What Is A Boat’s Beam?

If you are new to boating, there are many terms to familiarize yourself with.

You may have come across the term ‘beam’ and wondered, ‘’what is a boat’s beam?’’.

Well, you have come to the right place! I wrote this article to define a boat’s beam. I will explain everything you need to know about this aspect of your watercraft.

Later in the article, you will learn the importance of knowing what your boat’s beam is. Hint: it is all about your safety while in the water.

So, let’s dive in and answer the question: what is a boat’s beam?

Definition of A Boat’s Beam

Definition of A Boat’s Beam

A boat’s beam is the widest part of a boat.

The term ‘beam’ originated from the structure of wooden boats used in the past. Back then, boats were constructed so that large pieces of wood flanked the watercraft and helped strengthen the wooden boat. These wood pieces were placed on the boat’s rib, starting from the keel to the gunwale.

The boat’s deck was made of smaller pieces of timber. For ships with two floors, the underside of the deck acted as the ceiling of the first-floor cabin. You could see the wooden planks on the deck’s underside when on the first floor.

The pieces of wood on the deck’s underside were used to approximate the boat’s size. One could tell how big a boat was just by looking at the wooden planks underneath the deck, i.e. the roof beams.

Today, a boat’s beam is the measurement of the widest part of the vessel. Knowing how to measure or calculate the beam can help you decide whether your boat can navigate obstacles in water or pass through certain passageways.

Next, let us look at how to measure a boat’s beam.

How To Measure A Boat’s Beam

Measuring a boat’s beam is a simple skill you can learn quickly. You could always hire a marine surveyor to calculate the measurements for you. But, as with most things, it helps if you can do it yourself.

Still, if you need a boat’s beam measurements for official purposes such as state or federal registration, it is good to acquire a marine surveyor’s services.

Knowing your boat’s beam measurement can help you understand the vessel’s stability in water, storage capacity, and ability to navigate obstacles in the water or along certain passageways.

To measure the beam, you first need to identify the boat’s centerline. Then, you can measure the perpendicular distance from one gunwale to the other.

For this process, you will need:

  • A ball of string
  • Tape measure
  • Carpenter’s square
  • Duct tape
  • Pencil

Follow these simple steps to measure your boat’s beam.

Step 1: Measure the boat’s stern

Measure the boat’s stern

The stern is the rear end of the boat. Measuring the stern and the bow, which is the boat’s tip, will help you determine the centerline.

Use a tape measure to measure the width of the stern. Then, calculate the central point of the stern by dividing the width by two. For example, if the stern’s width measures 50 feet, the center point would be 25 feet.

Use a pencil to mark the center point of the stern. Place one end of the string at this center point and hold it down with duct tape.

Step 2: Determine the bow’s center

Determine the bow’s center

The bow is the front-most part of the boat. It is usually tipped and curved, so measuring its width using a tape measure can be cumbersome.

Instead, simply find the most pointed part of the boat. This should serve as the bow’s center point. Mark this point using a pencil.

Step 3: Determine the centerline

Determine the centerline

Once you find the stern and bow’s center, determining the boat’s overall centerline is easy.

One end of the string should be taped to the stern’s center. Then, pull the ball of string toward the bow’s center. Tape the string to the bow’s marked center point.

The boat’s centerline is the string’s entire length from the bow’s center to the stern’s center. Leave the string taped in place.

Step 4: Select the widest part of the boat

Select the widest part of the boat

Examine your boat carefully and identify the widest part from one gunwale to the other. A gunwale is the top edge of the vessel’s hull.

In most boats, the widest part is closer to the center of the watercraft. It is also where the waterline meets the hull.

Step 5: Measure the beam

Measure the beam

Once you visually identify the widest part of the boat, place the tape measure on one gunwale and stretch it out to the gunwale on the other side.

The measuring tape should be perpendicular to the taped centerline. Use the carpenter’s square to check that the tape measure is at a 90-degree angle to the centerline.

Step 6: Check the accuracy of the beam’s measurement

Check the accuracy of the beam’s measurement

The boat’s beam is the distance between the two parallel gunwales at the widest point. It is always good to take at least three beam measurements for accuracy.

Once you take the first beam measurement, with the tape measure still in place, move it slightly forward and take the beam measurement there.

Move the tape measure back to the first beam measurement. Then, move it slightly backward and take the beam measurement there too.

Check the widest measurement out of the three-beam measurements. The widest measurement is the most accurate boat beam measurement.

Why Does A Boat’s Beam Matter?

Now that you know the answer to the question: what is a boat’s beam? You might be wondering, “why does a boat’s beam matter?” How does this simple measurement help me?

A boat’s beam matters in many situations. But, the most important reason to know about the beam is to understand the vessel’s stability.

When handling a boat, you want to ensure that it is stable enough to navigate the waters you want to explore. A small, narrow boat lacks the stability to withstand deep waters with large waves; the boat will likely capsize.

The stability of a boat with a wide beam is different from a boat with a narrow beam. Let’s take a deeper look at how a boat’s beam affects stability.

1.  Wide beam boats

Wide beam boats

Boats with a wider beam are generally more stable. Stability, in this case, is a measure of a boat’s resistance to the center of gravity. As a boat moves through the water, the center of gravity may shift sideways due to the weight of the cargo in the boat, waves, or the wind’s direction.

Aside from these external forces, a boat’s stability will also depend on its volume, which is also a factor of the boat’s beam.

The vessel volume of wide-beam boats is distributed further away from the centerline, increasing the boat’s stability. When the vessel’s volume is concentrated toward the centerline, the boat’s stability generally decreases.

As the boat’s beam increases, its secondary stability decreases. A watercraft’s secondary stability is its resistance to capsizing if it is titled sideways.

Aside from the beam, the shape of the keel also determines a boat’s secondary stability. Boats with a curved keel have higher secondary stability but may lose their primary stability. On the other hand, boats with a square keel have lower secondary and greater primary stability.

Pros and cons of wide-beam boats


  • Higher primary stability, making them the best choice for deep-sea boating
  • More interior space so you can carry more cargo and passengers


  • Difficult to pass through narrow passages and get around obstacles
  • Finding marinas that accept large boats can sometimes be difficult, depending on where you want to dock

2.  Narrow beam boats

Narrow beam boats

Boats with a narrow beam are generally less stable and can easily capsize in the face of external forces such as waves and winds.

On the upside, narrow beam vessels move faster in water than boats with wider beams. Another advantage of narrow beam vessels is that they can easily pass through narrow waterways, and you can safely navigate obstacles in the water.

Pros and cons of narrow beam boats


  • Can easily pass through narrow passages and around obstacles, especially in coastal and inland waters where maneuvering large boats can be difficult
  • Moves faster, especially in small and calm water bodies
  • Easy and more affordable to access marinas


  • Lower stability
  • Smaller interior space. The amount of cargo and people you can have onboard is limited.

Summary: What Is A Boat’s Beam?

A boat’s beam is its widest point. When buying or renting a boat, the beam is among the most important factors to consider. Your vessel’s stability, space, and speed will depend on the beam. You should select a vessel that meets your boating needs, including whether you need more interior space and the types of waterways you plan on exploring to ensure that you navigate the vessel safely.

1 thought on “What Is A Boat’s Beam?”

  1. Interesting! I didn’t know that the beam of a boat refers to its width. Thanks for sharing this information on your blog post.


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