L'AC75 d'Ineos Team UK en entraînement à Auckland

What are the differences between the new AC75s ?

On Thursday 19th November Emirates Team New Zealand were the last team to launch their second AC75, the boat on which the Kiwis will defend the 36th America’s Cup over the period March 6 to 21, 2021. Three weeks before the start of the America’s Cup World Series in Auckland, Tip & Shaft interviewed Martin Fischer, co-design coordinator at Luna Rossa Challenge, and Benjamin Muyl, chief engineer for the Ineos Team UK challengers, to understand design differences between the new, second boats of the four teams.

Did you have enough time to learn much from the first AC75s to carry forwards to the design of the second boats which were launched about a year later?
Martin Fischer: Between the launch of the first boat and the freezing of the design of the second, we actually had have very little time. So we saw some things, but we didn’t really get a lot of experience out of it. On the other hand we learned a lot about the build, the experience of the first boat allowed us to see where we could gain or add weight. In terms of the shape of the hull, we stayed quite close, because we were convinced this is the way to go. And I think that we were right, because the Americans and the English have followed.
Benjamin Muyl: Our second boat is very different from the first so obviously our choices were not the same. What we can say is that after seeing all the first boats, with on one side the challenger of record and the defender, and on the other American Magic and we have revised our ideas, at least in terms of the hull adding what is being called a bustle under the hull.

These were already seen on the first AC75s of Luna Rossa and of Team New Zealand, what is the bustle for?
M.F.: First of all, it is used for take-off itself: we realized that when we started to take off quite quickly at a certain boatspeed, say 10-12 knots, because the foil is already lifting but after that its was getting difficult. We therefore designed this catamaran type hull so that once the boat has reached this fairly critical speed, it compensates for the deficit between the lift of the foil and the weight of the boat. The aim is to have the behaviour of a multihull, that is to say to accelerate more easily. And it is important for touchdown phases (see below).

What differences do you see between the hulls shapes of the different AC75s?
B.M: We can see that there are two families: Luna Rossa, Team New Zealand and us on the one hand, who went narrow and American Magic on the other hand which has a very V-shaped hull at the front with a lot of volume. Now they have removed some from the back to be able, on take-off, to turn the hull with the bow in the air by putting angle in their foils. We, on the other hand, we take off almost flat, a little down on the nose itself, to clear the back of the hull.
M.F: There are also differences in the volume of the bustle. The Americans seem to have very little volume, the English some more, while we and New Zealanders are somewhere in the middle. There where Team New Zealand is a little different, is that, much like the Brits, they completely separated this ‘thin hull’ from the rest of the boat, we really see a break between the two, as if it has been stuck underneath. We did it in a less extreme way. They have also tried to increase the freeboard of theirs so that it really needs to sink deep into the water before the rest of the boat really touches the water, especially at the front.

The lower you fly, the quicker you are”
What are the advantages and drawbacks of having a greater or smaller volume in the bustle?
 If you have a greater volume, it takes less lift from the foil to take off, while with less volume, the acceleration phase at low speed is a bit harder. But the advantage with less volume is that when you touch the water in a touchdown, you are close to the extra wetted surface, so you can afford to touch down more and take more risks flying lower. The lower you fly, the quicker you are. Ideally, the hull is practically touching the water.
B.M.: The other things at stake with the bustle are linked to aerodynamicsThe aim is to close the hull under the boat to act like a plate and limit the air circulation. We did it with a square shape, very vertical sides offering more volume than the others. Luna Rossa adopted a much rounder shape. The choices depend on how you see taking off and touching down. It’s not completely objective, and you need to imagine things.
M.F.: It is all a matter of compromise: you work out how much a gap of one centimetre more between the hull and the water costs and then the opposite, how much one centimetre less will increase the likelihood of touching a wave. Based on that, you decide the measurements of the bustle.The plate effect seems to have played a role too in the choice concerning the deck?
M.F.: Yes and here too, we can see differences: the Brits and Americans have a boom that you can see well above the deck, while our mainsail goes right down to the deck [like the Kiwis, editor’s note]. There is a structure underneath to take thee strain. It’s no longer really a boom. I can’t give you all the details about what it is exactly. It means we can create a hollow shape at the bottom of the mainsail and increase the plate effect. I think we have an advantage with that, because with a boom, you can’t develop that hollow effect at the bottom.
“The quality of manoeuvres will be very important”
Let’s talk about the foils: are there families of foils?
If you are talking about the foils being used at the moment, we have gone for a version without a bulb with a large surface. Luna Rossa has chosen a smaller surface but it is further forward with a small bulb. American Magic has a bigger bulb and a smaller surface, while Team New Zealand, after sailing for a long time with the largest foils in the fleet, now has small foils with bulbs. These are design choices. The foils must have a certain mass. If you have a smaller mass in the wings, you have to place it somewhere else to reach the minimum weight imposed by the rules. So you place it on the bottom and that has the shape of a bulb.
M.F.: The size of the foils is chiefly determined by the speed at which you want to take off. If you want to take off at a very low speed, you need a bigger foil, but once you have taken off, this means extra wetted surface, so that is a negative, but you do have a higher safety margin when carrying out manoeuvres. With a small foil, your wetted surface is reduced, but if you fail during a manoeuvre, you are likely to begin another take-off, which can cost you dear. For example, sailing upwind, it is impossible to take off. You have to swing around almost 90 degrees and once you have taken off, you luff up, while downwind it is the opposite. So that costs you a lot in terms of the distance lost during a poor manoeuvre. To finish this discussion about the differences between the foils, we can add that Team New Zealand is the only team to have gone for a perpendicular T-shape. There is no dihedral angle [the foil points a little bit downwards, editor’s note] at the bottom like the others. We have to stress that the teams have another two to work on with when looking at the foils. They can build six in all and the latest ones are currently being built.What about the rudder?
B.M.: Here too there is a lot at stake. The rules define the maximum size, but we have all gone down the road of a smaller rudder to reduce drag. But if you are looking at a small rudder, that means an increased risk in gybes for example, when you are flying fairly high with the rudder dropping out. You are heading for a crash and are likely to lose the race. So the best choice is for a small rudder but not go too far down that road. It is the same for the foils on the rudders. The bigger they are, the more drag there is. But when changing tack, they are under strain, so they cannot be too small as they can cause cavitation and you mess up your change of tack.
M.F.: This is a very tricky decision for all the teams. I think the differences are around 20 to 30 centimetres in length with the rudders for each team. The Americans have the longest, the Kiwis the shortest, while we along with the Brits are in the middle. But here too, as we can make four rudders, we haven’t yet seen everything. Each team has only shown two for the moment.To finish, have you drawn any conclusions with three weeks to go to the America’s Cup World Series about the performance of the others?
B.M.: It’s very difficult, as we are all in different development phases. We have a new foil, for example, a new mainsail system with a boom which we didn’t have before, so we have a lot of new things.
M.F.: It’s very hard to say, because we can’t line up with the others. The only thing we have seen is the quality of manoeuvres and I would say for the moment that we have an advantage. Our manoeuvres are carried out perfectly. That is very important, as if you are clear about your manoeuvres, you can afford to change tack for tactical reasons, while if you find it hard, you avoid that as much as you can. The ACWS will be very important as will the few days of training before the event. I think everyone will be doing their utmost to find out where they are, because if you have a deficit somewhere, you need to know as soon as possible to try to correct that before the Prada Cup.

Photo : C. Gregory / Ineos Team UK

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