Scarfs are the traditional way to join pieces of wood. There are lots of different types and this one is the regular or feather edge style. It maintains the strength of the two pieces being joined and provides enough gluing surface to make a good bond too. If using epoxy as the adhesive, the mating surfaces don’t have to be especially precise. Epoxy’s gap filling properties will take care of this.
The 8 to 1 ratio is standard for planking, but I've used 6:1 on occasion with no issues. Of course, this shorter ratio should be only used in areas where there's not a lot of bend, twist or heavy loading. In these areas the ratio should be increased, say 10 or 12 to one. In fact 12:1 is the common ratio for masts and spars in general. This is because they'll need to bend reliably and uniformly. Generally, you can use 6:1 on plywood planking, except in areas with a lot of bend, like that near the bow or a bilge turn in the after quarters of a powerboat. 8:1 is the usually standard, with 12:1 on spars and other highly loaded joints.
If the scarf will see water flowing past it, the slope of the scarf should be “with the flow” of water rushing by. This way water can’t be driven into the joint where it might cause issues. The same is true of scarfs placed where water might drip on them. Orient the slope of the scarf so it will drain any trapped moisture out of the joint. The Payson butt joint can eliminate many of the scarfs used, but sometimes you just need a scarf and it looks cool too.
A scarf isn't difficult to make, though it might intimate a few initially. If the scarf will be under paint, it really doesn't matter how you cut it, so long as the slope is about what you need. You could hack it out with a hatchet, butter it up with thickened epoxy, mash it together and under some putty and paint, no one is going to know of the wood butchery below. I usually employ a power plane to rough in the scarf, which just as easily can be done with a sharp hand plane or even a belt sander. Once it's close, I'll fine tune it with a chisel, hand plane or a sander.
You can use a jig of some sort and I have a couple I've made, but in reality, I find I can hack out a scarf faster using hand or power hand tools, compared to setting up a router jig. I do have a circular saw jig that I use for full width panel scarfs, but these aren't needed very often.
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| 6:1 scarf with slight notch for brightly finished (top) surfaces |
If the scarf is going to be finished bright (varnish or other clear coat), then you need to be precise and anal about the look, at least on the edge. When I have to do this type of scarf, I will use a jig, and I'll razor through the outer veneer (if plywood, as shown above) or about a 1/16" (3 mm) below the surface (if solid wood), just at the end of the cut. This helps prevent tear out and leaves a razor line to butt against the adjoining piece. I'll also cut the scarf a wee bit short, leaving a bit of a shoulder on the feather edge, again to preserve the razor cut edge, so under varnish, it looks good. These types of joints take some practice, but after a handful of scarfs, you'll get a feel for it.
In order to get a joined plank to accommodate the bends and twists in a hull, it's often necessary to cant the scarf joint a bit, so it better follows the ultimate shape of the boat. This brings up the crooked plank trick. Essentially you're aligning the grain to follow the hull's curves, yet with a flat hunk of wood. With plywood or solid timber, sometimes it's necessary to cock the adjoining piece to do this.
There's no special trick to this, just align the scarf or butt joint to move the grain in the direction of the curve. On the workbench this will look weird, but once bent around the boat, it'll look normal.
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