A centerboard's 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's similar but slides vert… (More on Centerboard)