Another soft-shaped hull with the flavour of American Magic but where the US team started with a no-compromise scow the Italians used as their baseline a traditional monohull concept that was then adapted into a powerful foiler. Above and below the water Luna Rossa is super-clean but performance has been patchy other than in light air when she looks the fastest boat of all. Luna Rossa was the only team to incorporate a keel on their first boat; it is also significant that of anyone the Prada design changed the least between boats 1 and 2. That is either a vote of confidence or a sign that this team was running out of scope for very much further development
The foils – an early video analysis During the pre-Christmas racing we looked as closely as we could at the effects of the foil designs where they meet the water. On Luna Rossa and Patriot the foils
enter the water creating a fine spray, limited in disturbance and leaving little turbulence in its wake. The foils of New Zealand create the impression of a fine penetration of the surface, closest to a knife blade and with the least disturbance. The foils used by Ineos before Christ-
mas produced the maximum disturbance. With the first Ineos foils several things were clear to me – and of course the team. l A rather fat and not very aerodynamic junction between the arm and the foils. l The fence on the foil arm is located approximately 30cm above the arm/foil junction – different from other teams. As this device is not submerged it interferes with the rising wake and returns it down to the foil, thus increasing the vortex. We are not surprised to see the British
foils that appear for the first race of the Prada Cup are completely different.
The foils in plan New Zealand, American Magic and Prada start the racing with similar slightly triang - ulated foils with a small flap, a geometry that can be identified as classical for use in the normal (subsonic) range of velocities. The flap allows us to modify the camber of the profile and thus to increase the Cz (lift) but also the drag (Cx). On the AC75 it is not allowed to modify the rake of the foil. Ineos have chosen another way with the
flap having a large surface area and a pre-curved foil profile which theoretically allows it to take off at lower speed. The profile thickness of this very triangular foil shape (with its large root chord) seems to be quite small which would explain its potential for a maximum speed a few knots higher than the other three AC75s. But we saw in the first races that the
team’s original choices (low speed take-off and higher maximum speed) result in a much narrower flight domain. In fact, when the AC75 reaches its maximum speed off the wind any attempt to head down to increase VMG results in a drop in lift, and thus implicitly in speed. Also in spite of the large curved foils the shape of the hull with its large wetted surface seems to contribute to surprisingly poor take-off. Increasing the camber of the foil
increases lift, but this gain is annihilated by a large increase in drag because of the tri- angular surface. It seems the only way for the first British foils to work is to increase the power of the sails which is impossible. Ineos also first opted for the lowest foil
l Tacking and gybing with the highest possible minimum speed l The ability to bear away quickly when loaded up l And, most important, a quick and easy lift-off and minimum drag on landing.
64 SEAHORSE Also the VMG differences downwind
are bigger than we thought when the first racing begins, the differences in angles sailed between the designs, particularly when trying to take off are quite surpris- ing. Clearly speed is not enough.
aspect ratio of 4.4, the other three AC75s are all above 5. The most extreme is NZ which works very well at 7.4. This is a high bar for the Challenger to meet before the America’s Cup Match in March. As aspect ratio increases marginal
vortices decrease, resulting in a reduction of induced drag. Choosing a foil with
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