top speed is higher and the first reach is the fastest part of the course, so they hoped to out-sprint ETNZ who had gone for their long tip configuration. If they could get to Mark 1 ahead they
had hopes of keeping the faster boat behind. They had seen this play out when BAR were able to keep ETNZ at bay by perfect manoeuvring and course manage- ment, though in slightly stronger breeze. Unfortunately for Oracle, ETNZ man-
aged to create two low-speed starts, with both boats approaching the line at less than 10kt. This meant that during the accelera- tion phase ETNZ had the lower-drag con- figuration and it was only when speeds got above 30kt that Oracle hit their sweet spot. By the time this speed was reached ETNZ had squeezed into a defendable position. Now Oracle had to labour on starboard tack with a board that was demonstrably wrong for the conditions. Somehow ETNZ had already managed
to disturb the psyche of the ‘Pitbull’ and his team to the point that they were already gambling. How did this happen? Was it a surprise? Could Oracle come back? Two things that were hinted at in the
early rounds were now clear: in winds less than 12kt none of the other Challengers or the Defender could match ETNZ. While sailing speeds displayed on the TV showed similar values during straightline sailing ETNZ routinely pointed 5° higher. This shows they had about five per cent less drag than their opponents – a huge amount. In previous Cups a drag excess of half a per cent was almost insurmountable. This drag difference doesn’t arise from an accumula- tion of tiny gains; they had dagger boards, rudders, elevators and wing trim that were all close to perfect for the wind range. Key to understanding the performance
differences are the daggerboards. Unfortu- nately the TV only shows the daggerboards in the raised position, when they are canted inboard to keep the tips from dragging in the water. In this position it looks as if there is a pronounced V in the shape of the ETNZ tips, but when the boards are low- ered and the boards canted out the inboard part of the tip is horizontal when not producing lift to support the boat weight. When sailing this tip probably deflects
up 300mm above horizontal. Also, the tip is as long as the class rule allows, which means that the drag due to lift is as low as possible even though you have extra wetted surface area. Keeping the tip as horizontal as possible means that the lift force from the tip is as vertical as possible and has vir- tually no component pushing to leeward. The more upward angle on the tip the
more the lift forces acting at right angles to the tip push the boat to leeward. If this lee- ward pushing force can be minimised, then the vertical part of the board (the bit being used as a centreboard) doesn’t need to do any extra work resisting the leeward push- ing component of the tip. Conventional considerations of hydro- foil flight stability show that angling the tip
40 SEAHORSE
up makes it easier to keep the boat at a constant ride height by making the system naturally stable. That is why the AC45F and the GC32 and so on have tips that point upwards – they are more draggy than need be, but the boat still foils and can be controlled without complex systems. For boats allowed a height-sensing wand like the Moths this drag vs stability compro- mise does not come into play. Complex systems are permitted in the
AC Class and ETNZ grasped this and went for a low-drag, low-stability solution. Like today’s inherently unstable fighter aircraft such a solution demands trigger-sharp con- trol. Clearly this approach was open to all of the teams, and once ETNZ showed their hand we all tried to adjust our daggerboard geometries to match. Matching the Kiwi control package was a different subject. ETNZ were able to steal a march on the fleet by building strongly on experience
from previous Cups. ETNZ had raced foil- ing catamarans in AC34, they had learnt lessons and retained the key naval archi- tects and structural engineers. They had the world’s three best A-Class cat sailors, at least according to the results in the 2014 worlds: Glenn Ashby, Blair Tuke and Pete Burling, two of whom were multiple world champions sailing together in the 49er. This was a group who were not afraid to
aim high. They were able to sail with low- drag daggerboards that other teams would have viewed as unsailable by giving control of ride height to a third pair of hands. All the other teams gave ride height control to the helmsman, using buttons or twist grips to adjust daggerboard rake angle. This is a natural solution, sailing a boat
with a helmsman and wing trimmer. It leaves you four grinders to give the controls motive power. If you ask a grinder to oper- ate manual controls that reduces his power
RICARDO PINTO
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