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Transforming the America’s Cup

Dec 8, 2017 News ,Technology

ETNZ’s concept design for the 2021 AC75 has elicited responses ranging from applause to derision. Mind you, the extremes of the spectrum are peppered equally with expletives, so they share some common ground. Words by Christian Stimson, images supplied.

When viewing for the first time the CGI video depicting the range of articulation in the foils for the various modes of sailing, one might think Optimus Prime would be an appropriate name for the vessel, given its resemblance to a Transformer er, well, transforming. Comparisons to a lizard on hot sand have also been made, as well as comments to the effect that it looks like it might crawl up the beach and lay its eggs.

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There have been cries of ‘that’s not sailing’ and ‘we wanted proper yachts’, with an underlying sentiment that Grant Dalton has come up short in his promise to return to a form that’s ‘more recognisable’ to the majority of sailors.

These views you can give a bit of credence. But the problem is you can’t put the foiling genie back in the bottle. It’s out – and it is faster than displacement sailing. So Dalts has an obligation to the future of sailing technology as much as the past.

Just as the internal combustion engine spelled the end of the horse-drawn era for general transportation, yacht racing in displacement sailing mode will ultimately be relegated to history other than for a few who seek to indulge in nostalgia. Horse and cart is a quaint way to get around a city as a tourist, but my car is faster. Period.

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So, the debate shifts to whether there is a tangible difference between the foiling AC50 cats and the AC75s, and should the cat class have been retained given the investment and experience gained. Once foil-borne, the hull is somewhat irrelevant in this discussion, other than to provide a platform for crew work, sail handling and to support the rig.

The class rule will leave hull design open to the teams to explore. And given the predicted ‘take off’ speed will be reached in around nine knots of wind (rather than  six knots for the AC50s), there will be design effort directed at striking a balance between a low drag (low stability) hull form.

A form that accelerates quickly and let the foils work sooner versus a high righting moment (higher drag) form that allows the power of the rig to be harnessed to drive the hull faster. Thus was ever the dilemma in mono-hull design.

While there will be differences in structural loadings (primarily a catamaran platform twisting transversely vs a mono-hull bending longitudinally), and in the way the foils attach and articulate, the fundamentals of an immersed foil supporting the weight of the vessel while opposing the heeling moment and side-force from the rig remain the same for both cat and mono-hull. So why fix it if it isn’t broken?

The AC75 as presented has no keel in the traditional sense, but two curved hinging foils with a T-foil on the tip of each and a single, centreline T-foiled rudder. The proposed design stated no dimensions – that’s to come with the class rules in March next year.

But scaling off the images provided and assuming 75ft/23m hull length, this indicates the beam to leeward out to the T-foil tips to be around 10m off centre and to windward its about 8m off centre. So, a nominal 20m beam on 23m length. Those are catamaran proportions.

The foiling IMOCA 60’s have a canting keel, with a large percentage of their total mass in a bulb on the end, and derive their stability partly through the bulb pulling down on the windward side, and partly from the foil lifting up on the leeward side.

15_71250   ©Th.Martinez / BPCE.  ILE DE GROIX - FRANCE . 17 Septembre 2015. Au large de l'ile de Groix, Brittany(FR), Monocoque IMOCA Banque Populaire XVIII, Skippeur Armel Le Cléac'h, co-skipper Erwan Tabarly.

15_71250 ©Th.Martinez / BPCE. ILE DE GROIX – FRANCE . 17 Septembre 2015. Au large de l’ile de Groix, Brittany(FR), Monocoque IMOCA Banque Populaire XVIII, Skippeur Armel Le Cléac’h, co-skipper Erwan Tabarly.

If the AC75 were to adopt a similar configuration it would require the yacht to carry ballast, and that would be counter to effective foiling, which requires the vessel mass to be reduced as much as possible.

There had been concepts of a 75ft version of a foiling Moth being the way to go, with a big T-foil on the bottom of a long vertical keel shaft. The problem with that is that the Moth works only because the crew weight provides such high righting moment.

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The rest of the craft is so light at sub-30kg compared to the 60-90kg crew sitting out on 2.25m wide wings. To scale that to 75ft would make the K-boat look mediocre and require quite high crew numbers, and/or very wide wings (making sprinters rather than cyclers the next cross-discipline athletes?)

One inescapable difference is that a catamaran is stable at rest, while a foil-stabilised mono-hull without a keel has no foil-derived stability if there is no flow over the foil. So, in a down-speed pre-start manoeuvre for example, there could be issues!

The AC75 has addressed this with ballasted foils, at between 1.0t to 1.5t each has been reported from ETNZ’s Technical Director Dan Bernasconi. When lowered they offer the same effect as a ballasted keel by lowering the vertical centre of gravity sufficiently to prevent the yacht from falling over.

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This would apply to a docking-in or docking-out, long tow-out to or back from the race area rather than a sailing mode, but could possibly allow the yacht to hold up head to wind in a dial up. It also gives the yacht the required degree of self-righting in the event it all goes pear-shaped

The two sailing modes are interesting, given the requirement for racing in a wider range of windspeeds and sea states than in Bermuda, with a high-speed sailing mode and a stable sailing mode described.

The high-speed mode involves the leeward foil being deployed to generate lift and side-force, and also oppose the heeling moment, while the windward foil is raised to reduce drag. Righting moment is generated by the lever arm between the T-foil’s lift acting vertically and the vessel mass acting downwards at the centre of gravity.

The slight differential in beam of the retracted and deployed foils may shift the vessel’s centre gravity to windward, which combined with the indicated 1,000kg crew weight being offset to windward should increase lever arm and hence the righting moment further.

It looks as if the T-foils are (nominally) set at about 20o off horizontal to oppose the result of the rig’s heeling force and the vessel mass acting vertically downward. A component of that T-foil lift is also acting horizontally, opposing the rig’s side-force to reduce leeway. These main foils can be canted up and down, in a gull-wing like motion, with the T-foils having trailing edge trim-tabs to vary their lift rather than raking the entire main board as was the case in the AC50 cats.

The T-foil on the rudder provides the fore-and-aft pitch stability, presumably being trimmed underway to maintain overall pitch control but, being only a single rudder, the AC75s won’t have the option for differential rudder forces to increase righting moment, where the windward one pulls down and the leeward one pushes up.

In stable sailing mode both foils are deployed such that the T-foils are horizontal, producing maximum vertical lift and importantly equally on both sides – so the yacht can’t fall over sideways, as long as it is going forward.

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With the rudder T-foil working to control pitch we have a high-speed three-legged milking stool that can corner aggressively in the same way that ETNZ’s AC50 could in Bermuda. (see diagram)

What we don’t yet know is to what extent the control of the foils will be manual or automated, or what sensors/inputs might be allowed in an otherwise manual operation to maintain ride height. Given the tightrope to be walked between embracing the future and constraining cost, ETNZ will recognise that measures to limit cost through limiting complexity can actually drive up cost as design effort is directed to circumventing the very rules intended to reduce cost.

We know there will be no internal combustion engine allowed to power the hydraulics, and that most likely a battery-powered system will drive the foils while traditional grinders will power the winches for sheets and halyards.

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The rig is currently undefined. The options being explored range from a conventional mast and soft sails to a wing mast, rotating and possibly canting, but with soft mainsail and headsails. The complexity and vulnerability of a full, solid wing sail is deemed too much to accept and will limit the operational window as well as making docking or berthing very risky. So, a canting, rotating wing mast would offer low drag and increased power over a conventional mast tube.

It is understood, though no confirmed, that fears of a high-speed leeward mark rounding becoming a case of ‘tin openers at dawn’ with foils slicing through a competitors under-belly as they vie for buoy room are being addressed with a proximity alarm and a virtual exclusion zone around each boat.

It is fair to say that this brings a level playing field as far as possible. ETNZ has a marginal advantage in being the creator of the rule, but has wiped out its undeniable edge in the AC50s for the sake of as fair a competition as possible.

My prediction is the nay-sayers, as was true for the AC50s, will eat their words once these beasts are seen in action.

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