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.