

V = [RPM × PP × (1 − (PS/100))] / [GR × C]

RPM = [V × GR × C] / [PP × (1 − (PS/100))]

GR = [RPM × PP × (1 − (PS/100))] / [V × C]

PP = [V × GR × C] / [RPM × (1 − (PS/100))]

PS = [1 − (V × GR × C) / (RPM × PP)] × 100

V is the speed at which the boat is traveling,

RPM is the crankcase speed (rpm),

GR is the number of revolutions the crankshaft needs to produce one revolution of the prop shaft,

PP is the blade pitch of propeller (inches),

PS is the index of propeller performance (as a percentage),

C is the constant to convert inches-per-minute of revolution to boat speed V ;

V (in mph), C = 1056 ;

V (in knots), C = 1215.2 ;

V (in kph), C = 656 .

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What Size Motor Is Right for Your Boat?

If you’re looking to get out on the water regularly, you’ll want to make sure you have a boat with the appropriate amount of power to give you the best trip possible. The weight and horsepower of your boat’s engine can significantly impact the boat’s performance and maintenance needs and can make or break your nautical adventures. Here’s how to ensure you power your boat correctly, according to Discover Boating .

How to choose an appropriately sized engine for your boat

#TGIF How are you spending this weekend? #searay #searaysummer #spx190 pic.twitter.com/jk97J5N7I5 — Sea Ray Boats (@SeaRayBoat) June 5, 2020

Although settling on an engine size may seem somewhat intimidating if you’ve never purchased a boat before, it’s fortunately a fairly straightforward process. The boats you look at should be rated for the maximum horsepower they can handle, and this is always the amount you should aim for.

While you certainly don’t want to get a motor that’s too big for your boat, it’s absolutely better to go bigger rather than going too small. In fact, Discover Boating reports that most often, low horsepower is the reason people experience a disappointing ride experience when they go out on the water. Speak to your dealer about the largest motor possible and go with that.

Motor size really does matter

Bigger motors and higher horsepower have a long list of benefits when it comes to boating. First and foremost, you’ll get more speed, making for more exhilarating rides. However, there’s much more to it than that.

Bigger motors provide better handling, especially at midrange speeds. This means that water skiing and other sports will be easier, and you’ll have a better time maneuvering in general. In rough weather, larger motors can prove particularly useful, giving you more control and security in choppy conditions.

Another perk of a larger motor is, counterintuitively, the fact that it uses less fuel, according to Discover Boating. While it’s easy to assume that a smaller motor will be more fuel-efficient, this is actually not the case. A motor that is too small will struggle to power the boat and will, therefore, eat up significantly more fuel. A larger motor may have a bigger fuel tank, but it will go through this fuel more slowly, as it is not struggling to power a boat that is too heavy.

What you should keep in mind, however, are official regulations. According to Formula Boats . You should be aware to base your motor selection based on the “rule of thumb,” which states that you should have between 40 and 25 pounds of weight per horsepower. Also, keep in mind the manufacturer’s plate capacity that is specified for your specific boat.

The manufacturer should be following federal guidelines to determine the maximum horsepower capacity, and that number should be listed for you on the boat. Any powerboat that’s less than 20 feet in length is required to have that capacity plate available for you.

Finally, always remember that it’s illegal to overpower your boat, so while bigger is better in a lot of circumstances, you can’t go too big.

The downsides of going too small

Related:  4 Classic Mistakes to Avoid When Towing a Boat

According to Discover Boating, you should be wary of a boat with a smaller motor that seems to do well during a test ride. Take note of the fact that during your test ride, you will probably have a limited number of passengers, and will have no extra drinks, food, or sporting gear. You may not even have a full fuel tank.

All of these factors lighten the load, making it so the watercraft can perform decently even with fewer horsepower. However, this isn’t necessarily indicative of how you will be using the boat the majority of the time. That is why it’s important to find out the maximum amount of power the watercraft can handle — this rating will account for how much weight the boat could potentially be taking on with a full passenger load, and will, therefore, be a more accurate gauge of the power you’ll need.

Which specific type of engine you end up with will depend largely on the type of watercraft and the areas in which you’ll be sailing. Horsepower can range anywhere from 2.5 hp all the way up to over 1,000 hp for certain types of outboard motors. 

Choosing the appropriate engine size for your boat will make it that much easier for you to enjoy your time out on the water. Make sure you go with the highest amount of horsepower that your boat can handle, and it’ll be smooth sailing all summer long.

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HOW TO USE THE BOAT PROP CALCULATOR TOOL

Knowing the theoretical speed for your boat can help you select the proper pitch.  To use our boat prop calculator, you need four inputs:

• RPMs :  Revolutions Per Minute.  Enter the high end of the operating range established by the engine manufacturer or the maximum RPMs you can attain with your current propeller.  The recommended operating range should be listed in your owner’s manual and can also be found online.

• Gear Ratio : This is the number of drive shaft revolutions for one revolution of the propeller.  You can find this in your owners manual or by searching online.
• Prop Slip : This is the relationship between the actual vs. theoretical distance your boat travels with each revolution of the propeller and is impacted by the hull design, weight of your boat, propeller design and other factors.  Most boats have between 5% and 25% prop slip and some slip is even necessary to thrust your boat forward.  An easy way to estimate your prop slip is with my prop slip calculator .

SHOULD YOU GO WITH A DIFFERENT PITCH?

All marine propellers involve a tradeoff.  If you prop your boat to maximize top speed, acceleration will be compromised and visa-versa. Therefore, the first consideration is whether to optimize top speed,  acceleration or some combination of the two.

To increase acceleration, consider reducing your pitch.  This makes particular sense if you are NOT hitting the max RPM level established by the engine manufacturer when running at Wide Open Throttle (WOT) with your current prop.  To see the potential impact on top speed, enter your current values for Max RPMs, Gear Ratio, Pitch and Prop Slip in the Boat Prop Calculator tool.  Then, decrease the pitch by an inch or two.  However, as you do this, you should increase your RPMs by approximately 200 for each 1″ reduction in pitch (until the maximum RPM rating is reached).

Conversely, to increase top speed, consider increasing your pitch.  This is especially relevant if you ARE hitting the max RPM level established by the engine manufacturer with your current prop.  However, it is hard to tell whether your RPM level is the best your engine can do or if it is being capped by the engine’s rev-limiter (to protect the engine).   If it is the later, then you likely have room to improve your top speed.  If it is the former, then it might not make much difference after you account for the fact that each 1″ increase in pitch will result in approximately a 200 decrease in RPMs.

Of course, most boaters will want something that is in between – good hole shot with acceptable top end speed.  Just be sure that whatever size prop you use your engine operates within its recommended RPM range.

If you liked our prop calculator, here are some other tools you might find useful:

• Prop Slip Calculator
• Fuel Consumption Calculator 
• Total Cost of Boat Ownership Calculator
• New Boat Price Estimator
• Boat Loan Calculator

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One of the most important things you need to know before setting out on the water is the maximum number of people and maximum amount of weight that your boat can safely handle. Not only is this an important safety concern, it's also the law.

Federal Law mandates that all powerboats less than twenty feet in length need to carry this information in the form of a Capacity Plate.

Each Capacity Plate includes the maximum number of adult persons, the maximum gross load, and the maximum size of engine, in horsepower, that your boat can legally carry.

The next time you're around a boat, look for its Capacity Plate; it should be permanently fastened near the steering area, or the helm.

Before any boat trip, you'll want to make sure that you are not taking more people onboard than is indicated by the Maximum Person number, and that you don't have more total weight than is indicated by the Maximum Gross Load. The Maximum Gross Load is the total weight your boat can handle, including people, equipment, stores, fuel, engine assembly and steering controls. If your boat doesn't have a Capacity Plate, you can calculate the number of people you can safely take onboard using the following equation.

Boat Capacity Calculation

If your boat doesn't have a Capacity Plate, you can calculate the number of people you can safely take onboard using the following equation and calculator.

Number of people = vessel length (ft.) x vessel width (ft.) ÷ 15

First, find out the length and width of your boat in feet, then use our calculator to find out your boat's capacity.

Note that personal watercraft do not have a capacity place. For P-W-C's, always follow the recommended capacity in the owner's manuel and on the manufacturer's warning decal.

Finally, the Capacity Plate will also indicate the maximum engine power for your boat, given in horsepower. This number applies only to boats powered by outboard engines; and it must never be exceeded.

Maximum Person Capacity

There are a number of variables that boat manufacturers consider when determining the maximum person capacity that appears on your Capacity Plate.

One of those variables is the weight of each person.

Boat manufacturers typically use an average weight of about one hundred and fifty pounds per person to calculate maximum capacity. It can be a little more or a little less, but if some or all of your passengers weigh over one hundred fifty pounds, you may have to decrease the number of passengers you can safely take on board.

Remember, the maximum person capacity is a guideline that you have to adjust given the weight of your passengers and the other supplies you are taking on board.

If you are carrying heavy equipment, you may have to further reduce the number of passengers.

Maximum Horsepower

If you don't have a capacity plate on your boat—which may be the case if you're operating a small, flat-bottomed boat—you can calculate the largest safe engine size in the following way.

Maximum Horsepower Calculation: Boat length x boat width = boat square footage

First, find out the square footage of your boat by multiplying its length by the width of the transom.

Then use our calculator and the table here, to find out your boat's maximum horsepower. For example, a twelve-foot boat with a four-foot transom width translates into a maximum engine size of fifteen horsepower.

Length x Width	Max Horsepower
35 feet or less	3
36 - 39 feet	5.5
40 - 42 feet	7.5
43 - 45 feet	10
46 - 52 feet	15

Overloading or Overpowering

Either overloading or overpowering your boat is extremely dangerous.

Putting an over-sized engine on your boat will cause your boat to sit too low in the stern , and that will make it much more susceptible to being swamped by its own wake or that of a passing boat. An overpowered boat is also hard to control.

What about overloading your boat? Overloading your boat, either with too many people or too many supplies, also makes your boat susceptible to swamping.

Even if you are within the maximum allowable weight, make sure that you distribute the load evenly, focusing the weight in the middle of the boat. This will keep your boat stable in the water and help prevent capsizing or swamping.

Finally, remember that in bad weather, you must be extra careful about how much weight you take in your boat. With higher waves, a heavy boat is harder to control and more susceptible to being swamped. Stay safe. Follow the guidelines for load capacity and always adjust for bad weather.

Beware of bad weather! Take much lighter loads in poor weather conditions to ensure boat stability.

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Choosing the power needed to motorize my boat

Choosing the power of your electric motor is something important. It must be enough to be able to get out of a bad situation in case of heavy seas or a gust of wind, but not too much as it will determine the size of the battery pack and therefore the final cost.

The power necessary to move a boat at a desired speed or to with headwinds depends on many factors :

• Size and shape of the hull(s) and annexes,
• theoretical calculations can give you an estimate of resistance/speed.

Other factors can significantly influence consumption :

• Propeller efficiency: 50/55% if you have a good propeller
• Mechanical losses (friction): 95% if you have good material
• The efficiency of the motor controller: 90/95% for a synchronous motor with permanent magnet and electronic commutation … Much less with asynchronous or brush-excited motors More information in our article Not all motors are the same   Tous les moteurs ne sont pas équivalents ?

In total, less than 50% of the power consumed will be used for propulsion. The loss can be much greater with the wrong choice of propeller, motor or equipment. Savings can sometimes become expensive when you have to increase the size of the battery pack to compensate for low overall efficiency.

Other factors can cause 2 identical boats to have very different performance :

• weight distribution
• the condition of the hull …

So how do you estimate the power required for your boat ?

There are different ways :

• Compare with the power of its existing thermal engine in CV or kW (1 kW = 1.4 CV) but beware that the comparison is limited because the powers are not calculated in the same way. A good basis could be to convert 1 KW to 2 HP concerning power, and therefore maximum speed, and 1 kW for 3 HP concerning torque, which allows the boat to move forward in difficult conditions. We explain why in our article How to compare thermal and electrical power? Comment comparer puissance thermique et puissance électrique ?
• Ask the boat manufacturer or architect for the boat’s resistance curve in water and against the wind . It will allow you to determine the power necessary to get out of a bad situation with the sea and the wind and to estimate the boat’s KW consumption in normal weather. You should leave a margin because the curves are theoretical with a boat that is often lightly loaded whose weight is ideally distributed and with a clean hull and propellers.
• To help you, we have developed a tool based on a theoretical calculation and on the feedback from sea trials. By giving us some information, we can provide you with a theoretical power curve in calm seas and with wind depending on the speed. This would of course be only a theoretical calculation that would take into account efficient inboard engines and these calculations must be interpreted in the light of experience, the state of the sea and the boat. The tool would show you how additional nodes are sometimes very expensive in terms of consumption.

Example of a power calculation

This tool could help you make a decision, but it is in no way a recommendation. Our experience shows that the loaded weight is often underestimated, we recommend you to leave a margin.

@ Send a request to estimate the power needed to motorize your boat

How to assess autonomy ?

A 1 kW motor will consume 1 kWh at full power over a period of 1 hour. We will double or triple the range by reducing the speed very little .

It is good practice to start with a ratio of 1 between kW (engine power) and kWh (battery pack capacity), even if it means leaving room for future expansion. Please note that this rule applies with LIFEPO batteries. Other chemicals require a larger pack. As for AGMs, it will be necessary to foresee that they should not be discharged more than 50%.

It is possible for a small boat to bring a small portable generator of 1 or 2 KW as a precaution. Connected to the charger, it will allow you to return to port at low speed even if the batteries are empty.

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How Much Horsepower Do I Need for My Boat? (Answered)

Written by Anthony Roberts / Fact checked by Jonathan Larson

Part of the joy of boating is feeling the wind slap your face as you travel at top speed. Unsurprisingly, many novice sailors ask, how much horsepower do I need for my boat? After all, horsepower translates to vessel speed.

Although no definite figures exist, one rule is the go-to for seasoned boaters. You can assign one horsepower for every 25 to 40 pounds of your boat’s weight. Hence, you can get a 32 to 50 HP motor for a 1,250-pound watercraft.

Of course, vessel weight is only one consideration, and there are others. So, kindly continue reading to learn more.

Table of Contents

What is Horsepower?

1. fuel efficiency, 2. passengers & vessel use, 3. boat manufacturer limits, 4. horsepower-to-weight ratio, 5. insurance considerations, 6. federal regulations, a formula to remember, 1. legal consequences, 2. risk of vessel damage, 3. risk of swamping, frequently asked questions.

Horsepower knowledge will help determine the most appropriate boat engine without referring to a boat horsepower chart.

Eighteenth-century engineers defined horsepower as engine output equal to 550-foot-pounds/second. Unfortunately, several flaws in the measurement were observed, prompting other experts to propose that 1 horsepower is equivalent to 746 watts.

These definitions were insufficient. Hence, engineers also tried relating horsepower to torque or “turning force.” This “horsepower” formed the basis for an HP rating for boats.

Boat engine manufacturers use a “dynamometer” to measure the power (or torque) an engine produces after load application. The device also measures the number of revolutions or rotations per minute (RPM).

That is why engines always have the following format: “__ HP at __ RPM.” For instance, you can have 50 HP at 4500 RPM. In this case, the engine has a peak horsepower of 50, measured at 4,500 revolutions per minute.

Still too technical? Here is the simplest definition we could offer. Horsepower is engine power.

So, what determines horsepower in boat engines?

How to Determine the Horsepower Requirements for Your Boat

Answering the question, “what size motor can I put on my boat?” requires considering several factors.

The conventional notion is that an engine with a high boat horsepower rating is a gas-guzzler. However, experts argue that everything depends on how you squeeze the throttle, just as how aggressive you are on a car gas pedal.

For example, if you maintain your diesel motor at 75% throttle or a petrol engine at 3,000 to 3,500 rpm, you can expect your high-HP boat engine to be more fuel-efficient than a low-HP motor hitting the redline or going 100% throttle.

Simply put, if you want better fuel efficiency when going at a fast speed, a high horsepower is preferable.

Vessel weight increases with the number of passengers. For example, a fully-loaded 17 foot boat will require a high-HP engine. However, if a similarly-sized watercraft only has to accommodate one person, a lower HP motor will do just fine.

So, boats for personal or family use will do fine with a lower HP engine than watercraft for towing water skiers, wakeboarders, tubers, and wake surfers. Similarly, higher HP output is recommended for hauling cargo and heavy items.

Always consider your vessel’s principal use when choosing boat engine size.

Your boat has a “capacity plate” specifying the vessel’s weight limitations and maximum recommended horsepower rating. You can check this information adjacent to the helm or your boat transom’s interior.

Alternatively, you can check the owner’s manual, especially for boats bought overseas. Online resources are also available. Reaching out to the boat manufacturer by going to their website’s homepage for contact information can also help you determine the best outboard motor size.

Trying to find the maximum horsepower for your vessel can be challenging, with so many variables to consider. One factor that can influence how much power your vessel requires is weight.

We already mentioned that the heavier the boat, the more power (or HP) it needs. So, how do we calculate maximum horsepower for a boat referencing its weight?

Calculating the ratio between horsepower and weight is straightforward. You only need your boat’s weight and its boat HP rating.

Suppose you have a 3,000-pound boat with a 200-horsepower outboard motor. Dividing 200 HP by 3000 pounds will result in 0.067 HP per pound (hp/lb). Here is a tip: the lower the “hp/lb” value, the faster the boat.

Knowing this value will help you choose the most appropriate HP rating in a boat to motor size chart. Of course, note that the boat HP rating must follow the 1 HP/25-40 pounds of vessel weight rule first.

Like everything else, higher-HP vessels demand higher insurance premiums. Most renter’s or homeowner’s insurance policy limits the coverage to boats with no more than 25 horsepower. Any vessel exceeding the HP rating requires different insurance coverage.

You might also want to know that most, if not all, insurers do not cover boats with horsepower ratings exceeding the manufacturer’s limits. You could reconsider mounting a high-performance engine only to plane a boat if you want adequate insurance coverage.

The Department of Transportation and US Coast Guard created the “Horsepower Capacity” for the Code of Federal Regulations, capping the horsepower for vessels no more than 13 feet long at 40.

Moreover, the federal government uses a “boat HP calculator” to determine a watercraft’s maximum horsepower to ensure safe boating. The good news is manufacturers adhere to these rules, so sticking by the manufacturer’s limit should suffice.

You can forget the online boat motor size calculator and memorize this formula instead.

• Boat horsepower = 25 to 40 pounds of vessel weight per horsepower

Let us assume you want an outboard motor for 20 foot boat weighing 4,000 pounds. In that case, the minimum horsepower rating you can get is 100 (4000 pounds ÷ 40 = 100 HP), and the maximum is 160 (4000 pounds ÷ 25 = 160 HP).

You can also use this formula on your sailboat, dividing its weight by 25 or 40 to get the ideal horsepower range. For instance, a 50-HP motor should be enough for a 2000-pound, 18 ft sailing vessel.

We mentioned the direct relationship of horsepower to speed – the higher the HP rating, the more powerful and faster the vessel. The next logical question is how boaters can convert HP to MPH.

Sadly, the formula for converting horsepower values to MPH equivalents does not exist. However, experts forward the following 1-horsepower equivalents.

• Personal watercraft (PWCs): 0.3 to 0.8 MPH/HP
• Small outboard engines: 0.8 to 1.3 MPH/HP
• Large outboard motors: 0.4 to 0.6 MPH/HP

For example, suppose you have a 20-HP outboard motor. You can safely assume your top speed would be between 16 and 26 MPH (0.8 MPH or 1.3 MPH x 20 MPH). Meanwhile, a 100-HP jet ski can go 30 to 80 MPH.

You can also multiply the MPH value by 1.609 if you want to convert MPH to KPH. For example, 50 MPH x 1.609 is 80.45 KPH.

Dangers of Overpowering

A boat HP rating guide exists to help you avoid exceeding your vessel’s horsepower limits and sidestep the following dangers of an overpowered watercraft.

The federal government limits boat horsepower requirements. Although boat manufacturers adhere to the Code of Federal Regulations, exceeding the vessel’s horsepower limits can open you to fines and other penalties.

States, cities, municipalities, and other jurisdictions might also have boating laws that prohibit overpowering watercraft.

That said, some states, such as Washington, California, or Oregon, may not have strict regulations regarding exceeding the HP limits. However, overpowered boats can lead to overspeeding, which can open you to civil and criminal liabilities.

Boat manufacturers do not “guess” the information on the capacity plate. They study these parameters for your safety while ensuring the boat’s structural integrity can accommodate the maximum horsepower.

Replacing your engine with a more powerful unit might put unnecessary stress or pressure on the transom. You risk damaging your boat’s hull, which can result in costly boat repairs.

Another reason boat manufacturers limit engine horsepower is the importance of outboard motor weights relative to HP values.

For example, a 15-HP outboard motor might weigh 95 to 140 pounds, and the manufacturer limits the boat to this HP value. Replacing the engine with a 40-HP unit increases the weight to about 205 to 220 pounds.

The weight increase makes your boat’s stern heavier than the bow, increasing the risk of swamping.

What boats are 2.0 to 3.6-HP outboard engines suitable for?

Two-horsepower to 3.6 HP outboard engines are ideal for portable watercraft, including inflatable boats, canoes, and tiny pontoons. These engines can propel small watercraft to about 5 to 9.5 MPH.

How fast can a 5.0- 6.0-HP motor go?

Twelve-foot inflatable boats, large canoes, and dinghies with 5HP to 6HP outboard motors can go 10 to 17.5 MPH with only one person aboard.

Determining the answer to the question, “how much horsepower do I need for my boat?” is never a walk in the park. Boaters must consider several factors in their boat horsepower decision.

Although boat manufacturers provide boat HP limits in capacity plates, fuel efficiency, passenger count, and watercraft purpose can still impact one’s decision. You might also consider the HP-to-weight ratio, insurance terms, and federal/state regulations.

We suggest sticking with the boat manufacturer’s maximum recommended HP ratings to ensure boating safety and avoid the pitfalls of an overpowered watercraft.

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Ten years of enjoying countless trips on boats never made me love them any less! So I am here to put all those experiences into good use for other boaters who want to have a safe and fun trip with their friends and families.

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The Sailboats Calculators below will enable you to calculate the main Sailboat Ratios, using data that you can retrieve from the Boat table or your own data.

We will be adding more calculators along the way and more in-depth explanations of how they work and what they can help you with., hopefully you will enjoy them and find them useful to search or understand the characteristics of your or any given sailboat ..

SA/D range of values

16 to 18 Heavy offshore cruisers 18 to 22 Medium cruisers 22 to 26 Inshore cruisers, racing boats 26 to 30+ Extreme racing boats

Ballast/Displacement:

A Ballast/Displacement ratio of 40 or more translates into a stiffer, more powerful boat that will be better able to stand up to the wind.

Displacement/Length:

The lower a boat’s Displacement/Length (LWL) ratio, the less power it takes to drive the boat to its nominal hull speed.

less than 100 = Ultralight;

100-200 = Light;

200-275 = Moderate;

275-350 = Heavy;

350+ = Ultraheavy;

Comfort Ratio:

This is a ratio created by Ted Brewer as a measure of motion comfort. It provides a reasonable comparison between yachts of similar size and type. It is based on the fact that the faster the motion the more upsetting it is to the average person. Consider, though, that the typical summertime coastal cruiser will rarely encounter the wind and seas that an ocean going yacht will meet.

Numbers below 20 indicate a lightweight racing boat;

20 to 30 indicates a coastal cruiser;

30 to 40 indicates a moderate bluewater cruising boat;

40 to 50 indicates a heavy bluewater boat ;

over 50 indicates an extremely heavy bluewater boat.

Comfort ratio = D ÷ (.65 x (.7 LWL + .3 LOA) x Beam^1.33), where displacement is expressed in pounds, and length is expressed in feet.

Capsize Screening Formula (CSF):

Designed to determine if a boat has blue water capability. The CSF compares beam with displacement since excess beam contributes to capsize and heavy displacement reduces capsize vulnerability. The boat is better suited for ocean passages (vs coastal cruising) if the result of the calculation is 2.0 or less. The lower the better.

Hull Speed Calculator

Hull speed calculator is a simple calculator that determines a vessel’s hull speed based on the length of the vessel’s waterline.

Boat Speed Calculator

The boat speed calculator calculates the top speed of a boat based on the boat’s power and her displacement. If you try to understand how fast a boat can go, this calculator will help you answer that. The boat speed calculator utilizes a constant known as Crouch constant which differs based on the type of the boat.  

FOR MULTIHULLS ONLY:

Bn – bruce number:.

The Bruce Number is a power-to-weight ratio for relative speed potential for comparing two or more boats. It takes into consideration the displacement and sail area of main and jib. 100% fore-triangle only, no overlapping sails.

Chris White, “The Cruising Multihull”, (International Marine, Camden, Maine, 1997), states that a boat with a BN of less than 1.3 will be slow in light winds. A boat with a BN of 1.6 or greater is a boat that will be reefed often in offshore cruising.

Derek Harvey, “Multihulls for Cruising and Racing”, International Marine, Camden, Maine, 1991, states that a BN of 1 is generally accepted as the dividing line between so-called slow and fast multihulls.

BN = SA^0.5/(Disp. in pounds)^.333

Kelsall Sailing Performance (KSP):

Another measure of relative speed potential of a boat. It takes into consideration “reported” sail area, displacement and length at waterline. The higher the number the faster speed prediction for the boat. A cat with a number 0.6 is likely to sail 6kts in 10kts wind, a cat with a number of 0.7 is likely to sail at 7kts in 10kts wind.

KSP = (Lwl*SA÷D)^0.5*.05

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• Sail Dimensions

What Sail Dimensions are Required to Calculate Sail Areas?

The required sail dimensions for calculating the area of any triangular sails are usually its height and the length of its foot. But that only works for mainsails and mizzens with no roach, and jibs with a 90 degree angle at the clew - and what about high-cut headsails, spinakers and cruising chutes? Read on...

Foresail and mainsail dimensions are universally referenced with the letters 'J', 'I', 'E' and 'P' approximating to the length of the foredeck, height of the mast, length of the boom and the height of the main sail - but more accurately defined further down this page.

Yacht designers need these sail dimensions to calculate thought provoking stuff such as the sail-area/displacement ratios of their creations, and sailmakers need them before they put scissors to sailcloth.

If our sailboat's sails were perfectly triangular then, as every schoolboy knows, their area would be 'half the height, times the base' - but with the possible exception of a mainsail with a straight luff, generally they're not. Here's how it works...

Main and Mizzen Sail Dimensions

These are almost right-angled triangles except for the curvature of the leach (the 'roach') which increases the sail area. 

It's usually calculated as:~

Area = (luff x foot)/1.8, or

Area = ( P x E )/1.8, where:~

• 'P' is the distance along the aft face of the mast from the top of the boom to the highest point that the mainsail can be hoisted, and
• 'E' is the distance along the boom from the aft face of the mast to the outermost point on the boom to which the main can be pulled.

For the mizzen sails on ketches and yawls , 'P' and 'E' relate to the mizzen mast and boom.

For more heavily roached sails, the increased area can be accounted for by reducing the denominator in the formula to 1.6.

Clearly calculating sail areas isn't going to be an exact science...

Jibs, Genoas and Staysail Dimensions

For a working jib that fills the fore triangle - but no more - and with a foot that's parallel to the deck, then you've got a 'proper' right-angled triangular sail, whose area is:~

Area = (luff x foot)/2, or

Area = ( I x J )/2, where:~

• 'I' is the distance down the front of mast from the genoa halyard to the level of the main deck, and
• 'J' is the distance along the deck from the headstay pin to the front of the mast.

Genoas, by definition, have a clew which extends past the mast and are described by the amount by which they do so. For instance a 135% genoa has a foot 35% longer than 'J' and a 155% genoa 55% longer. Areas are calculated as follows:~

Area (135% genoa) = (1.44 x I x J )/2, and

Area (155% genoa) = (1.65 x I x J )/2

High-cut Headsails

But these formulae don't work for a high-cut jib with a raised clew - unless you imagine the sail turned on its side such that the luff is the base and the luff perpendicular is the height.

It's still a simple calculation though, once you know the length of the luff perpendicular ( LP ), the sail area is:~

Area = (luff x luff perpendicular)/2, or

Area = ( L x LP )/2, where:~

• 'L' is the distance along the forestay from the headstay pin to the front of the mast, and
• 'LP' is the shortest distance between the clew and the luff of the genoa.

Spinnaker Sail Dimensions

Much like calculating foresail areas, but with different multipliers for conventional spinnakers and asymmetric spinnakers...

Conventional Spinnakers

Area = (0.9 x luff x foot), or

Area = (0.9 x I x J ), where:~

• 'I' is the distance from the highest spinnaker halyard to the deck, and
• 'J' is the length of the spinnaker pole.

Asymmetric Spinnakers

Area = (0.8 x luff x foot), or

Area = (0.8 x  I  x  J ), where:~

• 'I'  is the distance from the highest spinnaker halyard to the deck, and
• 'J'  is the distance from the front face of the mast to the attachment block for the tackline.

More about Sails...

Are Molded and Laminate Sails One Step Too Far for Cruising Sailors?

Although woven sails are the popular choice of most cruising sailors, laminate sails and molded sails are the way to go for top performance. But how long can you expect them to last?

Is Carrying Storm Sails on Your Cruising Boat Really Necessary?

It's good insurance to have storm sails available in your sail locker if you are going offshore, and these are recommended fabric weights and dimensions for the storm jib and trysail

Using Spinnaker Sails for Cruising without the Drama!

When the wind moves aft and the lightweight genoa collapses, you need one of the spinnaker sails. But which one; conventional or asymmetric? Star cut, radial head or tri-radial?

The Mainsail on a Sailboat Is a Powerful Beast and Must Be Controlled

Learn how to hoist the mainsail, jibe it, tack it, trim it, reef it and control it with the main halyard, the outhaul, the mainsheet and the kicker.

Is Dacron Sail Cloth Good Enough for Your Standard Cruising Sails?

Whilst Dacron sail cloth is the least expensive woven fabric for standard cruising sails, do the superior qualities of the more hi-tech fabrics represent better value for money?

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