Boat Beam – What Is It and Its Purpose?

According to the Merriam-Webster dictionary, a boat beam is the width of a boat at the widest part. It is the line where the hull of the boat meets the water. The stability of a boat largely depends on the width of the beam. This post centers on the boat beam – what is it and its purpose? Read on to learn.

Understanding a Boat Beam

The handling characteristics of a boat depending on the boat’s beam. The speed of a boat is directly related to the width of its beam. If the beam is narrow, the boat goes faster. And if the beam is wide, the boat goes slower.

However, in heavy waves, a boat with a wide beam performs significantly better than a boat with a narrow beam. A narrow beam may not provide enough stability when the waves are strong, and as a result, the boat will sway.

A wider beam gives a boat a wider cross-section, and as a result, the boat is more stable and can handle strong waves. But as a wider beam has to displace more water, it slows down the boat.

The widest point is referred to as the beam, and it is a very important design element of types of watercraft. There are two types of beams: BOA (beam overall) and BOC (beam on the centerline).

The centerline of a boat is an imaginary line across the bow and the stern. The bow is the front end of the boat while the stern is the rear end. The right side is the starboard and the left side is the port. In the following sections, we will explore more about a boat beam – what is it and its purpose?

Origin of the Term

The term “beam” can be traced back to the distant past when ships were exclusively wooden. In the early days, large timbers were used to make wooden ships strong and stable. These large timbers were placed on the ribs. They spanned the entire width of the ship or boat.

On top of a ship, there is a deck. Smaller wooden boards are used to build the deck. At the first level, the deck served as a ceiling. The interior of a ship more or less looked like a house. On the underside of the floor deck, the floor beams were exposed.

During those early times, people referred to the roof beams to determine the size of a ship. The width of a ship or any other watercraft was determined by these beams.

Measuring a Boat Beam

The goal of this post is to help you understand a boat beam – what is it and its purpose? The discussion of a boat beam is all about the size of the beam, and that is why it is important to learn how to measure one.

To measure a boat beam, you are going to need a carpenter’s square, a ball twine, some duct tape, and measuring tape. The process involved two steps. Here they are:

Determining the centerline

First, measure the stern’s width with measuring tape. To find the stern’s center, just divide the width by 2. Mark that point. Tape the twine’s one end to the marking. After that, tape the other end of the twine to the marked point of the bow.

To make the measurement more accurate, measure the bow’s width. Then divide the figure by 2 and mark the center. Stretch the twine. The line from the center of the bow to the center of the stern is the centerline of your boat.

Measuring the beam

Determine the widest part of the boat. You should be able to do that just by looking at the beam. Mount the measuring tape over the centerline, in a perpendicular position. A carpenter’s square will make it easier. Your boat’s beam is the distance between the hull’s outer edges.

Relation between Boat Beam and Stability

A boat’s beam determines not only the passage but also the stability of the boat. And if you understand how a boat’s beam determines the stability of the boat, you know everything you need to know about a boat beam – what is it and its purpose? 

To know a boat’s stability, specialists apply the principles of physics and mathematics. Now we will discuss how a boat’s stability is determined by its beam.

A wider beam provides more stability

A boat with a wider beam has a higher level of stability. This stability is basically a resistance that prevents the boat’s center of gravity from shifting sideways. The shifting of gravity may occur due to external forces such as wind and waves.

If the volume of a boat beam is shifted away from the centerline, it affects the beam’s stability. If the volume is mostly close to the centerline, the stability will be significantly reduced. This is the reason why a wider beam offers greater stability than a narrower one.

However, there are often issues with the secondary stability of a boat with a wide beam. A boat with a wide boat does not tip on either side, but it has a greater risk of capsizing.

The boat’s bottom shape also determines secondary stability. If a boat has a curved bottom rather than a square bottom, the boat’s secondary stability will be higher.

A narrow beam provides less stability but makes the boat faster

Understandably, a boat with a narrow beam is less stable but faster. A narrow beam also makes a boat safer, less slugging, and easier to maneuver. A boat with a narrow beam can pass through a narrow passage relatively easily.

Final Thoughts: Boat Beam – What Is It and Its Purpose?

You should now have a basic understanding of boat beams and their purpose? Technically, it is referred to as a BOA or beam overall. That is essentially the overall width of a watercraft – whether it is a boat or ship. We hope now you have a better understanding of this boat part.

Last Updated on August 25, 2021