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What is a Keel?

By Jonathan Stevens
Updated May 17, 2024
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A keel is found on most watercraft whether powered by wind or motor. It is commonly understood to be a fixed underwater extension protruding from the bottom of a boat although some versions are movable. This extension provides stability and resists sideways movement or drift. Sideways movements are caused by wind or cross currents and are not only countered by the shape and draft (depth) of a keel structure, but also by its weight (or ballast).

Movable keels are typically only associated with sailing vessels. Most move with regard to their being positioned in or out of the water, though canting keels refer to movement that occurs underwater while the keel is already in place. In these instances, it swings or angles more severely whenever the watercraft heels (tips) to extreme angles.

The adjustment in the angle of the keel will reposition the center point of its weight and allow for more advantageous hydrodynamics in service to speed through the water. One drawback of a canting keel is the resulting reduction of resistance to sideways movement. This is usually compensated for with additional fins or double-rudder systems designed to maintain resistance to sideways forces.

In the case of smaller sailboats, a keel may exist in a retractable form known as either a centerboard or dagger board. The centerboard is usually pivoted and is manually levered into position, whereas the dagger variation is simply thrust into position. Both are operated from within the vessel.

There are various modifications of keels as seen in the many compound uses of the name: dorsal keel, fin keel, winged keel, bulb keel, etc. Usually, these variations relate to its method of deployment or shape; however, there is another definition of the term that precedes the underwater extension definition. It refers to a structural element in traditional boat building and describes the timber traveling from stem (foremost part) to stern (rearmost part), and is the "spine" of the entire vessel. This center beam from which most other parts of a ship’s framework is connected to is properly known as the keel. Its primary function is to provide strength, integrity and centering balance.

The modern word descends from the ancient Anglo-Saxon word ceol, which means ship. The Latin version of the word is carina. Over the years, whenever a ship was hauled over on its side to clean the underlying hull, it was described as being "careened," an obvious variation of the Latin term.

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Discussion Comments
By kentuckycat — On Aug 24, 2011

@JimmyT - You are on the right track about the canting keels. They basically work like a long weight under the bottom of the boat, but instead of dangling, they are on a fixed pole.

If they dangled like a sinker on a fishing line, they could end up pulling the boat down farther if it started to tip. Since the weight is on the end of a fixed part of the boat, if the boat starts to tip, the center of gravity will shift toward the weight and stop it from going over completely.

The person or people in charge of controlling the boat would still have to take special actions in terms of steering that would help correct the tipping action.

On another note, I've heard the term keelhauling used to colloquially describe someone getting a harsh punishment. Does this relate to boat keels anyhow?

By matthewc23 — On Aug 24, 2011

The article mentions retractable keels, but what would be the purpose of these? Would it just be something you used when the waters were getting choppy or the wind was starting to pick up?

I guess the other option would be that it is something you could use if you were wanting to improve the stability while you were coasting along for relaxation purposes.

How do you go about operating the retractable keel from inside the boat? Is it something simple like a lever, or is it a mechanized system?

By JimmyT — On Aug 23, 2011

@stl156 - Maybe someone who is more experienced in ships can give some more insight, but your observation seems to make sense. Even if you think about pictures of the two types of ships, cruise ships are much more narrow as they go down. They are wider, as well, but I assume that is because they need more stability. Battleships tend to look flatter on the bottom, which would help them turn more sharply if they were avoiding fire from an enemy.

Could someone describe a little bit more about a canting keel? I'm not sure I completely understand it. From the description in the article, it sounds like maybe it is more like a weight tied to the bottom of the boat rather than being the sharp bottom of it.

By stl156 — On Aug 22, 2011

How far under the water does a normal keel go? Since having a deeper keel would affect where the center of gravity was, I would assume the deeper the keel, the more stable the boat was. I'm really curious about the difference in keels between something like a luxury cruise ship and something like a battleship.

Since a battleship would possibly need to be more maneuverable, it would make since that it had a more shallow keel, while the cruise ship could have a deep keel that would make the cruise smoother.

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