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North Sails Performance 2023 - LEADERBOARD

America's Cup: Dalton, Ainslie and Bernasconi on the new AC75 Class Rule

by Richard Gladwell Sail-World NZ 19 Nov 2021 04:43 PST 20 November 2021
No running backstays fitted - Luna Rossa Prada Pirelli - December 2020 - Xmas Cup - Waitemata Harbour - Auckland - 36th America's Cup © Richard Gladwell /

Improved performance and cost reduction are the keynotes of the new class rule for the America's Cup class to be used for the 37th Match for the premier trophy in sailing.

Not that the third generation of the foiling monohull will ever be simple. However they will be less complex, lighter, with a little less righting moment, and will foil earlier, than those seen in Auckland in AC36 and the Prada Cup.

And the bikes might be back.

The rule changes, in themselves, are not expected to make the third generation of boats look significantly different from what has gone before.

Any change in appearance is more likely to be a result of changed design thinking than being forced by a change in the class rule. Expect to see the same design refinement that occurs when the America's Cup class remains unchanged for multiple America's Cup - as a design consensus forms as to the optimum shapes and design trade-offs. The end result is that the boats all start looking very similar.

Obviously, this time there will be changes in hull shape, as designers encourage their progeny to lift clear of the surface as quickly as possible. Once the boats are efficiently foiling, drag considerations come into play, along with achieving the best interface between rig and hull.

"Reducing crew numbers has reduced the all-up weight of the boat. We are saving about 1,000kg of weight, which coupled with the increase in wingspan will drastically improve the light air performance. After Auckland we have agreed that is important," explains INEOS Britannia's CEO Ben Ainslie.

Grant Dalton agrees: "increasing the span of wings gives a bit more depth, so you can take weight out of the boat for a similar righting moment."

"Having fewer sailors on the boat reduces the all-up weight, along with reducing the budget. However, there is a limit as to how many sailors you can take off the boat," Ainslie explains. "This was something we discussed with all the teams from last cycle - in what was quite a collaborative meeting not long after the last Cup. That gave us the feeling of the group on the shape of the new Rule, and that is what we have stuck with."

The former AC75 Class Rule had a sailing weight, with a crew of 7834kg, which has been reduced to 6940kg in the new Rule.

That's a reduction of 894kg, achieved by removal of running backstays and their associated gear, lighter foil wing weight, and removal of three crew members. Also gone are the winch systems for the jib - replaced with a hydraulically powered self-tacker. A couple of grinder positions and their gear will also go.

Italians vindicated

Luna Rossa will be entitled to enjoy a quiet smirk after seeing the new rules that eliminate running backstays.

The Italians sailed without running backstays for much of the Prada Cup and the America's Cup World Series. They used the tension on their mainsail leech to take the place of the redundant running backstays.

The contentious issue went back and forth, and was only decided by the America's Cup Arbitration Panel on the eve of the the 37th Match,

"With the reduction in boat righting moment, the rig doesn't need the backstays," explains Emirates Team New Zealand designer, Dan Bernasconi. "Not having them saves weight, mainly in the hydraulics required to operate them, windage and crew power - which fits with fewer crew. There will be a reduction in forestay tension but the positives outweigh this."

"Removing the backstays was driven by reducing weight, cost, power and complexity… but it will have the benefit of making dumping the main easier. The change to the hydraulics rules is driven by modernising the boats, and removing restrictions that we're forcing teams to find workarounds."

Cyclors vs Grinders

"The cyclors versus grinders is another interesting move," says Ben Ainslie. "We saw that in Bermuda with the Kiwis doing a great job, and we'd expect it to go a similar way in the next cycle, given what happened in Bermuda. It is going to be fascinating to see how that develops - both in training and if people are brought in from other sports."

"If we go down the cycling route, we are lucky that we are partnered with one of the worlds leading cycling teams, the Grenadiers. That will have an impact on the sailor selection and athlete training," Ainslie explains.

There is no certainty that cyclors will be aboard, just because they were used in Bermuda. The option to use either cyclors or grinders has been created in the new Rule by removing the stipulation from the old Rule which said: "power from the crew may only be supplied by turning handles on the force input devices in a circular motion with the hands, and the radius of rotation shall be no more than 350 mm."

In the AC50 all hydraulic power was produced by the six-person crew's three and a half cyclors. There was no electric power onboard except for instruments, media gear and the like. Both the AC50 and AC75 the foils is are raised and lowered by hydraulics, however on the AC50 the power for the hydraulics was generated by humans, using leg power (cyclors) or by grinders. On all AC75's the hydraulic pump (for canting) is powered by an electric motor, powered by batteries.

The significant advantage ETNZ had with the cyclists in Bermuda was that they calculated their cyclors could generate, store and replenish hydraulic pressure using one less accumulator than the three accumulators used by the teams with conventional grinding stations. That gave a weight saving, which while all boats have to measure within a weight band, means the weight saved can be spent in other areas, within the rules, to make the boat go faster.

"If we are going down the cycling route, we are lucky that we are partnered with the Grenadiers, one of the worlds leading cycling teams. That will have an impact on the sailor selection and athlete training," Ainslie explained. The Brits had four grinding positions either side in the Prada Cup - eight sets of pedestals which gives plenty of scope for weight reduction.

The hydraulics onboard the AC75 now allow more closed-loop control system, giving more precise control. The question for all the teams is how to get the power of 11 crew from the eight now allowed.

"Teams are free to create whatever software systems they like that relate hydraulic pressures to valve controls, but you can't for instance detect a heel change and then move the traveller," Bernasconi explains in response to a question on one of two examples quoted in the new class rule which permitted for a control system to respond automatically to a "gust that causes an increase in mainsail sheet load .. and allows "a control system to use this information to make an adjustment to the traveller target position.."

Boat renovation limited

New teams will be allowed to purchase an AC75 from one of the established teams, who will likely retain their 2021 raceboat and build a new AC75 for the next Cup, plus acquire at least one AC40 for training and development, as well as participating in the Youth and Women's regattas.

They will not be allowed to alter the hull shape of the first generation AC75, but changes will be permitted to make changes to bring them into line with the latest version of the AC75 class rule.

"You only get the chance to design and modify one hull shape in AC37," Bernasconi explains. "If you buy a V1 hull, you could declare that as your new raceboat's hull and modify it as much as you like before launching.

"But if you buy it for training with the intention of building your own boat to race, you cannot modify it at all," he adds.

However, some modifications are allowed to the deck of a Version 1 AC75.

"There is a 12.5% allowance for the deck change on the V1 boats to allow for those boats to take off the winch and the runners and patching within the rig to accommodate that," explains Grant Dalton.

"The jib systems will also change because of the new Rule. Obviously, the changes made on the V1 boat will then flow into your new boat. If you go to cycling, then that is another change that will be accommodated within the 12.5%. There is no hull surface change allowed on the old boats," he confirms.

"For the Version 1 AC75's, there is an AC36 legacy wing allowance, but these wings cannot be modified," Dan Bernasconi notes. "There is no concept of carrying forward any unused allowance from AC36 into AC37," he adds.

There is no change in the hull measurements from the Version 1 to Version 2 of the AC75 class rule, meaning that the basic platform of the generation 1 & 2 boats will co-relate to the generation 3 hull, and the crossover from test boat into race boat should be reasonably seamless.

However, with the change in overall weight, and increase in wing dimensions, there has been an effective increase in beam.

"The distance of the Foil Cant Axis (the pivot point of the foil arm) from centreline hasn't changed (to keep existing hulls relevant), and the beam of the hull hasn't changed," Bernasconi explains. "The only slight difference is that the foil wing box is deeper, so the centreline of the bottom of the box will be slightly further outboard on the leeward foil. And the foil box is wider, so the extreme beam of the leeward foil has increased."

More Batwing mains?

Another crew who are entitled to a quiet smirk of vindication are American Magic who pioneered the so-called batwing main - which was a heavy air sail, and narrower in the top part of the mainsail - to get rid of excess sail area.

In the new class rule the mainsail girths, cross measurements have changed, where at the top (head girth) the sail must measure 2000mm - 3400mm, under the previous Rule, the range was 2600mm-3600mm. So at the top, a heavy air mainsail it can be half a metre smaller tapering down to the bottom of the sail where its maximum measurement, on the foot is 7400mm for both editions of the class rule.

Emirates Team New Zealand developed a more radical version of the batwing main, but it was used in only one training session, and winds during the Cup were not strong enough to justify its use.

"In top-end wind speeds teams were pushing to reduce mainsail area up high – hence the American Magic batwing," Dan Bernasconi explains. "Again, with a lighter boat and lower righting moment, we wanted to reduce the minimum mainsail area."

The controversial foil arms which were delivered several months late after failing initial structural testing, will still be a supplied component.

"We haven't specified the foil arm supplier yet," says Bernasconi. "They will be the same or very similar design. The Foil Control System is supplied and will be updated in some areas from AC36. Teams will not be able to modify the FCS software," he adds.

Still a tippy boat

The stability rules have been tweaked for the new boat weight, and to remove the requirement to measure the vertical centre of gravity of the mast and mainsail – which proved very time consuming in measurement.

Dalton agrees the AC75 will be just as tippy as ever.

"We probably capsized the most because we were the most radically difficult boat to sail. And there was no change to that."

"It is easier just to rip in with the chase boat - and pull the AC75 upright in a few seconds. There is now slightly less ability for them to self-right. We have seen now that the boats are so safe that we are not so worried about that," he adds.

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