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 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 centreboard isn't needed or when removing the boat from the water as when trailering. A daggerboard is similar but slides ve… (More on Centerboard)