Scarfing plywood

With fairing complete, the next step is putting the plywood planking first on the sides and then the bottom.  The sides require a piece of plywood approximately 3'x18' long.  Plywood this size cannot be bought.  Even two 4x8 plywood sheets would not be long enough to make this piece.  Fortunately, 4x10 marine plywood is available.  So, to make a long piece two sheets much be joined together.  There are two methods: one is a butt joint and the other is scarfed joint.  A butt joint consists of butting the two square edges together and then backing the joint with another piece of plywood that is glued and screwed to hold it together.

While the joint is easy to make, the problem with a butt joint is that the plywood will not curve uniformly across the joint.

The scarf joint requires tapering the plywood ends to a fine edge and then lapping the two pieces and gluing with epoxy. A scarfed joint will curve more naturally.  Here's a diagram of a scarf joint:

So, I decided to join the plywood with a scarf joint.  To make the task a little easier, I wanted to cut the 4x10 panels down to the minimum size needed to cover the sides.  To do that, I used roofing paper to make a template by taking a direct measurement off the hull as shown here:

The template showed that I needed a piece of plywood that was 32"x17' for each side.  I cut the plywood sheets down to 32" width and then proceeded to create the scarf joint.  To do that, I used a homemade scarfing jig for my planer that allows a shallow angled cut to be made.  This jig is a clone of the John Henry scarfing tool. Here is a link to an article on the jig and making a scarf joint.

http://forum.woodenboat.com/showthread.php?110517-John-Henry...-Easy-scarfing-and-glueup

Here is a joint in progress.  It takes many passes of the planer to get the joint down to where it needs to be.

Which yielded, with a little touching up with a sanding block, a sheet of plywood ready for gluing.

I laid the sheets on the garage floor and placed a piece of thick plywood under the joint so I could use screws to clamp the joint.  After coating with epoxy, I aligned the sheets and drove a couple brads through to hold their position.  I then screwed another piece of plywood over the top to clamp the joint while it dried.  Here is my clamping set-up.  On the good side, I also covered the plywood at the joint with packaging tape to prevent the glue squeeze-out from making a mess. 

The results were pretty good.  A little sanding touch-up and the joints will look pretty clean.

The next step is to trim these to size and attach to the frames.  While I could use the template I made, I plan to hang it on the frame and mark exactly.  I just feel safer doing it that way.