simplified, the clew area on the latest mainsails looks especially complicated. ‘When the rules were written for
the AC75 mainsails we had just been through two and half cycles of wings and we thought we needed all that control. The reality was that we didn’t need that much equipment up there and the twist could be achieved without it. But now we’ve got the additional complexity of two mainsails rather than one and there are several reasons for that. To understand the advantages, it’s best to compare with an equivalent wing. ‘One of the big disadvantages of
the solid wing was that it was very heavy. The twin-skinned mainsails are much lighter. We also realised that a lot of the time the wings were depowered in the upper sections and as a result they were just creating drag. That’s why the rigs for the AC75 look a lot shorter because we realised that aside from what’s required to get up onto the foils, once up and running we don’t need that much power. It’s why this generation of boats is so quick upwind, lower drag from the rig and only two foils in the water rather than three on the cats. ‘We also knew that when you
crash with a wing it can cause extensive damage, whereas the twin-skinned mainsails are very robust. And then at the end of the day you can drop the sail and dock the boat.’ So, given the return to
conventional control systems, why the double skin mainsail? ‘It's a drag thing. A single skin
on any mast is going to have a higher drag profile. Then, when you’re operating at high apparent wind speeds having the air between the two skins makes the sail very settled. You never see these sails flap like you would with a conventional sail. As a result, we're able to get much, much flatter cambers with a double skin sail than we could with a single skin. A single skin sail at five per cent camber would just be ragging all the time in the wake of the mast. That was one of the decisions we took very early in the AC36 cycle when we were brainstorming what sort of rig solution could work.’ Given that speeds have increased significantly in other classes outside the Cup, does he think there could be future for this setup aboard high-performance grand prix boats such as the Ultime class and the Imoca 60s? ‘There will be a crossover and
while those boats aren’t necessarily spending as much time upwind as a Cup boat, they do spend a lot of time
Top: AC40s have their sails close- hauled most of the time so the lower drag profile of a twin-skin mainsail is a distinct advantage. Above right: from the size and complexity of the kit in the clew region it is clear that the loads are very high in this area
reaching which keeps their apparent wind speed up so this configuration may well help.’ During the last Cup, another
detail to emerge was the decision on whether or not to have a boom. And while there were a variety of configurations, according to Fallow there are still pros and cons for each configuration. But for the AC40 there is no boom. ‘The arrangement is very similar
to the control system we had on Te Rahutai [ETNZ’s raceboat in the last Cup]. There’s an adjustable clew board on each of the clews of the two sails, so that’s effectively your outhaul control. So, if you want to flatten the foot you go forwards with the clew board. Because the mainsheet traveller is a fixed point on the deck, that makes the mainsheet angle more horizontal which flattens the bottom of the sail. Conversely, you move the clew board aft which makes the mainsheet angle more vertical and pulls down more on the leech which reduces the twist in the main.’ As photographs in this area of the sail highlight, from the size and
complexity of the kit in the clew region it is clear that the loads are very high in this area. ‘The thing to remember is that with such high apparent wind speeds and such narrow apparent wind angles we’re dealing with very flat sails and it’s those factors that lead to the high loads,’ he continues. The ingenuity and sophistication
of the sail control systems has clearly allowed more effective controls which in turn has helped to achieve more stable flight and ultimately higher, more consistent speeds, but there is still a key constraint - power. The AC40s do not have any
physically powered controls and are electro-hydraulically driven, yet there is still a finite limit on the amount of power available. But for all the high- tech advances that have been made since the Cup got up on foils, the very fact that a concept that many of the world’s experts found difficult to envisage working is now available for non-Cup teams to buy is not only impressive, but one of the biggest advances after just one Cup cycle.
www.northsails.com
❑ SEAHORSE 63
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