5/8th vs. 1/2 inch for Bolger Clam Skiff

[TomMcKinney]

I would like to solicit opinions on using 5/8ths plywood to build Bolger's Clam Skiff from Boats with an Open mind (also called a Work Skif on the payson instant boats site), instead of the called for 1/2 inch ply.

The reason for the switch is that I can find 5 ply 5/8ths--actually 19/32--at a good price but only 4 ply 1/2 inch material.The boat is designed to be built with lumber yard ply and not marine so grade is not the issue. However the extra thickness should help durability.

The downside would be extra weight and inhibiting the bending possibly required in the build. I do not think the extra 1/8 will inhibit the required bending but do not have the experience to know.


Thanks in advance.

Tom

[Cuyahoga Chuck]

One of the hallmarks of properly built plywood is that it ALWAYS,ALWAYS,ALWAYS, has an odd number of plys. The current products with even numbers of plies are destined to warp.
With the traditional odd numbers of plies every ply has a mate or two that has it's grain set at right angles to it. That limits wood movement and thus limits warping.
Plywood that warps from the moisture in the air is certainly not the thing to put in a boat hull.

[Bruce Hooke]

I think you will find that 5/8" ply will be a good bit harder to bend than 1/2" ply. However, it would not be hard to run a test. Mock up the section of the boat with the tightest curve and try to bend a sample piece of plywood around the curve. Make sure you use a big enough sample so that you get a representive sense of the amount of leverage you would have.

Keep in mind that the flexibility of a beam varies with the cube of the depth of the beam. In other words, that extra 1/8" makes 5/8" plywood about half as flexible as 1/2" plywood.

[George Roberts]

I believe that 4-ply is laid up in the following order: 0 (degrees), 90, 90, 0.

Plywood people are smart. Even numbers of layers warp no more than odd number of layers.

[Keith Wilson]

IMHO switching from 1/2" to 19/32" ply (.59 more or less) won't make any significant difference. The difference in the quality of plywood is more important than a small difference in thickness. Plywood folks may be smart, but they need to make a profit, and they make their product as cheap as they can get away with for a given application. The 4-ply stuff is designed for underlayment or wall sheathing where it is nailed down every couple of inches, and will often warp just sitting there. I'd be reluctant to use it for a boat.

The Work Skiff doesn't have a lot of sharp curves, and I bet the 5/8 would bend just fine, although with a bit more force. The bottom is doubled, right? Here's a picture. I like that boat a lot; a genuine no-BS design with no yachty pretensions at all.