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.
As the leech tension of the sail is inherent in the wing’s rigid structure, the mainsheet load is a fraction of that of a conventional rig, which reduces the amount of structure required in the catamaran’s aft crossbeam.
If trimmed properly, the wing is less inclined to stall. And there’s no luffing through tacks. “Normally [through a tack] you have 70 degrees of dead zone in a soft sail,” says Oracle Team USA designer Joseph Ozanne. “You don’t have this zone at all with the wing. You always have lift unless you are exactly head to wind. So it is very powerful during a maneuver.” Modern sailcloth may stretch minimally, but the wing, with its wider cross section is still more stable, allowing better repeatability of settings with a minimum of sail controls.
There are three primary controls for the wing. The traveller, which is often referred to as the mainsheet, controls the overall rotation of the wing and its angle of attack relative to the wind. The camber control dictates the depth of the wing. Pulling it tighter will flatten out the fore and aft elements, while easing it off puts more depth into the sail, just like an outhaul. Twist control adjusts the vertical alignment of the flaps along the wing’s trailing edge. Pulling on the twist control straightens the trailing edge, while easing it off allows the upper flaps to fall progressively farther to leeward.
Another advantage of the wing sails may seem counterintuitive. Being solid, with a fixed area, it would seem logical that as the wind strength increases wings get increasingly overpowered and hard to manage. In fact, this isn’t what happens. Using the wing’s twist control, the top flaps can be feathered, lowering the center of effort of the wing. It’s not much different than reefing a soft sail, except it’s much faster and infinitely variable.
Flying on foils
As advanced as the wing sails are, they’re not the most talked-about technological development on the AC72s. That honor goes to the foils, which generate enough vertical lift that the boats become airborne while sailing off the wind. Once airborne, the speed difference, compared with boats that do not foil, is noticeable.
“On paper we knew it would probably be better,” says Emirates Team New Zealand designer Guillaume Verdier of the foiling AC72. “From the start we tried both options.”
In this respect, the campaigns did not come out of the blocks equally fast. Artemis Racing’s first AC72 was not designed to foil. That boat was rendered all but obsolete during Artemis’s first extensive tuning sessions against Oracle Team USA’s foiling boat last February.
“We figured it wasn’t as important as it’s turned out to be,” says Schnackenberg. “The rule looked like it was conspiring against foiling, but it turns out that the way that it has been interpreted, you can.”
Since that time Artemis Racing has been trialing new foil shapes on its AC45. The team’s second AC72, which had yet to be sailed at press time, is expected to be an airborne one.
For Oracle Team USA, foiling was a natural evolution of the technology utilized by the large French offshore multihulls for more than a decade.
“Lifting the boat and reducing hull volume are natural gains for multihulls,” says Ozanne, a French aero/hydrodynamicist who joined Larry Ellison’s team in 2004. “Given the wind conditions and the power of the boat, it became apparent that we could lift totally the [leeward] hull out of the water.”
Once foiling was on the table, the most significant problem for all the teams became the design of the daggerboards, which need to essentially serve two masters, providing horizontal lift to combat leeway when sailing upwind and vertical lift when sailing downwind.
Further complicating the process was the AC72 rule makers’ bid to prevent AC72s from getting fully airborne. Vital to foiling successfully is being able to do so in a stable manner, so that the amount of vertical lift generated by the foils decreases as the boat rises farther out of the water. On foiling Moths, for example, a wand on the bow senses the height of the boat and adjusts accordingly a lateral trim tab on the daggerboard. Such trim tabs are prohibited by the AC72 rule.
But this has not stopped the design teams. The foils used on the AC72s have evolved a considerable way from the C-shaped boards first introduced a decade and a half ago on ORMA 60s (and currently still used on Nacra 17 and A-Class cats). The C-shaped board has a couple of significant flaws, notably the lower the board, the more vertical lift it generates. The famous S-shaped boards originally used on Alinghi 5 in the 33rd America’s Cup neatly solved this problem, the shape of an ‘S’ causing the board to become more vertical as it was lowered, minimizing leeway upwind.
For this America’s Cup, while trim tabs may be banned, there is still considerable flexibility in what teams can do with their boards. The AC72 rule allows these boards not only to be raised and lowered but to be canted laterally inboard, increasing their dihedral, and also for the overall pitch of the board (fore and aft rotation) to be altered. The big challenge for design teams has been working out the best shape to prevent leeway upwind and create stable, controllable vertical lift downwind, so that the boat has heave stability, meaning it can fly in equilibrium, neither launching nor nose-diving.
The best heave stability is found in foils with dihedral, such as the V-foils typically used on hydrofoil ferries, where flying equilibrium is achieved simply because the higher the out-of-the-water vessel moves, the less foil is in the water to provide vertical lift. However, the downside of such foils is excess drag and weight.
So the most popular foil configuration seen on the AC72s are variations on the L-shaped daggerboard, with the vertical part of the L preventing leeway, and the horizontal part providing upward lift.