Wing Sails Return to the America's Cup
Towering over Dirk de Ridder's head is the largest single airfoil section ever built. It's 190 feet tall, up to 30 feet wide and 6 feet thick, and weighs almost 4 tons. It represents nearly 40,000 man-hours of work, millions of dollars, and is capable of producing more power than perhaps any other single sail. Mounted on BMW Oracle Racing's 90-foot trimaran, it could be the key to winning the 33rd America's Cup. De Ridder is the wing's primary trimmer. That would seem like quite a load on one pair of shoulders, even if they happen to belong to a sturdy veteran of three Volvo Ocean Races, two America's Cups, and one Olympic regatta.
Actually, says the 36-year-old Dutchman, compared to the traditional-a.k.a. soft-sail configuration the team used previously, the hard sail makes his job easier. With the soft sail, de Ridder would spend his day clutching a mainsheet carrying 20 tons of load and, in many respects, the structural integrity of the multi-million-dollar craft.
"With the wing, you can do everything with two very small winches and four very small hydraulic rams," says de Ridder. "[With the soft sail] you were sort of in charge of the loading of the boat because the mainsheet transferred all the load to the headstay. Now navigator Matteo Plazzi and tactician John Kostecki are more in charge of the actual loading and how the mast sets up. So in that perspective my job has gotten a lot easier."
For all the science and technology in this remarkable sail, it is, in theory, a tried-and-true concept. "The two-element wing, I'm not sure when it was invented," says BMW Oracle Racing design coordinator Mike Drummond. "But it wouldn't surprise me if it was in the 1920s or thereabouts. Within the first 10 to 20 years after the Wright brothers flew, there was an enormous explosion of people developing the math and theory of airfoils and wings. Most of that is still applicable."
The basic design-looking at a transverse slice of the wing-is a forward section (the main element) with a hinged tail (the flap element) and a small gap in between. Angling the flap to one side or the other creates camber. That, combined with the angle of attack-the difference between the apparent wind angle and the centerline of the forward section-creates lift.
"With the two-element wing with the slot, we can achieve much higher lift coefficients [compared to a soft sail]," says Drummond. "So we can have a smaller sail area, if you like, and achieve the same power."
The wing has slightly less "sail" area than the mainsail used on BMW Oracle Racing's tallest conventional rig. And while the soft main was always accompanied by a head sail, the wing will likely only need one in very light air. Less sail area, of course, means less drag. Less drag means more speed.
No matter how basic the theory behind the wing; building it involved breaking a lot of new ground. No one had ever attempted to build a foil of this size and complexity. The rigid wing for Dennis Conner's catamaran challenge in 1988 was 108 feet tall. The rigs for the C-Class catamarans of the Little America's Cup top out at 40 feet. Outside the sailing world, the blade for the Enercon E-126 windmill measures 206 feet in length. But the chord is much shorter and the blade doesn't contain any moving parts.
