you should contact someone in the unicorn class in uk , unicorns are built from tortured ply , Gary Piper is current proffesional Unicorn builder and should be able to give advice , Bob Dorks too has built many . I think they use exterior ply as it bends easier than Marine when taking up shapes for a cat . above folk can be found through unicorn website http://www.unicorn-cat.co.uk .

I also had an A class cat built for me from a kit supplied by Roger Dewen a noted Unicorn and A class sailor, I think Roger got the ply he used manufactured to his own spec , the fella that put the hulls together for me - Bob Higgins claimed it was the easiest boat to pull together he had ever built , even though the A Class had more rocker (harder to form ) than the unicorns he had built , ply was 3mm thick , when wested both sides (no cloth) was strong enough , though I was always carefull when capsized to stand on bulkheads or deck edges .

Roger can be found on uk A class website in for sale section , he's selling a boat at the moment

http://www.aclasscatamaran.co.uk