Daggerboard

A daggerboard is a flat retractable foil that drops vertically through a trunk in the hull to resist leeway, the sideways slip a sailboat makes when driving to windward. Unlike a pivoting centerboard it is raised straight up, which keeps the trunk short but means it cannot kick up over an obstruction. Dinghies, beach catamarans, and foiling boats use daggerboards; on a foiler the board carries the lifting foil that raises the hull clear of the water.