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.

Sunset in calm waters from a boat with small outboard motor

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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The Best Outboard Motor for a Sailboat


Last Updated by

Daniel Wade

June 15, 2022

The technology of sailing has remained mostly unchanged for centuries. Since learning to harness the power of wind, sailors have been transiting the world’s oceans, expanding trade routes and exploring new cultures. Although nothing more than a renewable natural resource and a single sail is needed to move a sailboat along the water, there are times when it’s important (and in our modern age, convenient) to leverage off a motor to get you where you need to go.

Like any unique piece of equipment in the world of sailing, outboard motors come in a variety of sizes with features and options to fit any owner’s needs. But of course, one size doesn’t fit all. Every boat is different – even those that come off the production line at the same time – and every owner is looking for something specific when it comes to their sailboat. From the purpose of owning the boat (blue water sailing vs. racing) to the location and impact on maintenance (cold weather vs. tropical weather), an outboard motor is just one of the many elements that will define a sailboat’s function and performance.

Whether you’re a new owner, or a veteran sailor, it’s important to know the basic components of any outboard motor . You should also have an idea of what you want your outboard motor to do for your size and model sailboat.

Table of contents

Outboard Motor Size

A larger boat doesn’t necessarily mean a larger motor. Although there are different ratings for different classes of boats, a small power plant can be more effective than a larger one. Conversely, an outboard motor can easily overpower a small boat and create unsafe conditions at high speeds. Guidelines and requirements differ between motorboats and sailboats. And while there is some overlap, these considerations apply directly to sailboats.

Engine power has to do with how much water a boat displaces. For sailboats, smaller is better. If you’re a bit of a math geek, the exact formula is 4 horsepower for every 2200lb of weight. Coupled with a propeller size, which can be determined using a propeller calculator , you’ll get a rough estimate to use as a guideline to start shopping. This is a good first step, since size is essentially a fixed variable. Though it’s worth noting for those who are buying a sailboat directly from the manufacturer, that actual weight will increase after delivery – once all other rigging and outfitting has been completed.

Physical size of your outboard motor is an important consideration and is directly related to the design of your sailboat. An outboard motor is made up of three parts from top to bottom:

  • The Powerhead – Houses the engine. The bulbous part of the motor.
  • The Midsection – Houses the exhaust system. Varies in length and design.
  • The Lower Unit – Propellers attach to the gearbox. Submerged when operational.

Shaft length is an important design element and should be considered when purchasing a motor. A shaft that is too short will obviously prevent the propeller from being submerged in water, while a shaft that is too long will extend the propellers too far. Not only will it decrease the efficiency of your engine, it will create unnecessary drag. Know your transom length when looking at different models.

When an outboard motor is not being used, it should be stowed in its upright position. Some of the larger motors have an automated switch that will raise it out of the water, but some must be secured manually. Make sure everyone who sails with you is capable of lifting and securing the motor out of the water in case of an emergency.

Outboard Motor Power

Any kind of motor installed on a sailboat (inboard or outboard) should be viewed as a tool to help with maneuvering. Although there are some very skilled sailors out there who can sail into their slip without the aid of a motor, many harbors have restrictions that either don’t allow for the use of full sails, or there simply isn’t enough room to maneuver. A motor with both forward and reverse gears helps tremendously with docking.

While there is no exact correlation between boat length and total weight, the following is a rough guideline:

  • 1-4 HP for boats up to 20’ (approximately 1000-2000lbs)
  • 4-18 HP for boats between 20-30’ (approximately 2,000-10,000lbs)
  • 18-34 HP for boats between 30-40’ (10,000lbs or more)

There are some things to consider when deciding how much horsepower you need or want. Location and the type of conditions you expect you’ll be sailing in is one of the biggest factors. Heavy seas and high winds typically associated with open ocean sailing will put more strain on your engine, and in some cases overpower it, whereas an engine that is heavier than needed will add unnecessary weight when racing. If you plan on motoring for long distances, consider purchasing an engine that will stand up to a lot of use.

Less HP is required for:

  • Boat Design – Single hull boats made out of fiberglass require less power.
  • 2-Stroke Engines – This is due to an overall lighter weight engine and higher torque.
  • Diesel Engines – Diesel delivers more torque because the rate of compression is greater.
  • Bigger Propellers – More surface area means more water displacement.
  • Location – Motoring on lakes and rivers requires less power than open ocean.
  • Distance – A smaller engine is suitable for shorter distances.

More HP is required for:

  • Boat Design – Catamarans and heavier boats (regardless of size) require more power.
  • 4-Stroke Engine – Engine weight and an extra step of compression yields less power.
  • Gas Engines – The rate of compression for gas engines is much lower than diesel.
  • Smaller Propeller – A smaller propeller displaces less water.
  • Location – Open ocean, with tides and currents, will strain a smaller engine.
  • Distance – Cover more distance when wind conditions are poor requires a larger engine.

Outboard Motor Cost

There is no way to quantify how much you will pay for any given motor. But there are several costs associated with owning an outboard motor that are definitely worth considering when making your purchase.

Certainly, a lager, more-powerful engine is going to be costlier than a smaller engine with lower horsepower. But as mentioned earlier, size is not necessarily a guarantee of performance and efficiency. At the same time, there’s only so much you can get out of an engine before you exceed its capability. Larger engines tend to help with resale value should you choose to sell your boat at some point, but a boat outfitted with right motor to begin with will always perform better than a motor that’s large just for the sake of it.

Factor in maintenance costs and fuel when looking at models. You want to run your engine at around 90% of its max RPMs to balance proper fuel usage and with wear and tear. Making a few calls to marine mechanics to inquire about an engine you’re interested in will give you a lot of information a sales person won’t be able to give you. The good news about outboard motors is that most of them are portable, which means you won’t have the added cost of either paying a mechanic to come to you, or having to get your boat to the yard, which usually requires help from a very good friend willing to spend all day driving and sailing back and forth.

Owning a boat requires constant care and maintenance, so a little knowledge goes a long way. While an outboard motor is not required for sailing, it’s a convenient addition that can greatly increase your enjoyment out on the water. Being patient and spending time researching engines will not only help you make the correct purchase but will help you take advantage of a great deal when it presents itself. Whether you sail the Caribbean, or race off the coast of California in a catamaran, there is an outboard motor that’s just right for you.

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I've personally had thousands of questions about sailing and sailboats over the years. As I learn and experience sailing, and the community, I share the answers that work and make sense to me, here on Life of Sailing.

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  • Hunter 31 Sailboat

The Hunter 31 Sailboat Specs & Key Performance Indicators

The Hunter 31, a B&R rigged masthead sloop, was designed by Cortland Steck and built in the USA by Hunter Marine.

Hunter 31 sailboat

Published Specification for the Hunter 31

Underwater Profile:  Fin keel & spade rudder

Hull Material:  GRP (Fibreglass)

Length Overall:  31'4" (9.6m)

Waterline Length:  26'3" (8.0m)

Beam:  10'11" (3.4m)

Draft:  5'3" (1.6m)

Rig Type:  B&R

Displacement:  9,700lb (4,400kg)

Ballast:  4,000lb (1,814kg)

Designer:  Cortland Steck

Builder:  Hunter Marine (USA)

Year First Built:  1983

Year Last Built:  1987

Published Design Ratios for the Hunter 31

Sail Area/Displacement Ratio :  16.2

Ballast/Displacement Ratio:  41.2

Displacement/Length Ratio: 239

Comfort Ratio:  22.4

Capsize Screening Formula:   2.1

Read more about these  Key Performance Indicators...

Summary Analysis of the Design Ratios for the Hunter 31

eBook: How to Avoid Buying the Wrong Sailboat

1. A Sail Area/Displacement Ratio of 16.2 suggests that the Hunter 31 will, in the right conditions, approach her maximum hull speed readily and satisfy the sailing performance expectations of most cruising sailors.

2. A Ballast/Displacement Ratio of 41.2 means that the Hunter 31 will stand up well to her canvas in a blow, helping her to power through the waves.

3. A Displacement/Length Ratio of 239, tells us the Hunter 31 is a moderate displacement cruiser, which means she'll carry all your cruising gear without it having a dramatic effect on her performance. Most of today's sailboats intended for offshore cruising fall into this displacement category.

4. Ted Brewer's Comfort Ratio of 22.4 suggests that crew comfort of a Hunter 31 in a seaway is similar to what you would associate with the motion of a coastal cruiser with moderate stability, which is not the best of news for anyone prone to seasickness. 

5. The Capsize Screening Formula of 2.1 tells us that a Hunter 31 would not be as good a choice of sailboat for ocean passage-making, owing to the increased risk of capsize in strong winds and heavy seas when compared to a sailboat with a CSF of less than 2.0.

The Hunter 31 Sailboat: A Review

The Hunter 31 is a popular model from Hunter Marine, an American company that has been building quality sailboats since 1973. The Hunter 31 was first introduced in 1983 and was designed by Cortland Steck. It was produced until 1987, when it was replaced by the Hunter 33.5. In 2006, Hunter Marine launched a new version of the Hunter 31, which is sometimes referred to as the Hunter 31-2 or the Hunter 30/31. This boat was designed by Glenn Henderson and was in production until 2014. In this article, we will focus on the original Hunter 31, but we will also mention some of the differences and similarities with the newer model.

The Hunter 31 is a sloop-rigged cruiser that can accommodate up to six people in two cabins and a convertible salon. It has a fin keel, a spade rudder and a single wheel steering system. The boat has a self-tacking jib and an optional in-mast furling system for easy sail handling. The boat also has a Yanmar diesel engine with a low fuel capacity of 12 gallons.

The Hunter 31 is a boat that offers a lot of features and benefits for recreational sailors. Here are some of the main aspects of the boat that you might want to know more about:

Overview The Hunter 31 is a boat that combines performance, comfort and convenience. It is a boat that can sail well in light to moderate winds, thanks to its efficient hull shape and rig design. It is also a boat that can handle rougher conditions, thanks to its high stability and stiffness. The boat has a PHRF rating of 174, which means it is faster than some of its competitors in its size range.

The Hunter 31 is also a boat that offers plenty of space and amenities for living aboard. It has a beam of 10 feet and 11 inches, which gives it a roomy interior and a wide cockpit. It has two private cabins, one forward and one aft, each with a double berth and storage space. It has a large salon with a U-shaped dinette that can convert into another double berth, a navigation station with an electrical panel and instruments, and a galley with a two-burner stove, an oven, a sink, an icebox and ample counter space. It has a head with a marine toilet, a sink and a shower. It also has plenty of ventilation and natural light, thanks to its numerous ports, hatches and skylights.

The Hunter 31 is also a boat that is easy to operate and maintain. It has a simple and user-friendly layout, with all the controls and lines led back to the cockpit. It has an engine access panel under the companionway steps, which makes it easy to check and service the engine. It has a molded fiberglass hull and deck, which are durable and easy to clean.

Accommodation The Hunter 31 can sleep up to six people in two cabins and a convertible salon. The forward cabin has a V-shaped double berth with storage drawers underneath, shelves along the hull sides, an overhead hatch and two opening ports. The aft cabin has an athwartships double berth with storage lockers underneath, shelves along the hull sides, an opening port and an overhead hatch. The salon has a U-shaped dinette that can convert into another double berth with storage lockers underneath, shelves along the hull sides, four opening ports and two overhead hatches. The head is located on the port side of the salon, opposite the galley. It has a marine toilet with a holding tank, a sink with hot and cold water, a shower with a sump pump, storage cabinets, an opening port and an overhead hatch.

The accommodation layout of the newer Hunter 31-2 is slightly different from the original model. The forward cabin has more headroom and more storage space than the original model. The aft cabin has less headroom but more floor space than the original model. The salon has a straight settee on the starboard side instead of a navigation station, and a smaller dinette on the port side that can convert into a single berth. The head is located on the starboard side of the salon, opposite the galley. It has a separate shower stall with a folding door, which makes it more spacious and comfortable than the original model.

Hull and Deck The Hunter 31 has a solid fiberglass hull with a balsa core sandwich construction for the deck. The hull has a moderate displacement of 9,700 pounds and a ballast of 4,000 pounds. The hull has a fin keel with a draft of 5 feet and 3 inches, which gives it good performance and stability. The hull also has a spade rudder with a stainless steel shaft and bearings, which gives it good maneuverability and responsiveness. The deck of the Hunter 31 is designed for safety and convenience. It has a wide and flat foredeck with an anchor locker, an anchor roller and an electric windlass. It has stainless steel bow and stern pulpits, stanchions and lifelines. It has two dorade vents on the cabin top for ventilation. It has two large cockpit lockers for storage, one on each side of the wheel. It has a transom swim platform with a folding ladder and a shower. It also has a stern rail seat on each side of the cockpit, which provides extra seating and visibility.

The hull and deck of the newer Hunter 31-2 are similar to the original model, but with some improvements and modifications. The hull has a slightly longer waterline length of 28 feet, which increases its speed potential. The hull also has a shoal draft option of 3 feet and 11 inches, which makes it more suitable for shallow waters. The deck has a more modern and sleek appearance, with flush-mounted hatches, recessed handrails and an arch over the cockpit that supports the mainsheet traveler and the optional bimini top.

Mast and Rigging The Hunter 31 has a sloop rig with a deck-stepped mast and swept-back spreaders. The mast is made of anodized aluminum and has internal halyards and wiring. The mast height is 46 feet and 7 inches, which makes it suitable for most bridges and marinas. The mast also has an optional in-mast furling system for the mainsail, which makes it easier to reef and stow the sail.

The boat has a B&R rig, which is a fractional rig that eliminates the need for a backstay. This allows for a larger mainsail area and a smaller jib area, which improves the boat's performance in light winds and reduces its heeling tendency. The boat also has a self-tacking jib, which makes it easier to tack and trim the sail without changing sheets.

The boat has stainless steel standing rigging and low-stretch running rigging. The boat has two Lewmar self-tailing winches on the cabin top for halyards and reefing lines, and two Lewmar self-tailing winches on the coaming for jib sheets. The boat also has rope clutches, cam cleats, blocks, tracks and cars for adjusting the sails. The mast and rigging of the newer Hunter 31-2 are similar to the original model, but with some differences. The mast height is slightly lower at 46 feet, which reduces its windage and weight aloft. The mast also has an optional in-boom furling system for the mainsail, which gives it more sail shape control than the in-mast furling system. The boat also has an optional spinnaker or gennaker for downwind sailing.

Keel and Rudder The Hunter 31 has a fin keel which is bolted to the hull with stainless steel bolts and nuts. The keel draft is 5 feet and 3 inches, which gives it good performance upwind and downwind. The keel also provides stability and balance to the boat. The boat has a spade rudder with a stainless steel shaft and bearings. The rudder draft is 5 feet, which matches the keel draft. The rudder also provides maneuverability and control to the boat.

The keel and rudder of the newer Hunter 31-2 are similar to the original model, but with some options. The boat has a shoal draft option of 3 feet and 11 inches, which reduces its draft by more than one foot. The boat also has an optional wing keel or twin keels, which increase its stability at low speeds or when anchored.

The above text was drafted by using GPT-4 (OpenAI’s large-scale language-generation model) as a research assistant to develop source material; we believe it to be accurate to the best of our knowledge.

Other sailboats in the Hunter range include:

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  • How Much Horsepower Do I Need for My Boat?

1 How Much Horsepower Do I Need 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.

2 Horsepower Min Min

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.”

3 Dynamometer Min

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?”

How Much Horsepower Do I Need For My Boat V2 01 1

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.

4 Weight Min 1 Min

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.

5 Pulling Min

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.

6 Weight Min

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.

7 Illegal Min Min

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.

Determine The Maximum Horsepower Image 1

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.

8 Insurance Min

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.

9 Damage Min

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.

10 Maximum Min

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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How Sailboats Measure Up

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

sailboat horsepower

Sailboats by the Numbers

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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Max Horsepower Rating 20 ft sailboat

  • Thread starter CAP"NADAM
  • Start date Feb 11, 2021
  • Forums for All Owners
  • Ask All Sailors


Hello. Does anyone know the max horsepower rating for a RK20 or a comparable 20 foot fiberglass sailboat? A really thorough and therefore hopefully reliable used outboard guy wants me to tell him the max horsepower rating of my boat before he sells me a motor, which seems like a good idea. I can''t find the coast gaurd rating plate on my boat and can't find any information on the web other than (LxWx.8)-15 = hp, which might not convince him. So I thought I would try the experts, you all boat owners. Thank you.  


I had a Mac26S with a 5HP OB. 20 feet, you would be fine with something under 5hp. Maybe even electric or propane if you have restrictions in your area.  

Sailboats don't have HP ratings like motor boats do. A 20 ft sailboat will move quite nicely with a 4HP outboard. Sailboats are displacement hulls, adding more hp just means more weight, noise, and expense. Shaft length is more important than HP.  

Charlie Jones s/v Tehani

Charlie Jones s/v Tehani

Anything over about 5-6 HP would be a waste on that boat. ,My 25m footer runs hull speed at half throttle on the 8 hp, and my 21 footer uses 2.5 HP quite nicely A good friend of mine runs a RK20 with an old British Seagull one cyl  

Charlie Jones s/v Tehani said: A good friend of mine runs a RK20 with an old British Seagull one cyl Click to expand


I have a Tohatsu 6 HP sailpro. It is perfect for my boat which is a much bigger boat than yours. If you don't need the 6 amp alternator I'd go for a 4 HP but try to buy the long shaft version.  



dlochner said: Sailboats don't have HP ratings like motor boats do. A 20 ft sailboat will move quite nicely with a 4HP outboard. Sailboats are displacement hulls, adding more hp just means more weight, noise, and expense. Shaft length is more important than HP. Click to expand
Charlie Jones s/v Tehani said: ... an old British Seagull one cyl Click to expand

jon hansen

dlochner said: He's a brave man! Those things never start. Click to expand

Torqeedo Motors | Inflatable Boat Specialists

Thank you everyone for the information. This might be crazy but I'm thinking of having 2 outboards. Most of the time I wouldn't use both but having 2 makes me feel more secure. I want to go about 7 miles offshore to some islands eventually, so reliability and speed are important to me. My boat has a swing keel, so when lifted, and with mast and boom down, in an emergency, how much hp would be safe and how fast could I go? How much is hull speed affected by having the keel up?  

Maybe even a better argument for electric and more battery reserve. I don't think the keel up will change your hull speed. It will reduce drag though Hull speed or displacement speed is the speed at which the wavelength of a vessel's bow wave is equal to the waterline length of the vessel. As boat speed increases from rest, the wavelength of the bow wave increases, and usually its crest-to-trough dimension (height) increases as well  

rgranger said: Have you thought about a high thrust trolling motor? Or a smaller Torqeedo? Torqeedo Motors | Inflatable Boat Specialists We sell Torqeedo outboard motors and parts. Electric boating at its finest. If I were in the market for a new O.B., I'd consider an electric. On a small 20' boat, you could probably still get 100+ mile range on a charge. If you really wanted to go on longer cruises, you could add solar panels and stow a small Honda generator. But I really think you could plan a longer cruise with an electric O.B. Click to expand

Hull speed is a function of waterline length and hull shape. The centerboard will have a minimal effect on the top speed, but it will have a very positive effect on the boat's stability and help to stop any rolling or yawing. Top speed without planning or surfing will be in the 5.5 knot range. A 15hp motor only push to about that speed, so will a 4 hp motor. The only time a little larger, say 5 or 6 hp motor will be better is trying to motor into a seaway. That little extra power help the boat recover faster from being stopped by a wave. 7 miles seems like a lot, however, it is only about a 2 hour sail. In good conditions that should be easy to do. Just watch the weather forecasts.  


CAP"NADAM said: ... How much is hull speed affected by having the keel up? Click to expand
shemandr said: Enough so that it is worth doing while racing downwind in centerboard boats. But pulling the board up won't turn a displacement hull into a planing hull. I'm not sure why you would take the mast and boom down. Could you elaborate? Maybe you should look for a MacGregor 26X (I think), which is a cross between a power boat and a sailboat. It will plane with the Rx'ed engine and it can sail. Click to expand

At least you have sails to get you home. Imagine if you were in a stinkpot and it quit  

CAP"NADAM said: I figured that taking the mast and boom down, in the case that I had to motor to shore from the islands quickly, would help compensate stability wise for me raising the keel, thought it seems like my idea to raise the keel won't help me much. I guess I just like to think about options and ways to be safe because I am newer to the sailing side of boating. Click to expand
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Boating Hub

Maximum Boat Horsepower Calculator

You are currently viewing 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 Above0-3536-3940-4243-4546-52
Horsepower Capacity (All Other Boats)357.51015
Horsepower Capacity (Flat Bottom/Hard Chine Boats)n/a357.510

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

Determining your boat's maximum engine size

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

National Marine Manufacturers Association Certification Plate

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

sailboat horsepower

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Horsepower/Sail Area equivalence

Discussion in ' Sailboats ' started by bob the builder , Jul 26, 2009 .

bob the builder

bob the builder novice

good question, at what windspeed does 1 meter square sail give 1 horsepower? cheers, mal  


Guest625101138 Previous Member

Sails produce a force. Power is force times velocity. So you cannot equate the force developed by a sail unless you know the speed of the boat. The sail lift force is given by: Lift Force = 0.5 x rho x V^2 x Cl x Area (Perpendicular to apparent wind) Drag Force = 0.5 c rho a V^2 x Cd x Area (In line with the apparent wind) V is the apparent wind velocity. You have to do the vector sum of the forces. This will depend on the point of sailing. The maximum value for Cl with a sloop rig having sails with aspect ratio of 4 will be about 2.5. The Cd for this rig is around 0.5 at maximum lift. If you look past all the white noise on this thread you will see some images that give insight (same problem as your original thread): So working in metric. A boat doing 5m/s with apparent wind of 10m/s on a broad reach where the Cl is in line with travel, the force for every square meter in a sloop rig will be around: Force = 0.5 x 1.2 x 10^2 x 2.5 = 150N Power = Force x Velocity = 150 x 5 = 750W (Say 1HP)  

Manie B

Manie B Senior Member

Nice one thanks  
Manie B said: ↑ Nice one Rick thanks Click to expand...
My current project the "Microcruiser" is going to be the really good one to test the maths, it is the culmination of my experiences and approach within my current knowledge and capabilities My maths skills are nowhere near yours, so i appreciate your input this microcruiser is a down sized Transat mini, i really wanted to do this. The hull was modeled in freeship which also gave me the developed plates and the data as calculated in freeship. I did all the drawings in Turbo CAD16. Point is that i can answer design and drawing questions with accurate dimensions. I will also take good readings once on the water and then we can work backwards to determine what the maths says - this will eventually help me a lot, for future builds.  
Manie I am looking forward to the data. Will be an interesting exercise.  
1hp per meter, thanks rick!. got another question . . . (purely as a matter of interest, i can't follow you. not even past the first line.) Power is force times velocity? whats the word that means static power then? ie if the same sail boat is tied to a jetty? (with one meter of sail and pulling 75Kg on a scale tied to a bollard) if a weightlifter is perfectly steady holding 75 Kg above his head, he would dispute you saying he is using no power. so how much power is he using rick? if he's not moving that 75 Kg weight even a millimeter? isn't 750 N = 75Kg? so if the same boat has 30 m2 sail, why isn't there 30x75Kg force on the sail? (please don't use equations. use sock puppets and song please.) (also, don't answer unless you have the time. or if it's too exasperating.  

ancient kayaker

ancient kayaker aka Terry Haines

I did the calc for the downwind case: Power = sail force * boat speed sail force = sail area * pressure working in metric, the sail area = 1 pressure = ½ * (density of air) * (apparent wind speed)^2 * (shape factor) The density of air is about 1.25 kg/m3 The maximum shape factor occures when the air is stopped dead but that never happens as some wind flows around the object: lets assume an efficient sail that achieves a shape factor of 0.5 (a typical sail is probably half of that) wind velocity = V boat speed = S For the downwind case: apparent wind speed = V - S Putting that all together: power = 0.3125 * S * (V-S)^2 = 0.3125 (V^2*S - 2VS^2 + S^3) differentiating wrt S: dP/dS = 0.3125 (V^2 - 4VS + 3S^2) solving for max power: 3S^2 - 4VS + V^2 = 0 or: S = V/3 Thus: power = 0.3125 * S * (V-S)^2 = 0.3125 *(V/3)*(2V/3)^2 = 0.046 * V^3 = 746 (1 hp) or: V = cuberoot(746/0.046) = 25.3 m/s Using the wind pressures in the Beaufort scale yields 20 m/s It would be interesting to see at what point off the wind the power would maximize ...  


marshmat Senior Member

whats the word that means static power then? ie if the same sail boat is tied to a jetty? (with one meter of sail and pulling 75Kg on a scale tied to a bollard) if a weightlifter is perfectly steady holding 75 Kg above his head, he would dispute you saying he is using no power. so how much power is he using rick? if he's not moving that 75 Kg weight even a millimeter? Click to expand...
thanks marshmat so a normal person would say the tug boat is exerting 1 horsepower thrust on the bollard, 75Kg held in the air. and an engineer would say, "Sir, engineers use the same words to speak an entirely different language, and in our language you are speaking gibberish. the force exerted on the bollard is 750 Newtons, which is entirely different from 75Kg thrust" even though a 75Kg pull on a rope on a bollard is exactly 750 Newtons pull on a rope on a bollard is this right? mal (9.8=10 746=750 7.46=7.5 etc)  
Hi Bob, Using some nice rounding-off to powers of ten, yes, a 75 kg mass hanging on the end of a rope will put a tension of 750 N on the rope. The tugboat captain might brag about "My boat can do a bollard pull of 3,000 pounds". He'll then go on to say "That dinghy there weighs 200 pounds". What is omitted here, and what is really the source of most of the confusion, is that in imperial measure, there are three units with the name "pound". There is a pound-force, a pound avoirdupois, and a (rarely used) pound troy. Our captain, when he brags about his bollard pull, is talking in pounds-force. When talking about his dinghy, he's talking in pounds avoirdupois. Because at sea level on Earth, something that has a mass of one pound avoirdupois has a weight of one pound-force, the two are often treated as interchangeable. So we usually just say "pound" for both force and mass. In the SI system, different names are used for force and mass units, so that it is always clear which quantity is being discussed. One can mess around with nonexistent units like "kilogram-force" if he so chooses, but by far the easiest way is to just think of it in the proper units to start with: Newtons for "how much force, tension, thrust, etc." Kilograms for "how much material is that object made of"  
I redid my calc in post #8 which had an error; the original wind speed was suspiciously low.  


PAR Yacht Designer/Builder

In force 4 winds, 50 sq. ft. is about 1 HP.  
bob the builder said: ↑ ....... if a weightlifter is perfectly steady holding 75 Kg above his head, he would dispute you saying he is using no power. so how much power is he using rick? if he's not moving that 75 Kg weight even a millimeter? ......... Click to expand...
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daiquiri Engineering and Design

It is basically about the misuse of the word "power" in common talk. It is too often wrongly used as a synonym of "force", hence all these doubts.  


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Salt Water Sportsman

How Much Horsepower Does Your Boat Really Need?

  • By Alan Jones
  • September 4, 2023

Center console boat running with multiple outboards

This could be the shortest column in marine journalism history. “To determine how much horsepower to put on your boat’s transom, find the manufacturer’s maximum horsepower specification, and whatever number is listed there is the total power to install.” While that’s a good rule of thumb, there might be times when less is more. 

Go for the Max?

One good reason to get the most power possible lies in durability and longevity. The greater the horsepower, the less an engine has to work to keep the boat on plane with greater efficiency. Let’s look at an example. The Pathfinder 2005 TRS with a Yamaha F115 will have to operate at 5,500 rpm to reach 36.5 mph and will get 3.5 mpg. Power the same boat with an F150 and it reaches 35 mph at 4,500 rpm and will get 3.9 mpg, according to the manufacturer-­supplied performance data. This example also illustrates that the notion that less horsepower equals better fuel economy isn’t always true. Top speed is another reason to max out the ponies. More horsepower translates to a higher top speed. Sometimes the difference is dramatic. A Sundance FX19 flats boat with a Yamaha F90 will reach 41 mph, but it’s a 60 mph boat with an F150. Then there’s the resale value. If a buyer is looking at two identical boats and one has more power than the other, they will ­typically choose and often pay more for the more powerful boat.

Mo’ Power, Mo’ Money

Often, taking even a small leap in power can cost big bucks. The standard engine on the Blackfin 232DC is the Mercury 225XL FourStroke, but bumping it just 25 hp to the Mercury 250XL will cost $4,921 more. In this case, the big jump comes by moving from the 3.4L family of V-6 engines to a 4.6L V-8 for the 250. When reviewing the build-a-boat feature found on most manufacturers’ websites, compare the cost with different engines and look for the sweet spot where the least amount of money buys the biggest jump in ­horsepower. Often, the max-­power option is an exotic engine that comes with a larger-than-­proportional price tag. 

Read Next: How Much Fuel Capacity Do I Really Need?

Twin outboards on a fishing boat

Less Power Equals Less Weight?

Sometimes, adding more horsepower changes the weight drastically, but an increase in horsepower often adds no weight because it is done via programming the engine’s electronic control unit. For years, Mercury’s supercharged Verados all shared the same 2.6L inline-six-­cylinder block, even though the horsepower could range from 200 to 400, with a weight difference of just 38 pounds. 

In the case of the Mercury 225XL V-6 versus the 250XL V-8, the weight difference is 52 pounds, which might not seem like much but could be significant on a small flats boat. Using multiple engines just exacerbates the difference.

Moving to the largest engine can make a huge difference. Mercury’s new Verado 600 is a 7.6L V-12 behemoth with a host of technological advances, but it also weighs 1,260 pounds. Yamaha’s XTO Offshore 450 V-8 weighs 988 pounds. In comparison, the Mercury 450R weighs just 689 pounds. Although more weight can cause slower hole-shot times, often this is counterbalanced by the ­additional horsepower and the larger prop that a higher-horsepower engine can swing. Need an example? Take a look at the Mercury Verado 600 V-12 that turns a massive 18-inch-­diameter prop.  

Horsepower needs ­also change if only one or two people routinely use the boat. If you typically invite a crowd of friends to go fishing, powering up to the max might make more sense. Otherwise, the boat can be sluggish and slow to respond to the throttle.

Variables Matter

If a boat weighs more, it’s probably going to need more power, but factors such as the hull design also matter. A Carolina Skiff 21 SWS is a 21-footer that features a relatively flat hull bottom, weighs 2,351 pounds, and has a maximum horsepower rating of 150. The Regulator 23 has a steep 24-degree transom deadrise to help it slice through ocean waves and weighs 6,700 pounds with a single outboard. The smallest outboard you can buy it with has 300 hp.

If a boat has a transom deadrise of 20 degrees or greater and is run in waters that can get rough, err on the side of more power. During situations like running through a ripping inlet, having crisp throttle response is often needed to safely transit out or return.

If you’re not sure about how much power to choose, talking to a local dealer is a good idea. They have the experience with and information on the models they sell that can help you make the best possible decision when it comes to the question of, “How much horsepower do I really need?” 

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    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.

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