I can't see how you could biscuit joint 4mm plywood, there just isn't
enough material to cut a slot in is there?

There are 2 ways that I can think of that plywood can be joined end on
in boatbuilding, the scarf or scarph joint and butt joint.

The scarph involves cutting both ends to be joined at an angle so they
overlap (you finished plank would be less than 16' by the length of the
overlap) the longer the overlap the more area for the glue as John says,
so the stronger the joint. It is fiddly if you don't have a bench saw or
sander with suitable jigs to get the angles. An alternative way to make
the scarph is to cut down to 1/2 depth and then remove that half of the
material (count the veneers?) out to the edge - then you can overlap the
boards without getting a step. I'm not sure if this would actually be
easier than cutting the angle, I guess you could do it with a chisel,
plane and sander but it might be worth sorting your technique out on
some scraps first!

More commonly plywood is simply butt jointed, but obviously it needs
some reinforcement if you do this. In the stitch and glue method of
construction, the adjoining edges have holes drilled in through which
little bits wire are threaded and then twisted to draw the edges
together (this is useful where the "planks" are joined at chines but can
be used just as well for seams). Once everything is right fibreglass
tape is resined onto one side (a more modern method involves epoxy
filleting the inside joint to level that will hide the wires) and when
dry the wires can be cut on the other side and sanded flush before being
taped over. In the first case the original tape needs to be removed (hot
air gun works well) and the wires on that side pulled out or tidied up
before taping neatly.
As you don't have any shape and are presumably joining before bending
you might get away with laying them together and taping one side, and
then when it's dry, turning over and taping the other side.
Another effective method is to screw and glue a batten or say a 3" - 4"
wide strip of plywood over the butt to reinforce it. Obviously this does
not look as neat unless the batten/strip is going to form part of your
internal arrangement (on my dinghy the sheets are all joined at a frame
so not seen from inside), and obviously don't put the strip on the
outside which you want to keep smooth for resistance reasons!

JIM

Fred wrote:

advice wanted on joints in plywood. I am making a canoe out of 4mm plywood
and it mentions scarf joints, which I now know about. The question is could
I make the Joints biscuit joints and woiuld they be as strong?

(joining 2 x 8x4 to make one 16ft long plank)

fred


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