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America's Cup - Dan Bernasconi explains the foiling AC75 - Part 1

by Richard Gladwell on 22 Nov 2017
America’’s Cup AC75 - the foils and rig are well aft Emirates Team New Zealand http://www.etnzblog.com
Despite admitting that the foiling AC75 monohull is a first for the sailing world, Emirates Team New Zealand design team leader, Dan Bernasconi, is confident that the concept will work in practice.

The style of the foiling AC75 has never been seen before in sailing. The closest are the IMOCA60’s, but they are only using the DSS style foils to reduce hull drag and increase speed.

Under the AC75 concept, the whole hull will lift clear of the water, supported by two or three of the foils.

'We have done a lot of simulation work on it. We are very confident as to how the boat is going to behave. But of course, until we actually see it on the water, we won't understand it completely.”

In fact, Dan Bernasconi has such confidence in the simulation software that he doesn't think there is any point in testing in a prototype of the AC75.

'The Protocol allows test boats up to 12 metres long. So teams have the option of whether to build a test boat small scale. I think some teams will do that. But in terms of timing, it takes almost as long to design a 12-metre boat as it does to design a 75ft boat.

“It is not something that you could go through the full cycle of designing building sailing a small boat and then start work on a bigger boat. I think that reduces the benefits you would get from a test boat. However, it may be something that we or other teams consider.

The AC75 is coincidentally almost the same length as the International America’s Cup Class which was the class of choice for five America’s Cups between 1992 - 2007.


But instead of being a heavy displacement specialist match racing yacht which could be beaten around the track by most 50fters, the 2021 America’s Cup class will be one of the fastest yachts on the Planet.

'We are really excited about it. We've worked on dozens of concepts across the scale. We are really pleased that we were able to get enough confidence with this one which is at the real high-performance end of the scale. We were keen to push this concept forward, both with our team and the guys at Luna Rossa. In the end, it was an easy decision.'

Same speed or faster than AC50
After the 2010 America’s Cup when San Francisco’s Golden Gate Yacht Club announced that the 2013 Match would be contested in 72ft wingsailed catamarans, many questioned the direction being taken by the sport’s premier event.

With the change to a smaller wingsailed catamaran designed for foiling, the AC50 became the bench mark against which any new America’s Cup Class will be measured.

'In 20kts of breeze I imagine the AC75 will be close to the speed of an AC50,” Bernasconi told Sail-World from England where he is talking design with potential America’s Cup teams.

“The AC75 is a bigger boat, with a bigger rig and more righting moment. With the AC75, the righting moment stays constant as you go through the windspeed range - so in the mid-windspeed range, once you are foiling in 9-15kts of breeze there is definitely the potential to be quicker than an AC50. And at the top end of the windspeed, the two will come back into line and could be quite similar.

“On the other hand, we don't have rudder differential. The AC50's had the windward elevator pulling down, which at the top end of the windspeed range continued to increase righting moment”, he adds.


Righting moment and lift
The visually diverting aspect of the AC75 is the foil each side which will serve alternately as a hydrofoil generating lift and as a lever producing righting moment.

Foils aside the AC75 is a relatively normal monohull, with a conventional hull, a rig that is not dissimilar to other high-performance yachts and will be sailed with a crew of 12.

There is a sharp turn away from the direction of the AC50. Cyclors have been replaced with grinders, the use of hydraulics has been reduced, and the only concession to stored power is in the mechanism to raise or lower the side foils.

Having just begun to understand how daggerboard style foils worked, America’s Cup fans now have to grasp a whole new set of foiling concepts to understand what will make an AC75 tick.

'The leeward foil produces lift”, Bernasconi explains. “We expect that it will have trim tabs on it rather than changing the rake angle of the whole foil as we have done on the AC50. Those trim tabs would be controlled by one of the sailors to control the ride height.

“You still need lift control on the rudder, like in an AC50 or Moth. You still need control on both the main lifting surface and the rudder. Whether we rake the whole rudder as we did on the AC50 or whether we control trim tabs on the rudder which is fixed in rake is still something we are considering. We are looking at both options for that.”


Another notable innovation is the dual use of foils function as a canting keel on one tack and as a hydrofoil on the other. Bernasconi explains that it is still a work in progress.

'We are still working on all the details but expect that the weight will be between 1-1.5tonnes for the combined weight of the foil and any ballast, per side. It could be that most of the weight just comes from the horizontal itself. If you make the horizontal from steel, maybe with some lead, they will be in the region of 1-1.5tonnes per side.”

'We imagine that we will have batteries powering a hydraulic pump to change the canting of the foils,' he adds.

Conventional high performance rig
Traditionalists will be pleased to see a return to a more conventional style of rig, using soft sails. After an absence of four years, sail designers will be making a return to America’s Cup teams and will have a level of involvement last seen a decade ago.

'We are still working on the rig, so that is probably the least defined area of the boat at the moment, which is why we haven't said too much about it.

“We have decided that we don't want to have a wing that has to be craned in an out because we want the boat to connect more with sailors outside of the America's Cup. Although the wing is really efficient and gives you a lot of control, it is not something that is ever going to be the norm on club racing boats.

“We have been looking at ways of getting that same efficiency and controllability but with a rig that can be used without a crane - something that is hoistable.

“Some sort of wing is possible, but it is just as likely that we will stay with a conventional rig. We are looking at a few options, and by the time the class rule is announced at the end of March, we will have an answer on that.”

The return to soft sails will allow the AC75 to be towed to and from the race course, unlike the AC50’s which had to be manoeuvred using a technique known as “side-slipping”, when the cat had its tender strapped alongside bow to stern. That made for a long trip home after a mishap and required the boats to be tacked to get back into their berth inside the Royal Dockyard – a very tricky and slow process in some wind directions.

Code Zero adds grunt
Performance of any America’s Cup class at the bottom end of the wind range is vital for television coverage. The AC50’s ability to sail at four times windspeed in just 6kts of wind usually meant racing could commence on schedule.

The AC75 is not expected to perform at quite that level, but its performance should not be too shabby - particularly with the addition of a large Code Zero headsail to provide some extra sail area and grunt.


'I don't think they will be foiling in 6kts,” Bernasconi explains. “With ballast, they are carrying the AC75 is a lot heavier than an AC50, so the windspeed at which they will be foiling is higher than an AC50. But I still think these boats will be foiling in 9kts of breeze - and once you are foiling, you'll be foil tacking and foil gybing. It will certainly be a challenging boat to sail in that marginal condition between foiling and non-foiling.”

'Certainly when the boat is not foiling there will be a strong drive to use the Code Zero. In those marginal foiling conditions using the Code Zero in order to get up on the foils, I think, might become part of the game. If you can furl the Code Zero once you are up and foiling, that may be something we see. Once a boat gets to the middle of the wind range, 12kts and onwards, the Code Zero use will decrease. But it is early days, and we will have to see how the boats develop.

Bernasconi doesn’t yet have an answer to whether the Code Zero will be able to be deployed sailing to windward.

'In really light conditions that is a possibility. It is something that we are looking at, and I am sure it is something teams will learn as we gain more familiarity with the boat.

Ability to foil in waves
With a minimal ballast package and a mast height of around 90ft the AC75 would be expected to have some stability issues when floating upright, or alongside a dock

'When the boat is floating upright, the righting moment is quite low, due to the small keel weight for that size of boat (2-3 tonnes of keel weight)”, Bernasconi says. “These will be very overpowered boats until they start foiling. When you are foiling the righting moment is significantly more than an AC50.”


Without doubt, Auckland will turn on its beautiful sea breeze at some stage of the Prada Cup and America’s Cup. The 18-20kt breeze accompanied by a 1.5-metre chop a small swell, and an adverse tide, was one factor in the demise of the AC50, once Emirates Team New Zealand had won the America’s Cup.

“For sure these are going to be challenging boats to sail, but we are confident that they will be able to handle that kind of sea state that you can get in Auckland on a 20kt NE day. Obviously, the hull shape becomes very relevant and how you control the ride height. That was a criteria in the design brief, as conditions are different that Bermuda and we needed to design a boat that could cope with those.”

'You would still want to be foiling in waves, as getting the hull out of the water is easier than going through the waves. There is the possibility of lowering the windward foil into the water, rather than the normal mode which is to raise the windward foil out of the water for righting moment and reducing drag.

“I think it is something we will learn when we start sailing the boats - whether using the windward foil to improve the overall stability of the boat in difficult conditions, is something that we would want to do.'


In Auckland, there was every chance that the Kiwis pitchpole on the second day of racing in the Challenger Semi-Finals would be repeated. The Hauraki Gulf is quite a different prospect from the flat water of Bermuda’s Great Sound, and the albeit foiling monohull offers a more seaworthy option.

The foils and the rig are well aft, and the hull has substantially more volume than an AC50, and it is expected that the new AC75 will be a lot safer in that regard.

The design team expects a similar reaction from the AC75 as if it hits a wave full on in a conventional non-foiling monohull. There will be some bow-bury, but the boat should pop out intact – even if suddenly slowed.

Part 2 to follow on Friday.

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