Winging It To Victory
Sporting its sophisticated wing sail—more aircraft wing than traditional sail—USA-17 blew the competition out of the water in the recent America's Cup race
Just for a little while, in the second race of the America’s Cup regatta last February in Valencia, Spain, the defending champions thought they had it made. Two days earlier, the Swiss Société Nautique de Genève’s Alinghi 5 yacht had started the first race with a substantial lead, but the challenger, the USA-17 had come from behind to win. Now, midway through the first leg of the course, Alinghi 5 was again ahead. If it could hold on, it would even the score of the best-of-three contest and force a third and decisive race for the cup.
But Alinghi 5’s advantage proved as elusive as the shifting winds, and USA-17 soon overtook it just as before, “blowing the sails off” the Swiss boat in a decisive victory for the series and the cup. America’s BMW Oracle team, led by Oracle software mogul Larry Ellison, brought the cup back to the United States after an absence of 15 years.
Aerospace engineers helped to create a hybrid sailboat sail and airplane wing. At 20 stories high, it is more than twice the length of a 747 wing.
Not so unusual—after all, most of the champions in the 159-year history of the competition have been American. Both craft were sleek, 90-foot-long carbon-composite multihulls, both cost millions of dollars to build, both were sailed by expert crews, both were among the fastest things on sails. So what gave USA-17 its winning edge?
That becomes clear when comparing the craft side by side. Alinghi 5 is still obviously a sailboat, with big, soft sails. USA-17 has a giant airplane wing sticking out of it, as though someone had broken it off an airliner and attached it end-on upon the comparatively tiny hull, towering taller than a 20-story building above the water. This “wing sail” works just like an airplane wing—only horizontally rather than vertically. It harnesses the cutting edge of wind power over the water.
USA-17’s wing sail is the most sophisticated and largest yet built. Even a 747 wing is only slightly more than 100 feet in length, whereas the object rising out of USA-17 is over twice that. “It’s hard to describe how big it actually is until you stand alongside it,” says Mike Drummond, design director for the BMW Oracle team. “If we’d thought about it at the start, we might have been scared off.”
It may sound futuristic, but the basic concept has been around about as long as that of the aircraft wing. “Wings on sailboats go back 100 years,” notes David Hubbard, who led the design effort for USA-17’s wing sail and has created them for many other craft, including the 1988 America’s Cup champion, Stars and Stripes.
The precise origins of the idea, however, remain somewhat hazy. “The concept of something rigid [as a sail] goes back at least to a guy named Flettner,” notes Mark Maughmer, an aeronautics professor at Pennsylvania State University. Anton Flettner was a German engineer who exploited the Magnus effect, the generation of lift by a spinning cylinder. In the 1920s he designed and built a number of craft leveraging this phenomenon, including the Baden-Baden, which used two large spinning rotors to propel itself across the Atlantic in 1926. While Flettner’s applications worked, they proved inefficient when compared to other propulsion methods and were eventually abandoned.
Aerodynamics engineer Tom Sweeney had the idea of creating a sail that was actually rigid like an airplane wing.
Whoever first conceived the idea of wing sails, sailors looking to outrun their soft-sail competitors have been experimenting with them for years. Maughmer participated in substantial research on the concept at Princeton University in the 1970s under aerodynamicist Tom Sweeney. “He was an avid sailor and got the idea of sort of creating a hybrid sailboat sail and airplane wing,” Maughmer recalls. “It used a rigid leading edge like a mast, and it had a rigid tip rib and a root rib, and cable that ran between those along the trailing edge, and it had background sail cloth that wrapped around it.” The Princeton sail wing eventually found use on wind turbines and windmills after testing in NASA’s wind tunnels.
The Princeton work proved highly influential, and semirigid wing sails gradually began appearing on small boats. “They were quite good,” says Maughmer. “The only reason they didn’t show up on America’s Cup and racing is because it was illegal.”
By 1988, however, the game had changed. “For the first 100 years, it was 12-meter-yacht rules, which are very specific,” explains Maughmer. Then challengers and defenders alike began tweaking the rules of the race and the types of boats allowed. In the 1988 race, New Zealand’s Mercury Bay Boating Club challenged the San Diego Yacht Club amid a storm of controversy. The American defenders wanted to stick to a 12-meter boat, but the Kiwi team was able to get their much larger and more imposing yacht approved by the race administrators. That forced the San Diego sailors to quickly develop a new design.
“They decided that the best way to do it, given the time available and the speed potential, was to use a catamaran, which would be faster than the monohull,” says Hubbard. “Then to further improve their chances, they decided to use a wing.” Hubbard and another noted designer, Duncan MacLane, were brought in to design a wing sail for the American contender Stars and Stripes. Two identical catamaran hulls were built and tested, one with a conventional soft sail, the other with the wing sail. The latter proved to be faster and was adopted for the final model.
As further proof of the importance of aerodynamics in sailing, the noted aircraft and spacecraft designer Burt Rutan provided crucial assistance. “His crew built the main spar and so-called first element,” Hubbard relates. “Burt didn’t do too much himself, just a friendly looking over our shoulders sort of thing.”
“The group that won with Stars and Stripes was primarily a group of aerospace engineers,” points out Maughmer, who was also involved in the 1988 effort. Their wing sail “beat the sails off the big boat.” Traditionalists might still balk at the wing sail’s technological slickness, but the recent victory of USA-17 makes it hard to deny that if blazing speed over the water is your sailing goal, then this is the answer.
As further proof of the importance of aerodynamics in sailing, the noted aircraft and spacecraft designer Burt Rutan provided crucial assistance.
Just what makes a rigid wing better than soft sails? In its most common incarnation on airplanes, the wing produces lift primarily in a vertical direction, allowing the plane to overcome the force of gravity. By manipulating that lift in various ways, the aircraft can rise, descend, bank, and perform other maneuvers. But there’s no reason why lift—or more precisely, the pressure differential between the two sides of the wing created by its shape and motion through the air—can’t be generated horizontally as well.
Clueless landlubbers tend to think that sailboats are propelled mainly by the wind pressing on their sails. But that’s only part of it. Wind moving past the sail creates lift in the horizontal plane, and a sail’s shape and arrangement can be altered, engaging this to move the boat across—or even into—the wind. Balancing aerodynamic lift, drag, and the motion of the water to move the boat efficiently, quickly, and in the desired direction is what makes sailing both an art and a science.
In general, the wing sail unites greater lift with a higher degree of controllability. “It’s behaving like an airplane with the wing sticking upwards instead of horizontally,” explains Hubbard. “It all boils down to the amount of lift that you can produce for the amount of airflow that’s going past it with a minimum amount of drag. When you’re sailing in an easy ‘soldier’s breeze,’ which is sideways to the wind, anything can work. But a sailor also needs to beat upwind, when the angle of the wind is pushing the boat more backwards than sideways; then you need lift, which will push you sideways but with a small forward component.”
A rigid sail avoids the problem of luffing, the unwanted flapping of a soft sail when the airflow is disrupted, creating drag and slowing down the boat. The wing sail, says Maughmer, “holds its shape in all conditions . . . a rigid airplane wing is way more efficient than a sailboat sail.” Still, a wing sail has some problems of its own. “It has the disadvantage in that normally an airplane wing only has to lift in one direction, and the sailboat has to be able to equally lift in both directions. So they get around that by rotating the mast. That helps some. And there’s a little compromise there.”
As part of the compromise, many wing sail–equipped craft, including USA-17, also use a smaller triangular forward soft sail or jib, which allows a bit of the best of both worlds. “This foresail, which is a more conventional sail, gives them a lot of flexibility, and then they get high efficiency with a rear sail,” says Maughmer.
BMW Oracle’s Drummond confirms that the wing sail was chosen based on “just the belief that it would be fundamentally a better solution for the rig. The benefits range from having lower drag, being more controllable, and the loading being less.” Although the team had only about six months to design and build their wing sail, Drummond reports that it wasn’t a particularly troublesome process: “It was only a problem in that we needed to make very quick decisions on how we were going to build it, how we were going to handle it, raise and lower it, the shape, and so forth. We didn’t have very many design iterations. There were just the usual problems with a rushed program. If it didn’t work, we wouldn’t use it.”
“I was going to say it was textbook, but this hasn’t been done enough to have been written into a textbook,” reflects Hubbard. He worked with “a whole bunch of whiz-bang engineers” on the BMW Oracle team, “people who have CFD [computational fluid dynamics] programs . . . guys who did stress analysis and deflection and all the calculations of the stresses and loads and how much material you had to put in each component.” A mechanical engineer himself by trade, Hubbard observes that “they had all the tools, but no experience. I had the experience and not the tools. Or my tools were Stone Age.” Hubbard’s practical expertise in wing sail design helped keep the team on track. “They started sketching up wings and everything and I said, no, that won’t work, we tried that kind of wing back in ’78, and so forth. So I had kind of evolutionary experience. We worked very well together on it.”
The end result, towering above the carbon-composite trimaran hull of USA-17, is a 223-foot-tall creation of carbon fibers with a Kevlar frame. The wing sail is divided vertically into two elements: the single main section next to the mast, and a smaller section of nine flaps behind that. A slot opens between the two elements so that air can flow through the sail as well as around it. The flaps, mounted on hinges, can be individually adjusted to provide precise control of the airflow over the wing, so that the overall shape can twist as needed from top to bottom to maintain the best efficiency, even as the wind varies from the surface of the water to the sail’s towering crest. (The ability to use twist for control is one of the big differences between a wing sail and an airplane wing.) The main section and the flaps can be trimmed separately, allowing greater control than possible with a conventional soft sail.
The wing is operated through hydraulic actuators powered by a small onboard diesel engine (made by BMW, naturally). An onboard computer in the main hull monitors more than 250 sensors throughout the boat and sail rig, providing more than 26,000 data points 10 times per second on the wind, mechanical loads, and flaps and wing position. The computer is keyed to preferred settings according to wind levels and boat headings, with preset alarms to avoid the overloading of any component, the data being fed wirelessly to wrist-mounted PDAs worn by the crew. By pushing buttons on their PDAs, the crew members can control the appropriate hydraulic actuators to respond instantly to the ever-changing conditions and keep the boat in optimal trim. Although the yachts share the water with a flotilla of other craft during the race—safety boats, observers, and race officials—each crew is completely on their own. “The rules of sailing don’t allow outside assistance, so all computations and measurements are done onboard,” says Drummond.
Alinghi 5’s control system was similar. “Yacht instrument systems have targets for speed, heel angle, and wind angle stored on an onboard computer,” Drummond explains. “This isn’t new—yachts have had these systems since about 1970. The skill of the sailors is to interpret the theoretical targets and use their judgment to vary them depending on the wind profile and sea state.”
Even in trials before the actual race, USA-17 demonstrated that it wasn’t your average sailboat, recording speeds of up to 32 miles an hour (28 knots)—about three times faster than the prevailing winds. Not that it had to go quite that fast to beat Alinghi 5 in the cup match, however. Taking into account penalty turns and other tactical factors, most observers agreed that USA-17 enjoyed at least a 5 to 6 percent speed advantage, enough of a margin to convince even the most old-fashioned sailors that, as one member of the Alinghi 5 team admitted at a press conference, “The wing seems to be quite a weapon.”
Still, sailing remains one of humanity’s oldest endeavors, with a rich tradition and romance that some feel is threatened by such technological wizardry. Where’s the human element, the challenge, the primal contest of people against the sea? Is this all lost when space-age engineering and tons of cash become the deciding factor in such an age-old pursuit?
Drummond, for one, doesn’t think so, pointing out that “the America’s Cup has always been a combination of technology and sailing. Money helps, money counts, but it’s really the application of the technology that you can spend the money on that’s important. . . . The reality is, every day the wind is a bit different. And that’s where the skill of the sailors comes in. I don’t think those sort of skills will ever go away.”
Maughmer agrees: “I think the human element’s going to be there because ultimately you’ve got to skipper this boat well and use the winds you’ve got. The technology can get you even with the competitor, but it’s probably not always going to make you better.”
Just as the air races of the 1930s spurred advances in aviation, intense technologically based competitions such as the America’s Cup benefit more than moguls. “While this is a race and the sport of crazy millionaires, there could be by-products that help us all,” Maughmer points out. While they’re unlikely to become popular with the weekend recreational sailor because of their expense and handling difficulties (a soft sail can be folded and stowed, unlike a rigid wing), wing sails are under development for other purposes. Hubbard is working with a Seattle-based company, Harbor Wing Technologies, on autonomous wing sail vessels that could find military, scientific, and commercial applications. A Huntington Beach, California, company, Morrelli & Melvin Design & Engineering, is developing wing sail craft for ferry routes on San Francisco Bay. Others have proposed completely automated cargo vessels propelled by wing sails alone, or using wing sails on commercial cargo ships to supplement the main engines and save fuel.
Notwithstanding the traditionalists, it’s quite likely that wing sails will be seen again in the America’s Cup sooner or later. As a leading member of the team that will have to defend the cup against its next challenger, Drummond is a bit cagey about strategy. “Well, I think it’s up for discussion. The America’s Cup is the pinnacle of yachting alongside the Olympics, and in technology it’s certainly the pinnacle of yachting. If we really want that to be a true statement, then we should using any technology we like.”
And that includes the wing. “They use them on airplanes, I’m told,” he laughs.