sorry I didn't see this PM earlier. The height of the amas (outer pontoons) is determined by the desired use of the boat. Some trimarans do have both of the amas in water, like the Neel cruising trimarans, while the ocean racing trimarans have them several feet out of the water. I think the primary reason for having them so high out of the water is to prevent them from being hit by waves. You don't want to be flying along at 40knots and have that windward ama pound into a wave. The Neel is a much slower and heavier trimaran and when sailing the windward ama is out of the water by only a small amount. If it gets hit by a wave, the effects are minimal as the speed is low and the momentum is high. The designer didn't want the boat tipping to one side or the other when stopped at anchor as this is uncomfortable for a cruising boat people live on... for performance/reduce drag, you only want 2 of the 3 hulls touching the water at any time.

thanks for the feedback on my ol boat project....your the only one that actually gave me some calculations... I have a different question ....on a trimaran why do the outer pontoons sit a little higher? I mean why not have them a little lower than the main fusalage to have them up on water first.