Attached is a photograph of an Auscat MkV A-class under construction. This photo came from the Australian High Performance Catamarans Website:

http://www.ahpc.com.au

You will notice in the picture that the cross members (if that's what they are called) are made largely from polystyrene. It is partly this use of polystyrene that enables these boats to be built so light. The polystyrene of course acts as positive floatation.

I'm not sure about the A-class rules but the Taipan 4.9 rules (which have a similar construction) suggest that each hull can support 50kg when swamped.

Rob Wilson
Taipan 4.9 AUS175