There is no doubt that the AC75 is a remarkable boat; a monohull designed to fly, engineered to reach speeds that were inconceivable to sailors less than half a generation ago.

And not just in a straight line, but around a tight match racing course off the beaches of Auckland Harbour. It’s an revolutionary idea that’s about to move from the virtual world to become very real.

In this new series of America’s Cup podcasts, Sir Ben Ainslie, the skipper and team principal of British challenger INEOS Team UK, will be talking to Mark Chisnell exclusively about the technology and engineering that goes into the new Cup class yacht, and how one extraordinary idea – a T-foiling monohull – became a new class of extraordinary boats.

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It all started after the 35th America’s Cup, when the winner and the new Defender, Emirates Team New Zealand, began to work on the new class in conjunction with their chosen Challenger of Record, Luna Rossa.

The Italians wanted to return to a traditional monohull design and had extracted an earlier commitment from Team New Zealand that the next Cup would be in single-hulled boats.

The Kiwis understandably wanted to stick with the foiling technologies in which they had just proved to be the masters. The result was a convergence of foiling with a 75ft monohull to bring us the AC75.

“The Italian team wanted to have a link to traditional America’s Cup monohulls and the Kiwis married that with this new foiling generation of yacht design,” commented Ben Ainslie.” I think it’s a really neat concept. Certainly, sailing around in our T5 test boat has been a lot of fun. The boat handles really well, it’s very smooth. Upscaling to 75ft it’ll be fascinating to see how the boat performs.”

‘T5’ is INEOS Team UK’s 28ft test boat, based on the Quant 28 and launched in June last year, not long after the rule was published at the end of March. The rule, Ainslie says, “has been a work in progress ever since then with revisions and interpretations as it develops, people [realise they] missed things, or come up with different ideas and concepts on how it can get implemented.

“We’ve ended up with the rule as it currently stands, which is a monohull, it’s 20.7m long, with the bowsprit you get 22.76m or 75ft. It’s 5m in beam and weighs just over 7.5 tonnes with 11 crew.”

Apart from the obvious change to a monohull, there are other significant differences between the old and new Cup boats. One of the most important in performance terms is the change from L-foils to T-foils. The L-foil was a single solid piece, so the whole foil had to be moved to change the angle of attack of the foiling section, to change the amount of lift generated and keep the boat flying flat and fast.

In contrast, the T-foil has controllable flaps on the trailing edge to change its shape and the amount of lift generated. The whole of the T-foil will also move as it is rotated in and out of the water from one tack to another.

All about control

“You’ll change the cant of the foil arm and that will influence your side force and your vertical lift,” said Ainslie. “But the actual T-foils will be what are giving you the stability in flight… that will come down to the design of the foil and also to the control systems we use to be able to create that stable flight.”

Other significant changes (which we’ll explore later in this podcast series) include allowing stored electrical energy to control the hydrofoils. The solid wing of the AC50 is also gone, with a new one-design D-section mast and a double-skinned mainsail (the sails must be adjusted by the crew using a traditional grinding pedestal).

“This is an interesting, innovative design solution whereby you get more power created from the double-skin mainsail. With that comes extra weight from the extra cloth, battens etc. The idea is that it’s more user friendly compared with a solid wing mainsail. How you create the right sail shapes from a double wing mainsail and how that fits in around the mast section will be key for the performance of all of the teams,” said Ainslie.

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Design challenges

The other big thing is a philosophical change in the openness of the rule. The AC50 was tightly constrained in many areas, leaving the design battle to be fought over foil shapes and control systems. The AC75 is much more open.

The exceptions are the mast, which is a one design section, and the rigging, which is supplied by the organiser. The foil-arm systems that lift the T-foil in and out of the water are also ‘supplied equipment’. After that, the rest of it is up to the teams.

“For the America’s Cup purists, I think that’s a fascinating challenge,” says Ainslie.“With my Team Principal hat on I’m looking at the costs and the budgets for the campaigns and it does make it incredibly expensive.

“There’s a lot of scope there with this class, as always with a new class of boat. I think there’ll be quite big differences between the boats – certainly in the early days as the first generation gets launched, sailed and ultimately raced,” concluded Ainslie.

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 Chisenell about the innovations and technology behind the new AC75 foiling monohulls.