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Draft Dodgers

The America’s Cup does not take place until the summer of 2017 but the design race between the six competing teams is already raging fiercely. By the time competition begins on Bermuda’s Great Sound next year they will between them have splurged tens of millions of dollars in the hope of creating the world’s fastest 50-foot catamaran.

When it comes to designing a flying boat capable of winning the America’s Cup the designers’ holy grail is the elimination wherever possible of speed-sapping element known as drag. Oracle Team USA performance director Ian Burns explains for us the Cup defender’s approach to this complex challenge.

There are several ways of reducing a boat’s drag. Number one is to make it lighter, but in the America’s Cup that doesn’t necessarily result in a direct gain. It would have much more effect if we were in a free situation where we could take the weight off the boat completely, but the class rules constrain us from making the boat too light or heavy through a minimum and a maximum weight. 

However, reallocating that weight appropriately can make quite a big difference. Removing weight from places where it is not wanted - like the extremities of the boat – and moving it to places where it is more effective can produce a pretty nice drag saving, especially in specific conditions. 

You could also trade weight off against dagger board strength by removing some of the core in the middle of the board and putting in more carbon to make it stronger. Avoiding breaking a dagger board at a crucial point could feasibly win you the America's Cup – so if you had 10 Kg to spare, that could be a good place to use it. Alternatively, you could decide to put that 10 Kg into making the boat’s cross beams stiffer or stronger – an area where you are allowed to be in excess of the rules prescription. 

The other factor in drag reduction is trying to eliminate parasitic drag. That’s the stuff caused by the guys standing in the wind or the trampoline netting in between the hulls that all slows the boat down and decreases your efficiency. Any gains you make in those areas are just freebies because you are not reducing the power, just reducing the drag. You will see the sailors kneeling and generally staying low as much as they can. If they could get down any lower they would, but those hulls are quite tight and they already need some acts of contortion to get into some of those spaces at times.

There is a significant amount of drag generated from the cross beams and the interesting thing here is that you can either try to reduce the drag or make it work for you – either way there are some big potential savings in percentage terms. 

The beams are one design but the leading edge is an aerofoil shape and to a certain degree we are allowed to do what we like behind that, meaning there are opportunities to make some quite big gains there by fitting fairing panels. The question is whether you want to lift the boat, push the boat down, or just minimise the drag and not do any kind of force creation. 

You can use your fairings to roll the boat to windward so that you can pull the wing on tighter. To explain this, the boats are very finely balanced in that you have the combined weight of the crew and the boat being lifted by the leeward foil. That means that, effectively the righting moment - the force that stops the boat falling over when you pull the wing on - is pretty much fixed at that. 

If you can use the angle and shape of the fairing to roll the boat a bit more to windward, that’s a gain in righting moment and it will make you go faster. Achieving that is not a simple task but the flow is quite different on the windward and leeward sides so there is an opportunity to make a significant gain. 

Those are the sort of trade-offs that the teams are all researching right now. 

Since the America’s Cup went foiling the problems we are solving are highly complex and often multidimensional. What we had to deal with in monohull Cups were relatively simple compared to now – even though they didn't seem simple at the time and we spent a lot of time getting it right. Whichever team best gets their heads around the 3D challenges of designing a boat for the 35th America’s Cup could well end up passing around the trophy in Bermuda next year. 

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Susie Goodall: Old School Around the World

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