A centerboard is a retractable keel which pivots out of a slot in the hull of a sailboat, known as a centerboard trunk (US) or case. The pivoting ability allows the centerboard to be raised to operate in shallow waters, to move the center of lateral resistance to offset changes to the sailplan that move the center of effort aft and to reduce drag when the full area of the centerboard isn't needed or when removing the boat from the water as when trailering. A daggerboard is similar but slides vertically rather than pivoting.
A centerboard is used to provide lift to counter the lateral force from the sails. This is required for sailboats to move in directions other than downwind, since the force of the sail is never closer than 45 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 centerboards must use symmetric foil shapes so they will operate with equal efficiency on either tack.
The efficiency of a centerboard improves with increasing aspect ratio. A long narrow centerboard produces less drag than a short, wide one for a given amount of lift, resulting in a faster boat that can point closer into the wind. A pivoting centerboard can also be used to move the center of lateral resistance aft to match a change in sail plan such as furling or dropping the jib. A retracting centerboard 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. Other types feature a casing under the boat, which thus does not take up space but now has the problem of increased drag. For this reason, it is not uncommon to find boats with combination of shallow keel and centerboard (eg Randmeer).  The keel provides the housing for the centerboard, moving it out of the hull, but adds only a small amount of draft to the boat. The centerboard can then be lowered in deeper waters to increase the amount of lift. Ballast is usually provided in the keel, keeping the centerboard lighter and easier to handle.
Centerboards are often ballasted. Ballasted centerboards 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 centerboard trunk, rather than breaking it as might happen if the board were locked in place. The mass of a ballasted foil means that a system of pulleys may be required to allow the sailor to lift the foil, and a method of latching the board in the upward position is needed. A centerboard differs from a ballast keel in that centerboards do not contribute to the stability of the vessel; their purpose is to provide lateral resistance.
On larger sailing vessels, a similar design is sometimes incorporated to enable navigating into shallower water than a fixed keel would allow. In these situations the appendage is generally referred to as a "lifting keel" (which is usually pivoted but occasionally retracted like a daggerboard) or a "swing keel."
In such installations on offshore vessels, the keel should ideally be lockable in any position, so that it does not fall back into the keel well if the vessel is inverted.
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