It was close. The windward foil of Holcim PRB, sticking high up in the air, sliced open the genoa of Allagrande Mapei Racing, whose leeward foil was probably only not damaged because the keel of Holcim PRC was in the centre or even to leeward. If it had made contact, this foil would probably have broken. That could easily have meant the end. For one thing, it takes up to three quarters of a year to produce a pair of foils due to the complex manufacturing process. Up to 300 layers of carbon fibre are placed on top of each other, laminated and compacted. The component has to be tempered every three layers. The costs are also between 700,000 and 800,000 euros. And you can't just borrow a replacement foil from another team. For two reasons:
Foils are rarely compatible
The foil of one team rarely fits into the hull of another boat. The design of the retraction and tilting mechanism - the angle of attack of the wings can be varied by up to five degrees - differs from designer to designer and also varies according to the age of the boats. A replacement foil would only be possible from a real sistership, but even then it would have to be removed, transported and reinstalled. This would take a lot of time and money.
Different foils are prohibited
Even if the foil of another team fits, it must be the same as the other foil used. Wings with different characteristics, such as profile, span or immersion depth, are prohibited. There are various reasons for this ban. It would be easier to achieve faster technological progress if, for example, two different foils could be used in training. It would then be possible to directly compare the performance values of the boat in the same conditions with a different foil and the designers would immediately know which foil performs better.
If, on the other hand, only the same foils are ever used, performance data can also be collected and compared with other sessions in which different foils are used. However, the sailing conditions are never the same and this method requires more interpolation, which always falsifies the results.
Different foils would be useful
Another aspect in favour of banning different foils is to maintain equality of opportunity. Better-funded teams could build special foils for defined courses. For example, a special space wind foil could be installed on one side for long space wind passages in the Southern Ocean and an upwind foil on the other for the part when the course heads north again after Cape Horn.
The ultimate aim of both bans is to prevent the cost of an Imoca from exploding further and to keep older boats competitive.
Foil handling is crucial
Just as important as the right foil is its correct use. Paul Meilhat demonstrated this impressively with "Biotherm" directly after the start. The team sailed their Imoca in full foil mode, with the hull not touching the water at all or only very slightly aft. This seemed impossible for a long time, as so-called elevators are prohibited. These are T-rudders, normal rudder blades, but with an adjustable transverse wing at the tip. This would have the stern out of the water. Such rudders are used in the America's Cup, SailGP or in the Moth class. Actually everywhere where foiling is used. Just not in the Imocas.
This is forbidden by a class rule and again has the background of keeping costs in check and older boats competitive. This is why the Imocas always have their stern slightly in the water, even in foil mode. The fact that the "Biotherm" team still managed to maintain a 100 per cent flight mode was a masterstroke. This required relatively calm water, but above all perfectly adjusted foils and sails. The boat was thus in a balanced equilibrium, the axis of which was formed by the lee foil in conjunction with the keel fin. It was an extremely delicate structure, but the fact that the boat was able to get ahead of the competition showed what potential the boats still have.
However, it is uncertain whether elevators will be released in the future, which would mean another significant leap in performance. In addition to the aforementioned cost and fairness aspects, safety also comes more clearly into play when considering this. This is because a permanently flying boat could become so fast that the consequences of it crashing into a heavy sea would be neither predictable nor foreseeable.
Why there are different foils
The differences in airfoil shapes are still great, which is due to the age and time of development of a hydrofoil, but also to the different philosophies of the designers. For example, there are foils that are lower in the water and push the boats quite high out of the water at an early stage. On the one hand, these profiles are less susceptible to ventilation because they draw less air. Secondly, a foil is better protected against collisions with flotsam on the surface of the water. Other, narrower and at the same time more expansive foils, which lie closer to the surface of the water, can generate less resistance.
Many top teams have developed at least two foil versions for their current campaign, and some, like co-favourite Jérémie Beyou from "Charal", have even developed a third. This was partly necessary because the profiles have made another technological leap. For the first time in this Vendée, they are also adjustable by five degrees along the longitudinal axis. This means that they can be adjusted in the angle at which they press into the water - similar to a landing flap on an aeroplane. This was still prohibited in 2016.
The effects of development are enormous, as they in this special article can read.
