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:

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.

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.

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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.

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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.

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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 to Determine the Outboard Motor Size for Your Boat

How to Determine the Outboard Motor Size for Your Boat

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How to Calculate Canoe Capacity

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Will Charpentier

Your boat will tell you how much outboard it needs. Every boat has a maximum number of people it can carry, a maximum load of equipment it can carry and a maximum horsepower rating. The horsepower rating is based on the boat's "hull design speed." Even if your boating needs and circumstances may be different than those the designers used when doing their calculations, you can use their work as a beginning and, with a bit of math and a look at your boating habits, determine the right outboard motor size for your boat.

Items you will need

Locate the National Marine Manufacturers Association Certification Plate on the inside of the stern of the boat. The NMMA plate contains important information which you should know, including the number of people the boat can carry and the horsepower rating of largest motor you should use.

Ask yourself how many people are usually with you when you go boating. Use a calculator, if there are several passengers, to multiply the top horsepower rating by 1.10. If the plate says a 120 horsepower engine is the largest you should use, then: 120 x 1.1 = 132, or a 130 horse motor, since the horsepower of outboard motors is rated in multiples of 10. This adds ten percent to the horsepower and will make it easier for your boat to carry the load of several people more efficiently.

Work the equation the other way if you boat by yourself. Divide by 1.10: 120 / 1.1 = 109. Boating by yourself, you could get about the same performance from a 100 or 110 horsepower motor as you do from a 120 horsepower motor and save fuel.

Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.

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

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 .

sailboat motor size calculator

Diesel Marine Propulsion Engines, Generating Sets & Inboard Electric Propulsion Motors

Engines for yachts with displacement hulls.

The basis of the following guide is that the maximum theoretical displacement hull speed is to be achieved by a clean and polished hull in calm sea with no windage.

Racing yachts for example may wish to opt for smaller, lighter engines with more limited horse power as the engine is primarily only used to reach or leave moorings and is not required to achieve maximum theoretical hull speed. Propellers will be selected that deliver the least amount of drag when under sail rather than optimum efficiency when under power.

A motor sailor or yacht used in heavier seas and or stronger tides may often opt for an engine with up to 30% more power to ensure an adequate safety factor is available and easily be capable of achieving maximum theoretical hull speed in harsher conditions. Propellers will be selected for high efficiency when under power and drag when under sail is not such a primary consideration.

Engine power, speed (RPM), gearbox reduction ratio, propeller diameter and pitch are all variables that will affect a displacement hulls performance and as a consequence is a specialist subject that for new builds, the vessels designer should be consulted.

For vessels which require repowering, there are existing guidelines and or parameters that are already in place to help in making the correct replacement engine choice. The existing engine, gearbox reduction ratio and propeller performance compared to theoretical maximum hull speed for the vessel will be a good indicator on whether it can be improved or not.

Additional there may be some pre-set parameters such as the distance between the centre line of the vessels propeller shaft and the hull that may or may not allow a larger more efficient propeller to be used. If the propeller`s diameter is restricted, this can in part be compensated for by changing the propeller pitch, but there are limitations. For more detailed information on propellers visit Propeller Guidance .

Displacement Hulls

Beta Marine Ltd specialise in propulsion engines for use in; Seagoing Yachts, Motorsailors, Fishing & Work Boats, Inland Waterway Dutch Barges and Narrowboats.

A displacement hull will displace its own ‘Total Laden Weight’ * in the water, when it moves it has to displace and push an equal amount of water out of the way. As the vessel increases speed it has to push through more water in less time which increases the size of the wave building at the bow

* The ‘Total Laden Weight’ will include all fuel, provisions and crew / passengers.

Additionally as you approach the maximum design speed of the hull, the wave will grow so the crest remains at the bow and the stern wave comes up under the counter. At this point the vessel effectively digs a large whole in the water which traps the hull at the bow by a very large wave it can neither push through nor plan over.

sailboat motor size calculator

Heavy seas and or tides can also impact a displacement hulls ability to reach maximum speed.

The following example highlights the speed to power ratio required for a 25 foot yacht.

If absolute maximum hull speed is not a priority the above also highlights a significantly reduced power requirement and corresponding fuel saving is available for cruising at 6 knots.

sailboat motor size calculator

  • A general engine guideline for yachts with a displacement hull is the weight in tonne multiplied by 4 hp.
  • Theoretical maximum displacement hull speed in Knots = 1.3 (or 1.4 for a sleek hull) x √ Water Line length (in feet).

Beta Marine Limited will not accept any responsibility or liability for an incorrect engine selection.

PowerFlow Marine

Sailboat Motor Electric Calculator

Simulate the performance of your vessel under electric propulsion

Vessel Parameters

In the context of boats, displacement refers to the weight of the volume of water that the hull displaces when it is placed in the water. To calculate displacement, you should consider the boat’s fully loaded weight, which includes:

  • Full crew and passengers
  • All normal ship’s stores and gear
  • Two-thirds of the fuel and water tanks’ capacity
  • Two-thirds of the cargo being carried

A vessel’s length at the waterline is the length of a boat at the level where it sits in the water (the waterline). The waterline length will be shorter than the length of the boat overall.

Hull speed refers to the maximum speed at which a displacement hull can travel efficiently through the water. As a boat approaches its hull speed, the resistance from the water increases exponentially, requiring a disproportionate amount of power to achieve higher speeds. Therefore, it is generally advised to operate a displacement boat within or below its hull speed for optimal efficiency and performance.

Additional Notes

  • The provided calculator is designed specifically for displacement boats and may not be suitable or applicable to other types of boats or hull designs
  • While the calculator has proven to be valuable to many users, it is important to recognise that it proves only an approximation
  • Users often find that operating their boats at maximum motor output is rarely necessary. This understanding effects the calculations and estimation related to the duration of their boat’s performance
  • The equations used in this calculator were developed based on the research and expertise of Dave Gerr, as presented in his book “Propeller Handbook: The Complete Reference for Choosing, Installing, and Understanding Boat Propellers.” We have further refined the equations based on real-life examples.

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Sailboat Speed Calculator

How do you calculate the speed of a sailboat?

Sailboat speed is influenced by various factors, but one common way to estimate it is using the following formula:

Speed (in knots) = 1.34 * √(Sail Area in square feet) * √(Wind Speed in knots)

Please note that this is a simplified formula and actual sailboat speed can vary based on wind direction, hull design, sea conditions, and other factors.

What is the average speed of a 30-foot sailboat?

The average speed of a 30-foot sailboat can vary depending on wind conditions, sail configuration, and the boat’s design. In general, sailboats of this size might cruise at speeds between 5 to 7 knots.

How long does it take to sail 60 miles?

The time it takes to sail 60 miles depends on the sailboat’s speed. If the sailboat is traveling at an average speed of 6 knots, it would take approximately 10 hours to sail 60 miles.

What is the typical speed of a sailboat?

The typical speed of a sailboat can range from 5 to 10 knots, depending on the sailboat’s size, design, wind conditions, and other factors.

How long does it take to sail 1 nautical mile?

If a sailboat is traveling at a constant speed of 6 knots, it would take approximately 10 minutes to sail 1 nautical mile.

How long does it take to travel 1 nautical mile?

The time it takes to travel 1 nautical mile depends on the speed of the vessel. For example, at a speed of 6 knots, it would take about 10 minutes to travel 1 nautical mile.

Is a 30-foot sailboat too big for a beginner?

A 30-foot sailboat can be manageable for some beginners, especially with proper training and experience. However, beginners may find smaller sailboats in the 20 to 25-foot range more suitable for learning basic sailing skills.

How fast can a 40 ft sailboat go?

The speed of a 40-foot sailboat can vary depending on wind conditions and other factors, but it might typically cruise at speeds between 6 to 9 knots.

Can a 30-foot sailboat cross the Atlantic?

Yes, a 30-foot sailboat can cross the Atlantic Ocean. Many sailors have successfully completed transatlantic crossings with boats of this size. However, it requires careful planning, appropriate safety equipment, and experienced crew.

Do longer sailboats go faster?

Generally, longer sailboats can achieve higher hull speeds and might perform better in certain conditions, but various factors, including sail area, hull design, and weight, also influence a sailboat’s speed.

How far can you sail in 8 hours?

The distance you can sail in 8 hours depends on the sailboat’s speed. For example, if the sailboat is traveling at an average speed of 6 knots, it could cover approximately 48 nautical miles in 8 hours.

Can you sail in 20 mph winds?

Yes, sailboats are designed to handle different wind conditions, including 20 mph winds. However, the actual ability to sail comfortably and safely in such winds depends on the sailboat’s size, crew experience, and sea state.

What is a safe sailing speed?

A safe sailing speed depends on various factors, including the sailboat’s design, sea conditions, and the crew’s experience. In general, maintaining a moderate speed that allows for control and maneuverability is advisable.

In which position does a sailboat travel the fastest?

Sailboats typically travel fastest on a broad reach or downwind position with the wind coming from behind the boat.

How long should a sailboat be for the ocean?

A sailboat that is at least 30 feet or larger is generally considered more suitable for ocean cruising due to its seaworthiness, storage capacity, and stability.

Why do ships use knots instead of mph?

Ships use knots as a unit of speed because it is based on nautical miles per hour, which is more practical for maritime navigation. Nautical miles take into account the curvature of the Earth, making it easier to measure distances accurately at sea.

How fast is 25 knots on a boat?

25 knots on a boat is equivalent to approximately 28.8 miles per hour (46.3 kilometers per hour).

How far is a nautical mile?

One nautical mile is approximately 1.15 statute miles or 1.85 kilometers.

How many nautical miles can you sail in a day?

The number of nautical miles a sailboat can cover in a day depends on various factors, such as wind conditions, boat speed, and crew endurance. Experienced sailors might cover 100 to 150 nautical miles or more in a day of continuous sailing.

How fast is 10 knots on a boat?

10 knots on a boat is equivalent to approximately 11.5 miles per hour (18.5 kilometers per hour).

Why is boat speed called knots?

The term “knots” for boat speed is derived from the traditional method of measuring a ship’s speed using a device called a “common log.” The log consisted of a line with knots tied at regular intervals. The number of knots that passed through a sailor’s hands in a given time was used to determine the ship’s speed.

What size sailboat is best for 2 people?

For two people, a sailboat in the 30 to 40-foot range is commonly considered suitable. It provides enough space and amenities for comfortable cruising.

What is the largest sailboat you can sail alone?

The largest sailboat that one person can sail alone depends on the sailor’s skill, experience, and the boat’s design and equipment. Experienced sailors have been known to solo sail boats in the 30 to 40-foot range and even larger, but handling a boat of this size alone can be very challenging.

What size sailboat is best to single hand?

For single-handed sailing, smaller sailboats in the 20 to 30-foot range are more manageable, as they require less physical effort and are easier to handle alone.

Is it faster to sail upwind or downwind?

Sailing downwind is generally faster and more comfortable than sailing upwind. However, the speed and performance of a sailboat also depend on its design and the specific wind conditions.

Can one person sail a 50-foot sailboat?

Yes, it is possible for one person to sail a 50-foot sailboat, especially if the boat is equipped with modern sailing systems like roller furling, electric winches, and autopilot. However, handling a sailboat of this size alone can be challenging and may require significant experience and skill.

What is the fastest angle for a sailboat?

The fastest angle for a sailboat is typically achieved on a broad reach or downwind position, where the wind is coming from behind the boat.

What is the minimum size sailboat for the ocean?

The minimum size sailboat for ocean sailing depends on the sailor’s experience, the boat’s seaworthiness, and the intended route. However, most experts recommend a sailboat in the 30 to 35-foot range as a minimum for safe ocean voyages.

Is a 30-foot sailboat a yacht?

The term “yacht” can be used broadly and is not strictly defined by size. In general, many 30-foot sailboats can be considered yachts, especially if they are well-equipped for cruising and recreational use.

Which ocean is easiest to sail?

The Atlantic Ocean is often considered one of the easier oceans to sail due to its generally predictable weather patterns and a wide range of available ports and resources.

How can I increase my sailboat speed?

Several factors can affect sailboat speed, such as proper sail trimming, using the right sail combinations, optimizing hull shape and weight distribution, and making sure the bottom of the boat is clean and free from marine growth.

Can you leave a sailboat in the water year-round?

Leaving a sailboat in the water year-round is possible, but it requires proper maintenance, anti-fouling measures, and protection from harsh weather conditions. Many sailors prefer to haul their boats out of the water during the off-season for maintenance and storage.

Do sailboats tip over easily?

Sailboats are designed with stability in mind, but they can capsize or heel over in extreme weather conditions or due to improper handling.

What is the best sailboat length?

The best sailboat length depends on individual preferences, intended use, and sailing goals. Many sailors find boats in the 30 to 40-foot range to be a good balance of size and performance.

Why do sailboats not flip over?

Sailboats are designed with a keel or a ballast to provide stability and prevent excessive heeling. This design helps the boat remain upright and minimizes the risk of flipping over.

What is the lifespan of a sailing ship?

The lifespan of a sailing ship can vary significantly based on maintenance, usage, and materials. Properly maintained wooden sailing ships can last for several decades or even centuries, while modern fiberglass sailboats can also have long lifespans with proper care.

Can you sail while sleeping?

Yes, sailors on long passages often adopt a watch system, where crew members take turns to sail the boat and rest while others keep watch.

What happens if you sail too close to the wind?

Sailing too close to the wind can cause a sailboat to stall or lose forward momentum. This position is known as “in irons.”

What is the furthest you can sail in a straight line?

The furthest you can sail in a straight line is limited by the Earth’s curvature. On a long ocean passage, sailors must consider the curvature of the Earth and plan their routes accordingly.

What is the most comfortable point of sail?

The most comfortable point of sail for many sailors is a broad reach or downwind, where the wind is coming from behind the boat.

What is an unsafe height of a wave?

The height of waves that is considered unsafe varies depending on the size and type of vessel. In general, large waves with heights exceeding 10 feet can be hazardous for smaller sailboats.

What wind speed makes water choppy?

Choppy water is often caused by wind speeds between 15 to 25 knots or more, depending on local conditions.

What is the 50-90-100 rule in sailing?

The 50-90-100 rule is a rule of thumb used in sailing to estimate the amount of headsail to unfurl in different wind conditions. It suggests using 50% of the headsail in winds up to 10 knots, 90% in winds between 10 to 15 knots, and 100% in winds above 15 knots.

What is the rule of 12 in sailing?

The rule of 12 is a simplified method used to estimate wind direction when sailing. It states that if you look at the water, you can estimate the wind direction by finding where the ripples end and then adding 45 degrees to that direction.

What is Rule 17 of sailing?

Rule 17 of the International Regulations for Preventing Collisions at Sea (COLREGs) governs the action that a sailing vessel should take when encountering a vessel not under command, restricted in ability to maneuver, or constrained by its draft.

What is the slowest point of sailing?

The slowest point of sailing is typically close-hauled or sailing into the wind.

How do you slow down when sailing?

To slow down when sailing, you can reduce sail area by reefing (reducing the size of sails), furling (rolling up sails), or changing to smaller sails.

How do you know when someone is sailing too close to the wind?

Sailing too close to the wind is evident when the sails begin to luff (flap) and the boat loses forward momentum.

What is the most popular sailboat size?

The most popular sailboat size can vary by region and market trends. In many regions, sailboats in the 30 to 40-foot range are commonly popular for cruising and recreational sailing.

How often should you haul out a sailboat?

The frequency of hauling out a sailboat depends on factors such as the boat’s hull material, usage, and the environment it operates in. In general, sailboats with fiberglass hulls may be hauled out for inspection, maintenance, and bottom painting every 1 to 3 years. However, this can vary based on individual circumstances.

What is the best size sailboat to cross the Atlantic?

The best size sailboat to cross the Atlantic depends on various factors, including the sailor’s experience and comfort level. Sailboats in the 30 to 50-foot range are commonly chosen for ocean crossings due to their seaworthiness and accommodations.

Why is a knot 47 feet?

A knot is not 47 feet; it is a unit of speed used in navigation that represents one nautical mile (approximately 1.15 statute miles) per hour.

How many knots does a military ship go?

The speed of military ships can vary significantly depending on their type and purpose. Some warships can reach speeds of 30 knots or more, while larger aircraft carriers might have a top speed of around 35 knots.

How many knots does a Navy ship go?

Navy ships have various speed capabilities based on their class and purpose. Some Navy ships can reach speeds of 30 knots or more, particularly modern destroyers and cruisers.

Is 20 knots safe for boating?

Boating in 20 knots of wind can be safe, but it depends on factors such as the size and type of boat, sea conditions, and the experience of the crew.

Is 20 knots good for sailing?

A wind speed of 20 knots can be favorable for sailing, particularly for experienced sailors. It provides good boat speed and performance.

Why is a knot longer than a mile?

A knot is not longer than a mile. One nautical mile is approximately 1.15 statute miles, making it slightly longer than a land-based statute mile used for measuring distances on roads.

How far can you sail in 12 hours?

The distance you can sail in 12 hours depends on the sailboat’s speed. For example, if the sailboat is traveling at an average speed of 6 knots, it could cover approximately 72 nautical miles in 12 hours.

How long does a sailboat need to be to cross the ocean?

A sailboat that is at least 30 feet or larger is generally considered more suitable for ocean crossings due to its seaworthiness, storage capacity, and stability.

Is 12 knots good for sailing?

A wind speed of 12 knots can be considered good for sailing, providing moderate conditions for enjoyable sailing.

Is 15 knots fast for a sailboat?

A wind speed of 15 knots is considered a moderate breeze and can be a favorable sailing condition for many sailboats.

How fast is a knot vs. mph?

One knot is equivalent to approximately 1.15 miles per hour (mph).

What is 30 knots on a boat?

30 knots on a boat is equivalent to approximately 34.5 miles per hour (55.5 kilometers per hour).

Can a 30-foot sailboat cross the ocean?

Yes, a 30-foot sailboat can cross the ocean, but it requires careful planning, preparation, and experienced crew to ensure a safe and successful voyage.

Can you sail a 40-foot sailboat alone?

Sailing a 40-foot sailboat alone is possible, but it can be challenging and may require advanced sailing skills and experience.

Can you live on a 40-foot sailboat?

Yes, many people live on 40-foot sailboats as their primary residence. With proper amenities and storage, a 40-foot sailboat can offer comfortable living conditions.

How big a sailboat can a couple handle?

A couple can handle a sailboat in the 30 to 40-foot range comfortably, especially if the boat is equipped with modern sailing systems and conveniences.

What is the minimum size sailboat to live on?

The minimum size sailboat to live on depends on individual preferences and comfort levels. Some people choose boats in the 30 to 40-foot range for full-time living aboard, while others may opt for smaller boats.

What is a good size for a first sailboat?

A good size for a first sailboat depends on factors such as sailing experience, intended use, and budget. Beginners often find sailboats in the 20 to 30-foot range to be more manageable for learning.

Which wind direction is fastest for sailing?

Sailing downwind with the wind coming from behind the boat is typically the fastest point of sail for most sailboats.

Which sailing direction is the fastest?

Sailing downwind with the wind coming from behind the boat is generally the fastest direction for most sailboats.

What size boat to sail to Hawaii?

Sailing to Hawaii requires a seaworthy boat suitable for offshore passages. Sailboats in the 30 to 40-foot range are commonly used for voyages to Hawaii.

What size sailboat for 2 people?

What is a good hull speed for a sailboat?

Hull speed is a theoretical maximum speed that a sailboat can achieve based on its waterline length. For displacement hulls, the hull speed is approximately 1.34 times the square root of the waterline length in feet.

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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.

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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Loss of ship's power and stiff current may have led to bridge collision, experts say

Scott Neuman

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Jackie Northam

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The container ship Dali after it ran into and collapsed the Francis Scott Key Bridge on Tuesday in Baltimore. Win McNamee/Getty Images hide caption

The container ship Dali after it ran into and collapsed the Francis Scott Key Bridge on Tuesday in Baltimore.

Details are still scant on what might have caused a giant container ship to collide with Baltimore's Francis Scott Key Bridge early Tuesday, sending the span crashing into the water.

While authorities have said a "momentary loss of propulsion" could have caused the incidents, videos appear to also indicate a loss of electrical power aboard the nearly 1,000-foot ship as it careened into the structure's support.

The Singapore-flagged, Grace Ocean-owned vessel, Dali was departing Baltimore harbor destined for Colombo, Sri Lanka , at 1:27 a.m. EDT Tuesday, when the collision with the bridge span occurred.

More from WYPR in Baltimore:

  • Construction worker says friends, colleagues missing in bridge collapse
  • Federal government pledges full support to rebuild FSK bridge, reopen port

For the latest from member station WYPR in Baltimore head to

The Maritime and Port Authority of Singapore issued a statement Tuesday saying that the ship's management company, Synergy Marine Pte Ltd, reported "that just prior to the incident, the vessel, Dali had experienced momentary loss of propulsion" and that "As a result, it was unable to maintain the desired heading and collided with the Francis Scott Key bridge."

If propulsion was lost on the Dali just as the vessel was maneuvering in a tight channel in Baltimore's Patapsco River, it may well have triggered a cascade of events leading to the collision, experts tell NPR.

The Francis Scott Key Bridge in Baltimore collapses, 6 feared dead

The Francis Scott Key bridge in Baltimore collapses after a ship crashed into it

"Generally, believe it or not, today's ships do not have much redundancy, especially these big ships," says Basil Karatzas, the CEO of Karatzas Marine Advisors.

Matthew Collette, a professor of naval architecture at the University of Michigan notes, "there's one propeller on the ship. It is the only engine that can turn the propeller."

However, Collette speculates an electrical failure suggested by the video of the accident could have been a contributing factor. The lights on the ship could be seen switching on and off several times before it struck the bridge.

Photos: Baltimore's Key Bridge collapses; search and rescue efforts continue

The Picture Show

Photos: baltimore's key bridge collapses; search and rescue efforts continue.

He says the engine has redundant systems, but the fuel system requires electricity to keep fuel feeding it. A ship the size of the Dali "would have either three or four independent diesel generators" to provide electricity and another above the main deck ready to "automatically start in a blackout condition," Collette says.

"But it's not instantaneous. It might take 30 seconds or 60 seconds" to start them and restore power to the ship, he says. Without electrical power, both the engine and the steering system could have been disabled in the critical moments leading to the collision, he says.

While, the steering system is triply redundant, Collette notes, each of the backups is dependent on electricity to run pumps that then pressurize the hydraulics and activate the ship's massive rudder.

At the time of the collision, an ebb tide was also running, meaning that the current would have been flowing out of the harbor — the same direction the vessel was traveling. When a tide is running with the ship, it makes maneuvering more difficult, according to Collette. Compounding that difficulty, the collision occurred just one day past a full moon , when tidal current velocities are especially strong .

In its statement, the Singapore port authority said the Dali dropped its anchor before the collision.

"It's standard practice in this situation to try to anchor the ship," to stop it, Collette says. "If they have room and they have channel depth."

"That's an evolution that's going to be taking minutes," he says.

The ship reportedly had aboard two harbor pilots — personnel with specialized knowledge of the port who assist with navigation — to assist in transiting the narrow channel leading to the Chesapeake Bay. With an engine and/or a steering failure, there is little else the crew could have done to prevent catastrophe, Karatzas says.

"I suppose they were praying just to get power," he says.

The collision also occurred at night, which could well be another factor, according to Jonathan Roach, a container market analyst at London-based Braemar ACM shipbroking.

The entrance to the harbor is "quite narrow," he says. "And, it's quite late at night. It's dark. You have to make allowances for that."

Correction March 27, 2024

An earlier version of this story said the container ship Dali is owned by Maersk. The vessel is owned by Grace Ocean and was being chartered by Maersk when it crashed into Baltimore's Francis Scott Key Bridge.

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Chris Baraniuk

Why the Baltimore Bridge Collapsed So Quickly

The steel frame of the Francis Scott Key Bridge sits on top of a container ship after the bridge collapsed Baltimore...

Just shy of half past 1 in the morning, the MV Dali , a giant container ship, was sailing gently out of the port of Baltimore when something went terribly wrong. Suddenly, lights all over the 300-meter-long vessel went out. They flicked on again a moment later, but the ship then began to veer to the right, toward one of the massive pylon-like supports on the Francis Scott Key truss bridge—a huge mass of steel and concrete that spans the Patapsco River.

The Dali ’s lights went out a second time. Then the impact came. The ship plowed into the support, with large sections of the bridge’s main truss section instantly snapping apart and falling into the river. It took just 20 seconds or so for the structure to come down.

Now, a major US port is in disarray, and several people who were working on the bridge at the time of its collapse are missing. A rescue operation is underway. President Biden has called the disaster a “terrible accident.” Ship traffic is currently stuck on either side of the crash site, and a major roadway through Baltimore has been cut off.

“It’s a dreadful tragedy and something you hope never to see,” says David Knight, a bridge expert and specialist adviser to the UK’s Institution of Civil Engineers. But commenting on footage of the bridge collapse , he says he is not surprised by the manner in which it crumpled.

Large steel structures may seem invulnerable, but steel, explains Knight, is relatively lightweight for its size. As soon as it is pushed or pulled the wrong way with enough force, it can fold like paper. In this case, the Francis Scott Key Bridge was a “continuous,” or unjointed, bridge that had a 366-meter-long central truss section. (Truss bridges use steel beams, arranged in triangular shapes, to support their load.) The central truss was made up of three horizontal stretches, known as spans, with two sets of supports holding these above the water. It was the third-largest structure of its kind in the world.

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“When you take a support away, there is very little in the way of robustness,” says Knight. “It will drag down, as we saw, all three spans.” The separate approach spans remain standing. There is nothing in Knight’s view that immediately suggests any structural problem with the bridge. An engineering firm, Hardesty & Hanover, confirmed to WIRED that it performed an inspection of the bridge in 2019, and that other inspections have been carried out since, but did not provide any additional details on the state of the structure. WIRED has approached H&H for further comment. In June last year, the US Federal Highway Administration rated the condition of the bridge as satisfactory .

The immense force of the container ship impact should not be underestimated, adds Knight. Such vessels require a lot of power and time—perhaps many minutes—to come to a complete stop. The Francis Scott Key Bridge was completed in 1977. In more recent decades, bridge engineers have commonly incorporated defenses to reduce the potential damage by ship strikes when bridges are erected in similar locations, Knight says. These include hydraulic barriers and additional concrete around the base of bridge supports, for instance. However, even with such fortifications in place, heavy strikes can still cause devastating damage.

It is not clear why lights turned off and on again on the Dali , a Singapore-flagged ship built in 2015. “That is an indication of a massive problem,” says Salvatore Mercogliano, a maritime historian at Campbell University in North Carolina and a YouTuber who has analyzed the crash .

At the time of the accident, two pilots—mariners who board a ship to help it navigate particular stretches of water, including in and out of ports—from Baltimore were on board. The Dali was broadcasting its position publicly via the automatic identification system (AIS) and was traveling at a speed of over 8.5 knots. It then slowed to around 6 knots in the moments before the crash, according to AIS data .

Both pilots and all crew members on the Dali are accounted for. There are no reports of injuries, the ship’s management company, Synergy Group, said in a statement on March 26.

ABC News reports that the crew of the vessel made a desperate mayday call in an attempt to warn transport officials that the crash was about to occur. A report from the Cybersecurity and Infrastructure Security Agency, seen by ABC, says the Dali “lost propulsion” and that the crew were aware they had “lost control” of the ship. Maryland governor Wes Moore told reporters that , thanks to the mayday call, officials were able to stem the flow of traffic over the bridge, an intervention that he says “saved lives.”

Mercogliano says it is very difficult for ships of this size to make rapid adjustments to their trajectories. Video footage shows a sudden outpouring of smoke from the vessel’s stack, indicating a change in engine activity of some kind. What is particularly disturbing is that, in this case, the vessel ends up plowing straight into one of the key supports for the bridge, clearly off course. No information as to why this happened has become public.

Photographs of the aftermath show the bow of the ship pinned beneath fallen sections of the bridge . The anchor chain is visible, meaning that at some point the anchor was dropped, though it is not certain whether this happened before or after impact. The chain appears to be at an angle, however, which Mercogliano says could be a sign that it was dropped shortly before the crash and dragged for a brief time.

Lawyer James Turner of Quadrant Chambers in London specializes in, among other things, ship collisions. He says that there would have been no automated systems on board a merchant ship of this kind able to prevent the impact. Information from radar, AIS, and visual observations would have been available to the crew, however.

But data-collecting systems may now reveal exactly what happened. As on airplanes, commercial ships have data and audio recorders on the bridge, which are often a key source of information for investigators post-incident. “The master will hit a button and that ensures that the last two hours of audio recording are preserved, as well as all the data from the various parts of the ship, like the engine and steering and so on,” explains Turner. “That can be downloaded and queried.”

He adds that estimates of the ship’s speed at the time of the incident as recorded by AIS are likely “99.99 percent accurate.”

For now, the focus of responders will be on locating survivors from the fallen bridge. Two people have been rescued, one of whom is in the hospital. Six construction workers remain missing .

The disaster has come at a difficult time for shipping, with drought afflicting the Panama Canal and Houthi attacks striking multiple vessels in the Red Sea in recent months. Somali piracy is on the rise again , also. The grounding of the Ever Given in the Suez Canal is very much still within recent memory—it occurred a mere three years ago.

The Port of Baltimore insists in a statement that it has not been shut down—road vehicles are still operating within the port—however, all ship traffic in and out is suspended until further notice. AIS data reveals around a dozen commercial vessels at anchor outside the port, their entry now blocked by the stricken bridge and the Dali . It will take some time for the US Army Corps of Engineers to remove the steel pieces of the bridge, which present a significant threat to passing vessels, from the river.

“Whatever ships are in the port are now stuck,” says Mercogliano, who notes that Baltimore is an important port in terms of car deliveries and coal exports.

Overall, he argues, maritime operations are extremely safe today, though the volume and velocity of trade mean that when things go wrong it can be especially serious.

“We move goods a lot faster than ever before, and there’s very little margin for error,” he says. “When there is a mistake, the mistakes tend to be very large.”

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Watch CBS News

What caused the Dali to slam into Baltimore's Francis Scott Key Bridge? What we know about what led up to the collapse

By Kerry Breen

Updated on: March 27, 2024 / 7:43 PM EDT / CBS News

Shocking video showed the moment a massive cargo ship collided with Baltimore's Francis Scott Key Bridge early Tuesday morning, sending parts of the decades-old suspension bridge, along with people and vehicles, into the Patapsco River. 

Six people who were on the bridge are missing and presumed dead , officials said late Tuesday. Two others were rescued from the water. All eight were construction workers who were repairing potholes on the bridge, officials said. There were 22 Indian nationals, including two pilots, aboard the cargo ship. 

Investigators and officials are now crafting a timeline of events, including what caused the Singapore-owned vessel , called the Dali, to hit the bridge just minutes after leaving port . Here's what we know so far. 

What caused the Dali to slam into the Francis Scott Key Bridge? 

The Dali, which was chartered by shipping giant Maersk and operated by Synergy Marine Group, hit the Francis Scott Key Bridge shortly after leaving the Port of Baltimore. 

An unclassified memo issued by CISA, the Cybersecurity and Infrastructure Security Agency, said the ship reported losing propulsion . Maryland Gov. Wes Moore said the ship's crew reported a "power issue." A spokesperson for the National Transportation Safety Board, which is investigating the crash, said the agency still needs to verify that the Dali lost power before striking the bridge column. 

Key Bridge Accident

Two U.S. officials told CBS News multiple alarms rang out on the ship, alerting pilots and crew to an issue on board. The crew ran several system tests to attempt to remedy the loss of propulsion from the motor, but the tests proved unsuccessful. At that point, the ship's pilots alerted the Maryland Department of Transportation and the Maryland Transit Authority. 

That alert allowed local officials to stop traffic on the bridge and likely saved lives , officials said. 

The ship's crew tried to deploy the anchor, though it remains unclear how much progress was made, multiple officials said. The massive ship is over 900 feet long and was moving at about 8 knots, or just over 9 miles per hour. Authorities said that speed is considered "very rapid." 

Captain Michael Burns, executive director of the Massachusetts Maritime Academy's Maritime Center for Responsible Energy, told CBS Boston  that stopping a cargo ship is difficult, especially in such a short time. 

"It's extremely challenging, and takes years of experience and training in order to be able to do this safely," he said. "It can take up to a mile for some of these ships to get stopped, depending on the circumstances, so we really need to think well out, miles ahead of the ship."

Why did the Dali lose propulsion? 

It's not clear what caused the vessel to lose propulsion, officials said. 

A spokesperson for the NTSB told CBS Baltimore that it had collected the ship's data recorder, and would review and analyze the material there to determine what happened aboard the vessel in the moments before the collision. 

That data recorder will also be used to establish a timeline of events. 

What happens when a ship loses propulsion? 

James Mercante, the president of the New York Board of Pilot Commissioners, told CBS News that a ship that has lost steering and power is essentially "a dead ship just being carried by the current or its own momentum." 

He highlighted a moment in the video of the crash that appears to show a "big, big puff of black, real dark black smoke" which might indicate that the vessel's power was "restored at the last minute" and that the pilot was "attempting to make an emergency maneuver" to avoid hitting the bridge. However, he emphasized that it would be difficult to stop the massive cargo ship, especially in such a short time. 

"It would take quite a while — probably the length of five [or] six football fields — to bring that ship to a stop, even after dropping the anchors, because of its power and momentum," said Mercante. "This is a behemoth." 

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  • Bridge Collapse

Kerry Breen

Kerry Breen is a reporter and news editor at A graduate of New York University's Arthur L. Carter School of Journalism, she previously worked at NBC News' TODAY Digital. She covers current events, breaking news and issues including substance use.

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The collapsed Baltimore bridge didn't stand a chance against such a huge cargo ship, engineers say

  • Engineering experts sought to explain how the Francis Scott Key Bridge in Baltimore collapsed.
  • They centered on one factor: the sheer size of the cargo ship that hit it.
  • Faced with such momentum, a total collapse was inevitable, they said.

Insider Today

The Francis Scott Key Bridge in Baltimore stood no chance against the huge impact from a cargo ship Tuesday morning, engineering experts told Business Insider.

The bridge was destroyed in the early hours of Tuesday, collapsing in a violent fashion after a large cargo ship, the Dali, hit one of its support pillars.

This video shows the impact:

BREAKING: Ship collides with Francis Scott Key Bridge in Baltimore, causing it to collapse — BNO News (@BNONews) March 26, 2024

Understanding why and how the bridge collapsed could have big implications for safety, both in the cargo shipping industry and in civil engineering.

The physics of the impact are "pretty clear," said Leroy Gardner, a professor of structural engineering at Imperial College London.

"There's a heavy impact from a cargo ship into one of the piers," he said in a call with BI. "Once that collapsed, then the rest of the bridge followed soon after."

Experts told BI it was unlikely that any defects in the bridge's structure were relevant to the collapse, given the scale of the impact.

The Dali is a substantial vessel, 300 meters long and with a gross tonnage of roughly 95,000, according to the website .

It's of comparable size and weight to a Nimitz-class aircraft carrier in the US Navy. The Dali was almost as tall as the bridge it was trying to pass under.

"All bridge piers will be designed to resist impact from a vessel. And I think this will be unquestionably no exception to that," Gardner said.

"I think it's the magnitude of the force in this case, which is extremely unusual, which has caused the problem for this bridge," he said.

Related stories

The Francis Scott Key Bridge opened in 1977. Its design is a steel truss bridge. The bridge was resting on concrete piers before the impact, experts said based on footage shared online.

"The support is a very, relatively, flimsy structure when you look at it — it's a kind of trestle structure with individual legs," Ian Firth, a structural engineer and bridge consultant in the UK, told the BBC . "So, the bridge has collapsed simply as a result of this very large impact force."

The impact from the Dali seemed to knock out one of the concrete piers, a "critical, significant part of the bridge," Gardner told BI.

"Structures generally are typically designed to have a certain amount of robustness. So if there's damage to a small part of it, the rest of the structure can remain intact," Gardner said.

"I think losing such an important element, I would expect the entire bridge to collapse, which is what happens," he said.

It wasn't immediately clear what caused the crash. It also wasn't clear from the footage how quickly the Dali was moving, experts said. It had only recently left the port.

Barbara Rossi, an associate professor of engineering science at the University of Oxford, told BI in an email that the impacting force "must have been immense to lead to these massive (concrete) structures to collapse, leaving the superstructure without one of its supports."

Long investigations are likely to follow, said Mark Richards, director of NESTA Consulting Engineers.

"It's inevitable that the engineering community will look at this and investigate what happened very carefully, to learn lessons from it — if there are lessons to be learned from it," he told BI in a call.

"The bridges are designed for events that are considered by the wider community unit of engineering to be appropriate or probable. Maybe this event just steps outside of that," he said.

"It may be that those events could not be foreseen, and so we can't forget that," he said.

In an emailed statement to BI, a Danish engineering and architecture consultancy called COWI said that bridges are not usually designed to withstand a direct impact from a ship.

Instead, engineers would create structures near the bridge supports that a ship would hit first, absorbing the impact.

Then "the failure would be linked to that and not to the bridge itself," said Lorna Wharton, Head of Press and Public Affairs for COWI.

It was not immediately clear what defenses the Francis Scott Key bridge had, the experts said. COWI and Richards said that something may have been there.

Engineers will likely be considering not only the protections for the bridge itself, but how the broader dynamics of what routes ships took under and around the bridge, the experts said.

Correction: March 27, 2024 — An earlier version of this story mischaracterized the cargo ship that hit the Francis Scott Key Bridge. It has a gross tonnage of around 95,000, not a mass of around 95,000 tons.

Watch: The container ship that destroyed the Francis Scott Key Bridge has crashed before

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The Dali was just starting a 27-day voyage.

The ship had spent two days in Baltimore’s port before setting off.

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The side of a large ship, painted blue, with the words “Dali” and “Singapore,” sitting at a port.

By Claire Moses and Jenny Gross

  • Published March 26, 2024 Updated March 27, 2024

The Dali was less than 30 minutes into its planned 27-day journey when the ship ran into the Francis Scott Key Bridge on Tuesday.

The ship, which was sailing under the Singaporean flag, was on its way to Sri Lanka and was supposed to arrive there on April 22, according to VesselFinder, a ship tracking website.

The Dali, which is nearly 1,000 feet long, left the Baltimore port around 1 a.m. Eastern on Tuesday. The ship had two pilots onboard, according to a statement by its owners, Grace Ocean Investment. There were 22 crew members on board, the Maritime & Port Authority of Singapore said in a statement. There were no reports of any injuries, Grace Ocean said.

Before heading off on its voyage, the Dali had returned to the United States from Panama on March 19, harboring in New York. It then arrived on Saturday in Baltimore, where it spent two days in the port.

Maersk, the shipping giant, said in a statement on Tuesday that it had chartered the vessel, which was carrying Maersk cargo. No Maersk crew and personnel were onboard, the statement said, adding that the company was monitoring the investigations being carried out by the authorities and by Synergy Group, the company that was operating the vessel.

“We are horrified by what has happened in Baltimore, and our thoughts are with all of those affected,” the Maersk statement said.

The Dali was built in 2015 by the South Korea-based Hyundai Heavy Industries. The following year, the ship was involved in a minor incident when it hit a stone wall at the port of Antwerp . The Dali sustained damage at the time, but no one was injured.

Claire Moses is a reporter for the Express desk in London. More about Claire Moses

Jenny Gross is a reporter for The Times in London covering breaking news and other topics. More about Jenny Gross


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  1. How to Calculate Outboard Motor Size for Sailboats

    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.

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    It seems so complex to pick the right motor size for your sailboat. I was done the complex calculations and tried to make i best here. Like to pick the right outside motor magnitude for your sailboat? To get the right amount of horsepower needed to efficiently propel one sailboat, dividing this displacement out the boats (in lb) by 550.

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  5. How to Calculate Outboard Motor Size for Sailboats

    I like to use this simple pattern: HP = displacement (lb) / 550. So 1 HP for any 550 lbs displacement, the 4 hp per 2200 lb. Here, HP are the amount of human you demand to reach one maximum hull speed. This your in optimal conditions. So you have smooth water, no windage, a cleaning press polished hull, the so on.

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    How the choosing the law outboard motor size for your sailboat? To get the right amount regarding horsepower needed to efficiently propels a sailboat, divide one volume of which boat (in lb) by 550. You need approximately 1 HP per 550 lb off dislocation or 4 HP per 2200 lb. Most sailboats don't need a motor with more than 30 HP.

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    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.

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    For instance, a fiberglass 15 foot boat and 16 foot boat usually weigh around 1,700 pounds, meaning you should get an engine with a horsepower of 34 to 68. However, an aluminum 17 foot boat, despite being longer, will only tip the scale at 670 to 1170 pounds. In other words, the ideal motor size for this boat falls within the range of 26 to 46HP.

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    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 ...

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  26. The Five Minutes That Brought Down the Francis Scott Key Bridge

    When a ship of this size loses engine power, there is little to be done to correct its course, even dropping an anchor down. And the Key Bridge was particularly vulnerable. As long ago as 1980 ...

  27. Why the Baltimore Bridge Collapsed So Quickly

    Steel structures aren't as strong as you might think—and the immense power of a container ship shouldn't be underestimated. Just shy of half past 1 in the morning, the MV Dali, a giant ...

  28. What caused the Dali to slam into the Francis Scott Key Bridge?

    The crew ran several system tests to attempt to remedy the loss of propulsion from the motor, but the tests proved unsuccessful. At that point, the ship's pilots alerted the Maryland Department of ...

  29. Baltimore Bridge: Huge Size of Ship Made Collapse Inevitable, Experts Say

    An image of a chain link. It symobilizes a website link url. Copy Link The Francis Scott Key Bridge in Baltimore stood no chance against the huge impact from a cargo ship Tuesday morning ...

  30. Dali Ship That Hit Key Bridge Was Destined for Sri Lanka

    The Dali was less than 30 minutes into its planned 27-day journey when the ship ran into the Francis Scott Key Bridge on Tuesday. The ship, which was sailing under the Singaporean flag, was on its ...