Like many in the sailing community I watched with interest the recent America’s Cup action in Valencia. It’s been a long time coming and while the racing wasn’t exciting in the usual tactical and sporting sense the intrigue about whose boat would be faster more than made up for it.

Prior to the race I was of the opinion that Oracle’s boat should be quicker upwind so long as it could get the middle hull out of the water, due to it’s higher righting moment, and Alinghi’s boat, presumably lighter would have an advantage downwind. The big unknown, other than the weights was the wing – it had to be more efficient but would it make the difference? Well the answer is yes.

There are two reasons that a wing is better than a sail. The first is it has a better lift to drag ratio so for a similar lift it should have less drag and hence be faster.

The second, and I think more important in the conditions the cup took place in, is that a wing has a higher maximum lift coefficient. For a sail the highest lift coefficient that could reasonably expect to be achieved is around 1.5 and for a single element airfoil this might get a little higher. But a slotted wing, like Oracle’s, is capable of much higher lift coefficients.

In light wind, when the yachts crave power cl max is crucial and this week it was clear Oracle were able to use their wing to generate higher lift – get their hulls out of the water and sail away from Alinghi.

Another method that needs to be mentioned in any discussion of CFD in Naval Architecture is the Michell integral for the wave resistance of thin ships. First published in 1898 by Australian mathematician J.H Michell, it uses a centreplane source distribution proportional to the rate of change of local beam to represent the potential flow field. It assumes that the change in local beam must be small, an assumption that only holds true for thin ships, hence it is often known as thin ship theory.

While it does have limitations the Michell integral is a very useful tool particularly in multihull design and wash studies. See whole thread

CFD has a bit of a reputation for eating up time and dollars. Sometimes it can take days to generate a solution and then, after all that time, you realise there was a mistake in the setup.

A good way around this is to start your simulation with a small number of large cells, that way it’ll run quickly and you can iron out any setup and physics errors without having to wait days for the computer. Then when you’re ready put in more cells and get an accurate answer!

This won’t avoid every problem as some issues, particularly with convergence, don’t show up until the grid is finer but it’ll get you off to a good start. There’s nothing worse than waiting overnight for some precious results before realising you forgot to scale the geometry! See other tricks