The America's Cup has very strict design restrictions. For example, in the latest edition, they weren't expecting catamarans to be able to foil because they had disallowed moving parts on the control surfaces. The Kiwis realised that you could actually move the entire control surface to get the desired control. It was awkward, which is why they had so much trouble actually controlling the foiling.

Anyway, yes, there are a lot of design restrictions in America's Cup yacht design.

That surprises me - I would have thought they would have disallowed the catamaran entirely. It's a radically different design. Are the restrictions for safety, or to cap some kind of performance?

The winner of the Americas Cup determines the rules for the next one. In this year's Ellison wanted catamarans. Here is some more info on the some of the rules http://www.cupinfo.com/en/ac34-americas-cup-2013-ac72-class-...