The apparent wind on the new AC50 catamarans comes diagonally from the front on every course, even downwind. So at top speeds of up to 50 knots, it is constantly blowing heavily from the front over the boat. This has made aerodynamic resistance more important than ever before.
As the shape of the fuselage, the beams and the wing are predefined, the work of the aerodynamics experts is mainly limited to designing the fairings of the beams and the crew positions. They have a free hand in this. However, the beam fairings must be permanently installed and may not be changed while sailing. However, they may be changed before each race day, and the entire boat may be re-measured before each race day.
This option can be significant, as the fairing of the beams is not streamlined, but has the shape of an aerofoil that generates upward lift. This additionally lifts the boat and helps when turning.
The AC50 catamarans have become so fast and have such low resistance that for the first time it is possible to stay completely on the foils during a tack, i.e. to make a so-called power tack. These tacks have an interesting effect: when the helmsman luffs and the boat goes into the wind, the wing brakes at the top as it no longer generates propulsion if the angle of attack to the wind becomes too small. However, due to its inertia and high speed, the boat continues to travel under the wing, so to speak.
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As a result, the bows start to rise. The cat therefore tries to go skywards with the bows when tacking. This is primarily prevented by the beam fairing aft. As it generates lift and raises the cat aft, presumably more than the fairing at the front, the entire boat rises. This reduces the weight on the foils so that they manage to keep the catamaran in the air despite the lower speed when tacking.
This aerodynamic lift at the stern has nothing to do with the obvious longitudinal trim of the catamarans on the bows, i.e. that they sail higher with the stern than with the bows. This trim is generated solely by the rudders. The additional drag that occurs is significantly less than if the aerodynamic fairing were to generate this lift.
Bow-heavy sail trim
There are two reasons for leaning the sail forwards. Firstly, the rudder blades are as far out of the water as possible, which reduces their resistance. The second reason is the "bowsprit", the triangular platform that supports the genoa and closes it off at the bottom. This component is usually standardised and inclined two degrees upwards in relation to the waterline. If the boats were sailed parallel to the waterline, this plate would be slightly upwards and would have increased resistance. This resistance is reduced by leaning forwards.
There are only two distinctly different crew positions. Apart from the New Zealanders, all teams use grinders and the four crew members who operate them stand in the boat. The New Zealanders, on the other hand, sit on wheels and work with their legs. As well as providing more power, this constellation is also said to be more aerodynamically favourable. As with a four-man racing bike, the four men form a closed unit, which is somewhat more exposed than with the grinders, but causes less turbulence than four people standing alone.
The New Zealanders' obsession with detail was recently evident once again. The four cyclists were given "humps", raised areas on their clothing in the neck area, similar to those used in motorbike racing. These are intended to ensure an even more undisturbed flow.
The big Cup guide: In the America's Cup special in YACHT 12/2017 you will find all the information and background to the sailing event of the year. From 24 May at the kiosk or digital here.
