Re: plywood joints
It is much stronger because, as Eric and Christopher noted, the bulkhead is not wide enough to keep the sides bending fair, and prevent the seam from opening up, which would be both a cosmetic problem immediately and a structural problem pretty quickly. But also because when you have the joint over a bulkhead, it forms a hard point and so anytime you have a stress on the side, when a wave hits, for instance or just bending the sides on, the bulkhead behind the joint essentially becomes the fulcrum to a lever and wants to break the sides just at that point, a butt joint, where they are at their weakest. You might not get joint failure immediately, but you would have a crack in the paint at the very least that could lead to other problems down the road.
The other thing is that even if you back the joint up with a butt block, and made the butt block wide enough so it didn't matter in practice, you would still have to complicate your life and make the frame narrower to account for the width of the butt block or you would have a bulge at that point.
With well made scarf joints in plywood, the wood in theory becomes one piece and so it wouldn't matter where the joints fall. But it would seem a good idea to try to keep them away from the frames anyway, at least in boats where the joint runs the whole width of the sides/bottom.
Cheers, Brian
The other thing is that even if you back the joint up with a butt block, and made the butt block wide enough so it didn't matter in practice, you would still have to complicate your life and make the frame narrower to account for the width of the butt block or you would have a bulge at that point.
With well made scarf joints in plywood, the wood in theory becomes one piece and so it wouldn't matter where the joints fall. But it would seem a good idea to try to keep them away from the frames anyway, at least in boats where the joint runs the whole width of the sides/bottom.
Cheers, Brian
--- Inbolger@yahoogroups.com, "dickpensil" <dickpensil@...> wrote:
>
> why are the joints on the sides and bottom not on one of the bulkheads. is it really stronger being held with a piece covering the seam but not directly on a bulkhead.
>
Yes, it is much stronger. You need at least an eight to one scarf. 12 to one is much better. This can be achieved either by a butt block backing up the seam or by a tapered scarf on each sheet of plywood. Bolger generally calls for a butt block backing, but scarf seams are fairly easy to make, especially if you have several to make. Stack the plywood so the ends don't meet by the length the scarf needs to be and then plane the plywood from a point that many inches back from the top sheet of plywood to the bottom front edge of the bottom sheet of plywood. A power plain makes quick work of it. From what I have already said I am sure you have already realised that the width of a bulkhead is no where near wide enough to scarf two pieces of plywood together on.
Eric
Eric
--- Inbolger@yahoogroups.com, "dickpensil" <dickpensil@...> wrote:
>
> why are the joints on the sides and bottom not on one of the bulkheads. is it really stronger being held with a piece covering the seam but not directly on a bulkhead.
>
On stick and frame plywood boats, the plywood is often fastened to the
longitudinals (Chine, sheer, stringers and keel) only not the frames.
Fastening to the frame provides a hard point, the fiberglass/epoxy taped
seam provides a long joint with a lot of area.
HJ
dickpensil wrote:
longitudinals (Chine, sheer, stringers and keel) only not the frames.
Fastening to the frame provides a hard point, the fiberglass/epoxy taped
seam provides a long joint with a lot of area.
HJ
dickpensil wrote:
> why are the joints on the sides and bottom not on one of the bulkheads. is it really stronger being held with a piece covering the seam but not directly on a bulkhead.
>
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Chris
dickpensil wrote:
why are the joints on the sides and bottom not on one of the bulkheads. is it really stronger being held with a piece covering the seam but not directly on a bulkhead.
why are the joints on the sides and bottom not on one of the bulkheads. is it really stronger being held with a piece covering the seam but not directly on a bulkhead.