Saturday, December 16, 2006
Not so scary scarf joints
The one thing about building from plans vs buying the kit that I was most apprehensive about was cutting and gluing the scarf joints. These are used to join two sheets of plywood end-to-end so that a boat longer than seven or eight feet is possible. I was encouraged by chapter nine, Scarf Joints in Chris Kulczycki's book, which I mentioned in my initial post. He explained the process in a way that almost made it sound as easy and enjoyable as it actually was.
The 9mm sheets are marked, stacked and lined up for a 3" ramp (scarf) to be cut with a very sharp block plane. The edge of the bottom sheet needs to be supported on the edge of the work bench to prevent the plywood from breaking off when it gets thin. I also clamped a couple of pieces of MDF on top to prevent the panels from shifting or bowing while I planed across the width of the plywood. Keep the plywood veneer bands parallel, and cut with smooth, long strokes. The 6mm sheets are done in the same way, except that a 2" scarf is cut. I did attempt to practice cutting scarfs on some scrap luan and found that the luan was much more prone to tear-out than the okoume was. I had similar difficulty with some scrap fir plywood I practiced on.
After the scarfs are cut, the sheets are glued end-to-end with epoxy thickened with Silica. The 9mm and the 6mm panels can be stacked and glued at the same time if you lay plastic wrap between the sheets where the joints are. In retrospect, I think I would not have tried to stack them, and only glued one at a time. This is because it is much more difficult to perfectly align four sheets on two different planes than two sheets on the same plane.
I placed another layer of plastic wrap on the top, and then placed a 4x4 beam across to use as a clamping bar. It is very good advice not to apply too much clamping pressure. All you need, is to tighten until you begin to see the epoxy squeeze out. Again, this would be much easier to determine if you only glue one set of panels at a time.