JEM Watercraft
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linear joints for wood strips/boards
http://www.jemwatercraft.com/forum/viewtopic.php?f=5&t=3995
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Author:  LesForgue [ Fri Jan 23, 2015 12:40 pm ]
Post subject:  linear joints for wood strips/boards

Seen much info here on scarf and butt joints,
but what about this kind (which I have no name for)
Would it be strong, stable, etc?
Why does it not seem to be commonly used?
Thanks.

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Author:  jem [ Fri Jan 23, 2015 6:36 pm ]
Post subject:  Re: linear joints for wood strips/boards

A lapped or rabbeted joint?

It should work with the proper dimensions (8:1 of the wood thickness is traditional) and epoxy. Tough part is the wood used on canoes and kayaks is pretty thin to start with. Not easy to mill the rabbet joints.

Author:  Mark J [ Fri Jan 23, 2015 9:08 pm ]
Post subject:  Re: linear joints for wood strips/boards

Matt, as you know some of the pre cut boat kits these days are coming with panels having a "puzzle piece" shape cut into them for interlocking.
I've been meaning to try it in my shop using a dovetail fixture. Not necessarily the dovetail shape itself but more along the lines of a simple finger joint which could easily be set up and accomplished with even a small trim router on this thin plywood. You could actually do a joint that wouldn't be as noticeable as some I've seen along these lines. For that matter you could make a simple pattern to router cut the "puzzle piece" shape into your panels.

This spring, I might tinker around with the router and develop a simple fixture to use.
I always have 2 or 3 projects going on at the same time in the shop anyway. No need to open up the shop and do an hour's worth of work on a boat and close it back up until epoxy cures.

Author:  OnkaBob [ Fri Jan 23, 2015 10:35 pm ]
Post subject:  Re: linear joints for wood strips/boards

I'd call that a halved lap joint and would estimate it as being close to half as strong as the original ply as well as considerably weaker than scarf or butt block joints.

The problem with it is that in 2 places there is a butt joint extending half way through the ply and since butt joints have very little bending strength it is like being reduced to half the original thickness at those points.

There is a butt joint when using butt blocks but the backing piece is the full thickness of the ply so as long as it is wide enough the joint will be close to original strength .

With the scarf there is a gradual reduction of one side as the other gradually increases so at no point is there a significant reduction in effective thickness.

As long as the joints are glued well the scarf and butt block joints are going to last but the halved lap is likely to crack at the outside half butt joint when flexed. So, for the amount of work involved to get away from butt blocks you may as well do a scarf.

Author:  LesForgue [ Fri Jan 23, 2015 10:51 pm ]
Post subject:  Re: linear joints for wood strips/boards

Thanks for the replies.

Had been wondering since I was considering buying sticks too short for the gunnels and joining them,
now if ever I do that I would try scarfing probably,

but when I seen how much clearer and straighter the 16 foot sticks are at the store, I will buy a 4 or 6 inch by 16 foot piece to rip my gunnels from. (for the JEM Trapper 15-3)

Thanks.

and Thanks Matt for this forum even being here!

Author:  jem [ Sat Jan 24, 2015 10:15 am ]
Post subject:  Re: linear joints for wood strips/boards

Puzzle joints work to both join and align. The tolerances of the cut have to be very precise to achieve that. We're talking like 1/32" precision. That's why they typically limited to CNC or specialized bench tooling. The panels also have to be lengthened a bit to compensate for the length of the interlocking teeth.

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