Best guess

ORMA 60 designer Marc Lombard gets specific about what the new America's Cup multis will probably be like

This article continues on from part 1 published yesterday

The power versus beam issue is a fascinating one for multihulls. In theory you could go for maximum sail area, counteracted by maximum beam far larger than the length of the boat. There are spurious rumours for example of BMW Oracle building a 64m or a 58m tall spar. However Marc Lombard doesn’t reckon this is the winning option. “Probably we would come up with a relatively moderate beam because we want to fly the main hull as soon as possible.” While the massive sail area could get your maxi-multihull powered up in lower wind speeds, the moment the wind exceeds this, the extra sail power becomes redundant, extra drag, its extra weight actually slowing the boat down.

Lombard reckons (and bear in mind this is without him carrying out any of his own R&D) that the beam range to investigate would probably be around 70-80ft. “[Going too wide] was a mistake we made on the ORMA multihulls, because it was giving us good answers in the VPP models. In fact the problem is that you need to fly a hull quite quickly if you want to achieve good pointing ability and so on.” He points out that this was one of the secrets to Groupama 2’s success inshore - a narrower beam allowing her to fly her centre hull in lower wind speed.

On 31.5m long Groupama 3 the mast height is 38.4m or 121% of the LOA, Lombard can imagine it will be taller on the AC multis, but not 200% as the rumours over the new BMW Oracle boat suggest. “I would test probably up to 160% of the length of the hull, but I am not sure I would choose it. The limit is the longitudinal righting moment. You can make the boat wider and wider and you have more and more drive forward, but if you increase that drive forward up to more than a reasonable amount you will capsize longitudinally. In any case you cannot image a boat that is as twice as wide as it is long because it would not use the extra stability to push the boat forward, even with foils, because with those are also limits. So this means clearly that a configuration where the mast height is twice the LOA would only be of interest if you can carry that sail. That is a real limit.”

Obviously it would need researching, but Lombard reckons, based on experience, that the optimum upwind sail area might be 500-600sqm upwind ( Groupama 3 carries around 557sqm upwind) and “probably not very much more, because then you’d end up with such a heavy mast that you would decrease the longitudinal stability and the boat would go slower.” So a similar sail plan to Groupama 3 only on a boat 15ft shorter and perhaps a third lighter.

Given that in a Deed of Gift match there are no limitations on crew numbers or their weight, and that manoeuvring the new boats efficiently is more important than it was on the ORMA multis, Lombard reckons that the boats could end up being sailed by a small army. This would allow the crew to carry out efficiently the huge number of tasks on board during manoeuvres (not only moving the sails from side to side, but dealing with the foils, mast rotation, mast cant, rake, daggerboard trim tab, etc). The large crew would also contribute to the stability – bear in mind that 12 burly Cup sailors weigh about a tonne so 24 would represent something like 15-20% of the boat’s displacement. “That is another reason not to go for an extra wide multihull,” warns Lombard.

Manoeuvrability is also another reason, in Lombard’s view, for going to a trimaran as opposed to a catamaran. On two hulls are you much more likely to get caught irons, especially if racing is taking place in light wind. Even if a D35-style cat was chosen with a central fore and aft beam out of the water, the trimaran with its main hull in the water would still offer better stiffness along this plane to withstand the rig and forestay loads.

While it is certain the sail plan versus length ratio will be substantially higher on the AC multihulls than it would be for an offshore equivalent, the biggest variation is likely to be in displacement. As the boats are not designed to go offshore, and therefore won’t have to withstand repeated massive wave impacts, there is the possibility of removing a substantial amount of structure. However given that the boats will be sailing upwind at 20 knots and will be capable of exceeding 40 knots downwind (the opinion of both Lombard and Alain Gautier) the structure will still need to be substantial. There is also the consideration of the Deed of Gift course which includes a 20 mile beat, that is likely to take the boats some way offshore where there is a higher probability of larger waves.

“You still need to be able to carry the sailing loads and that is very important,” says Lombard. “So there is a limit and I believe it would be difficult for them to design something that is under 12 tonnes. I am not sure of the exact figure but something around 11-13 tonnes, depending upon the beam. The more beam they have the heavier it will be because you place more righting moment on the cross arm. So the cross beams won’t be that much different to the offshore boats because the righting moment you can carry if you dig into the water downwind is enormous and that can happen in flat water too. So therefore the risk of being very very light on the structure might not be the key if you want to finish! So there will be a limit to how light you can go.”

While the structure will be lighter, potentially the volume of the hulls could also be smaller, further pairing down weight. But Lombard is not convinced about this, at least with the floats. “When you fly a hull upwind and you have the entire weight of the boat on the float, so you need a certain volume. If you have 100% [enough buoyancy to carry the weight of the boat] in the float you cannot achieve good drag figures. So I don’t think you can go under 160%. I wouldn’t be very interested in trying very small volumes and semi-submersible floats because of structural problems. And you have to keep a boat that is fairly rigid, that doesn’t transfer the energy of the sails into bending. So they will be probably come up with floats that are 160-250% - which will make for very fine floats anyway because the boat will be light.”

Solid wing?

Looking back to the 1988 Cup and both camps are likely to have at least considered solid wing sails, however this time around the game is very different. The challenger has announced its intention to race a 90ft long multihull, not a giant monohull. In 1988 the Stars & Stripes catamarans were 66ft long, and according Dirk Kramer this was long enough to beat the big monohull but small enough so that they could build a C-Class cat style solid wing rig at a manageable size. “Otherwise the rig would have been an enormous challenge. It was plenty of a challenge even then.” Significantly Conner also chose to build two platforms enabling him to test the solid C-Class cat style wing they ended up using, versus the conventionally rigged boat. With the latest Cup there has been no indication that either camp plans to build two boats to carry out this type of testing. The teams will be hard pressed to get one boat fully functional.

Lombard agrees: “The solid wing is a very interesting answer. We have designed in the past C- Class cats and it is very efficient. The problem is development time and difficulties because if you capsize once it is finished and you lose the America’s Cup. So that is something you have to consider before building a boat, knowing that time is short.” However he adds that if, as Brad Butterworth has threatened, the 34th and future America’s Cups might be held in 90ft long multihulls then with time this avenue of development would certainly be worth pursuing.

Fully foiling?

And with time - what about a fully foiling boat, a match racing L’Hydroptere? Despite being pro-foils, Lombard reckons this is a non-starter. “ L’Hydroptere is not a very good boat upwind or downwind and most of the AC will be raced upwind or downwind so therefore there is no point in doing a pure foil configuration. The idea of L’Hydroptere is very good for record or offshore speed trials and things like that.”

Where foilers become a lot more difficult to design is when they must operate in a wide variety of wind strengths. When there is just 5 knots of wind for example - what happens when you can’t retract the foils? You don’t move.

“Even with the ORMA 60s, when you are doing 15 or 16 knots, foils are not efficient - floats are more efficient. But once you are doing 30 knots then they are very efficient. But if you pass 45 knots, then you have other problems with cavitation and so on...”

Windward-leeward

Other avenues of development will certainly be attempting to improve the upwind and downwind ability of the new maxi-multis. Thus we may see the new boats fitted with longer daggerboards with more efficient trim-tab arrangements. It is possible that teams could end up fitting retractible boards in the floats that are optimised for upwind work. Lombard points out that an additional effect of using his curved foils in the floats is that they are a very good way of moving the centre of lateral resistance. “That is one of the most important keys to the foils, because when you are downwind and reaching, the aerodynamic force is more forward, therefore if you don’t use foils the boat tends to bear away all the time, so the best way to balance the boat is through the foils. That is what we also discovered in the ORMA class.”

Then there is the tacking angle of the sails. On the ORMA 60s Lombard says it is 5- 6 degrees, but given the high upwind speed and smaller apparent wind angle off the bow, it will be tighter still on the new AC boats.


Expect to see many windage reducing devices on the new multis

Windage will certainly a major consideration too on the new maxi-multis given their high speeds. Even in single figure wind speeds, the boats will be potentially sailing at three times wind speed for example. Lombard reckons that the new boats will be fully powered up and flying their main hull in just 7-9 knots. So it is entirely possible that we will see more work done on the fairings for the cross beams and fairings to streamline the cockpit and crew areas.

“Probably the key will be to have the abiltity to create your own wind very fast - that is the reason they will probably try to keep the boats light and probably there will be more difference in light air conditions than there will be in rough conditions, because in rough conditions manoeuvring will be the key to winning, because the boats are very physicial to handle and if you are reaching and downind under genniker, doing 30 knots, the legs will be very short,” says Lombard.

So in summary it is likely that the boats the teams produce will be similar to Groupama 3 but on a diet, with less displacement, the biggest variations being in sail area and beam, depending upon whatever the team has found works best in the expected wind strengths. While the CFD boys will be have reams of evidence urging the teams to go taller and wider, we suspect the Iren/Cabarets and van Peteghem/Lauriot Prevosts will be the voice of reason, urging them to backpedal.