What Is The Beam Of A Boat (Full Breakdown)

Probably the most common way that people describe the size of a boat is in its length, measured at the centerline from the bow to the stern.  You’ll hear boaters talking about their 21, or 19 footers but hardly ever do people focus on the boat’s beam, although it is equally important.  So, what is the boat’s beam, and why should you care?

The boat’s beam is the width of the boat measured at its widest part.  As a rule of thumb, wider boats are more stable (up to a point) and have more storage space, while narrower boats are faster and easier to maneuver.  On the other hand, wider boats may also be more uncomfortable and “choppier.”

Deciding on a narrower or wider beam will depend on your personal preference and what you intend to do with the boat.  Understanding the effect of wider and narrower beams is vital to ensuring you get the most out of your boat.

The “Beam” Of A Boat

The term “beam” originates from the earliest ships constructed from wood.  The hull of those early ships kind of resembled an upside-down roof construction.  Large wooden beams formed the ribcage-like support structure on which builders fixed the hull. 

The center beam was usually the longest and thickest of the lot.  Builders used the length of this beam to determine the width of the ship as it was the widest point, requiring the most substantial beam. 

As a result, the ship’s width became known as “the beam.”  This further developed to refer to the general side of the vessel.  So, where the front is called “the bow,” the back is called “the stern,” the left and right sides became “the port beam” and “the starboard beam,” respectively.

The Relation Between A Boat’s Length And Beam

It’s probably a pretty obvious statement, but boat builders usually work around pre-determined ratios between a boat’s length and beam when building a new vessel (unless you are accidentally trying to build a barge, that is.)

For example, for boats less than 20 feet long, builders typically use a ratio of 2:1, meaning that the beam is around half the boat’s length.  However, as the boat increases in length, so does the ratio.  For example, thirty feet boats have a length-to-beam ratio of around 5:1. 

Apart from the boat’s length, the type or purpose of the boat will also affect how broad they build the beam.  The average beam of a center console boat is smaller than that of the average motor yacht.  Similarly, catamarans will have a wider beam than monohulls because they are essentially two hulls wide.

Wider Vs. Narrower Beam – Which Is Better?

Although there are ratios of length-to-beam as mentioned above, there are variations between manufacturers.  Meaning that you may find vessels with everything else being equal, but one broader than the other. 

In any case, you have to understand the difference between a broader and narrower beam to make the right choice for your particular use and needs. 

1. Wider Beams Are Generally More Stable

Think of the stability of a catamaran vs. a speed boat.  You are less likely to bob around in the wider catamaran in rolling waves and foul weather.  In addition, its wider beam acts as a strong foundation, making it less of a rock-and-roll experience. 

This added stability is one of the reasons why cats are top-rated options for sport fishing vessels.   Being anchored at a deep-sea fishing spot for hours on end is just more forgiving in a boat that isn’t rolling quite as much with the waves.  It also means that a sudden wave to the beam is less likely to capsize the vessel.

However, you should be aware that although wider vessels are generally more stable, it is not a hard and set rule as other aspects come into play, such as the hull design, the center of gravity, etc. 

A deeper hull, or chine, will add to stability, while a shallow hull coupled with a broader beam may be more unstable than if the beam were narrower. 

2. Narrower Beams Are More Comfortable

The flip side of the increased stability is that wider beams tend to have a more uncomfortable and bumpier ride than narrower hulls.  This is because that stronger foundation means that every bump in the sea is transferred through the boat’s hull, whereas a narrower hull is better at cutting through those bumps. 

Even when anchored, if the sea is choppy, you may find yourself bouncing around a little more in the wider hull.  So, you must decide whether you prefer bouncing or rolling.

3. Wider Beams Have More Space

It should go without saying, but if you have a wider beam, you have a wider boat, which means you will have more storage and cabin space. 

Keep in mind that I’m comparing apples with apples here – meaning that a wider monohull will have more storage than a narrower monohull, and the same for two cats. 

Try not to compare the beam of a cat with a monohull because their interior layouts are very different from one another.

4. Narrower Beams Are Faster

If you have two boats with the same specs, but one is narrower than the other, the speed prize will go to the narrower vessel. 

Narrow-beamed boats have smaller contact areas with the water, meaning they can cut through the water better than wider boats. 

This smaller surface area also translates to their maneuverability, meaning that (at least in the docks) they are going to be easier to maneuver in tight spaces.  Conversely, you may find them slightly harder in very few instances because of their decreased stability. 

5.  Wider Boats Have Size Limitations

OK, sure, for your average sport fishing boat, adding a foot across the beam isn’t going to force you to raise the “vessel with a restricted ability to maneuver” day shape, but it does come with a few considerations. 

Narrow channels can become more difficult to traverse with wider boats.  Especially river channels focused on small, holidaying vessels.  Similarly, the wider your boat is, the more daunting the task will be of mooring into a tight space.  You may even be required to pay more for a wider mooring if your vessel is too wide for a standard one.


The boat’s beam refers to the boat’s width at its widest point.  Having a wider beam generally means that your boat will be more stable and have more space.  However, it also means that your boat may be slower, more uncomfortable when riding, and harder to maneuver.

However, I need to add that all these pros and cons should be taken simply as guiding points in your consideration.  If you are stuck between two vessels, the best choice is to take both out to sea and choose the one that fits your needs and use best.  In the end, the boat you enjoy is the boat for you.

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Project Boating Editorial Staff

My name is Brad Visser the chief editor and owner of Projectboaing.com. We have an amazing team of writers that contribute to our website. This team is passionate about boating and have years of experience not only in boats, but in writing helpful, informative articles to answer questions you may have.

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