A centreboard (UK) or centerboard (US) is a retractable keel which pivots out of a slot in the hull of a sailboat, known as a centreboard trunk (UK) or centerboard case (US). The retractability allows the centreboard to be raised to operate in shallow waters, to move the centre of lateral resistance to offset changes to the sailplan that move the centre of effort aft and to reduce drag when the full area of the centreboard is not needed or when removing the boat from the water as when trailering. In a case that the centreboard consists of just a pivoting metal plate it is called a centerplate. A daggerboard is similar but slides vertically rather than pivoting.

A centreboard (often called a foil in a modern racing dinghy) 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, when sailing upright, 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 more a yacht heels the more asymmetric its hull shape becomes.

David Van Cortlandt Crosby (born August 14, 1941) is an American guitarist, singer, and songwriter. In addition to his solo career, he was a founding member of three bands: The Byrds, Crosby, Stills & Nash (who are sometimes joined by Neil Young), and CPR. Crosby has been inducted into the Rock and Roll Hall of Fame twice: once for his work in The Byrds and once for his work with CSN.

David Crosby was born in Los Angeles, California. His parents were Aliph Van Cortlandt Whitehead and Floyd Crosby, an Academy Award winning cinematographer. He is also the younger brother of musician Ethan Crosby. Growing up in California, he attended several schools, including the University Elementary School in Los Angeles, the Crane Country Day School in Montecito, and Laguna Blanca School in Santa Barbara for the rest of his elementary school and junior high. At Crane, he starred in HMS Pinafore and other musicals but was asked not to return due to lack of academic progress. He graduated from the Cate School in Carpinteria, completing his studies by correspondence. In 1960, his parents divorced, and his father was remarried to Betty Christie Crosby.