Scarfing: How boatbuilders make short planks longer

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In this video, I will show you how to scarf two pieces of wood together. 

Scarfing is a technique that is used a lot in boatbuilding, because it allows the builder to use shorter pieces of wood

And because it makes it possible to use straight grained wood instead of crooks for curved boat parts. 

 - Mikkel Pagh

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Video transcript:

Sometimes, the wood you have available is not long enough for the part that you want to make. Or perhaps you need a curved piece of wood but you only have straight boards available.

Both of these are the case for me now that I'm making the garboard planks for the Fiddlehead canoe. Here's how I did the scarfing of my garboard planks.

First, I'm placing my boards on the bench, one on top of the other. Then I'm placing my plank pattern on top of that. I'm trying to figure out how to align the wood to avoid the splits at the ends of the planks, avoid the sapwood on the bark side of the plank, and to get the grain to run as straight as possible along the pattern.

In this case, I want the scarf joint to be right at the mid frame. This will allow the grain to follow the pattern nicely, and it will allow me to steam the ends of the planks without compromising the glued scarf joint.

So I have drawn a line at the mid-frame mark, and now I'm measuring out 12 centimeters from that line to give me the extra wood I will need for the scarf joint.

The math of it is like this: I want to plane a 12-to-1 slope on my scarf, because this will create a joint that is as strong and as flexible as the rest of the plank.

The plank is around 2 centimeters thick, so the total length of the scarf will be 24 centimeters - or 12 centimeters on each side of the mid-frame mark.

Then I'm cutting to this line. Note that this cut is not at 90 degrees to the edge of the plank. It is at a bit of an angle, as defined by the cardboard strip at the mid-frame mark on the plank pattern.

Then, I'm putting the pattern back on top of the boards. Once I'm satisfied that the shape of the planks matches the pattern, I'm drawing a line along the edge that I just cut off.

Then, I'm measuring 24 centimeters away from that line, to get the length of the scarf joint, and then I cut to this line.

Again, I'm replacing my pattern just to make a final check that everything looks OK.

With the pattern in place, I'm making rough marks with a piece of chalk to guide me in cutting off the bulk of the waste before making the actual scarf joint. This will save me a bit of sweat when I'm moving the planks and when I'm planing the joint.

I'm also making a mark on both planks that will help me get the correct alignment before cutting the scarf.

And I'm marking the surfaces that will need to be planed to produce the scarf joint. It is very easy to mix up once the planks are flipped around.

Now, I've cut off the waste outside the chalk lines, and I've also cut off the ends with the splits. My next job is to align the two planks so that the 24 cm line on the lower plank matches the edge of the upper plank. I'm also aligning the center mark. This mark will disappear once I'm done planing, so to make alignment during the glue-up easier, I'm making new alignment marks along the edges of the upper plank.

Now, the planing can begin. I've aligned the leading edge of the lower plank with the edge of my workbench. This will allow me to plane the plank all the way down to a feather edge while the wood remains supported. I've secured the planks to the bench with a couple of clamps.

To begin with, I'm using my scrub plane to knock off the corners of the planks. A scrub plane has a blade that is ground to a curve, and it has a large mouth opening. This makes it very efficient for removing large amounts of material quickly.

The scrub plane is the old-world alternative to a handheld power planer. If you do have a handheld power planer, though, it will work very well for this job. And you'll perhaps get the job done a little bit quicker.

But a well-tuned scrub plane is actually quite enjoyable to use, and cutting one of these scarf joints with hand tools will probably give you about as much cardiovascular exercise as a good jog around the neighborhood.

As you can see, I'm planing directly across the grain. This is the most efficient way of removing large amounts of wood. It leaves a very rough surface and the edges may chip off a bit, but it doesn't matter, because it will all be refined later, anyway.

By the way, you definitely don't need a scrub plane to do this job. It can be cut with a block plane or any other hand plane, but in that case perhaps the initial wood removal will take you just a little bit longer.

Once I'm getting closer to the cut lines, I'm beginning to take diagonal passes with my plane. This will help flatten the joint. Remember, the plane only cuts the high spots. So if there is a low area in your joint with higher areas around it, the plane will ride the highs and leave the lows untouched.

Now that I'm getting closer to the end goal - a feather edge on each board - I'm switching to my jointer plane. Again, it is not necessary to use a jointer plane to cut scarf joints. It can be done with any type of plane. But, if you have one, this would be a good time to use it. Because the jointer is so long, it makes it very easy to knock off all the high spots, so that the joint becomes perfectly flat.

I'm cutting both diagonally and along the grain here to get rid of any un-evenness. Once I have created a feather edge on both boards and I've reached the two pencil lines, the joint is done.

But in this case, actually, I'm planing a bit past the pencil line on the upper board. This is because it turns out the upper board is just a little bit thicker than the lower one. So to get the same angle all the way across the joint, one of the "ramps" must be a bit longer.

OK! Now it's time to take the planks back on the long workbench and try them with the pattern again.

I want a good alignment of the joint, and I want - of course - to make sure that the plank shape matches the pattern nicely.

Here you can see how the scarf joint on the upper plank is a bit longer than the one on the lower plank. I'm trying to get the joint perfectly flat towards the benchtop, leaving the extra wood at the top. Once the glue has cured, I'll plane off the excess wood from the upper plank.

To make sure that I will position the planks correctly during glue-up, I'm marking the ends and the edges of the planks with blue masking tape.

Then, I'm applying the glue. I'm using polyurethane, but epoxy would work very well too, especially if you are not sure your joint is 100% flat.

This glue is very slippery, so to keep the planks from sliding apart, I'm securing them with a few clamps.

Then I'm clamping up the joint, using a few blocks of wood to make sure the pressure is evenly distributed.

Polyurethane glue likes high clamping pressure, but if you're using epoxy, the clamping pressure should be relatively light. Just enough so that there is thickened epoxy squeezing out at all edges of the joint.

Next day, the glue is dry, and I'm giving the plank a few passes through the thickness planer. To make sure the wood at the scarf joint is not torn out, I'm putting the plank into the machine with the feather edge pointing towards me. In that way, the machine will cut away from this this edge, eliminating the risk of any disasters.

Now I'm ready to cut out the plank to the shape defined by the plank pattern.

Once that's done, I can split the plank in the middle on the band saw, which will give me a set of identical planks, one for each side of the boat.