What Is A Boat’s Beam?

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There are many nautical terms that are like an alien language to people unfamiliar with seamanship. All those bows and sterns can drive you crazy!

Well, add another term to the list of those that you should learn – the beam. This one’s pretty crucial, and it is the nautical term that we are going to talk about today.

What Is A Boat’s Beam?

A boat’s beam can be defined in several ways.

The definition that is used the most common is the one given by the Merriam-Webster dictionary – the definition fof the word “beam” says that a beam is the ship’s extreme width at its widest part.

Wikipedia also defines a boat’s beam as its widest point at the ship’s nominal waterline, i.e. the line where the boat’s hull meets the water.

Aside from that, Wikipedia also gives definitions for two types of beams – beam overall (BOA) and beam on centerline (BOC). BOA is simply the beam of the ship, which is the term we’ve just explained.

BOC is used for multihull vessels like catamarans or trimarans. It is measured as the perpendicular distance from the centerline of one hull to another. The centerline, if you didn’t know, is the imaginary central line going from a boat’s bow to stern.

How To Measure A Boat’s Beam?

Knowing the beam of a boat or ship is crucial since it allows you to determine if passage around an obstacle can be made safely. It may be useful in a number of other situations as well.

If you need precise beam measurements, then follow the steps outlined below. You may alternatively have a marine surveyor do all the measurements. In fact, if you don’t really know what you are doing and need to do boat measurements for federal documentation or state registration, addressing a professional is preferable.

Besides, all the necessary measurements may be in your boat’s documentation, so you may consult it instead of doing measurements.

Precise measurement of the beam

You will need to do two steps for beam measurement – determine your boat’s centerline and then measure the beam. You will need a ball of twine, some duct tape & measuring tape, as well as a carpenter’s square to do this.

Determining the centerline

So first of all, measure the width of the boat’s stern (the back end of the boat). Determine the center of the stern by dividing its width in half and marking it. Tape one end of the twine to the center marking. Then, stretch the twine towards the boat’s bow and tape it to the bow’s point.

For added precision, you may also measure the width of the bow and then mark its center. You would then stretch the twine from the center of the bow to the center of the stern to mark the centerline.

Measuring the beam

Visually pick the widest part of your boat. Then, pull measuring tape across the gunwales (the top edge of a boat’s hull). The tape needs to be mounted perpendicular to the centerline (i.e. at a 90-degree angle to the centerline), which you may determine with a carpenter’s square. The distance between the outer edges of the hull will be your boat’s beam.

Do your first measurement and then move the tape ahead and to the rear of the original point of measurement. Measure the tape’s length again at both points. If any of them is wider than in the original point, then you will need to move your point of reference there. Repeat the process again to determine whether the point of measurement is right or not and until you find the widest area of your vessel.

Quick measurement of the beam

This one’s a much less precise but quicker way of measuring your boat’s beam. You may use it as a rule of thumb when accurate measurements are not required.

Wikipedia suggests that the following formula may be used for many monohull vessels:

Beam = LOA2/3 + 1

Here, LOA is the vessel’s length overall, which is the maximum length of a vessel’s hull parallel to the waterline. Note that the above formula has all units in feet.

The exponent 2/3 means that you need to find the LOA’s cube root (which is 1/3) and then square it (which gives 2/3). For example, to calculate the beam of a 125 feet boat:

  • You find the cube root of 125, which is 5.
  • You find the square of 5, which is 25.
  • You add 1 to 25 and get 26 feet.

This formula will not work for all vessels, and if you need precise measurements, you should instead measure your vessel’s beam manually.

Boat Beam And Stability

A vessel’s beam is important not only for determining its passage – it’s also important from the point of view of its stability.

There is plenty of physics and mathematics involved in boat stability, so we will not cover this subject in depth. We will only glance at the general things related to the beam and stability.

Wider beam

Generally, the wider a vessel’s beam, the higher its initial stability. The initial stability is the resistance of a vessel to the center of gravity moving sideways (right or left). The center of gravity may move sideways from an external force – for example, wind or waves, as well as due to the movement of cargo inside the vessel.

Aside from the beam, what also affects stability is how the vessel’s volume is distributed away from its centerline. If most of the volume is close to the centerline, then the stability of the vessel will be lower. But since wider beams allow the volume of a vessel to be distributed farther away from the center, wide-beam boats can inherently be more stable than narrow-beam boats.

With that said, the secondary stability often suffers as the beam gets wider. The secondary stability of a vessel is its stability when tipped on its side. A boat with high initial stability may be hard to tilt to the side, but once it gets tilted, it will capsize relatively easily.

The secondary stability of the boat, to be fair, also depends on the shape of a vessel’s bottom. A more curved shape generally means higher secondary stability, but the primary stability also gets lower. More square bottoms tend to have higher primary stability and lower secondary stability.

Narrower beam

On the other hand, we have narrower beams which are pretty much the inverse of wide beams. Vessels with narrow beams are less stable, but they tend to be a little quicker (though this depends on other factors like boat length).

A major thing that’s better about narrow-beam vessels is that they are less sluggish than wide-beam vessels. Not only that, but narrow vessels are safer to maneuver around obstacles, not to mention that they can get in narrow waterways that are inaccessible for large boats.

Boat Beam And Interior Space

Another thing that the beam of a vessel impacts is the interior space available. Needless to say, a wide-beam vessel will likely have more space than a narrow vessel. With that said, the length of the vessel also plays a role in interior space –a long but narrow boat may have the same or more interior space as a short but wide boat.

In some cases, people would prefer shorter and wider boats than longer and narrower ones. The main factor influencing this is how cargo will be placed in the vessel. If you need to place the cargo across the vessel, then its beam may be more important to you than its length.

The Pros And Cons of Narrow And Wide Vessels

If you are planning to buy a boat – no matter the purpose, be it commercial, recreational, or living –then you should consider the desired beam of your future vessel.

If you don’t know the beam dimensions you should aim for, let us introduce you to the pros and cons of narrow- and wide-beam vessels. We’ll give a quick rundown of what we’ve already overviewed,and we’ll also give some additional details along the way.

Wide-beam vessels


Increased stability: This is perhaps the most apparent benefit that comes with wide vessels. You will notice the benefits of a wider beam with any ship type and size, be it a compact kayak or a huge cruise ship.

When it comes to stability, you’d choose the width of the boat based on water conditions. For example, if you were to kayak in a lake, you could go for a narrower kayak since lakes are relatively calm water bodies. For ocean or whitewater kayaking, you may want to go for a wider kayak.

And generally, for ocean seafaring, you’d want to go for a wider vessel. The increased stability of a wide-beam ship would imply improved safety of the cargoon cargo ships or increased comfort for passengers on cruise ships, for example.

Your needs are also important when it comes to boat stability. For example, if you want to fish while standing, you would want to get a wider kayak or canoe.

Increased interior space: The increased interior space is another benefit we’ve talked about. However, we’ve only talked about the usefulness of the increased width in cargo transport. The use of wide ship hulls goes well beyond that.

An area that will probably interest more people is that the increased interior space provided by wide hulls is greatfor full-time boat living. You have more space for furniture, and you have more space for essentials and other stuff that you may need.

And since living on a boat isn’t the most pleasant experience due to the lack of space, the benefit provided by a wide hull is undeniable.


Decreased passage in narrow areas:Wide hulls are bulky and will thus have a more difficult time going around obstacles. This won’t be a problem for ocean or high sea travel, but it probably will be a big downside in inland or coastal waters.

Limited number of marinas with enough space:Another problem may be the lack of marinas with enough free space for large and wide boats. This will be a particularly big problem if you are living on a boat full-time.Some marinas may have no issues with taking a large boat, while others may have some limits on boat dimensions.

Narrow-beam vessels


Increased compactness: Narrow-beam vessels are compact and thus are easy to get around obstacles and get into narrow passages. This benefit is especially great for boats in inland and coastal waters where larger boats may have quite a trouble.

Generally higher speed & maneuverability:Narrow vessels tend to be quicker and maneuver better. This is another thing that’s great for boats traversing in smaller water bodies.

Better accessibility of marinas:While larger boats may have trouble with marina accessibility, smaller and narrower boats are very unlikely to have trouble with dimension requirements. This is great if you are heavily relying on marinas during your travel.


Decreased interior space:Perhaps the biggest downside of narrow vessels is the decreased interior space.This could be a serious downside for any kind of boat, be it a fishing boat, a cargo vessel, or a boat that you are living in full-time. But for certain boat types and activities, the lack of spacemay be a bigger problem than for others.

Decreased stability:And the second downside of narrow vessels is their decreased stability.

This won’t be too big of a downside for everyone. It will be a pretty serious problem though if you will be traveling in ocean or sea waters. These waters can get pretty rough at times, and narrow vessels can get very unstable there.

Final Words

Your boat’s beam will not necessarily be crucial for your trips, but it’s definitely a thing to consider.

If the information given in this post has provided you with a new insight in nautical beams, then be sure to make use of it. It may be especially important if you are considering to buy a boat, so make sure to know your needs so that you choose properly.