Scarfing Jig

I built this jig to help me make consistent 12 to 1 scarfs for my planking. I made an extra wide base for my router from scrap 9mm plywood leftover from the planking stock. The base is stiffened by two 1/2 inch wide strips set on edge. The base is set into rabbits in the edge strips and glued. Some brads hold things in place while the glue sets. The base of the jig is made from 3/4 inch plywood, leftover from making the building frames. The rails are cut from 2-by stock, cut to a 12-to-1 slope on the band saw and cleaned up with a plane. The rails are screwed through the base into 2-by-fours set on edge. The screws are deeply countersunk, so the base of the router can ride on the sloped rails. In addition to giving a secure base for the screws, the 2-by-fours hold the base off of the workbench, leaving room to fit a clamp under the plywood.

Here is what it looks like when it's set up. You can see the pencil line 4 1/2" from the end of the plank that marks the start of the scarf. Some scraps of wood are stacked on the plank to raise a short 2-by-four high enough to clear the rails. This cross piece is clamped to the base, holding the plank in place.

The pencil line on the plank marking the start of the scarf is aligned with a line on the jig. This ensures the scarf is aligned correctly with the jig.

I cut the scarf with a few passes of the router, increasing the depth gradually to creep up on the line. I find that if I just take out the pencil line, and bring the front edge down to a feather edge, just a little ragged, the joint is pretty easy to clean up after gluing.

It's not a perfect setup. If the plank doesn't want to lie flat or has some twist in it, the scarf may not be perfectly even, or the cut may be too deep at the feather edge. In practice, it works pretty well, though.

I really was only think of the planking when I made this jig, but it might also be useful in a few other situations. I might find that I want to scarf material together for the sheer clamps, or when I start building spars.

Shaping the Final Plank

Here is where we left things in the last construction post:

The patterns for the sheer strake had been cut out and shaped. Now it is time to get out the marine plywood. This seems like a big step, a momentous occasion. I'm really working on my boat again. When people ask me "How is the boat coming along?" I no longer have hang my head and admit that it is not coming along at all. I can look them straight in the eye and say "I'm working on the sheer strake. After that it's the stem and stern posts and the keel. Pretty soon, I'll be turning it over!"

Well, maybe I shouldn't get ahead of myself.

I traced out the patterns onto the plywood. After that I cut them out with my jig saw. I know that lots of people say that the jig saw won't cut a straight line, that it wants to wander. It's certainly true that I can't follow a line as closely with the jig saw as I can with my band saw, but it seems more like my fault than the saw's. It goes where I direct it, it's just harder to direct than the band saw. In any case, I saw about 5-10mm away from the pencil line. The fibers of the top ply tend to chip out, especially when cutting across the grain.

Here we see an "action shot" of my jig saw. I don't know how old it is, but it was my grandfather's before it was mine, and it is a heavy duty piece of machinery. No plastic body on this one! It runs fine, though it gets a bit hot when I run it for a long time, and it takes a while to cut out these planks. In the end, I use the jig saw to separate the planks from each other, and cut close to the line using the band saw. I didn't happen to get any pictures of that, but it you imagine someone using a band saw... that's pretty much what it looks like.

Finally, I used my router to trim the planks to the exact size of the pattern. The natural thing would be to use a template bit to do this. I don't happen to have one of those. I do have a flush trim bit, though. It's basically the same thing, but with the bearing on the bottom instead of the top. It's not quite long enough to cut both planks (port and starboard) at the same time. Oh, well. I like doing this, right? I get to do twice as much. In the end, I have two planks that exactly match each of the four patterns.

Next step... Cut the scarfs and glue the planks up to their full length.

Epoxy Test Pieces

When I began my boat building project, twelve years ago, I ordered two gallons of epoxy and two gallons of hardener from System Three Resins. I figured that would be enough to get me trough coating the hull.

In response to the question "What is the shelf life of your epoxy resin products?" The System Three product FAQ has this to say:

All solvent-free epoxies have essentially unlimited shelf lives so long as they are stored in sealed containers. The resin may crystallize or the hardener may darken but this does not affect its performance. If the material is more than a year old do a test to satisfy yourself that it cures properly.

Twelve being more than one, I did just that. I mixed up some epoxy and glued up some test pieces. I mixed a batch of straight epoxy, and a second batch to which I added wood flour (basically sanding dust, much finer than saw dust) until it had the consistency of honey. (The consistency of epoxy mixed in this way is measured on a scale from honey, through molasses, and all the way to peanut butter.) I glued up two test pieces, one of solid wood (pine), and one of the meranti marine plywood I'm using for the planking. In each case, both sides of the joint got a coat of the unthickened epoxy. Then I coated one side with the thickened epoxy and joined the pieces, clamping until there was some squeeze out all around the joint.

A couple days later (it was a little chilly, I wanted to give the epoxy all the time it needed to cure) the epoxy left in the mixing bowls had cured as expected. After some encouragement, I was able to pop out two epoxy disks.

The foam brush and tongue depressor are, of course, permanently embedded in the epoxy. You can see the tip of a tongue depressor in the disk of unthickened epoxy. It wasn't strong enough to break the bond of the epoxy with the plastic bowl. There was a small patch of uncured epoxy - just a thin film less than a square centimeter - left behind in the bowl of unthickened epoxy. I assume it was not completely mixed.

But what about those joints? How strong were they? I rigged up a destructive test using my face vise and a pipe clamp. The picture below was taken just after I heard the first "crack" from the joint.

Not long after that, the piece broke, but you can see that the plywood separated between the plies, not at the glue line.

In the second picture, you can see that the joint in the pine also split along the grain, not at the glue line. I rate this test a success! That's good, because I still have about $200 worth of this stuff. I'd hate to have to replace it, but even worse, how would I throw away three gallons of epoxy and hardener that wouldn't cure? It's hard enough getting rid of old paint. It's good to be able to file this under "Problems I Don't Have to Solve."

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.