Every designer in the America's Cup has probably dreamed of drawing a boat with a canting keel because it adds so much upwind performance, but such a thing is expressly prohibited in the America's Cup Class rule.
As Brad Butterworth, skipper of Alinghi, a Kiwi himself and someone who can play the rumour mill with virtuosity, says: "The rules say you are allowed only two moveable surfaces beneath the water and as anyone can see, we have a rudder that moves and a trim tab on the back of the keel that moves. That makes two."
What has been going is far more subtle and is a continuous thread of development through the last three or four Cups. Just as the sail designers know how advantageous it is to let the mast twist to match the curvature in the wind, so hull designers know that keel fins work much more effectively if some of the efficiency losses caused when they sag to leeward as the boat heels can be clawed back.
Precisely because the designers would love to make the mast and keel fin do things, the rules prevent them from using complex mechanisms to make this happen. What teams can do is use controls to limit or harness what happens when normal sailing forces are applied.
Any piece of smart, agile engineering that harnesses natural forces acting on the rig and fin keel is beneficial.
What is noticeable about the Sui 100 is that its keel fin shape varies over its four-metre depth. This suggests it will react differently as it 'flies' through the water. But Alinghi are not alone among teams in trying this idea.
But why are the rumours of a 'secret weapon' sweeping through the Cup community now? The source will be one camp or another. It does not seem likely that Alinghi would put their optimum keel on show two months before they have to defend the America's Cup.
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