Tech Review: Maximum Lift
Tech Review: Maximum Lift
Speed in a sailboat is a matter of balancing lift and drag. Thanks to two significant technological advancements—one above the waterline and one below—no race boat has ever done it better than the AC72.
Calling the AC72 the most technologically advanced sailboat is not hyperbole. Upwind, on their slowest point of sail, the 72-foot catamarans can hit 25 knots, speeds well above the outright capabilities of most sailboats. Downwind, the AC72s fly on impossibly narrow foils, going so fast that through the jibes the wind passes around the bow, rather than the stern.
But this giant technological leap has come at a price: The financial resources required to design, build, and develop the new boats is tremendous, and has kept all but four teams—three sponsored by billionaires—from competing. Even more significant, at the time of this writing, the AC72s had suffered two catastrophic capsizes, the latter ending with the tragic drowning of British double Olympic medalist Andrew Simpson.
Catamarans are not new to the America’s Cup. Dennis Conner campaigned the wing-sailed Stars & Stripes to victory in San Diego in 1988, while Alinghi 5 was beaten handily by BMW Oracle Racing’s trimaran during the 33rd America’s Cup off Valencia, Spain. But those Deed of Gift matches were all about speed. The specified courses were either two or three legs and 40 miles long.
The 34th Cup is the first time in the event’s history twin-hulled vessels will be used by mutual consent between the defender and challenger of record.
Winning this Cup, on inshore, multiple-lap courses constricted by artificial course boundaries will require a boat that is as nimble as it is fast.
“Maneuvering speed is a very big part of it,” says Artemis Racing’s Tom Schnackenberg, whose America’s Cup experience dates back to the 12-Metres. “You lose several boatlengths in a good tack and more in a bad tack. Race modeling and working out the time around the course has been a big part of the design process.”
Nonetheless, it’s the shocking speed that even the most casual spectator will instantly appreciate. And that speed has two vital components: the 130-foot rigid wing sail that powers the boat, and the groundbreaking foils that—when going downwind—lift both hulls clear of the water, at the same time.
The rigid wing sails fitted to the AC72s are similar to an aircraft wing, but there are differences, starting with their vertical orientation. An aircraft wing only needs to generate lift in one direction. The AC72 wings must be capable of generating lift on either side, so the wing components are symmetric and the mechanics doubly as complex.
The AC72 wings typically have—working front to back—a main element, an optional hinged tab off the back of the main element, a slot, and then a series of flaps that form the wing’s trailing edge.
Camber—basically the depth in the sail—is created by hooking the aft section to windward. The greater the angle between the two elements, the more power the sail can generate, but like an airplane wing, only so much camber can be put in before the sail stalls.
The angle of the rear flaps, relative to one another vertically, determines the twist, which allows the rig to match the difference in the apparent wing angle between the top and the bottom of the wing, and plays a key role in depowering the sail. On some wings—Emirates Team New Zealand’s second wing being one example—twist can also be induced up the length of the front element.
A tab on the trailing edge of the forward element—Oracle Team USA’s wing has one, for example, ETNZ’s does not—helps smooth out the camber, but at the price of an added level of complexity.
Vital to the efficiency of the wing is the slot, which allows wind from the high pressure windward side of the wing to blast through to the leeward low pressure side, enabling flow to remain attached over more of the wing’s surface area. This raises the wing’s lift coefficient, making it more powerful downwind compared to an equivalent soft sail. But the design is complicated as regulating the size of the slot is essential: Upwind the slot is nearly closed, while downwind, when there is more camber in the wing, it is open.
In the buildup to this Cup, wings have taken some criticism. They are cumbersome, if not dangerous, to step and take down, and heavy. Oracle Team USA design team member Tom Speer, a former Lieutenant Colonel with the U.S. Air Force, says keeping the weight of the solid wing close to that of a similar soft-sail set up—as they were able to do with the 223-foot wing used in the 33rd America’s Cup—is the most significant challenge in building a rigid sail.
If these disadvantages can be tolerated—a very capable shore crew is a must—they bring a wealth of advantages over soft sail rigs.
First, compared to conventional mast and sail combinations, wings are vastly more efficient aerodynamically and create less drag. The lift coefficient, used to rate the efficiency of a wing, for an AC72 wing is around 2 to 2.5. The lift coefficient for a state-of-the-art traditional sail set up is approximately 1.5 to 2.
“Basically a soft sail rig can be comparable to a wing sail section at a particular operating point,” says Speer, “but the wing does much better at conditions away from there. It operates over a much broader range, and as the wind is always fluctuating, that is important. It has a higher maximum lift as well.”