Left: this Finite Element Analysis (FEA) Mesh detail of a generic twin-skin AC75 mainsail taken from the North Membrain program also clearly demonstrates the way in which the two skins will ‘flow’ seamlessly off the aft face of the relatively simple D-section spar. Above: the top of the sail is now inverted to depower and reduce heeling moment without increasing drag. The AC50 wings could achieve this by trimming the top flap the ‘wrong way’ – lowering the centre of effort and even adding righting moment if necessary
anything that stops us wanting to do this?”’ This involved adapting North’s own
Membrain software to work with the double-skin mainsail. Fallow says this required work but was made easier as they had already developed a version to work with the solid wings. Also vital to the virtual analysis was the
role of North Sails and Luna Rossa’s CFD guru Michael Richelsen, who adapted his FEA work on wings to the new AC75 rig. Fallow explains: ‘Michael had to adapt his aerodynamic Flow program to accept the thick mainsail which was basically contigu- ous with the mast tune. And the control systems – Michael already had those mechanisms coded that he could adapt. ‘We needed FEA to answer big questions
like “how is load shared between the skins?” “How do they react to the control system?” “How far down the sail does that work?”’ And – drum roll – the number crunch-
ing to date has shown that the twin-skin mainsail/D-section mast provides an improvement of around 15 per cent in total Lift Force over a conventional soft mainsail and headsail package. Why only 15 per cent? Fallow puts this
in perspective: ‘In any other area of boat design 15 per cent is simply a huge jump.’ The solid wing provides such high lift
co-efficients due to its ‘twin elements’ (much like an aircraft wing) and the all- important slot in between. This allows some of the air flowing across the high- pressure windward side of the wing to escape to the low-pressure side of the rear element(s) where it reattaches the wind flow (or ‘re-energises the boundary layer’), effectively making a greater area of the wing productive. Could this not be emulated by somehow
fitting slots into the new mainsail set-up? ‘We did look at it and there are a couple of
examples out there where people have tried to do that,’ says Fallow. ‘But there are two difficulties: firstly, it’s heavier even than a wing, and if you’re dealing with a foiling boat reducing overall weight is attractive because it means a lower take- off speed – among many other factors. ‘Secondly, absolutely key to the aero -
dynamics of the wing is the gap between the two elements. Physically it is very small and where the front edge of the trailing element ends up relative to the back of the front one is really critical to the flow between the two elements. If you haven’t got exact control over that (ie a very strong and well-controlled spar running up that space) it’s not going to be effective.’ But the wing’s advantage is not just high
lift co-efficient, but how it is depowered. ‘With the wing you can manipulate where the lift is vertically – we’ve picked up that feature by being able to invert the top of the AC75 mainsail,’ says Fallow. And the control system for this opera-
tion does come directly from the AC50 wings. Fallow explains its benefit: ‘A big part is to do with controlling heeling moment. You can do that by just depower- ing the entire sail, but that’s not efficient. ‘With this you can create a lot of power
down low in the sail plan or do the opposite and do the negative up high – so it’s very efficient from a drag point of view and in many other aspects as well. On a conven- tional square-top mainsail all that can hap- pen is the sail gets eased until it matches the apparent wind angle when it feathers up top. With this you haven’t got that sort of feath- ering-type arrangement – it’s much more stable if you invert the top and have that effectively set on the other tack while the bottom is set on the tack you’re sailing on.’ So could this work on a conventional mainsail? According to Fallow, it might be
possible: ‘Our impression was that it was less stable vertically – you could see it going from one tack to the other up and down the sail. It wasn’t bad and there’s possibly something in that, but it would need more work.’ Teams can apply their own wing-control
mechanisms as appropriate to the twin-skin mainsail but only in the ‘mast upper zone’, encompassing the top 4m of the 26.5m mast and the ‘mast lower zone’ (<1.2m above the ‘foot’). The rest of the mainsail must behave passively like a conventional sail, albeit one with two skins supported by full-length battens. ‘Between the bit at the bottom, where your boom is, and the top where our control arms are, we want the two sail skins to be reacting to what’s hap- pening at the top and bottom,’ says Fallow. This whole area will be prime for AC
teams to vent their creativity, along with how the skins attach to the mast and how they converge towards the leech. The rule permits battens to attach to another batten or skin but only within 0.4m of leech or luff. A tethered connection between battens and batten pockets can be no longer than 0.6m and must not be able to take com- pressive forces. Aside from those within the two ‘zones’, battens cannot be adjusted. ‘We don’t know if we actually will need
to physically attach the skins or not,’ says Fallow. ‘We do know that there are differ- ent tensions in the two skins. It may just be that with the way the tensions work out the leeches just end up laying on each other. Even if they don’t it won’t require a difficult mechanical attachment.’ This control over mainsail shape is
especially vital on flying boats due to their huge acceleration as they lift clear of the water, causing the apparent wind to swing forward very quickly. ‘With this system mast rotation and sail camber are
SEAHORSE 37
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