Tuesday, November 13, 2012

Gluing the Scarfs

Well, this

is where I left things in the previous construction post. From the start of construction, I had planned to paint the hull. Consequently I had not been concerned with matching the grain or color of the planking stock.

Interestingly, the "meranti" plywood I'm using seems to alternate between layers of lighter and darker wood. The lighter wood seems a little less dense than the darker - almost like the ocume plywood I used to build a kayak, once upon a time. Most sheets have the darker wood on the outer face, but some have the lighter. You can see the difference in the pictures that show the hull. The color of the darker plies can vary considerably, too, even on opposite faces of the same sheet.

Well, the thought had come to me that I might like to leave the sheer strake finished bright, while painting the rest of the hull. I very carefully chose plywood that had face veneers with matching color, at least on one side. Unfortunately, though, when I traced the patterns for each section of the plank, I didn't pay attention to which side was up. Once I cut out the sections, there was no way to line them up so the colors matched correctly. Oh, well. Paint it is.

Now you may have seen in the post about my scarfing jig how I cut the scarfs to join the four parts of each plank together. So, the scarfs having been cut, it is time to start gluing the planks to full length.

If you look in the lower right of this picture

you can see a pencil line on the edge of the plywood. This marks the center of the scarf, and, when gluing up, aligning these pencil marks makes sure the plank will fit properly on the boat.

The process is pretty straight forward. First, I coat each face of the scarf with unthickened epoxy. Then I add wood flour to the remaining epoxy until it is about the consistency of honey or molasses. One face of the scarf gets a coating of this thickened epoxy and then the scarf is lined up...

...and clamped into place. The small silver clamps keep everything aligned, while the larger C-clamps put pressure on the joint. Wax paper between the joint and the scrap wood keeps the whole assembly from sticking together, and also leaves a fairly smooth surface that requires only a little cleaning up. First I use the small clamps to hold one side of the joint against a board underneath (with the scarf face up). After applying the epoxy, the other side is lined up, and clamped to the same board, holding the joint steady. Then the larger clamps are added, squeezing the joint between two pieces of scrap large enough to cover the area of the joint.

That's the theory, anyway. It's worked beautifully for the first 40 or so joints. Halfway through this final plank, though, something went amiss. I don't know if my pencil line was off, or if the setup slipped while I was gluing it up, but when I checked the next-to-last scarf to see how they fit, they didn't. Aligning the section of the plank with frames 3, 4, and 5 put things out of alignment by a couple inches at the bow. No way to finesse that!

So I cut one end of the joint off, shaped another plank-piece using the same pattern as before. (Is there a term for the sections of plywood that are glued up to make the full length plank? It seems like there ought to be.) Recut the scarfs. (It was sort of interesting to cut down to the glue line in the old piece.)

I didn't want to make the same mistake again, so I decided to glue the last four scarfs on the boat, like everybody else does. It wasn't as bad as I feared. It required a little more clamping ingenuity, and a little more effort to clean up the squeeze-out. All in all, though, not too bad. The full length planks seem to have a slight kink around the joint that I had to cut apart, but on the boat it looks pretty fair. I may have to adjust the sheer a little after I turn the boat over. Oh, well. I'm building a boat to use, not a museum piece, right?

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