What do you do when you can’t get lumber long enough to fill the need? You go get the board stretcher! Any neophyte woodworker has heard that one. But the board stretcher I’m referring to really does exist. It’s called a scarf joint.
Boatbuilders have been scarfing together lumber for a long time. It’s a method of joining two boards so they have continuous sides as if the two boards have grown together to form one long board. Before the days of the great glues we have today, the scarf joint was made by overlapping the boards with a long diagonal cut, and then pegging a backing block to the joint. But I’m using epoxy. And if done properly, the glued scarf joint is stronger than the wood fibers themselves, obviating the need for a backing block.
In this case, I’m scarfing together two planks to make a blank that is 16 ft. long by 6 inches wide by 1-1/2 inches thick. That’s a serious piece of white oak! But it needs to be, it’s the backbone of the boat.
So I start with an 8 ft and a 10 ft plank. The scarf joint will be a diagonal overlap at a 12:1 ration. This means that since the planks are 1-1/2 inches thick, the joint will be 18 inches long. The joint needs to be so long in order to provide plenty of faying (glue-able) surface. The more faying surface, the stronger the joint. A structural joint like this one requires a 12:1 ratio.
I first line out the part of the board that will be cut away. Look closely in the picture at left, and you’ll see the diagonal line I’ve drawn on the edge of the board.
Then I stack the two boards on top of each other, offsetting them so I can make one long slanted cut with the power plane at the angle I want. I finish the cut with the hand plane to make for a good fit.
Then wet out the glued surfaces with straight epoxy. This lets some of the epoxy soak into the pores of the wood so the joint won’t be starved of glue. Then slather on a nice thickened mixture of epoxy and clamp, leaving overnight for it to partially cure.
When the joint is set, but the epoxy is still “green”, it’s time to plane it down to the finished thickness and clean up the joint.
Then lay out the outline of the new blank from measurements taken from the old keel. Since the edges are a curved taper, they have to be lined off with a batten as shown in the photo.
Finally, the blank is cut out on the bandsaw. And there it is.
- 7. The finished blank is cut out on the bandsaw.
The next step is to cut the rabbet (the groove along each edge where the bottom planking rests. That’s another story…