Multihull Sailor

The AC72 Teams Makes Foiling Catamarans Look Easy

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There was something surreal about watching Oracle’s AC72 catamaran lift itself clear of the waters of San Francisco Bay and fly, with roostertails of spray firehosing off the daggerboard and rudders the only indication of speed: that, and the fact that the chase boat on which I was perched had to strain to keep up with the big black 72-footer as its speed nudged toward and then beyond 30 knots.

There’s nothing new about foiling multihulls—beach boat builders Hobie and WindRider both built car-toppable foilers once upon a time, and the big French foiling tri L’Hydroptere continues to set records. The concept also got a recharge a few years back when foiling Moth dinghies appeared on the scene, quickly dominating the class. When Emirates Team New Zealand got their boat sailing on hydrofoils last year, the chemistry of the 34th America’s Cup was changed overnight.

Any displacement boat must push water out of its way in order to make forward progress, and its speed is limited by a combination of wave action and the drag of the water along the “wetted surface” of the hull. By elevating the hull(s) out of the water, hydrofoils allow a boat to go faster because the drag on the foils is much less than the drag on the hull. 

It is incredible to see such a boat as an AC72 supported on one or two skimpy carbon fiber foils only a few inches wide and a few feet long. These foils, basically daggerboards with a sharp bend in them, work in the same way as the wings on an aircraft—they generate lift—but because water is 1,000 times denser than air, the foils can be much smaller than an aircraft wing, and the boat does not have to be moving all that fast before they lift the hull out of the water. 

Foil design is complicated because of the need to control roll, pitch and yaw, the results of which are plain to see in videos of AC72s foiling. If the foils leave the water, they cease to generate lift and the hulls drop back into the water, sometimes abruptly. The vertical element of the daggerboard also ceases to prevent leeway. AC rules allow crews to tweak daggerboard trim only to a certain extent—ideally you would have a number of controls like trim tabs and moveable winglets to fine-tune the foils, the way Moth sailors do. But these kinds of controls were deliberately proscribed by the rules in a vain effort to prevent the boats from full foiling. 

Nor are they allowed to alter the trim on the rudder winglets while racing, which would be a big help in keeping the foils where they should be—in the water. The height at which an AC72 can “fly” is limited by the depth of the daggerboards. If part of a foil breaks the water so that air gets under the lifting surface, the consequent “ventilation” causes the foil to lose lift and drop down into the water, where the process of generating lift must start all over. Another problem is cavitation, which is common with propellers—so much air is mixed in with water flowing over the blade that pressure drops. Lift is lost and the boat again thumps down into the water. 

To avoid the ventilation and cavitation problems, L’Hydroptere has very long, adjustable foils that reach deeper into the sea so there always is some foiling surface immersed even in a rough sea state. Other foiling boats have stacked “ladder” foils intended to overcome the same issues. No such luck for the America’s Cup teams.

Keeping a boat sailing fast and true on foils like those of an AC72 is no easy matter. You want to keep the foils immersed at all times and in a San Francisco Bay wind-over-current chop, that’s a real challenge. It’s scary, and it’s dangerous. When you see these crews gybing, on foils, at speeds of 25-plus knots, you should have your heart in your mouth. I bet they do. 

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