In recent weeks we have seen the first AC75s start to fly, but the engineering and sailing techniques needed to get these new monohulls to foil are very different to the catamarans of the previous Cup cycles

America’s Cup teams first flew in San Francisco in 2013. The use of hydrofoils to lift the AC72 catamarans out of the water was a Team New Zealand innovation, despite a class rule that was written to keep the boats in the water. This time around the AC75 rule definitely intends us to fly, and the foils will be a focus for the team right from the launch.

“The hydrofoils on a Cup boat work exactly as on an aircraft wing: you have the asymmetric flow around the wing or foil, which creates a pressure difference. That then generates a force that’s trying to equalise the difference. The force lifts the plane off the ground – or the boat out of the water, in our case,” explained Ben Ainslie.

The central innovation in those early days was shaping the daggerboard into a foil that could provide both lift and leeway resistance. The problem was how to control the lift sufficiently well to enable stable flight. The AC72 design rules forbid any part of the foil to move independently, so the Kiwis couldn’t use flaps to control lift. These are the adjustable trailing edges, just as a pilot uses to control the lift generated by an aircraft wing.

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Trailing edge

The design rule for the AC50s in Bermuda also forbade the use of flaps, but this time around, they’re allowed. This is one of the biggest differences with the AC75 rule; it has ushered in a new era in foiling Cup-boats by specifically allowing flaps on the T-shaped foils.

“We will have direct control of the flap on the trailing edge of the T-foil,” explained Ainslie. “Moving the flap down will create more lift and more drag, but the lift is good for creating take-off. Moving the flap up to come more in line with the rest of the foil will reduce the lift and drag, which is then good for higher speed. It’s finding that trade-off really.”

The flap is a much more efficient way to control lift than moving the whole foil, and it’s no surprise that it’s been allowed for the AC75. Now we’re back to racing monohulls, which have a lot less stability at rest and slow speeds when the foils are not supplying any righting moment. The control systems will need to be more efficient to fly these boats compared to the AC50.

“It’s [the AC75] got some form stability from the hull,” said Ainslie, “but after that it’s reliant on the lift from the wings and the foil arm to give the boat the righting moment. We think – we hope – we’ve got a good idea of how this is going to play out and what sort of speed we need to create righting moment and ultimately to take-off.

“Without a lead keel you’ve got to be very, very careful about how you power up the boat, how you create the speed, what angles you sail to create the speed. The simulator has been really critical to developing the techniques, both for the designers and for the sailors,” explains Ainslie.

“What sorts of forces do we need to get the boat flying in the first place? What techniques do we employ? What settings do we employ? We can then incorporate that [information] into our test programme and flight programme for getting RB1, as we call the first AC75, out on the water and sailing and foiling.

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“We’ve seen through simulation and the test boat that when you start off you’re very gingerly trimming in the sails trying to get the right heel on the boat to create speed. Then as the righting moment kicks in you’ve got the power, you trim on, you lift out of the water and then you’re up to speed and the boat is more locked in.

“In that sense it will be quite a neat challenge for the sailors out on the water. Very similar to the high-performance dinghies you see. I think our sailors are very well prepared for that. Obviously, time on the water will be critical, as will time in the simulator, so that we get as much time together as a group and really understand how we get maximum performance from these boats.”

About the author

Ben Ainslie is the most successful Olympic sailor of all time, and Team Principal of the British America’s Cup challenger. INEOS Team UK will be challenging for the 36th America’s Cup in New Zealand in 2021. Each month he’ll be talking to Mark Chisnell about the innovations and technology behind the new AC75 foiling monohulls.