If you email me your address, I'd be happy to send you a sample of the 4 and 6 oz. cloth I use. Sounds like your using a cloth made for extremely structural aplications - especially since it won't completly wet out.
Or... contact the provider of the plans and have them send out some glass samples.

With a stitch and glue, you really don't need a lot since the plywood is very strong in itself. Early Chesapeakes were built with no glass skin other than in the filets. I don't know what they suggest now.