Spiling: How to make a plank pattern for a lapstrake canoe
Jan 23, 2022Do you want to build a wooden boat?
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This video is a free excerpt from my course Building the Fiddlehead canoe.
It's about one of the things that mystified me the most before I started building boats myself: How to find the correct shape for a boat plank.
I've added a transcript of the video below.
- Mikkel Pagh
Video transcript:
Learning how to plank a hull is perhaps the skill that separates boatbuilders from other types of woodworkers and craftsmen.
Planking is what turns the skeleton of the boat into a structure that can actually keep water out and float. It's what transforms your assembly of wooden parts into a vessel that can take you to places where you otherwise wouldn't be able to go. It's what makes the beautiful, sweeping, ever-changing curves of the boat come to life.
But planking is also shrouded in a lot of mystery. After all, a boat is quite a complex, three-dimensional shape. So how do you take a flat board of wood and cut it to the correct shape? How do you get a good enough fit on your boat that it will actually keep water out?
When I started out in boatbuilding, this seemed almost like a super-human level of skill to me.
But again, as in all of boatbuilding, planking boils down to a sequence of relatively simple steps that I think anyone will be able to master. It takes some time to learn, and most of us will most likely make some mistakes along the way. But once you get the hang of it, I think you will find that planking is one of the most fun and satisfying parts of the entire boatbuilding process.
So I've already prepared my Fiddlehead canoe for the garboard planks by beveling the bottom, the frames, the bulkheads and the stems.
The next thing I need to do is to figure out the shape of the garboard planks. I will do this by making a pattern for the plank. In boatbuilding terminology, this process is known as "spiling".
Spiling can be done in many different ways, using different tools and techniques. But the end goal is always the same: To get an accurate pattern that you can place flat on your planking stock so you can mark your plank shape, cut it out and install it on the boat.
The method that I will show you today is one that I think is efficient, easy to learn and which works well for lapstrake planking.
First, I want to record the width of the plank at the frames and the bulkheads. For this, I'm using strips of cardboard that I'm cutting to length. I'm cutting them so that one end of the strip is at the edge of the bottom panel and one end is at the knuckle on the frame or bulkhead.
Then, I'm tacking each strip in place with a couple of nails.
You could use thin pieces of wood instead of cardboard, but I think cardboard is easy, because it can be cut with scissors or a knife.
Now, I want to record the shape of the stems. I'm doing that by holding a piece of cardboard over each stem and tracing around it with a pencil. I'm making sure that the cardboard piece is large enough to cover entire area where the garboard plank will lie. Then, I'm cutting out the shape and testing it on the boat, before I tack it into place. With a few nails in place, I'm cutting off the cardboard right at the line on the stem that indicates where the garboard plank should end.
I found the nails a bit difficult do drive in, so I'm using a backing iron to provide some resistance.
And I'm bending the nails in order to make sure that the cardboard is pulled tight against the bottom and the stem.
Now I have made cardboard patterns for the frames, the bulkheads and the stems. Each one of these patterns will give me the shape of the plank at certain points along the length of the boat. What I want to determine now, is how these shapes relate to one another.
This is done by connecting them with thin strips of wood. These strips of wood are also knows as "spiling battens".
In this case, I'm using some 2mm veneer that I happened to have on hand. This veneer is a little bit to the flimsy side - 3 or 4 millimeter plywood would probably be a better choice because it’s a bit stouter and less prone to sag.
The strips shouldn't be too narrow. Around 10 cm or 4 inches works well for the Fiddlehead canoe.
I'm using a hot glue gun to attach the wood strips to the cardboard. I'm making sure that the strips land somewhere between the nails that I drove in at the edges of the cardboard strips.
The garboard plank has a lot of curve to it, so I'm using three strips of wood to connect all the cardboard pieces. Here, I'm tracing with a pencil around the area where I need to apply my hot melt glue.
One thing that is very important to keep in mind when attaching the spiling battens is to avoid edge set. Edge set is the batten moving sideways, usually due to gravity. The spiling batten is so thin that it can easily be moved quite a bit, both upwards and downwards. This will result in an inaccuracy in the shape of the pattern, and consequently also in the shape of plank.
What I'm aiming at is to get the batten to fall into place as effortlessly and naturally as possible, without pulling it upwards or downwards.
If possible, it is usually best to attach each batten at its middle first, and then attach it towards the ends. This will balance the batten so that the ends will sag as little as possible.
If you suspect that the weight of the batten is making it droop a little, you can consider making a slight correction by pulling it upwards just a little bit before gluing it in place.
Regardless how careful you are, there will probably be a little bit of edge set in your finished plank. But as long as you are mindful about keeping it at a minimum, you should be OK. The final plank will have some flexibility, too, which will allow you to tweak it back into the correct shape when you install it.
I'm using a glue gun to make my plank patterns because it's quick and easy. But don't worry if you don't have one. Instead of cardboard and hot melt glue, you can use wood and screws. Just cut your patterns out of wood instead, tack them in place, and apply the spiling battens using thin screws and wood glue. Once the glue is dry, the screws can be removed and the pattern can be taken off.
So now, my plank pattern is done, and I'm pulling the nails so that I can try the pattern on my planking stock. In this case it's a board of 19mm, or 3/4 inch larch that I have scarfed together at the middle to get the curve and length I need for the plank. This board will be split on the bandsaw once the plank shape has been cut out, so that it will give two identical planks of about 6 mm or 1/4 inch thickness.
I'm aligning the pattern on the wood so that I avoid sapwood and other defects. And I'm making sure that the pattern lies as naturally and effortlessly on the plank so that no edge set is introduced.
Then, I'm marking the ends of the cardboard strips, and the forward edge and the corners of the stems. To get the boat as symmetrical as possible and to weed out any slight inaccuracies, I'm flipping the pattern around end-to-end and then I'm marking the edges of the cardboard strips once more. As you can see, there is a variation of perhaps 6 mm or a quarter inch at some of the stations. I'm settling on a compromise, making my final mark between the two.
Then all the points need to be connected. I'm using a flexible batten for this, nailing it to the outside of each of my pencil marks along the length of the plank.
Then, I'm checking to see if the curve looks nice and fair. There's a little bit of a hollow around the second nail, so I'll try to pull that. Now, the plank jumps into place, making the curve much smoother.
The same is the case at the other end. There, it jumps into a much better shape.
Especially with lapstrake boats, it is important that the plank edges are fair. If they are not, it may be very visible. Of course, I want to be as close as possible to the marks I got from my pattern, but in a lapstrake boat I would always give fairness of lines the top priority.
In lapstrake, the planks obviously overlap, so the plank width is not quite as crucial as in carvel planking. If the overlap is a little bit less in one place and a little bit more in another, it doesn't really matter. No-one will see it, and the boat will work just as well. And if the plank lines are fair, the boat will certainly be a beauty.
Now I'm checking the curve once more before I draw the line.
Then I'm doing the same thing at the other edge of the plank. This is the edge that is towards the bottom of the boat. I don't need to fuss too much about this edge, because it doesn't need to be precise. It can be cut too large because it will be trimmed off once the plank is installed on the boat.
So now I have my lines. Because the curves are quite gentle, I can cut them out with the circular saw. A jigsaw or a bandsaw would also work well for this task. I'm cutting the edge of the plank that will be towards the bottom of the boat a bit too large, so I have a some extra wood to work with case adjustments need to be made.
I'm leaving the edge towards the bottom of the boat rough, but the other edge, I'm planing to the line. This is a "final edge", so I want it to be nice and fair. I'm starting out with a block plane and then I'm finishing off with a finely set smoothing plane.
The next thing I'll need to do is to split the plank in two on the band saw, so I'll get an identical plank pair.
But before I do that, I'm marking the ends of the stems at each end. This will help me get the correct alignment for both planks when they are installed on the boat.
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