Why are racing catamarans' hulls not teardrop shaped? Looking at the America's Cup 72 class catamarans, I was wondering about their hull shape, which has a sharp wave piercing bow. My question is: since an airfoil shape experiences less drag than a sharp triangular one, why does the AC72 catamaran design have the latter? Does it have something to do with wave drag or with the speed of those boats?
 A: I think piercing the waves gives more pitch (backward/frontward) stability. If a big wave hits the bow then you do not want your bow rapidly forced upwards and you really don't want your bow to be forced downwards which can lead to pitch pole capsizing.
The most common capsize in sailing is for the boat to be pushed onto its side, but sailing boats can nose-dive into waves and pitch-pole capsize. The bow goes into the water and the stern goes up in the air. Pitch-pole capsizes are, I believe, more nasty than regular capsizes, particularly on such complicated catamarans.
A: If you look at pictures of the boats in action you'll notice that they have hydrofoils and so the hull shape is only important at slow speed: in maneuvers or very light wind.
Having said that I would guess that the hull shape is to help minimise wave drag, which is fairly significant, especially as you approach foiling speeds.
Firstly, a more pointed shape helps pierce the waves and generally reduces wave making drag.
Secondly, having an inverted tilt on the bows could help increase the effective length of the boat and the period of any wave produced. Wave friction scales rapidly with speed/length. Therefore, increasing the length of the boat helps decrease wave friction (obviously at a cost of increased skin drag and weight).
When near to planing/hydrofoil speeds wave drag is probably dominant, as it scales much faster than skin drag which is the other main drag component. Therefore it is most beneficial to minimise the wave drag.
