The first races have been completed and the teams have already shown almost everything they have up their sleeves. And it looks very different. In three episodes, we explain how the new AC75 racing cars work and where the differences between the teams lie. It's an insight into complicated engineering work and the interplay of sailing physics on a new level.
Part 1: The hulls
First a few key data: The overall length of the hull is specified to be between 20.60 and 20.70 metres, which corresponds to 68.5 feet. Adding the bowsprit results in 75 feet - hence the designation AC75. The hull width must be between 4.80 and 5 metres, the total weight between 7785 and 7815 kilograms.
The bow shape and the stern are defined by minimum widths. The bow must be narrower than 1.6 metres and 1.0 metres at two points, measured 17 metres and 19 metres forward from the stern. The stern must be at least four metres wide.
The hull must have a minimum volume of 70 cubic metres and also have a certain righting moment in a 90-degree capsize test. A minimum hull stability is also specified. These properties can only be achieved with a corresponding freeboard. Otherwise, the hulls could be flat as a board.
The take-off
A distinction must be made in the design between two phases, the displacement mode with the so-called take-off, the take-off of the boats, and the pure flight mode. The actual displacement mode is almost unimportant, as the boats will hardly ever sail in this mode. The lower wind limit is 6.5 knots, the upper limit is initially 21 knots (later 23). The aim is therefore to get out of the water at 6.5 knots, be it after a duel in the pre-start phase or a splashdown, i.e. after falling off the foils, for example after a manoeuvre. Ideally, however, this should not happen at all during the race. Apparently, the designers have interpreted the take-off a little differently.
The most striking of these are defence team Emirates Team New Zealand and challenger Ineos Team UK. Both teams have given their hulls so-called bustles, which look like long keels but have nothing to do with them. They are not ballast or effective against drift. The aim is to reduce the wetted surface area and thus the frictional resistance of the hull as quickly as possible during take-off. In both boats, the hull lifts, but the bustle remains in the water and provides buoyancy with reduced surface area and volume. In principle, the hulls of the two other challengers, American Magic and Luna Rossa, work in a similar way. The only difference is that a slightly rounder frame was chosen in each case, without the pronounced hump, which means that the transition from displacer to semi-plane is more of a smooth one in their case, whereas in the other two it is sudden.
The same applies the other way round. If the fuselages are flown a little too low or sink when the pressure is lost, only a small area of the British and New Zealanders is wetted when they sink slightly, which does not slow them down as much. However, the Bustle provides some lift, which can prevent further sinking. With the other two, there is more of a risk that the fuselage will get stuck when lowering and become fully submerged, as a larger area is wetted more quickly. They need to be flown more carefully and are less forgiving of sinking.
The Italians and British also have a kind of edge in the former. However, this has less to do with take-off and more to do with achieving the fuselage stability required by the regulations. This requires a certain volume.
In the first races, it was noticeable that the New Zealanders get onto the foils very quickly. This is due to the shape of their hull. The frame in the rear hull area is almost flat (the hull must not be hollow anywhere). This is actually a bad thing because the stern tends to get stuck in displacement mode. On the other hand, the centre of the fuselage can be flatter because the tail area contributes to fuselage stability. During take-off, the New Zealanders lift the wing strongly at the tail with a strong angle of attack. As their hull is relatively voluminous at the front and therefore offers a lot of buoyancy, the boat does not dive away at the front, but suddenly lifts out of the water as a whole. The British boat, for example, tends to oscillate around the more voluminous centre of the boat in this phase, as do the boats of the other two teams, which have rounder stern sections and therefore rounder centres.
The flight phase
Once the hulls are out of the water, the main thing is to minimise their aerodynamic drag. They could be flat as a board, but the rules prohibit that. The boats should still look like boats and not like some kind of sailing equipment.
All teams have made significant progress in this area from their versions of the first boats to the current designs. The New Zealanders are again the most striking. Like a Le Mans racing car with full-length wings, the crew is positioned in two elongated capsules. The helmets barely protrude beyond them. These closed capsules are very aerodynamically favourable, but increase the overall volume of the hull and thus the aerodynamic resistance. However, the deck in this area was drawn very flat in order to compensate for the higher drag of the two capsules by reducing hull drag. Overall, the hull probably has a similar drag to the other teams, but the crew does not. The crews of the other teams, on the other hand, are much more exposed to the wind, which at 30 knots upwind speed with 15 knots of true wind is easily over 40 knots.
Bustle also plays a major role in flight mode. Although the Italians and Americans do not have this as pronounced as the New Zealanders and British, they achieve a similar effect with a skeg, a vertical surface from the bow to the stern or just before it.
The aim of this is to prevent pressure equalisation between the leeward and windward side not only in the mainsail, but in the entire boat system. To achieve this, not only the genoas rest on the deck, but also the mainsails.
This closed surface should extend as far as possible to the waterline; no air should circulate under the hulls. This is why the boats are always sailed as close to the water surface as possible.
The teams also try to heel the boats upwind during the flight phase and tilt them as far down as possible with the nose. This also has something to do with aerodynamics, but even more so with the foils. But more on this in an upcoming episode.
In the following episodes, we will explain the different designs of the foils and the operation of the sails. There are also interesting and striking differences in these areas.
