Building the Rushton Catboat

Scarfing the Planks

I scarf the planks with a 10:1 ratio, meaning that my scarfs are 2 and 1/2 inches long. First I cut the angle on the ends being scarfed, half my total from the spiling batten. In other words, if my spiling batten showed an eight degree angle then I put a four degree angle on my plank ends. To get as much accuracy as possible, instead of relying on a protractor I calculated a triangle formed by that angle. My bench is 78 inches long. I lined up the edge of my planking at one end and took the tangent of my angle and multiplied it by 78 inches. That resulting figure was the distance up the side of the bench where the planks had to lie to be at the precise angle. You can get the tangent of any angle by simply Googling "tangent of X degrees."

I roughed out my scarf with my belt sander. I worked the material down to within about an 1/8 inch of the line before reaching for my plane. The layers of plywood are an obvious indicator as to whether or not you are removing material evenly. When it comes to the final planing I make sure that the iron is SHARP. The glues in the ply are very hard on the blade's edge, and it is necessary to sharpen your tools much more frequently when working with plywood. The last thing I do is separate my pile and fine tune each scarf individually before proceeding with gluing.

I set up a gluing bench using a sixteen foot plank on sawhorses. I made sure it was relatively flat. I struck a centerline across the middle of the plank and then lines 78 inches (the width of my bench) to either side of the centerline. I used my tangent formulas again to mark the precise spot where my planks should lie for each angle.

Since I was planning to varnish the inside of the hull it was imperative that I get the very best scarf that I could, with the narrowest glue line possible. Glue would inevitably squeeze out of the scarf joint and it is hard to sand or scrape this hardened epoxy and not sand through the top veneer of the plywood. What I did was to use masking tape and tape some heavy paper across the edges of the scarf joint. Any squeeze out would flow on to the paper and tape and not the plank. I glued up my planks as a pair, and of course I put sheet plastic between the planks, and over the bench, so that things didn't get glued together.

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