Left: Sunnucks and Eeles enjoy level flight in trials of the new ‘Vampire’, a Marström M32 with an early trial T-foil configuration and a wand control mechanism similar to the benchmark foiler Moth system. Left to its own devices, a J-foil layout (below left) will promote the porpoising common in AC45Fs; continual human intervention or a very sophisticated feedback system is needed for level flight


a year’s worth of development – but William wanted it done in six weeks…’ Ellway and Smith set about using the tools they had developed for the Exocet Moth and applied them to the Sunnucks project. These included: a) CFD hull drag analysis b) 3D CFD foil lift and drag analysis c) Hull and appendage windage drag analysis d) 3D CFD rig analysis to derive drive and heel forces and associated moments e) Non-foiling cat VPP (to provide a reference to check accuracy of hull and sail models) f) Lift-out analysis to determine foil sizing and thrust (wind) required to foil – this model uses the predicted hull and foil drags from a) and b) g) Simple dynamic stability analysis (basic checks on flight stability) h) Full foiling VPP analysis to aid design and tuning i) 2D and 3D CFD foil design to optimise foils within design constraints j) Cavitation analysis (because the boat is capable of just shy of 40kt in flat water) k) Primary structural analysis for foils l) CAD design for aluminium foil tooling ‘We were pretty pleased with what we managed to achieve in less than 10 weeks,’ says Ellway, ‘although we’d have liked to have been able to find time for a few other things too.’ In an ideal world, these would have included: a) 3D dynamic simulations (basically how the boat reacts to waves). It’s a bit like a virtual test tank with waves b) Fewer design constraints c) A full CFD flying windage analysis (highly computer time intensive) d) Develop a rig properly suited to foiling e) The hull structure would probably also be modified to include endplates (as on the AC72s)


Despite not covering all the bases, Ellway says the analytical models con- firmed the design would benefit if canted foils were used; in fact, it turns out that for Sunnucks’ boat the optimum cant angle is about 30°. The VPP analysis also con- firmed the boat would be ‘a potent per- former… definitely faster than a Moth.’ Smith explains the rationale behind the canted foil: ‘Any sailing yacht is subject to two large forces. The lift (buoyancy) to keep it above the water surface and the


lateral force of the sails. Normally it is the keel or centreboard that opposes the side force. Once up on foils the most efficient way to oppose these two forces is with a canted single foil.’


Ellway adds: ‘If you think of any other of the foiling cats the horizontal part of the foil isn’t canted down, and optimally it should be. But the challenge is that if you are going to use canted T-foils you can’t really put them through the hulls because the inboard side is going to always be sticking well below the hull; and when you come into a shallow beach you end up wrecking the inboard foil tip.


‘As an alternative, we came up with the idea of gull-wing foils, which are just pivoted about the outer gunwale of the boat. This allows the foils to be raised completely above the water while allowing the flap control mechanism to remain


Sir


I was interested to read A Lateral Question – Part 1, in which the question is put, ‘why none of the multihull community have yet applied the tried and tested foiling technology of the International Moth’. Actually, the writer attributes this question to Will Sunnucks, who apparently was underwhelmed by the foiling performance of the C-Class cats at Falmouth in 2013.


I too visited Falmouth to drool over the boats and was surprised at how difficult they seemed to be making everything, but it is certainly not the case that multihullers had not applied foiling Moth technology (if anything it was the other way round); so I am curious to know how ideas and knowledge get transferred (or lost) as each successive generation discovers the joys and pitfalls of foiling.


Veterans of early Weymouth Speed Weeks will know that the implementation of controlling the elevator of an inverted T-foil from a trailing surface sensing wand first appeared on Mark Simmonds’ A-Class catamaran Rampage in 1977, while Philip Hansford’s little trimaran Dot demonstrated truly stable flight, including through gybes, from 1985.


In January 1999 I was lucky enough to be invited to present a paper to a sailing


permanently attached.


‘This arrangement has a further advan- tage in that it actually makes the boat sub- stantially wider than the nominal beam of the platform. This is because the head of the foil is against the gunwale, and the “bulb” of the T-foil, because it’s angled out, is substantially further outboard than the leeward hull. ‘So we managed to make the boat wider – increasing righting moment – as well as having a method whereby the foils could be raised. ‘Needless to say, there are disadvan- tages in terms of getting the foils up and down, which are proving a bit of a trial, but that was Will’s design brief. The idea was also that you could stick this system onto, for example, an old Tornado cat, or other existing cats of similar proportions. A better solution is available but time con- straints didn’t enable us to implement it’. q


science conference in Hobart. My paper was about the design and development of a 4.9m foiling catamaran, Calliope, which my father George and I had built some seven year earlier. I think this was around the time the Moths were first experimenting with surface-piercing foils but whether there was any transfer of ideas to the Moths down under I’ll probably never know.


In 2003 I wrote a second paper about our larger foiling cat Ceres for the Chesapeake Sailing Yacht Symposium, in which I had started to look at the maths of foiling in waves. Again, I do not know if this influenced anyone’s thinking, but by this time the inverted T and trailing wand configuration was the preferred option for Moths.


By 2007 Fred Eaton’s foiler Off Yer


Rocker had borrowed heavily from the Moths, so the technology transfer had gone full circle back to multihulls. Sadly the Rocker’s performance was relatively poor (probably too much ‘stuff’ underwater) so for a while it looked as though fully flying hydrofoil technology would be limited to small monohulls. The consequences of the revolutionary developments by ETNZ for the last Cup have changed everything and are yet to fully play out… I look forward to reading about them in Seahorse.


Joddy (Edward) Chapman, Devon SEAHORSE 39


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