Monday, October 25, 2010

More Ultralight.

We've started a new session of classes in my cedar-strip canoe building class. I have two new students joining the class this year along with previous students returning to complete boats that are in-progress. One thing I am pleased about is that my two new students are experienced woodworkers. One is a retired contractor and the other still works in the trade running his own business.

These two students are building the Wee Lassie II design to Mac McCarthy's plans. After our successful use of box-beam construction last year for the Prospector Ranger we opted to use this strongback style for the Wee Lassies as well. We've also removed material from the station molds to remove weight there as well.

One thing to remember is that a strongback and station molds need to be robust. Most builders do not move the forms once built so that the shape of the canoe doesn't change as it is being built. For our class, we need to move the boats on a weekly basis so the forms need to be both light and rigid. The box beam does this for us.

The box beam consists of a 1/2" thick plywood deck 1/4" thick Luan ply sides and 2x6" blocking. It is remarkably stiff once assembled and has the advantage of being stable with temperature and moisture changes.

We rip the 1/2" plywood deck to be 7-1/2" wide and the 2x6 blocking to match. Because the ply is only 8' long, we glue and screw the blocks to the underside of the ply with a block beneath the center joint.

The Luan is ripped to be the width of the 2x6 (actually 5-1/2") and the thickness of the ply (*close* to 1/2", but not exact), so it is nearly 6" wide. Again, because the sheet goods aren't long enough for a continuous piece the sides have a patch that is 5" wide and 2' long that gets glued to the side panels at the joint. We stagger the joints so that they are not in the same place, weakening the structure.

Once the sides have cured, the whole structure is glued and screwed (or in this case stapled - one of the students had a pneumatic stapler which made life very easy) to assemble the beam. When the cut edges are lined up, the structure is straight and flat. The large surface area that is glued up when the sides are assembled makes the structure very rigid.

We then mark the centerline and lay out the location of station molds. The station molds get screwed (but NOT glued) to the 1/2" plywood deck.

You will note the unsupported ends which have a taper cut on them. The taper is to clear the hull that we'll be building on the forms. We're also avoiding fiberglassing the hull to the strongback with this clearance cut. It is likely that we will support this with a small piece of 2x4 screwed into the end blocking and to the deck so it isn't flapping in the breeze. You will also note that the stem molds are not in place yet - we use them as forms to laminate the stems before they are installed on the strongback.

We have decided that there are a few flaws here, however. The old method, which used dimensional lumber, gave us a "ledge" which we could clamp to when screwing down the station molds. I'm concerned that we could crush the 1/4" sides if we clamped to them, so I need to either design in a ledge, or have a removable block to fit under the deck to clamp to when screwing down the molds. The other concern is how to support this at a reasonable work height. Small sawhorses seem to be the best solution for this at the moment. The Luan is also a bit splintery, but not bad - a little sanding at the top edge seems to solve that problem.

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