Scarfing Strips
by Philip Jacobs
One of the first surprises about building an MCA style canoe is that not all of the strips run the full length of the hull. Smaller strips are used for 'footballs', 'stair-steps', or 'herringbones'. At other times, available, shorter pieces are combined to fill a full length space. Many builders follow the method in the MCA builder's manual, butting strips end-to-end, preferably, over one of the form locations. I chose to scarf strips, hoping to avoid tell-tale dark lines where the ends meet and the joint often fills with resin. A scarf is simply a tapered joint between two pieces of wood, providing increased gluing area and a gradual transition. In theory a scarfed joint is stronger, maintains more of the wood's character and flexibility, and yield fairer curves. Scarfs are referred to by the ratio of the length of the joint compared to the thickness of the wood being joined. For example, 1/4 inch strips joined over a length of 3 inches would be a 12:1 scarf. (Note, however, that joining 10 foot and 6 foot pieces with a 12:1 scarf will yield a completed strip only 15 feet, 9 inches long).
As a practical matter, once the the wood is sandwiched between layers of fiberglass and resin, the difference in strength is probably academic. However, scarfing strips before glue-up allowed me to incorporate smaller pieces into 'full-length' strips, and to stagger joints anywhere on the hull. This was a definite advantage when building a canoe without the use of staples. It also allowed me to remove large knots and defects in longer strips, rejoining the parts in sequence, in order to reduce waste and scrap and maintain a continuous appearance.
I developed a simple fixture for cutting scarfs on a stationary disc sander. Since the exact angle of the cut is not important, but must be the same on both pieces, the fixture simply attaches to the table of the machine and guides the strips against the disc at a consistent angle. The soft cedar virtually melts against the spinning abrasive disc (note: good dust collection and ventilation required!), providing a smooth, uniform surface for gluing. Speaking with another builder revealed that he had developed an almost identical method for lengthening strips. It may also be possible to cut the scarf angles using a fixtured belt sander, or with a sliding fixture on a band saw or router table, although care must be taken to prevent chipping of the thin, fragile ends.
The prepared strips were glued together using a short piece of masking tape to keep them from slipping lengthwise, and a pair of spring clamps to apply clamping pressure. Careful alignment is required to glue the strip straight, and a simple alignment jig might be made out of a grooved piece of scrap wood. After drying, the completed strip is lightly sanded along the edges of the joint to remove any ridges or dried glue. Using this method, stock up to 1/2 inch thick can be scarfed on a 12 inch disc sander; gunnels up to 3/4 inch thick can be scarfed at an 8:1 ratio by first cutting them to the approximate angle with a hand or band saw, and then cleaning up the cut on the disk sander.
The results included another interesting surprise. I had expected the tapered scarfs to make the glue joints almost disappear. What I discovered was that, although the joints were feathered in appearance versus the abrupt, straight line observed with butt joints, the visible glue lines on the scarf joints were actually wider than those on butt joints in some cases. In retrospect, the reason is clear: sanding along the face of the strip cuts across the glue line of a scarf joint at a shallow angle, exposing a wider face than that of a butt joint sanded at 90 degrees.
Overall, I think that appearance of the scarf joint is more appealing than the butt joint, despite the slightly wider glue line. Once scarfed, the longer, composite strips reduce glue-up time, compared to piecing, and reduce waste of wood, provided that an easy method of cutting consistent scarf angles is available.
