Scarfing Begins

In the 'stitch and glue' method of building the hull, the bottom and sides must be single pieces.  On a 18'-6" boat this means the bottom and sides are far larger than a standard 4x8 foot sheet of plywood.  Scarfing is the technique of joining sheets of plywood together to make really big pieces of plywood.

After a couple of weeks of testing both 'stepped scarfing' and the traditional 'tapered scarfing', I chose traditional tapering.  It has two advantages; setting router bit height is easier than the stepped scarfing method, and the flat taper is far easier to spread epoxy glue on.

There are numerous ways of cutting the taper.  Since I already owned a good router I chose this method and made a jig to router the taper at a consistent angle.  Searching the internet shows many jigs for router scarfing but most are for narrow boards.  I need to scarf the entire 8 foot length of my plywood and pondered some days away to design this carriage sliding along a groove in a straight edge.

 This jig is specific to 12mm plywood, cut to a 3 inch wide taper.  The angle the router runs up and down at is determined by the height of the carriage runners.  The bottom runner slides directly on the table and the upper runner slides in a channel on the straight edge.  

I did a test on some scrape plywood in my basement shop. This gave me an important piece of knowledge.  With this jig setup and method it is very important that the table, plywood, and straight-edge guide be at precise heights.  I accomplished this by screwing the two sheets of plywood to the table every 16 inches and screwing down the guide every 8 inches, holding the whole works tightly together.  Yes, this puts many screw holes in my boat, but putty and paint cures many things.

Side notes; Carriage and straight-edge guide are made from an old piece of pergo flooring.  To prevent the carriage from bending to the weight of the router I nailed and glued two short walls on either side just under the router handles.  Also, I placed the bottom stop precisely to prevent the router from cutting into the table, which in my case is other pieces of plywood stacked up.

Cutting is done in three stages.  The first two takes off about 5mm each stage, then the third time the router bit-depth is carefully adjusted to do the final cut.  Even with the vacuum attached to the router, I had to stop a half dozen times to vacuum away the piles of sawdust.

The first 4 feet cut.  The guide is then moved to the remaining 4 feet.  The plywood remains screwed in place. 

First 4 feet of the 8 foot length.

 The finished cut, unscrewed from the table and separated.

The finished cut.

 There are 12 scarf joints to be done before the bottom and side panels can be cut out.  There are four 8-foot scarfs, and eight 4-foot scarfs.  A total of 64 feet of scarfing to do.  I managed to get two 8-foot scarfs cut on Saturday and two more on Sunday, that's 4 of 12 scarfs and half the total distance.  

Today I rest.