Centerboard

A centerboard is a retractable fin lowered through a slot in the bottom of a sailboat's hull, providing lateral resistance against the sideways push of the wind so the boat can sail effectively upwind, and able to be raised in shallow water. British Royal Navy officer John Shuldham is often credited with an early patent for a sliding keel of this kind in 1774, though various forms of retractable keel were also developed independently by American sailors working in the shallow waters of the Atlantic coast. The centerboard remains common on small sailing dinghies and shallow-draft sailboats today, distinguishing them from deeper-draft boats with a fixed keel.