Using Marine Ply in replace of Oak Sawn frames

[CSJ]

Hello,

Further to my last post; I am trying to assemble the frame components of a boat using marine ply components rather than traditional sawn lumber.

My reasoning behind this is to save a bit of time & tooling along with the hassel of having materials sent to the NWT Canada (N of 60).

The plans that I am working from show 7/8" x 2-1/2" sawn oak frames. Can I simply replace these frames with 7/8" marine ply?

What considerations should be made for weight & strength of the plywood vs the oak frames??

Thanks for your thoughts!!

Steve

[JimD]

Steve, I can't see how any plywood is going to be equal to oak. Not even close. Can you say more about the design, please?

[Bruce Hooke]

This is not a simple replacement for a couple of reasons:

1. The big virtue of plywood is that it is strong in both directions, whereas the strength of oak is focused in one direction. For example, if you where to cut out 7/8" x 2 1/2" by 24" pieces of oak and plywood and then tried breaking both I think you would find the plywood easier to break except if you tried to break the oak in a way that results in it splitting along the grain. This test would be easier to run on 1/4" material if you are just trying to understand what is going on.

2. If the plans call for running fasteners into the frames to hold the planks in place then you should be aware that fasteners into the side of a piece of plywood are not going to hold with nearly the strength as the same fasteners running into oak. However, if the primary structural connection comes from epoxy then you are in better shape.

To deal with issue #1 you would have to increase the depth of the frame a bit (say to 4"), but I do not have enough experience (or anywhere near enough information) to say how much that increase should be. At the same time it might well be possible to drop the thickness to 3/4" becuase 7/8" plywood is likely to be hard to find.

My father spent many summers doing research in the interior of Baffin Island so I am somewhat familiar with the challanges of getting specialized supplies up to the far north! Good luck with your project!

[Hal Forsen]

Fellow forumite Bill Childs is building Bartenders and selling kits (with Mr. Calkins approval) using frames cut from ply with CNC.


HF