Butt Joints (in Plywood)

One of the major reasons that I built a Puddle Duck Racer, and then a Mouse Boat was that both of these boats are under 8 feet long. That meant that I could easily purchase lumber that was as long or longer than the length of the boat. I did this because I assumed that joining pieces of plywood or timber was difficult.

It certainly seemed difficult. Most books on boat building spend a significant amount of text on the subject, and the Internet is full of jigs and strategies for scarfing lumber and plywood. Pre-built kits get around the need for scarfing plywood with fancy cutouts that allow the pieces to fit together like puzzle pieces. A bit of fiberglass and resin over the top and the joint becomes the strongest part of the sheet. Of course, I knew right away that this sort of precision was right out for me. Not to mention the fact that I want to steer clear of epoxy so that I feel more comfortable getting kids involved.

I knew when I started building a Lazy Weekend canoe that I was going to have to join lengths of plywood. Once that was decided I simply had to decide how I was going to join the plywood. In the end, I decided that I would try the least technical of all the plywood joining techniques. The butt joint. A butt joint basically consists of two pieces of plywood butted up next to each other, with a third piece of plywood on top that holds everything together like this:

Traditionally this joint would have been reinforced with clenched nails or bolts, but modern glues are stronger than the wood and make the reinforcement unnecessary. At least that is what I hope.
Traditionally this joint would have been reinforced with clenched nails or bolts, but modern glues are stronger than the wood and make the reinforcement unnecessary. At least that is what I hope.

Under most boat building situation this apparently works just fine. The biggest problem with the butt joint is that it is very ugly. Not only does it tend to leave a crack on the outside of the joined plank, and a big ugly bandage looking spot on the inside of the joined plank, it also bends funny because it is twice as thick as the normal plywood.

This is what making butt joints in plywood looks like. At least this is what it looks like when I do it.
This is what making butt joints in plywood looks like. At least this is what it looks like when I do it.

Originally, I had planned to put a strip (or three) of fiberglass drywall tape bedded down with Titebond II over the outside of the plank. However, the joint turned out so well that I am probably just going to paint over it now. I made the butt part of the joint over-wide, but that doesn’t really hurt my feelings.

Inside of butt joint, under bent to approximate final position.
Inside of butt joint, bent to approximate final position.

Apparently I don’t have a picture of the outside of the joint, but that’s OK, there really isn’t much to see. The crack is a tiny hair line well filled with Titebond II. As long as you are joining the factory edges of 5mm thick plywood I don’t think that you have anything to worry about. If you don’t mind the way that these joints look, then they are very simple to do.

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