The America’s Cup never fails to impress. The very fact that the boats can sail three or even four times faster than the wind that’s powering them is enough to stop spectators in their tracks. And new technology helps racing yachts sail faster than wind that propels them.
Don’t be surprised if you watch the America’s Cup and see a recorded wind speed of 12-15 knots while the boats reach more than 52 knots. That is equivalent to 60mph and still nowhere near maximum speed.
It might seem like magic but the state-of-the-art 50ft long, 80ft tall America’s Cup Class (ACC) boats are able to harness the wind with devastating efficiency.
Land Rover BAR’s America’s Cup boat’s secret weapon is a giant 23-metre wing ‒ as long as a single wing on a Boeing 737. And it works exactly the same way, by generating lift, only in this case the “lift” is horizontal.
That lift would push the boat right over if it were not for the ACC boat’s complex daggerboards. Just like the removable keel in a basic dinghy, the two daggerboards of an ACC have to stop the boat falling over so that lift from the wing pushes it forwards instead.
They play another important role, too. These gracefully curved daggerboards create an additional, central wing and also generate lift: enough, despite their modest size, to lift the three tonnes of ACC and crew clear of the water.
Nicknamed “foils” or “boards” and occasionally “dangerfoils”, they are the defining technology of the 35th America’s Cup, having first appeared four years ago in the 34th in San Francisco harbor. Daggerboards are now appearing on smaller, one-man sports boats and even on the giant 60-metre boats that contest the Vendée Globe around-the-world race. In Paris last month, a prototype for a foiling water taxi made its debut.
Off a boat and out of context, foils are abstractly beautiful; expensive too, costing around £500,000 for a pair and taking three months each to make in many layers of directional carbon fibre. They are a complex shape on ACC boats, like two exaggerated, inverted question marks. This is to ensure that when they’re dropped into the water ‒ in the America’s Cup, outside a strict 10-second window during manoeuvres, only one foil is allowed in the water at any time ‒ their lift is as far away from the centre of the boat as possible to keep it stable.
Perfecting the shape and construction of the foils and the wing (also made from carbon fibre, with thin plastic skin stretched tight over a series of ribs) is key to success in the America’s Cup.
Using tools sourced from the aeronautical and competition automotive sectors such as computational fluid dynamics (CFD) and finite element analysis (FEA) ‒ analytical software that examines the flow of air and liquids around a solid object.
So, while it might seem like magic when a boat sails faster than the wind, it is all down to physics and good, solid mathematics.