3/8 or 1/2 inch ply?

[robertgrandbois]

I am restoring an old plywood boat. The frame is all red oak and in very decent shape. There was 3/8 plywood on it before, but I can get 1/2 inch on sale at half price but I am scared it will not bend at the bow. Can I do the bow with two layers of 1/4 and the rest with 1/2 inch?

[alan white]

You're right to be worried. The thicker plywood isn't just 50% stiffer. It's probably about twice as stiff as the 3/8. Unfortunately, short of running the finished piece through a drum sander to make it thinner, you'll have to bite the bullet and buy the more expensive plywood.
You could, as you mentioned, use two layers of 1/4" plywood, but you will use a lot of epoxy becuae the whole piece ought to be 1/4", or else you'll have to scarf it in, which is a lot of bother, to say the least.

[robertgrandbois]

that is what i tought but was hoping to save a few bucks. i will go with the 3/8 plywood. Now could i use the 1/2 inch for the top? it does not seem to have much curves to it.

[alan white]

Yes, of course, at the cost of some extra weight.

[tom28571]

You can scarf a piece of 3/8 to the 1/2 for the bow section. Works fine and this method is used by many.

[robertgrandbois]

ok tom i but i was thinking of doing a but joint would that work?

[lewisboats]

no...the transition needs to be as strong as possible. To use a butt block you need to have the same thickness to butt together. That option really needs a 12:1 scarf.

[alan white]

Originally Posted by tom28571

You can scarf a piece of 3/8 to the 1/2 for the bow section. Works fine and this method is used by many.

That's true. It would work fine and would be practical if a scarf was required anyway to lengthen the panel. If not, buy the 3/8" plywood and save the hassle.

[robertgrandbois]

I got the ok to get all 3/8 from upper management(wife). There originally was a scarf but I was thinking of a butt joint instead be faster.

[alan white]

A butt located where there's not much curve or twist, with a backing plate, is not a problem. Especially if the exterior will be glassed.

[ancient kayaker]

Material stiffness varies as the cube of thickness. 4/8 thick ply compared to 3/8 ply would be stiffer by a factor of (4/3)^3 = 2.37

Thats assuming uniform material. However if the ply has relatively few, thick veneers, it would be more flexible in one direction, so perhaps you could use it "cross-grained" and still get around the bends.