Charlie -

I think for a boat of less than 16 feet, you migh be building a bit "heavy". I would reference our class of high performance (speed) multihulls. The 18 Square Meter class had a lot of builders building with plywood, and the "norm" was 3mm nominal thickness. 4 oz. cloth for covering with 9 oz. tape at seams. These boats are 18 feet long, 12 feet wide, and there are considerable torsional loads. I can't put my finger on the info at the moment, but when you start getting up to heavy stock over 1/2 inch, you are probably looking at scantlings for 28 foot plus sail boats. If in doubt, just add extra glass to hull to spread mast and centerboard stress to a wider portion of the hull. The 18 Square hulls were coming in at about 42 lbs. and many of the home builders felt even at that they were over-built.

Easiest way is to send a email to Gougeons (WEST SYSTEM) technical guys. They are great at sharing information. Good luck