One of the biggest innovations for the new AC75 America’s Cup boat is the soft wingsail: a double-sided mainsail where each of the two ‘skins’ are attached to the back edges of the D-section wing mast. Sir Ben Ainslie explains all

It’s not a completely original idea – it has been tried in the past, but never with the resources that four big, well-funded America’s Cup teams can bring to the development. So if it’s ever going to work, it will be now.

The AC75’s 26.5m mast is stepped on a ball so that it rotates, and it has a single set of spreaders and running backstays. It’s a one-design section with the mast surface defined by the rule, along with a minimum laminate specification; so teams can further stiffen or strengthen sections if they wish.

The rigging is supplied and there are also rules defining the mast fittings, spreaders and so on. The boat is allowed up to six full-length battens and another six battens shorter than 1m that must all finish at the leech. The rules say they can’t be inflatable or hinged.

That is what is controlled. But there are other areas where designers have been given a lot of scope, with the goal of making this new soft wingsail as effective as possible. This is important because it has to generate the speed to lift a 7.5 tonne monohull out of the water.

The more efficient hard wing used on the AC50 multihull had to elevate a boat that was five tonnes lighter, shorter and with less wetted surface – albeit with a completely different and less efficient foil system.

“The class rule identifies two areas, one at the top and one at the bottom of the mast, called the upper mast and lower mast zones,” Ben Ainslie explained. “And in these areas the teams are allowed to develop whatever systems they think are going to control the sail the best.”

The laws of physics, practicality and the cost of the engineering are the limiting factors here, not the rules. So what are the INEOS team trying to achieve in the lower zone?

“The intent in the rules is to allow the teams to have the mainsail sweeping across the deck to create an effective end plate,” explained Ainslie. “And that has massive aerodynamic advantages, so the designers are really pushing for it.”

The end plate effect is a well-established fluid dynamics phenomenon, where the aero- or hydrofoil ends in a perpendicular surface. This reduces the loss of the pressure difference between the windward and leeward sides of the foil, and reduces the formation of a tip vortex. Both improve the efficiency.

The other area is the upper control zone – the top four metres of the sail, so it’s a big sail area. “We saw in the last Cup, particularly with the Kiwis, that sail twist was becoming a really effective power control. And so opening up the top of the rig here in this class of boat allows control of that twist,” said Ainslie.

“Then it’s really a balance of twist control over traveller control, again having that openness in the rule so you can control both effectively. It’s down to the teams to decide which one they think is more effective to give you control of the boat… and the greatest straight-line performance.”

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Headsail cross-overs

Forward of the mast the AC75 rule also allows both a jib and a Code Zero, and there has already been plenty of discussion about if and when we will see both sails used.

It’s possible to imagine a situation where the Code Zero is the right sail to accelerate the boat to take-off speed, but once the boat is foiling the apparent wind speed and angle will change dramatically, and the headsail might be the right sail. Is it possible to change sails while foiling, and even if it is possible, is it fast?

“How do you handle that sail, how do you hoist it, drop it, trim it? All of that obviously is going to take power,” said Ainslie. “And once you’re going through that hoisting and dropping process, you’re going to be losing power in terms of guys on the handles trimming the sails… they’re all interesting trade-offs that the teams have got to go through and work out.

“We spend a lot of time monitoring and observing the other teams to learn from them as much as we’re developing from our own work. That’s part of the game really. And frankly, it’ll be the team that develops the fastest that will probably come out on top.”

Sir-Ben-Ainslie-Team-Principal-and-Skipper-headshot-bwAbout 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.

First published in the December 2019 edition of Yachting World.