Will it fly?
The divided opinions regarding the new AC75 design for the 36th America’s Cup show no signs of converging after the recent publication of the Class Rule.
Analysing the recently-released AC75 Design Rule.
Whether the images of the yacht with one foil raised/one lowered remind you of a peeing dog or an aggressive praying mantis, the fact is there is no reference point on which we can lean to understand the viability of the proposed design. Because it is so far removed from anything seen thus far. Even the most advanced of the current crop of IMOCA 60s only become partially foil-borne when reaching.
Without doubt the ETNZ and Luna Rossa design teams are among the best and the brightest, with the most sophisticated simulation tools at their disposal. If they believe the numbers to be true, who are we to doubt them? To my mind Schrödinger’s Cat would be applicable here.
In his 1935 thought experiment, Erwin Schrödinger presents the scenario of a cat in a sealed box. Until the box containing the cat is opened, and its fate known, the cat may be regarded as simultaneously both alive and dead. I think until we actually see an AC75 sailing, both possibilities exist simultaneously – they will either be an amazing spectacle or an incredible flop.
I’m banking on the former. But why might they be a flop, with all that brain power directed to ensuring they’re not?
Well, fundamentally, in setting out to do for monohulls what the AC50S did for multihulls – raising the technology level with foils, wingsails and control systems – the concept of a fully-foiling monohull has to overcome a number of unavoidable encounters with the laws of physics.
Foils don’t aid stability until they are moving forward through the water – they need flow over them. The force generated varies with the speed of flow squared. If you double the speed you get four times the lift.
The converse is also true. If you halve the speed, you get a quarter of the lift. So the transition from displacement mode to foiling will see huge changes in speed and lift, as will falling off the foils or losing lift through either ventilation or cavitation.
In the event of losing lift through ventilation, the righting moment will ‘switch off’ but the heeling moment will not (or at least not instantaneously, no matter how fast the main is eased), so we could see capsizes.
To keep the yacht upright before the foils generate enough lift, and to prevent the yacht from inverting if capsized,
ballast is required. Unlike the catamarans, which have high initial form stability at rest, the monohulls need a low centre of gravity to keep the rig pointing skywards.
The mass of the ballast – around 2.3-tonnes shared equally in the two foils – is added mass that needs to be both accelerated and lifted, increasing the loads on the foils and making them more sensitive.
Certainly, scale effects and inertia will esnure things happen slower than on a foiling moth, and Dan Bernasconi has explained how, in the simulator at least, a partial loss of foil area to ventilation is not catastrophic, leading only to loss of ride height, slowing a pitch down by the bow.
In addition, the top of the mainsail has a ‘gaff’ with a yoke that allows twist control without high leech tension. In this way heeling moment can be controlled quite precisely to mitigate against capsize.
Again, as explained by ETNZ, the added mass (approx three times that of an AC50) actually makes the yacht more powerful at higher speeds, even though the lighter AC50 is quicker on the foils in lighter winds initially.
So it could be argued that adopting the monohull is solving a problem that didn’t need solving: a catamaran is already known to be a good option for a foiling race boat. Being light, stiff and carrying no parasitic ballast weight, with wetted area and drag that reduces by half with a small increase in heel angle, the multihull option is hard to beat.
Why not just stick the new, double- skin mainsails on the AC50S and solve the one big negative aspect of operating those cats?
The counter-argument goes along the Kennedy-esque lines of ‘we do this – and the other things – not because they are easy, but because they are hard’. Why dismiss what the future of monohull sailing might become?
At some point in the future displacement sailing will be seen in the same light as square-riggers and old gaffers – a quaint, sepia-toned nostalgic indulgence. The real business of sailing efficiently (because burning fossil fuel will be outdated too) will be foiling, no matter what the configuration of vessel.
Now, I’m not saying the cruising yacht of the future will look like an AC75, but just as the Wright Brothers’ Kittyhawk bears little resemblance to a Gulfstream corporate jet, or the Model T Ford to a Formula 1 car, both can trace their lineage back to those points in time. We can identify the steps on the way that were necessary to reach the current state of design.
We’re only at the beginning of this particular evolutionary journey.
THE AC75 RULE – A LOOK UNDER THE HOOD
It returns to a box rule as we had before the AC50 cat, in which a box of dimensions sets the envelope, and teams optimise their design within that envelope. This ensures closeness of racing, by limiting how far a team can depart from the rest of the fleet while leaving enough room for creativity (and cunning) to play a role.
Some parts are specified in their geometry (such as the rig) and some are supplied (such as the foil arms) to limit the degree of spending – both in terms of money and design hours – allowing teams to focus on the areas that do make the biggest difference.
The hull length is set at 20.7m (68ft) plus a 2.16m bowsprit, taking the length to 22.86m (75ft). Beam is set at between 4.8m and 5.0m. The weight (ex- crew) is set at 6,195kg, while the D- section mast is 26.5m long.
The pivot points for the foils are set at 4.10m apart and 0.52m above the measurement water plane (MWP) within a zone 10m to 12m from the transom reference plane (TRP).
Foils may only cant through a range that does not cross the centre line, but there is no outboard limit other than that set by the allowable range of articulation of the Foil Cant System (FCS).
Unlike the AC50S, foil rake is not adjustable. Foil forces are varied by changing the global angle of the yacht (by raking a single 3m-wide rudder T-foil) and adjusting a trailing-edge foil flap to induce camber.
The foil arms are supplied, as are the batteries and rams in the FCS, but foil wings and foil tabs are open in terms of design and must fall within a boxed area that limits span to 4m and chord to 1.1m.
Foil wing/flap assemblies must be symmetrical about the foil arm and weigh around 1,150kg each. These form a ballast keel to hold the
RIGHT Articulated foils ensure the AC75 will be selfrighting in the event of a capsize.