how much does a laminate spring back

[pvanv]

I need to replace a sliding companionway hatch for a sail boat.

The hatch is about 32 inches wide and just under 40 inches fore to aft. The original is a piece of 1/2 inch plywood with thin teak adhered to the surface for appearances and wear. I can probably salvage the teak "planking". The entire piece is mounted to white oak tracks of a sort, and the tracks seem to be salvageable. The unit is curved up in the center, about 1-1/2 inches. The aft end is supported by a cross piece that matches up to the companionway "washboards", so it will hold the curve very well. The forward end is not supported at all, so the curve must be nearly perfect.

I was thinking of laminating 4 pieces of 1/8" plywood or 3 pieces of 3/16 plywood to make the curve. I am thinking of using either epoxy or resorcinol to glue up the lamination. Probably epoxy as it is more forgiving of light clamping pressures.

I know from experience that the piece will spring back when I take it out of the clamp. The question is "how much?". I would rather not make 4 or 5 of these as tests to get the curve right.

Any help appreciated. Thanks

[jonathan]

There is a formula which goes something like this, for laminated beams : Rjig = Rfinal*(n-1)/n where Rjig is radius of your jig, where you set up your wood to be glued and Rfinal is the final curve radius, the one you want for the finished part. And n is the number of laminates. So for example, to do a beam with radius 5m from ten laminates, you would have to build a jig with a radius of : 5*(9/10) = 4.5 m
Good luck

[pvanv]

Thanks Jonathan. I had seen that formula before, and was wondering: Will that formula apply when using plywood? Also, does anyone have a similar formula for steam bending?

Thanks again,

Paul Van Voorhees