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Keels

A keel is a device that is attached to a sailboat or any other type of large commercial ships and used to keep the boat on a straight track and from moving side to side. A keel was first used in the time of Vikings as early as the 800�s. The word keel comes from the word ceol. A ceol was a singled masted, rectangular ship that the Vikings used to travel. Keels over time have been made out of different kinds of materials: some hard woods such as oak, pine, cedar, and white oak, as well as from fiberglass, carbon fiber, and aluminum.

A keel is used for more then one purpose. Predominately it is used to counteract the force of the sail and provide stability for steering. It is also used to keep from flipping some ships over and it also gives the boat some drag so that it doesn�t reach dangerous speeds for the boat. For example if there wasn�t a keel on a boat it could drift sideways and flip very easily.

Keels are different from centerboards in that keels are often fixed (though some are retractable) and are often made of heavy materials to provide ballast to stabilize the boat. A keel is a foil that converts lateral slipping of the boat into a lateral force that resists the slipping. This is required for sailboats to move in directions other than downwind, since the force of the sail is never closer than 90 degrees to the apparent wind. Since most sailboats are symmetric along their axis of motion, the lateral force can come from either side, which means that keels must use symmetric foil shapes so they will operate with equal efficiency on either tack.

Other advances of the keel include morphing into a daggerboard, which is retractable and removable and is advantageous to small boats in shallow waters (i.e. your typical sunfish or laser owner). And also into a centerboard, which is unlike a daggerboard in that it raises by pivoting around a point.

About.com: Daggerboard or Centerboard?
   http://sailing.about.com/od/sailboat_parts/a/dag_or_cent.htm
Lulea University: Origins
   http://home.freeuk.net/elloughton13/origins.htm
Humber Keel and Sloop Preservation Society: What's a keel?
   http://www.humberships.org.uk/html/what_s_a_keel_.html
Wikipedia: Centreboard
   http://en.wikipedia.org/wiki/Centerboard
Wikipedia: Daggerboard
   http://en.wikipedia.org/wiki/Daggerboard
Wikipedia: Keel
   http://en.wikipedia.org/wiki/Keel
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