Neck on the line
Having previously bravely scored the generation one fleet of new AC75 designs Dave Hollom looks death in the face to pass judgment on the second generation of designs that will shortly be racing for the America’s Cup
Hulls In Sense and Sensibility (issue 483) we talked mainly about AC75 hulls and their relationship with the rig. But they were first-generation hulls and things have moved on. So what of the improvements? Well, I said in that article that to my
mind the boat with the most elegant and efficient solution to the new rule was Luna Rossa and low and behold their second boat is a development of their first so they, presumably, must be of the same opinion. The skeg, the long appendage that runs
almost the full length of the boat, and which some persist in calling a bustle, appears to be a little deeper which makes sense. It allows the hull to ride a little bit higher from the water surface which makes it less likely to dip into the sea with the attendant drag while still keeping the smallest gap between skeg and sea to maximise rig efficiency.
60 SEAHORSE As regards the foils, ride height is impor-
tant but probably not critical to speed in these boats. It’s a bit of a trade. Firstly, the nearer the foils run to the water surface the less of the arm is immersed and the lower its drag. But there is a limit. The closer to the surface the lift foils run the more they disturb that surface and the higher the drag that disturbance causes and also the more likely it is to break the water surface, lose its lift and cause a crash. Riding higher reduces arm drag. Flying lower reduces foil drag. So there will be an ideal ride height for minimum drag – it is important but probably not critical. There are other reasons for varying ride
height. As mentioned in the previous article, the ideal will be to produce all the lift forces, lift to fly the boat and side force to resist the rig forces, on the horizontal (or nearly so) lift foil. To do that it must be angled at about 20° to the horizontal. As the lift arms rotate about an axis on the side of the hull that angle can be achieved by adjusting the amount of rotation of the arm but that will also alter the ride height of the hull which, as already mentioned, is important in making the rig more efficient.
The Universe is under no obligation to make sense
to you – Neil deGrasse Tyson
Obviously getting that ride height to a
minimum and the lift foil at that 20° angle the right distance below the water surface to minimise drag should be worked out at the design stage. But if it’s a little out a slight alteration to the depth of the skeg could be a satisfactory fix, increasing or decreasing the distance between it and the sea surface to keep the arm and horizontal foil at the right angle. Also, of course, the further the arm is around its arc of rotation the higher the ride height and the smaller the lateral distance between foil lift centre and the lateral centre of gravity (LCG) of the boat and the lower the righting moment. Additionally,
the higher you ride
because of arm rotation the greater the distance between rig vertical centre of pressure and foil vertical centre of lift, the longer the heeling arm and the greater the heeling moment. Rotating the arm more than absolutely necessary is therefore a double whammy as regards lateral stability. Rotating it as little as possible without the hull coming into contact with the water too much, the greater the power the rig can exert and the faster you can sail. It is difficult to be certain, as until they
reached Auckland the challengers man- aged to keep published photos of their work to the minimum, but Prada’s first boat did appear to have a distinct concave V-sectioned skeg while the new one appears more rounded, presumably to
IVOR WILKINS
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