The Tongass!
Step 7) Scarfing the Bottom Panels
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My plywood was not square (!) so I had to square up the ends. The plywood also needed to be cut to 32" wide. For this task, I made one of my usual cutting jigs for the circular saw. To make one, you just screw a straight edge (cedar 2x4 in this case) to a piece of scrap plywood, then run the saw along the straight edge to cut off the excess plywood. You just clamp this big jig on your work, aligning the cut edge of the plywood perfectly with your cut line, then run the saw along the straight edge ...voila! A perfectly straight cut that is precisely where you wanted it. This same jig can be made for any kind of hand-held saw, e.g. jig saw, circular saw, etcetera. Note that the offset on opposite sides of a saw are rarely the same, you need mark the direction that the saw should travel on the jig for an accurate cut (see black arrows). |
| After cutting out 2 each 32" x 59-1/2" x 1/2" and 32" x 8' x 1/2" panels, and 4 each 32" x 8' x 1/4" panels, I was ready to scarf. I'm using an 8:1 ratio rather than the 7:1 ratio recommended by Renn ...just because it's what I always do. To cut the scarfs, I first used a Bosch power hand plane and a Stanley low-angle block plane to produce a template set of plywood (see top and bottom douglas fir plywood in picture). These provide a surface for the planes to ride on while I scarf the wood that goes into the boat. Next, I did the usual marking off and stair-step stacking of the wood to ready it for scarfing (see Renn's instructions on marking). Then I again used the Bosch power hand plane to get within 1/16" of the finish, and then finished it up with the low-angle block plane. The block plane iron is forced to cut against grain and across grain when scarfing, so make sure you keep it sharp. I use a Japanese industrial diamond ultra-flat sharpening flat for this. Iterate checking your scarf by observing that all lines from plies are parallel, straight, and perpendicular to the edge of the plywood and using a straight edge to verify the 8:1 angle and flatness of the scarf joint. I leave a very slight hollow in the middle of each scarf surface to it'll hold a tad bit more epoxy than otherwise. |
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Here's the obligatory close-up of the scarfing (everyone seems to like scarfing, then showing their work...lots of fun!) |
| For gluing up the bottom panel scarf joints, I am following Renn's plastic, scrap plywood, and drywall screw method for clamping the joints. In this picture you can see the wetted-out scarfs, ready for milled-glass-thickened epoxy adhesive. I used WEST Microfiber filler plus about 25% silica to smooth it out and make it thixotropic (non-sagging.) NOTE: Another Tolman builder had a plywood scarf pop open on him, and I myself had a handrail scarf pop open on another boat I built ...the moral of the story is a) make darn sure your epoxy ratio is right, and b) make darn sure you pre-soak the heck out of the scarf cuts before applying thickened epoxy for gluing. I love my perfect-ratio producing Sticky Stuff Dispensor by Michael Engineering, and I gave the epoxy a full 15-20 minutes of pre-soak with unthickened epoxy, going back over it several times with fresh epoxy to guarantee no more would soak in (mahogany can suck up a lot of epoxy.) All my scarf joints were wet looking and stayed that way, prior to putting the thickened epoxy on. I used a home-made notched squeegee for applying the thickened epoxy. |
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Shown here is the plastic-covered (and under) scarf joint clamped for curing with scrap plywood and drywall screws. As suggested, I used 1-5/8" screws and put them in on a 6" schedule. (More coming after the scarfs cure ...clean up of joints, then off to lofting and building the boat bottom.) Notice that I laid 4x8s of plywood on the building jig to make a long scarfing table rather than do my gluing on the floor or sawhorses. I used end-to-end very-straight cedar 2x4s for straight edges (screwed to the table) to align all the sheets of plywood. |
| The finished scarfs. They didn't turn out too bad, just need the usual light scraping and sanding and they're good to go. NOTE: Every time I put a hole in something, I use a permanent marker to draw a circle around it. It's a lot easier to find the holes to fill later when they are circled like this. |
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© Copyright 2004 Brian Dixon
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