Update
loadings generated at sailing speeds of more than 40kt. The ballast is not just there to provide righting moment to weather, however, but also to permit the AC75 to self-right after a capsize by canting the windward foil. Typically when sailing the weather T-foil will be used like a
conventional canting keel to provide righting moment, albeit lifting clear of the water to eliminate hydrodynamic drag. Meanwhile, the degree the leeward foil is lowered will depend on the point of sail. According to Davies, it will provide the vertical lift required to elevate the boat out of the water in around 11kt TWS (although he anticipates this could eventually drop as low as 8kt). The exact rig format remains undecided. For practical reasons
it won’t be an AC50-style wing. However, Davies says it will be some- thing more ‘wing-like’ than a conventional multihull-style rotating wingmast with soft sails. Several hybrid wing systems are in devel- opment independently at present but Davies says none has proved up to scratch, although conceptually they are interesting: ‘One of our drivers is for the sailing community to be able to relate to these boats and have some of the technology trickle down into other classes.’ One option is a wingmast with a big chord and a double- luffed mainsail, perhaps converging further aft into a single skin. He says they are keen to transfer the AC50 twist control mech-
anism used at the head of the AC50 wings to drive the head of the sail to leeward, providing righting moment from the top of the sail. ‘It would be a big breakthrough to have a working strut up there.’ Also yet to be decided is how much will be one-design. According
to Davies, parts certainly will be, but less than with the AC50. ‘One way of curbing costs is to go one-design. We could go one-design for the shaft of the T-foil and let teams do their own foils at the end of it. We might do a one-design mast. But we are going to
keep the hulls open – that was a push from Luna Rossa.’ Usually the weather foil is raised out of the water while how much
the leeward one is lowered depends on the point of sail and the mode required. ‘The more you have it up the more lateral resistance it has,’ says Davies. ‘The design is all about getting the angle right for the horizontal section of the foil. The shaft isn’t doing a lot of work so if you cant it up it lifts the whole horizontal section more.’ However, there are occasions when the weather foil will be lowered.
‘In survival conditions it becomes like a training wheel – it is incredibly stable with both down, good for the upper wind ranges. Also in the pre-starts above 15kt it looks good with both foils down – you can just keep going in a foiling circle. It will be much more manoeuvrable than an AC50 where the foils were opposing each other if they were both down – that’s not the case here.’ Instead of altering the rake of the entire foil (as on the AC50s), the AC75’s T-foils will each have a trim tab/flap running along their trailing edge. The AC75 only has a single rudder but with a minimum length
to ensure it remains immersed. According to Davies, this removes a lot of complexity as well as cost. It is likely that instead of having an elevator flap the rake of the entire rudder can be changed. Compared to an AC50, the AC75 is certain to be more sticky in
sub-8kt conditions but Davies says it has the potential to be faster in winds above 15kt. ‘As soon as you are foiling they will be very efficient. The wing on the AC50 was very efficient and we are not going to match the performance of a wing. Otherwise aerodynam- ically they are almost better than the catamaran. You have a lot of righting moment and only one rudder so less resistance.’ Other - wise the polars will be quite similar to the AC50, Davies reckons. As to how the boat will be sailed, it will have 12 crew which might
seem few for a conventional 75ft monohull but a lot compared to an AC50 or other boats with few sails capable of giant speeds. As Davies observes: ‘It will be very physical. With any foiling boat you need to be active on the mainsheet in and out. You might have six grinders locked into the mainsheet. It will be quite loaded...’ Given this Cup boat will actually have sails, it will require personnel
to hoist, lower, trim and even move them… Controversially, they seem to not be of a mind to make life easy and will be avoiding self-tacking jibs. ‘Just to give the crew a bit more to do!’ says Davies. On an all-up displacement of around seven tonnes, crew weight
(12 x 87.5kg = 1,050kg) will play a considerable part in the trim. Given this you might think that the beam of the boat would be large – but large beam equates to unwanted extra weight to get foiling. However, Davies reckons beam will still end up around 5-5.5m.
YACHTS… AND REEFS – Don Street Jnr I have been in the Caribbean for 60 years. I have been writing guides to the Caribbean for 51 years and have worked with Imray developing and updating the Imray Iolaire charts to the eastern Caribbean for 38 years. The arrival of super-accurate electronic navigation has not
reduced insurance claims. I wrote an article in Caribbean Compass in 2016: ‘Over-reliance on electronic navigation supports the local salvors.’ There was a photo of a new Swan 90 aground on the western side of Spithead Channel, Antigua, caused by the skipper relying on his chart plotter rather than eyeball navigation. The grounding cost the underwriters $US800,000. The problem is worldwide. Two races ago a Clipper
round-the-world racer was lost in Indonesia; in the last Volvo one boat was lost; this Clipper round-the-world race another boat was wrecked. In all cases the electronic navigation was working, the losses were caused by human error. Electronic navigation is a wonderful aid to navigation.
It must be backed up by commonsense, good seamanship and eyeball.
14 SEAHORSE
‘You do need some form stability, otherwise it will just tip over in a gust. In fact, you need that initial stability but then you don’t need it at all (in other words, when it is flying), so you could end with quite a refined hull shape.’ Verdier’s own experience with keel-less boats finishing the Vendée Globe may have been useful here! Also yet to be decided is how the trim-tab control on the foils (the
‘flight-control system’) will be adjusted. There is a complete pan- theon of options from fully automated, which seems unlikely to be adopted, to fully manual with no electronics. Davies says in between there is the same option for the semi- automatic system that clearly worked so effectively for them on the AC50. There’s even the possibility of a wand-style system similar to a Moth, ‘but there’s the potential to damage that and then you’ll be out of the race’. And as to whether there will be stored energy on the AC75, Davies
hopes not – although they might reserve the right to add it later on. ‘It will come out in the rule, whether we allow some green energy. There is amazing technology out there and the Cup should be a technology race.’ The trim tab will require substantially less hydraulic power to operate compared to the rake controls on the AC50. In terms of manoeuvres, Davies says that tacking should be
straightforward and again they are aiming for the least energy burn by the crew: ‘The windward foil can drop down with gravity – it only has to drop by about 30° to be in the water at a good angle. You have to lift the old one up, but hydrodynamically that wants to lift anyway and then you just pull the last bit out of the water. There isn’t a huge amount of energy required.’ Downwind, Davies reckons the AC75 will withstand going through
w
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