Opposite: Luna Rossa shows off her fair underbody in striking contrast with the very aggressive shape of Ineos Team UK’s Britannia 2 (left). By halfway through the final of the Prada Cup it was clear that while the performance of both designs had moved forward the performance bias had not – Prada again proving the easier AC75 to lift out in marginal conditions. The brutally chopped shapes on Britannia suggest an emphasis on higher speeds which she demonstrated on the windy days during the Prada Cup round-robins. Everything about Luna Rossa suggests a smooth, buoyant platform for lift-off with low wetted area. Britannia’s square keel always looked potentially sticky – making any drop off the foils particularly costly. Below: American Magic’s rebuilt Patriot was in trouble from the very start of her semi-final duel with Luna Rossa, suffering control problems during manoeuvres and constantly nervous in flight. An amazing effort to get back on the water fatally compromised by the complexity of the systems needed to manage these boats
foiling moth, minus the elegance. One person sails the moth, and a single T-foil daggerboard works on both tacks. The boat can be sailed with extravagant heel to windward, so the daggerboard foil is providing the vertical force to hold the hull out of the water and a horizontal compo- nent of that (heeled) foil also resists the heeling force from the sails. The helmsman has the following controls
to hand: he balances heeling moment and righting moment with his body position and sail trim via the mainsheet; he controls heel to get the best balance between vertical and horizontal force from the daggerboard and wing; he can control the global angle of attack of the daggerboard foil with his fore and aft position; and the manipulation of the rudder foil angle of attack; he can control sail shape for best aerodynamic performance; and then he can think about picking his way round the racecourse and avoiding collisions. His only assistance with this plate-
spinning act is the surface sensing wand on the bow that is connected to the flap on the daggerboard T-foil. When the boat rises the wand drops and the flap reduces the camber and vice versa. This is a simple but crucial mechanical feedback loop. Scaling up this light and delicate craft is
as if the performance die is cast. There are some clear differences. The
Luna Rossa hull looks to have less frontal area, and a much lower-volume skeg. I don’t think the Ineos boxy style is helpful in terms of getting unstuck on take-off. Much is made in commentary about the
differences in the foil wing area as the source of performance difference – it probably isn’t. The key design feature of the foil is the wingspan. There is a fixed maximum set by the class rule, everyone has gone to that number. The foil wetted area is less important, and bound up with the section shape, thickness and chosen optimum lift coefficient. Sail control is coming to the top of the list as the commentators run out of things
to talk about. It does seem that Luna Rossa have more control over sail camber with their boom-less set-up. More camber is important for take-off and the down- wind legs and is pretty much a free lunch provided you can quickly flatten the sail for upwind work. If Luna Rossa can trim the sails to get their aerodynamic centre of pressure lower than their opponent, with- out increasing induced drag too much, then they will have more thrust available. More thrust more speed. In the first four races of the Prada Cup
final we have seen more tacking and this has highlighted the different approaches to sharing control of the boat. First some background as to why this issue has arisen. The AC75 is in essence a scaled-up
just like trying to scale up an insect: nothing works as well when you try to make it bigger, weight goes up as scaling factor cubed and area as scale squared. Soon once the scale factor gets much more than 2 it’s turned to custard. You need two people to sail it because one person doesn’t have the strength, and you have to start looking for workarounds. The scale factor moth to AC75 is about
7 so the workarounds are now major. One central keel won’t work because the inte- gration of the control systems to keep the boat flying with windward heel is imposs - ible. That problem is solved by having a keel and its attendant control mechanisms on both sides, adding to the weight. Now the AC75 is constrained to sail pretty
SEAHORSE 49
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