grob,

Cool idea with some interesting challenges to pull off. One challenge everyone with a cat faces is how to keep your hull sealed to keep water out never mind presurized to the point where you are requiring it for structural stability. It would not be too bad if traditional catamaran construction did not require so many attachments.

The second issue is to provide a hull shape consistant with good performance. Weight is not the sole factor in how efficient a hull form is hydrodynamically. To provide adequate stiffness increase to a thin skin material like polyethelene, you will end up creating a tube. This shape would be more efficient fully submerged, but not on the surface.

I would be really curious as to how this ends up working. Please keep us informed as you go.

Good Luck

Matt