Builders' Forum Archives
Re: Ply thickness substit
Posted by Laszlo on Aug 16, 2007
I've had 4 boats with 1/4 inch (6mm) bottoms. The one that was designed to take the most stress was a planing garvey that could do in excess of 25 mph. This is way more stress than a Jimmy Skiff would see.
After all, when the boat is in the water, it's the water which supports passengers & cargo. The bottom just keeps them dry. Think inflatables.
So while we should wait for the designer to weigh in, I personally would feel totally comfortable building a Jimmy Skiff with a 1/4 inch bottom. For those who want something better than an empirical gut feel, check out Arthur Edmunds' book Designing Power & Sail. Chapter 6 gives the formulae for calculating framing and bottom thicknesses. The Jimmy Skiff with 3 adults moving at 6 knots should only have around 2 psi of pressure on the bottom. That's well within the capacity of 1/4 inch plywood supported on an incompressible fluid.
On the other hand, the seats, dagger boards and transom give the boat its transverse stiffness. There I wouldn't use thinner wood. Éric, if it's an availability problem, just cut out 2 of each piece, laminate them into 1/2 inch thick and use that. The weight gain will be offset by the lighter bottom. There shouldn't be any stability problems either, since the crew is the ballast.
Ron, nice boat.
In Response to: Re: Ply thickness substit by Ron Paro on Aug 16, 2007
- Re: Ply thickness substit by Ron Paro on Aug 16, 2007