Winged keel
A winged keel is a sailboat keel, usually of high aspect ratio, that uses a nearly horizontal foil, the "wing", at the bottom to provide additional performance. The horizontal foil serves two purposes: it acts as an winglet on the foil, effectively doubling the aspect ratio, and it provides downwards lift to increase the stability of the boat. Winged keels are generally found on high performance offshore sailboats, if they are not prohibited by class rules.
The winged keel made its first appearance in the 1983 America's Cup Australian challenger Australia II, and is considered the decisive factor that allowed the Australian team to win the cup. The increased stability afforded by the winged keel allowed Australia II to be as short and light as possible under the 12 meter rules, and still carry enough sail to beat the American entry. The wings were angled downwards at about 20 degrees, which, since they were lifting downwards, acted as a dihedral.
Most winged keels also incorporate a significant amount of ballast where the wings join the keel. This feature makes these winged keels also a type of bulb keel.