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Rod Davis


Party time (at last)


Interest in the 36th America’s Cup and the new foiling 75ft ‘monohulls’ has finally taken hold. So I thought it would be a good time to break down these new boats into the challenges all the teams are dealing with in creating the best package to win the Cup. Firstly, I will state my self-interest – none! I


have no affiliation with any team, nor the running of the America’s Cup. For the first time in 40 years I am just like you and watching the show from the sidelines. Whether you like the new boats or not, whether it’s sailing or


flying, good for the Cup or bad, the fact is each of the teams is pushing the envelope to its limits. That is pretty cool, in a clean sheet of paper, experimental kind of way. As we all know, if you are not foiling you lose, and lose badly.


That is clear enough, but there is foiling and there is fast foiling. Fast meaning knots faster than just getting foiling. Fast foiling requires smaller, lower-drag wings; whereas just getting up foiling requires lots of lift (and drag). Picture it by thinking of a glider with its very long wings and


maximum speed of 60kt. A Spitfire has short wings that can’t even take off at 60kt but once it does it can fly at 250kt. The teams can only use one set of foils per regatta so every trick needs to be used to get foiling on the smallest foils possible. Let’s have a look at how that is going to happen, starting with


the hulls. In basic terms there are three top considerations in hull shapes – quick lift-off, good touch’n-go shape and an aerodynamic flying shape that is low drag. All the teams have gone for a bustle (bulge) running down the


centreline. This bustle allows the boat to rise up fast, almost solely sailing on the bustle. The effect of that is reducing the wetted surface of the whole thing so the boat gets to ‘fly’ much more easily. Voilà! Next step the whole boat is out of the water, flying on the leeward foil and rudder. Water is 600 times denser than air. Think of water as a sticky


34 SEAHORSE


glue. The less hull touching the water, dragging across the glue, the better off (like way way better off) you are. Thus this bustle not only helps with that, but helps in the touch-down after a tack or mistake, like a touch and go. The boat is able to sail on the bustle for a split second and be up and flying again quickly. The other big factor (there are hundreds of little factors) is a


low-drag shape when the boat is flying. All the boats have different takes on the best shape, but they all agree that the shape needs to have rounded surfaces and few sharp corners. This is where they are looking more like racecars than boats. Some of the boats have a further fin-like skeg running down the


centreline of the hull in addition to the bustle. This is solely part of the aero package. The thought is to seal the rig/boat with the water, end-plating the whole boat. This requires the boat to fly very low, close to the water, making the air flowing under the boat run fore and aft, then out at the back, rather than across the bottom. Crazy, but this is the modern America’s Cup! There is a rule that all the boats must have a certain amount


of stability. This rule is there for the simple reason you don’t want the boat to fall over when sitting at the dock. If not for this rule the boats would look very different. In real terms this means every boat has to have waterline beam


(WLB). That is how wide the boat is where it touches the water. The interesting thing is that the three challengers placed this maximum WLB in pretty much the normal place for a sailboat. Team New Zealand have pushed this static stability all the way to the back of the boat. The effect of that is the first half of the Kiwi boat has less volume, thus a nicer shape to get flying and has a better aero package. At least that is the thinking. Let’s have a think about the decks. Cutting drag caused by the


wind, and sealing the sails to the deck, are the focal points. The air going across the deck is travelling close to 100kph. Stick


your head out of the car window when you’re going down the motor- way. What you feel, as the air is trying to peel the skin off your face, is the drag the teams are trying to minimise. Oh, just for fun, and


MAX RANCHI


GILLES MARTIN-RAGET


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