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A centreboard 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 centreboard to be raised to operate in shallow waters, to reduce drag when the full area of the centreboard isn't needed, or to remove the boat from the water as on a trailer. A daggerboard is similar but slides vertically rather than pivoting.

A centreboard 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 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.

The efficiency of a centreboard improves with increasing aspect ratio. A long narrow centreboard 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 centreboard 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 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. Ballast is usually provided in the keel, keeping the centreboard lighter and easier to handle.

Centreboards are often ballasted. 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 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.

• leeboard
• bilgeboard
• daggerboard

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