What ply?

Do you really need marine grade ply for a smallish dory?

 

You could get away with using exterior plywood, but it will de-laminate sooner. If you're talking about a small tender that spends most of it's life out of the water, then normal ply is fine. I would personally build it in exterior ply and then "paint" it in an epoxy-resin from SP-systems make sure all the joints are epoxied, and it is "painted" inside and out. Then do a nice paint-job on it (car spray paint works well) and you should have no problems.

Please note that if it is to spend all it's time in the water, use marine-grade ply. Anything else is a false economy.

Good Luck,

Tim B.

 

Yeah thanks Tim.
I thought as much but it is only a cartopper in 6mil and i was going to sheath the entire boat with epoxy after glassing the joints/hull bottom etc:
The only thing is "Exterior grade" here is just the crap you use to brace the framing on a house and I think if you used 6mil exterior it might distort whilst bending to the shape of the hull,Am I being a big wooos"jesse" about this or is 6mil thick enough to handle the torsion without bulging on one side and looking like crap?

 

You could always laminate 2 sheets of 3mm ply for better (material) stability. Also, try looking in a local model-shop (Hobby-Lobby or the like) for 3mm ply. It's higher grade than exterior (more expensive, but not ridiculous) so it would probably go around highly curved geometries better. Two layers of that and you'll have you're 6mm skin that should be ultra-stable (in terms of shape). Also, do you really need 6mm or was it a convenient thickness? I'd have guessed 4mm with some re-inforcing around the keel and chines, myself. It would be a little lighter that way... may not be as robust though. In fact, you could probably get away with 3mm on the sides, if you want to reduce the weight. I'd then cover the whole lot in glass cloth (woven) at +-45 degrees to add to the torsional strength. The thinner the ply is, though, the better the quality must be.

As with all these things it's a trade-off, between weight (and performance) and durability.

Good Luck,

Tim B.