Centreboard
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A centreboard is a form of movable keel on a small sailing boat or dinghy which can be moved to lower the draught (or depth) of the vessel. Unlike a daggerboard, a centreboard is usually pivoted around a point - Daggerboards lift vertically.
A centreboard 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 centreboards must use symmetric foil shapes so they will operate with equal efficiency on either tack.
Since long, narrow foils are more efficient than short, wide, foils, a long, narrow centreboard will produce the least drag for a given amount of lift, resulting in a faster boat that can point closer into the wind. Long foils mean deep foils, however, which is why centreboards retract; if they did not, then the boat would be unable to traverse shallow waters. A retracting centreboard is more complex than a fixed keel, and most take up space inside the hull of the boat that could otherwise be used for passenger accommodations. For this reason, it is not uncommon to find boats with combination of shallow keel and centreboard. The keel provides the housing for the centreboard, moving it out of the hull, but only adds a small amount of draft to the boat. The centreboard can then be lowered in deeper waters to increase the amount of lift.
Centreboards are often ballasted to provide additional stability when they are lowered. The ballast is placed in the lowest part of the foil, where it will provide the most stability. For this reason, ballasted centreboards are generally not locked in place when lowered; the mass of the ballast keeps them down. This also provides a measure of safety should the boat run aground--the force of impact will push the foil back into the centreboard trunk, rather than breaking it. The mass of a ballasted foil means that a system of pulleys is required to allow the sailor to lift the foil, and a method of latching the board in the upward position is needed.
Some boats have two foils mounted off centre to provide lift. These are called leeboards when they are mounted at the sides of the hull, and bilgeboards when they are mounted between the sides and the centre of the hull. The advantages of leeboards and bilgeboards is that the windward foil is lifted, which means that more efficient asymmetric foils can be used.