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Keel laid for the USS United States in drydock The keel plate of USS United States (CVA-58) being laid in a construction dry dock on 18 April 1948.jpg
Keel laid for the USS United States in drydock

The keel is the bottom-most longitudinal structural element on a vessel. On some sailboats, it may have a hydrodynamic and counterbalancing purpose, as well. As the laying down of the keel is the initial step in the construction of a ship, in British and American shipbuilding traditions the construction is dated from this event. [1]



The word "keel" comes from Old English cēol, Old Norse kjóll, = "ship" or "keel". It has the distinction of being regarded by some scholars as the first word in the English language recorded in writing, having been recorded by Gildas in his 6th century Latin work De Excidio et Conquestu Britanniae , under the spelling cyulae (he was referring to the three ships that the Saxons first arrived in). [2] [3]

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.

Structural keels

A structural keel is the bottom-most structural member around which the hull of a ship is built. The keel runs along the centerline of the ship, from the bow to the stern. The keel is often 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 may mark the start time of its construction. Large, modern ships are now often built in a series of pre-fabricated, complete hull sections rather than being built around a single keel, so the shipbuilding process commences with the cutting of the first sheet of steel. [4]

The most common type of keel is the "flat plate keel", and this is fitted in the majority of ocean-going ships and other vessels. A form of keel found on smaller vessels is the "bar keel", which may be fitted in trawlers, tugs, and smaller ferries. Where grounding is possible, this type of keel is suitable with its massive scantlings, but there is always a problem of the increased draft with no additional cargo capacity. If a double bottom is fitted, the keel is almost inevitably of the flat plate type, bar keels often being associated with open floors, where the plate keel may also be fitted.[ citation needed ]

Hydrodynamic keels

Hydrodynamic keels have the primary purpose of interacting with the water and are typical of certain sailboats. Fixed hydrodynamic keels have the structural strength to support the weight of the boat. [5]

Sailing yacht with a fin keel MariahQuarterView.jpg
Sailing yacht with a fin keel
Lateral resistance effect of a sailing keel Capsizing effect of keel.svg
Lateral resistance effect of a sailing keel
Righting effect of a keel, where A is the center of buoyancy and G is the centre of gravity (hypothetical example). Segeln Gewichtsstabilitaet.svg
Righting effect of a keel, where A is the center of buoyancy and G is the centre of gravity (hypothetical example).

Sailboat keels

In sailboats, keels serve two purposes: 1) as an underwater foil to minimize the lateral motion of the vessel under sail (leeway) and 2) as a counterweight to the lateral force of the wind on the sail(s) that causes rolling to the side (heeling). As an underwater foil, a keel uses the forward motion of the boat to generate lift to counteract the leeward force of the wind. Related foils include centerboards and daggerboards, which do not have the secondary purpose of being a counterweight. As counterweight, a keel increasingly offsets the heeling moment with increasing angle of heel.

Moveable sailboat keels may pivot (a swing keel), [6] retract upwards (retracting keel), [7] or swing sideways in the water (canting keels) to move the ballasting effect to one side and allow the boat to sail in a more upright position. [8]

See also


  1. Man, Myth & Magic: The Illustrated Encyclopedia of Mythology, Religion and the Unknown. M. Cavendish. 1995. p. 2364. ISBN   9781854357311.
  2. "Gildas, The Ruin of Britain &c. (1899). pp. 4–252. The Ruin of Britain".
  3. G. W. Whittaker (1970). Collected Essays. Ayer Publishing. p. 44. ISBN   0-8369-1636-0.
  4. Walton, Thomas (1901). Know Your Own Ship. London: Charles Griffin & Company. pp. 57–60. ISBN   9783861951643.
  5. Streiffert, Bo (September 1994). Modern Boat Maintenance: The Complete Fiberglass Boat Manual. Sheridan House, Inc. p. 173. ISBN   9780924486715.
  6. Spurr, Daniel (2004-07-02). Your First Sailboat: How to Find and Sail the Right Boat for You. McGraw Hill Professional. p. 10. ISBN   9780071778770.
  7. Kent, Duncan (2011-02-04). The Insider's Guide to Choosing & Buying a Yacht: Expert Advice to Help You Choose the Perfect Yacht. Fernhurst Books Limited. ISBN   9781119999188.
  8. Slooff, J. W. (2015-04-25). The Aero- and Hydromechanics of Keel Yachts. Springer. p. 190. ISBN   9783319132754.


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A daggerboard is a retractable centreboard used by various sailing craft. While other types of centreboard may pivot to retract, a daggerboard slides in a casing. The shape of the daggerboard converts the forward motion into a windward lift, countering the leeward push of the sail. The theoretical centre of lateral resistance is on the trailing edge of the daggerboard.

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Proa Type of multihull sailboat

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A leeboard is a form of pivoting keel used by a sailboat in lieu of a fixed keel. Typically mounted in pairs on each side of a hull, leeboards function much like a centreboard, allowing shallow draft craft to ply waters fixed keel boats cannot. Only one, however, the leeward, is used at a time, as it does not get lifted from the water when the boat heels under the force of the wind.

Ballast is used in ships to provide moment to resist the lateral forces on the hull. Insufficiently ballasted boats tend to tip or heel excessively in high winds. Too much heel may result in the vessel capsizing. If a sailing vessel needs to voyage without cargo, then ballast of little or no value will be loaded to keep the vessel upright. Some or all of this ballast will then be discarded when cargo is loaded.

Winged keel Keel type

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Bulb keel

A bulb keel is a keel, usually made with a high aspect ratio foil, that contains a ballast-filled bulb at the bottom, usually teardrop shaped. The purpose of the bulb keel is to place the ballast as low as possible, therefore gaining the maximum possible amount of leverage and thus the most righting moment. An example of a class of boats that use a bulb keel is the International 110 racing class, which uses a 300 lb (136 kg) cast iron bulb keel on a boat whose minimum racing weight is 910 lb (414 kg).

Hobie Cat

The Hobie Cat is a small sailing catamaran manufactured by the Hobie Cat Company. Hobie's line of products includes surfboards, sailboats, kayaks, stand-up paddle boards, and pedalboards, although the Hobie Cat Company is known worldwide for its catamarans. Hobie also designed a successful monohull, the Hobie 33.

Canting keel

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