The winged keel is a sailboat keel originally adopted by Ben Lexcen. It made its first appearance on the America's Cup winning 12-metre class yacht Australia II in the 1983 America's Cup races. Although the Australia II team claimed that Ben Lexcen was the designer, there is ample evidence that it was conceived, proposed and designed by a Dutch aerodynamicist.
Along with Australia II's efficient sail design, this winged keel was one of the factors contributing to Australia II's success. Upwind, it offered 30% less resistance due to side force, as well as increased stability. The increased stability was due mainly to the extra low-positioned lead in the wings producing a very low centre of gravity. This allowed Australia II to be as short and light as possible under the 12 metre rules, and still carry enough sail. The wings were canted downwards at about 20 degrees, in order to promote proper hydrodynamic loading (lift) on the wings when sailing to windward. The total advantage offered by this concept on the race course was about 1 minute per upwind leg.
The lateral wings of a winged keel are usually of moderate aspect ratio, forming a nearly horizontal foil, the "wing", at the bottom to provide additional effective span, in the same way as the winglets on an aircraft. Note that, contrary to classic configurations, the keel is "upside down" under the hull (the root chord is smaller than the tip (bottom) chord) in order to minimize the loss of side force due to the proximity of the water surface. Each wing acts as a winglet, effectively doubling the keel aspect ratio and reducing lift-induced drag. Because the yacht is heeled over when sailing upwind the leeward foil attains more draft, which reduces the loss of efficiency that always occurs under heel.
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 with minimum resistance are important, and when the draft is limited by the class rule or the requirement to be able to sail in shallow water. 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 cruising in shallow waters.
- Barbara Lloyd, "Who really designed Australia II?", Nautical Quarterly, Spring 1985 Issue, NY
- Joop Slooff, Australia II and the America's Cup, the Untold, Inside Story of The Keel, 2016, ISBN 978-1530590230
- Piers Akerman, "Flying Dutchman who won America's Cup with a Keel", The Australian, April 2, 2016
- J. W. Slooff, The Aero- and Hydromechanics of Keel Yachts, Springer, 2015, ISBN 978-3-319-13274-7