I've been suspended on rocks with a double layer of 4 oz under me to good effect. I would not use just one layer, but two should be good, or you could compromise and use 4 oz for most and double up between the chines with a layer of 6 oz over the 4. Everything is a trade off you need to decide how important strong vs light is. Remember that plywood is a little harder than cedar strips and as such will not compress as much under pressure. This will make it less susceptible to compression damage, but more susceptible to some slicing damage.

: This Sat. I found myself caught on top of a smooth rock pile with the waves
: going out from under me etc. 1 1/2ft seas. basically calm conditions. I
: put some nice deep scratches into the bottom glass that won't re-wet out.

: Question or concern: I'm building a Sea Spirit S & G hybred and was going
: to try to use 4 oz. cloth throughout to make it light. After this Sat. I
: reafermed my belief in 6 oz. with an added 6 oz. football on the botom. I
: know many use 4 oz. and I'm convinced that it's a fals sense of security
: or for lake paddling only. If I had 4 oz. on my Outer Island, I would have
: been into the wood. I think it's cool to carry a super light boat on your
: shoulder down to the beach but to take a pounding on a rock, I just don't
: think 1 layer of 4 oz. will protect the hull at all.

: I scratched up mine good and remember, - calm conditions, no real big waves -
: just laying on some smooth rocks and having to lift the hull with my hands
: to free myself.