I've used .8mm and 2mm it was the go to formula for boats of around that size here when folks still raced wooden boats, it worked well.
I also raced against 1 guy who used .4mm and 1.5mm, very fast boats but he treated them as disposable, any contact or even an unlucky blowover or stuff and it would be done and he would have to build a new boat.
There is no need to glass a birch plywood boat. Balsa does need the surface hardening for anything other than experimental prototypes, 25gsm E glass cloth and epoxy is commonly used for this.
I don't think it worth doing for a boat that size, but I have laminated balsa into thick balsaply over temporary birch plywood formers, and used that to make a 36" cat with the only birch ply retained at the end being the ride pads and transom, it resulted in a very stiff and very light boat, but was also pretty expensive and a PITA to make.
Paul Upton-Taylor, Greased Weasel Racing.
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