Tuesday, September 8, 2009
As I've mentioned before, there has been great interest in from future students to build tandem canoes. For the most part, we've built solo boats because they are smaller, lighter and use less material and therefore are less expensive. One of the unique challenges that my class has is that we have no "fixed" workshop to leave our boats in as they are built. We need to carry the boats in from a storage barn and back out again every week. To be able to do this without altering the shape of the canoes as they are built, the strongback and forms must be very rigid and strong enough to take the abuse of moving them around.
While the 15 foot long canoe would only about 1-1/2' longer than the Wee Lassie II, it is about 7" wider. Also, the 15-footer is probably at least a third heavier in both forms and boat. Not insignificant weight. For this reason, I have shied away from offering a tandem canoe design.
This year, I'm finally relenting and trying a 15' canoe. I'm doing everything that I can think of to put both the forms and strongback on a diet. First, is the strongback. I'm thinking of using a box-beam design. It is a lighter-weight approach, but I need it to be rigid for moving the boats. Box-beams are just that - a beam with a box-shaped cross section and are usually hollow. Because of this configuration, they are not always rigid in torsion (read: twisting). I'm trying to avoid this twisting by using rigid foam for the beam's core.
The picture at the top shows the materials that I'm using. At the top is 5.5mm thick Luan plywood sheet. In the middle is a 2" thick foam insulation board. At the bottom is a piece of 2x4 framing lumber. Some 1/2" plywood (not shown) will be the top for mounting the patterns.
First, I squared up the foam's edge (it had a tongue-and-groove feature) and then cut pieces 3-1/2" wide that were 8' long. There were 6 pieces. I then took two of the full length pieces and cut 2' from the end. The goal here is to stagger the joints when I laminate the foam together. In the picture below, I've pulled the joints open to emphasize their location and show the spacing. There are three 2" wide laminations showing here.
The next step was to cut some blocks from the 2x4 for the bottom of the beam - about 5-3/4" long blocks - these are to keep the beam from spreading and to give something rugged to clamp to and screw to when in use. I'm envisioning this beam sitting on workbenches or sawhorses as the canoes are being built. The picture below shows the pieces sitting on the foam. At this point, the beam is upside-down.
The next step was to rip the Luan sheet. I made the sides 5-1/2" wide to allow for the 2x4, the foam and the 1/2" plywood deck. There are 4 pieces. As the butt joint between is in the middle of the beam, I made up some 5" wide 2 foot long butt blocks to glue to the sides. I made it 5" wide so that I had room to put a square up against the sides of the beam for setting up patterns. The bottom edge of the butt block will be at the bottom edge of the beam. (Where the 2x4 is...)
This picture shows the dry-fit of the beam in its proper orientation- foam core, Luan sides, plywood deck and 2x4 bottom blocks. Tomorrow, I'll assemble the pieces using the foam adhesive in the tube below. The sides will be screwed to the deck and the blocks to keep things together while the adhesive sets.