This will be interesting
For the first time in a long while at least one aspect of the development required to make a success of the latest America’s Cup yacht should genuinely benefit the rest of the sport. James Boyd talks twin skins with Cup-winning sail designer Burns Fallow…
The AC75 hoistable wing While we are yet to be convinced that twin flippy-up, agitated gecko-style keel-foils will find their way onto regular yachts, what the Defender and Challenger of Record have conjured up for the rig pack- age seems much more likely to filter down. The base philosophy of the new AC75 is
for it to be ostensibly a ‘conventional yacht’ compared to the flying catamarans that are apparently alien craft to which average Joe yachties couldn’t relate. This effectively means a monohull and, at rest and with gecko legs submerged, the new AC75s should indeed resemble a regular yacht. When the AC75 rule was first unveiled
in general form last autumn the exact rig configuration was not finalised but was already being mooted: a D-section mast which is one-design, as are the spreaders and fittings, with supplied standing rigging. The mast’s flat trailing edge will be around 400mm across, with a mast track running up each side for the mainsail’s twin ‘skins’. These skins, like two mainsails hoisted side by side, will meet towards the leech to create a tack-able ‘wing’. In accordance with the AC75 design
philosophy, the twin skins must be hoisted and dropped like conventional mainsails, eliminating the wings’ post-race crane-out that has been mandatory in the last few Cups. Thus, tied up to the dock, the AC75
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will still have a mast, albeit one with a highly unusual section. So wow – cool new technology – let’s get
excited? Let’s get excited, for sure, but the idea is far from new. In fact, L Francis Her- reshoff (son of the legendary Nat) filed a patent for a double-skin mainsail back in… 1925. Herreshoff’s patent also included a double-skin headsail (prohibited under the AC75 Rule – boo!), a rotating mast sitting on a ball and a ball-bearing headsail mech- anism resembling a primeval roller-furler. Where the Herreshoff patent fundamen-
tally differs from the AC75 set-up is in spec- ifying that the twin skins be sewn together towards the leech, which presumably would have resulted in some unwanted crinkling in the high-pressure side forward. Ninety three years on, modern materials
and sailcloths make this set-up much more achievable and for the first time the Cup teams’ PhD-filled design departments can take up the challenge of developing this potentially useful tech in earnest. The basic AC75 rig format was conceived
in a matter of minutes between a group comprising Emirates Team New Zealand, Luna Rossa, North Sails and Southern Spars. Among them was North Sails’ Burns Fallow, a veteran of Kiwi AC campaigns dating back to 1995: ‘We wanted something different, a softsail that mimicked, to some degree, the features of the wings.
‘We were never going to get to the
performance of a two-element hard wing – that’s just impossible – but what we were trying to look at was how far could we could go and still have something reason- ably practical, hoistable and understand- able by regular sailors that might transfer to other forms of sailing. We wanted to get back to “mast and sails”.’ Development of the new rig took two
parallel paths: one in the virtual world while the other comprised three test rigs of increasing complexity. For the most rudi- mentary, engineer Tim Meldrum conjured up the simplest of outline jigs using some old AC72 battens, plywood and hose clips. ‘One of the great things about that was it had a total cost of NZ$7!’ The second was fitting a scaled version
around 7m in height to a trailer. ‘That was just to see how the thing would set up and what sort of shapes it could generate. It was very simple – some aluminium box section for the mast and a couple of panel sails.’ Finally Southern Spars built a proper-
scale model which they fitted to a former team member’s Farrier F-22 trimaran. ‘It wasn’t a proof of concept because by that time we’d already done a lot of CFD devel- opment and had created an aero model that could be fed into the VPP to give an idea of the performance difference we were going to get. It was a case of, “is this showing us
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