Tuesday, October 2, 2012

Sheer Strake Patterns

With the building frame leveled, it's time to begin work on the sheet strake. I know some folks have had success making full length patterns by running two battens to mark the edges of the plank and attaching cross braces to make a rigid structure. I tried that early on, and it didn't seem to work well for me.

The Eun Mara is a 20' long boat. The planks are made from 8' long sheets of plywood. This means that each plank must be built from several sections and scarfed together. Now many people have had great success gluing scarfs on the boat. I never actually tried it. Early on I tried a dry fit this way. It was hard enough getting everything lined up, that I couldn't imagine doing it again with epoxy all over, trying not to make a mess, and hoping to get everything into place before the epoxy goes off.

I'm sure both these approaches are very sensible, and that if someone were to show me the correct way, I'd say, "Oh, that's how it's supposed to be done. I didn't have anyone to show me, though, so I had to work out something for myself.

I ended up making patterns out of 1/4" plywood, as described in Iain Oughtred's CLinker Plywood Boatbuilding Manual. But with an extra twist to help get the scarfs lined up correctly. My method is as follows:

A sheet of 1/4" plywood is bent around the boat, roughly where the plank will be. The 1/4" plywood from the home center is much lighter and easier to bend into place than the 9mm (about 3/8") marine grade meranti plywood. I climb under the boat, mark the plank lands at each frame, that is the corners on the frames that mark where each plank ends. These will be joined up with a fairing batten after the plywood comes off of the boat. I also trace the edge of the previous plank. This will not mark the edge of the new plank. The planks need to overlap, so another line, slightly above this one needs to be drawn. I didn't get under the boat to take pictures of this, so hopefully this all makes some sense. I also marked the location of the edge each station along the plywood. This helps with lining everything up correctly.

Early on in the construction, I made this gauge to mark the location of the plank lands. Here you can see how it is used to mark the lands on the previous plank, so the edge can be beveled correctly to mate with the next plank.

Here you can see the same gauge being used to mark the edge of the new plank. It is placed upside down on the line traced from the edge of the previous plank. You can see the traced line running under the block. Then I make a pencil mark in the same corner as was used to mark the line in the previous photo. I mark several locations, then put a brad in each and lay a batten so it rests against each of the brads.

Here is the batten. It's held to the curve of the brads with a couple clamps. It doesn't always want to lie exactly along the brads, and some adjustments have to be made. If it's pretty close (within a pencil line) I wouldn't sweat it. It it's more than that one of the marks may be off. I never had too much difficulty. If you're having difficulty, well, use your best judgement. It's likely to be better than mine!

The pencil lines in this photo are a little hard to make out. The lower horizontal line marks the end of this section of the plank. The center horizontal line marks where this plank will lay across one of the station moulds. The upper horizontal line marks where the start of the scarf for this joint. Each end of each plank is marked like this for a 12-1 scarf. Those who know more than I do say that a scarf of 8-1 or 10-1 is sufficient, but 12-1 can't be any worse, and doesn't really use any more material. The two nearly vertical lines mark the edges of the plank. (It looks like there is another nearly vertical stray line.) The center lines, marking the location of the station, are marked onto the edges of the planks after they are cut out. Lining these marks up ensures that the glued-up plank will lay correctly on the boat.

After the pattern is outlined, it is cut out. First I rough cut with a saber saw, then I cut close to the line on the band saw. Here are the patterns for the four sections of the sheer strake.

Marking the scarfs in this way has one drawback. Because all the scarfs must lay on one of the stations, it limits how the planks can be divided into pieces. Some planks that could have been glued up from three almost-eight-foot sections end up being made from four shorter sections. It's not too much extra, though. I'd say I only ended up making six or eight extra scarfs on the whole boat. And some of the work is easier, involving smaller pieces that are easier to cut out.

Next I trace the patterns onto meranti plywood, rough cut them with a saber saw, and use a pattern bin in my router to give them their final shape.

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