Winged keel
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A winged keel is a sailboat keel, usually of moderate aspect ratio, that uses a nearly horizontal foil, the "wing", at the bottom to provide additional performance. Note that the wing is upside down with relation to the boat. The horizontal foil serves two purposes: it acts as a winglet on the foil, effectively doubling the aspect ratio by reducing lift-induced drag, and they produce additional lift, but downwards. Because the yacht is heeled over when sailing upwind the leeward foil is closer to vertical, and provides additional side force hence making the boat sail upwind more efficiently. The windward winglet is closer to horizontal and hence produces a force directly downward, which gives a small benefit to the vessel's stability.
Winged keels are generally found on high performance sailboats, if they are not prohibited by class rules. They are only of benefit for yachts sailing upwind where stability and the ability to produce side force are important. Downwind the extra skin friction drag is a hindrance. Besides the performance benefits, winged keels can also be applied to pleasure boats as a way to reduce draft, allowing for greater versatility when gunkholing.
The winged keel made its first appearance on a 12 metre class yacht in the 1983 America's Cup Australian challenger Australia II. Along with Australia II's radical sail design, this keel designed by Ben Lexcen was one of the factors contributing to Australia II's success. The increased stability afforded by the winged keel, due mainly to the extra lead in the wings producing a very low centre of gravity, 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, helmed by Dennis Conner. 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.