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JOBBER
11-04-2016, 06:38 PM
I’m not sure it the below has real merit, or it’s just a mind exercise. I tried Googling the below, but only found how to make a scarf joint.

1/4” [6mm] meranti marine plywood is 5 layers.
The 2 face layers and the center layer have the grain parallel to the 96” [2440mm] length.

For a boat bottom (42” amidships), is there any advantage to cutting the sheets in half (48”x48”) and making the scarf parallel to the face grain?
The assumption is there would be less bending with the maximum number ply sheet grains going the short way.

The above presents the 2nd question. If the OAL length is 120”, this would require 2 scarfs in the bottom.
Would all equal sections, or 2 equal and one odd be a better pattern choice?

BBSebens
11-04-2016, 06:42 PM
You are way over thinking it. There is little, probably nothing, to be gained from what you are suggesting. Use the plywood to get the most efficient use out of its dimensions, and you will be just fine. No need to go creating more work for yourself.

There is some wisdom is staggering scarf joints, but that matters more in solid lumber than plywood construction.

Tom Lathrop
11-04-2016, 08:53 PM
Not certain what your real question is, but the ply will be stiffer with the outer plies grain across the width of the boat. Requires more scarfs but 42" is a lot of unsupported span for 6mm ply but this orientation is stiffer. Stiffness varies by the cube of thickness so the outer plies are most important. All this depends on whether there is a keel/keelson or other frame structure.

David G
11-04-2016, 10:31 PM
You are way over thinking it. .

This.

There are only a few exceptions.

One example - if you are cutting planking for an area that has a LOT of bend, and you are using one of the stiffer species (meranti, fir) you might be served by making you cuts on the bias, as a sheet well bend a little easier with plies running at 60/30 degrees or 45/45 than at 0/90. But for the vast majority of applications... no reason to dink around.