The winged keel is a sailboat

keel The keel is the bottom-most longitudinal structural element on a vessel. On some sailboats, it may have a hydrodynamic and counterbalancing purpose, as well. As the laying down of the keel is the initial step in the construction of a ship, in Br ...

layout first fitted on the

12-metre class The 12 Metre class is a rating class for racing sailboats that are designed to the International rule. It enables fair competition between boats that rate in the class whilst retaining the freedom to experiment with the details of their designs. ...

yacht ''

Australia II ''Australia II'' (KA 6) is an Australian 12-metre-class America's Cup challenge racing yacht that was launched in 1982 and won the 1983 America's Cup for the Royal Perth Yacht Club. Skippered by John Bertrand, she was the first successful ...

'', 1983 America's Cup winner.

Design

This layout was adopted by

Ben Lexcen Benjamin Lexcen AM (born Robert Clyde Miller, 19 March 1936 – 1 May 1988) was an Australian yachtsman and marine architect. He is famous for the winged keel design applied to ''Australia II'' which, in 1983, became the first non-American ya ...

, designer of ''

''. Although Ben Lexcen "had tried the winged keel idea before",Bruce Stannard, ''Ben lexcen, the man, the keel and the cup'', Faber and Faber, 1984, there is conjecture that it was computed and designed by a Dutch aerodynamicist at the Wageningen towing tank (Netherlands Ship Model Bassin).

Wings

The lateral wings of Australia II are of moderate aspect ratio, forming a nearly horizontal

foil Foil may refer to: Materials * Foil (metal), a quite thin sheet of metal, usually manufactured with a rolling mill machine * Metal leaf, a very thin sheet of decorative metal * Aluminium foil, a type of wrapping for food * Tin foil, metal foil ...

, the "

wing A wing is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is expres ...

", at the bottom to provide additional effective span, in the same way as the winglets on an aircraft. Each wing acts as a

winglet Wingtip devices are intended to improve the efficiency of fixed-wing aircraft by reducing drag. Although there are several types of wing tip devices which function in different manners, their intended effect is always to reduce an aircraft' ...

, effectively increasing the keel aspect ratio therefore reducing the

lift-induced drag In aerodynamics, lift-induced drag, induced drag, vortex drag, or sometimes drag due to lift, is an aerodynamic drag force that occurs whenever a moving object redirects the airflow coming at it. This drag force occurs in airplanes due to wings or ...

. Because the yacht is heeled over when sailing upwind, the

leeward Windward () and leeward () are terms used to describe the direction of the wind. Windward is ''upwind'' from the point of reference, i.e. towards the direction from which the wind is coming; leeward is ''downwind'' from the point of reference ...

foil attains more draft, which reduces the loss of efficiency that always occurs under heel. The wings were canted downwards at about 20 degrees, in order to promote proper hydrodynamic loading (lift) on each wing when sailing to windward. Upwind, it offered 30% less resistance due to side force.

Upside down keel

Note that, contrary to classic configurations, the keel of this boat is "upside down" under the hull (the root chord is smaller than the tip (bottom) chord) in order to minimize the hull-keel interaction and the loss of side force due to the proximity of the water surface. The extra low-positioned lead in the upside down keel and wings gives a very low centre of gravity, increasing the righting moment (lateral stability) and allowing to carry more sail area.

Shorter boat, more sail area

Under the 12 metre class rule, the allowed sail area is an inverse function of the boat length and weight. Her stability advantage allowed ''Australia II'' to carry more sail although the boat was lighter. Along with ''Australia II''s efficient sail design, this winged keel was one of the factors contributing to ''Australia II''s success. Total advantage offered by this concept on the race course was about 1 minute per upwind leg.

Application

Winged keels are generally found on high-performance sailboats if they are not prohibited by class rules. They are especially advantageous for heavy yachts with a lot of sail area (as 12-metre class boats), sailing upwind when the draft is limited by the class rule or by the requirement to be able to sail in shallow water, because in that case high righting moment and efficient side force are difficult to obtain. Downwind, the extra

skin friction Skin friction drag is a type of aerodynamic or hydrodynamic drag, which is resistant force exerted on an object moving in a fluid. Skin friction drag is caused by the viscosity of fluids and is developed from laminar drag to turbulent drag as a f ...

drag is a hindrance. Besides the performance benefits, winged keels can also be applied to pleasure boats as a way to reduce

draft Draft, The Draft, or Draught may refer to: Watercraft dimensions * Draft (hull), the distance from waterline to keel of a vessel * Draft (sail), degree of curvature in a sail * Air draft, distance from waterline to the highest point on a vesse ...

, allowing for greater versatility when cruising in shallow waters.

References

{{reflist, refs= 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 Australian inventions Shipbuilding Sailboat components