July 26, 2021

I, Science

The science magazine of Imperial College

How a sailing boat can move three times faster than the wind behind it!
SBTP- Americas Cup_1200w
America’s Cup victory: Oracle Team USA drop off their hydrofoils as they cross the finish line ahead of Team New Zealand

Last September British sailing legend, Sir Ben Ainslie, guided Oracle Team USA to America’s Cup victory in one of the most astonishing sporting comebacks of all time. Hosted in San Francisco, this 34th America’s Cup Series saw USA and New Zealand race head-to-head in state-of-the-art 72ft catamarans, battling to rack up a winning total of nine victories. Trailing the Kiwis 8-1, Oracle drafted in Ainslie as their tactician in a last-ditch effort to avoid defeat on home waters. In Ainslie’s words, Oracle had “a mountain to climb”.

Almost unrecognisable as sailing boats, the ‘AC72’ catamarans of this Cup Series pushed the limits of aerodynamic and hydrodynamic technology, reaching speeds of up to 55mph – nearly three times the wind speed that propelled them.

Sailing faster than the wind

The secret to this surprising feat lies in the AC72’s unique sails. In place of traditional fabric sails, the AC72 sports a gigantic rigid panel, similar in size and shape to the wing of a 747 jumbo jet. Not only does this catch a huge amount of wind, but its asymmetric profile also pulls the boat forwards through the water. This works in the same way that an aeroplane wing generates lift: wind passing over the rear of the sail moves faster than wind passing over the front, generating a pressure difference that sucks the boat forwards.

The speed produced also lifts the 7-tonne boat out of the water, onto its carbon fibre hydrofoils – surfboard-sized projections extending downwards from the hulls. Gliding on its foils, the boat flies over oncoming waves rather than forcing its way through them, reducing drag and increasing the boat speed by 10-15%.

And the magic ingredient? Dinghy-sailing champion, Ainslie, calling the shots. Seven days after he joined the crew, Oracle lifted sailing’s most prestigious prize. The final score: 9-8.

Image & words: Ellie Pinney