How to design the perfect yacht

By Suzanne McFadden

There are three things that make an America's Cup yacht go faster.

1. If it's longer. 2. If it's lighter. 3. If it has more sail area.

But the International America's Cup Class rule, introduced after the cat-and-dog cup debacle of 1988, does not let you have all three of those at once.

Team New Zealand structural designer Mike Drummond explains that it is a trade-off. So if you want to be longer, you have to be heavier or have less sail area.

"The rule formula must equal 24m, which is about the length of the boat. The mast can only be a certain height, the draft can only be 4m, and it can't weigh more than 25 tonnes.

"The rule is a little bit flawed in that most boats are at the heavy end. So in practice, it's really a trade-off between boat length and sails. Other important decisions are what type of bow shape to use and the beam [width] of the boat.

"The bow can be sharp but short, or have a longer overhang and be blunt. The sharp bow doesn't perform as well in large waves."

In 1995, the black boats were narrower for the light winds, and had longer, blunter bows.

"A narrower boat has less drag, but also has less stability, which limits how much sail area can be carried. A car with narrow tyres can't use all the power of the engine - a narrow boat can't use all its sail area.

"Narrow boats are fast downwind though, because the boats are running away from the wind and don't heel."