Go-faster foils have changed the face of the America’s Cup.
These beefy blades raise the hulls out of the water, reducing drag, and allowing the boat to “fly” at speeds of more than 40 mph.
The foils act exactly like an aeroplane wing and work by generating lift force when the boat speed reaches about 18 knots.
“The reason to foil is quite clear,” says Land Rover BAR’s chief technical officer Andy Claughton. “If you look at the average yacht sailing around, it might go at 10 knots on a good day. In the same wind we can go at 35 knots. We’re getting rid of 75% of the drag of the boat by riding on our hydrofoils.”
Adjusting the angle of the foil, by changing “rake”, is a role performed by the helmsman with a panel of buttons. Changing rake alters the amount of lift.
Try sticking your hand palm-down out of a car window when moving.
Then adjust the angle of the front edge and feel it pushed up or down by the air. That’s what the foils are doing, while lifting about 2.5 tonnes – or about the weight of a range rover - out of the water on a plank the size of a small kitchen table.
Wings on the rudders also help with the lifting like the tail plane on an aircraft, without these the boat would be completely uncontrollable.
The crew, especially the grinders, must generate the hydraulic power to adjust the foils and wing.
"These things are heavy," says grinder David "Freddie" Carr. "It takes the three front guys a lot of hard work to get the daggerboards up and down and in and out of the water."
An ideal state is for the hulls to be flying about a metre out of the water on a level plane.
“The key to speed in the America’s Cup is stable flight,” says Land Rover BAR team principal and skipper Ben Ainslie.
“We must work as a team to achieve stable flight but I’m in charge of making small adjustments that will keep us going in the right direction.
“And I have to adjust the rake to get just the right amount of lift to keep the boat flying.”
Designing the shape, width and thickness of daggerboards is a trade-off between speed and stability. It is also influenced by wind conditions and planned race tactics.
- Foils that bend (or are canted) far below the hull increase stability, but reduce speed.
- Foils with a short cord (width) are more aggressive, working well in low winds, but are less stable.
- A daggerboard that forms a V-shape in the water will help keep the hull at a constant height, but increases drag.
Foiling. The new fast.