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Last Updated on August 23, 2024 by Paul Clayton
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What Is A Boats Beam
A boat’s beam, a term often used in maritime circles, refers to the widest part of the boat. Understanding a boat’s beam is crucial for boat builders and users as it impacts the vessel’s balance, speed, and ability to carry cargo or passengers safely. It is an essential aspect to consider in boat design and selection.
- It is measured at the widest point, a crucial dimension.
- The beam is typically measured in feet for clarity.
- Beam size significantly affects the boat’s stability.
- Wider beams offer more stability on water.
- Speed impacted by beam width, design challenge.
- Beam size dictates navigable waterways and docking spaces.
Many nautical terms are like an alien language to people unfamiliar with seamanship. All those bows and sterns can drive you crazy!
Add another term to the list of those you should learn – the beam. This one’s pretty crucial, and it is the nautical term we will discuss today.
The definition used the most commonly is the one given by the Merriam-Webster dictionary – the definition of 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 defines two types of beams – beam overall (BOA) and beam on the centerline (BOC). BOA is simply the ship’s beam, 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. If you didn’t know, the centerline is the imaginary central line going from a boat’s bow to stern.
How To Measure A Boats 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 several 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. If you don’t 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, and a carpenter’s square to do this.
Determining the centerline
So, first of all, measure the width of the boat’s stern (the boat’s back end). 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 its point.
For added precision, you may also measure the width of the bow and then mark its center. You then stretch the twine from the bow’s center to the stern’s center 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 measurement point. Measure the tape’s length again at both points. You must move your reference points there if they are wider than the original point. Repeat the process to determine whether the measurement point is right 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 measure your vessel’s beam manually.
Boats Beam And Stability
A vessel’s beam is important not only for determining its passage but also from the point of view of its stability.
Plenty of physics and mathematics are 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- due to cargo movement 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, the vessel’s stability 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 also depends on the shape of the 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 tend to be 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. Also, narrow vessels are safer to maneuver around obstacles, and they can get into narrow, inaccessible waterways 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. That said, the vessel’s length 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.
Sometimes, people prefer shorter and wider boats over longer and narrower ones. The main factor influencing this is cargo placement in the vessel. If you need to place the cargo across the vessel, its beam may be more important than its length.
Does the Width of the Boat Matter?
A boat’s width, or beam, significantly influences its speed and stability. Narrow hulls are more efficient in cutting through the water, reducing friction and displacement, and allowing greater speed. However, wider beams enhance stability, particularly in rough waters, due to a broader base that improves buoyancy and lowers the center of gravity.
Boat designers must balance speed, stability, and capacity – to achieve optimal performance. Modern high-speed ferries and racing yachts demonstrate advanced hull designs that effectively combine speed with stability.
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 narrow and wide-beam vessels’ pros and cons. 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
Pros:
Increased stability: This is perhaps the most apparent benefit of wide vessels. You will notice the benefits of a wider beam with any ship type and size, whether a compact kayak or a huge cruise ship.
Regarding stability, you’d choose the boat’s width based on water conditions. For example, if you kayak in a lake, you could go for a narrower kayak since lakes are relatively calm water bodies. You may want to go for a wider kayak for ocean or whitewater kayaking.
And generally, you’d want to go for a wider vessel for ocean seafaring. The increased stability of a wide-beam ship would imply improved cargo safety on 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 want to get a wider kayak or canoe.
Increased interior space: The increased interior space is another benefit we’ve discussed. However, we’ve only discussed 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 great for full-time boat living. You have more space for furniture, essentials, and other stuff 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.
Cons:
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 live 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
Pros:
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 great thing 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 unlikely to have trouble with dimension requirements. This is great if you are heavily relying on marinas during your travel.
Cons:
Decreased interior space: The only downside of narrow vessels is the decreased interior space. This could be a serious downside for any boat, be it a fishing boat, a cargo vessel, or a boat you live in full-time. But for certain boat types and activities, the lack of space may be a bigger problem than others.
Decreased stability: 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 travel in ocean or sea waters. These waters can get rough sometimes, and narrow vessels can get very unstable there.
Final Words
What Is A Boat’s Beam? It refers to its width at the broadest point, typically measured in feet for clarity and consistency. This is not a mere physical attribute but a determinant of several crucial aspects of a boat’s performance and usability.
Also, the boat’s speed is impacted by the beam width, presenting a design challenge to engineers and manufacturers who must balance stability with speed. A narrower beam might increase speed but compromise stability, while a wider beam enhances stability but might reduce speed.
This tightrope act of balancing these two factors makes boat design a complex yet fascinating endeavor.
As a result, understanding a boat’s beam is essential in comprehending a boat’s overall functionality and maneuverability.
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