Good to see someone prepared to go out a a limb and try something different regardless of commen sense! I've been a lurker here for awhile but am interested to see if this project goes well. As Gary mentioned some bigger cats have WRC composite construction, I built a 15metre cedar/epoxy/glass (not carbon) mast for my 11metre Schionning catamaran. The mast was designed by Geoff Schionning for either glass or carbon construction. The WRC component however is only 4mm thick planks stripplanked over a male half mold, glassed with varying orientations of unidirectional cloth inside and out then two halves joined / faired and painted etc.
I only did this project as I like making stuff and the time it took meant I had a motor cat instead of a sailing cat for 6 months. There maybe some info at schionningdesigns.com.au as I believe he is designing an all carbon A cat as well as his usual bigboats.
Personally I think a thin ply wall would be a better option done in similar fashion with glass or carbon, maybe do-able as a one piece if all the glass goes on the outside and blocking as needed is added before gluing the ply to fwd and aft stringers.
