Keel

From Wikipedia, the free encyclopedia

1 Structural keels
2 Hydrodynamic keels
3 Sailboat Keels
4 Etymology
5 See also

For other uses, see Keel (disambiguation).

In boats and ships, keel can mean either of two parts; a structural element, or a hydrodynamic element; these parts overlap.

[edit] Structural keels

A structural keel is a large beam which the hull of a ship is built around. The keel runs in the middle of the ship, from the bow to the stern, and serves as the foundation or spine of the structure, providing the major source of structural strength of the hull. The keel is generally the first part of a ship's hull to be constructed, and laying the keel, or placing the keel in the cradle in which the ship will be built, is often a momentous event in a ship's construction--so much so that the event is often marked with a ceremony, and the term lay the keel has entered the language as a phrase meaning the beginning of any significant undertaking.

[edit] Hydrodynamic keels

A hydrodynamic keel is a foil on the bottom of the hull that is used to give the ship greater directional control and stability. In traditional boatbuilding, this is provided by the structural keel, which projects from the bottom of the hull along most or all of its length. In non sailing hulls, the keel helps the hull to move forward, rather than slipping to the side.

[edit] Sailboat Keels

In sailboats, keels use the forward motion of the boat to generate lift to counter the lateral force from the sails. To counter the large lateral force generated by the sails, sailboats have much larger keels than non sailing hulls. Keels are different from centerboards and other types of foils in that keels are made of heavy materials to provide ballast to stabilize the boat. Keels may be fixed, or non-moveable, or they may retract to allow sailing in shallower waters. Retracting keels may pivot (a swing keel) or slide upwards to retract, and are usually retracted with a winch due to the ballast. Since the keel provides far more stability when lowered than when retracted (due to the greater moment arm involved) the amount of sail carried is generally reduced when sailing with the keel is retracted.

There are several types of fixed keels including: full keels, fin keels, winged keels, bulb keels, and among other designs.

Non-fixed keels are known as canting keels or Swing keels. These are found on racing yachts such as those competing in the Volvo Ocean Race. They provide much more righting moment for a lot less weight, as the keel moves out to the windward-side of the boat. The perpendicular distance from weight to pivot is increased, therefore a larger righting moment is produced.

[edit] Etymology

The word "keel" comes from Anglo-Saxon cēol, Old Norse kjóll, = "ship" or "keel".

Carina is the Latin word for "keel" and is the origin of the term careen (to clean a keel and the hull in general, often by rolling the ship on its side). An example of this use is Careening Cove, a suburb of Sydney, Australia, where careening was carried out in early colonial days.