Japanese Boat Design

by Douglas Brooks

One of the most important elements of our work is recording the design of the tenmasen. Bansho san works squarely in the Japanese tradition and works from a 1/10th scale drawing on a plank of wood. These drawings are called itazu (literally “plank drawing”) and the one we are using was originally drawn by Bansho san’s father. Interestingly, Bansho san is the very first traditional boatbuilder I’ve met in Japan who uses the metric system. He said his father converted to it after the War, abandoning the shakkansho system which originated in China.

The itazu or plank drawing Bansho san uses to build the tenmasen. Photo Credit: Douglas Brooks.

Bansho san’s drawing, consisting of a profile and plan view, would be familiar to any boatbuilder. What might surprise a Westerner is that he uses only three unevenly spaced stations for his boat. He has a forward station, a middle station at the widest point of the boat, and one he calls the koshi (“hip”), located aft where the bottom is bent upwards to meet the transom.

The station layout of the tenmasen. Drawing is not to scale.

Bansho san’s drawing is quite comprehensive, which is unusual in Japan, where boatbuilders’ drawings are intentionally incomplete to protect each craftsperson’s secrets. Contrast Bansho san’s drawing with this copy of one of my former teacher’s drawings.

The itazu for a seaweed gathering boat from Tokyo Bay called a bekabune. The drawing is intentionally missing some lines so that only he can build the boat. Photo Credit: Douglas Brooks.

You can see the profile, which is complete, and even an expanded transom. But in plan view, there are no lines for the chine and sheer. These were my teacher’s secrets and he relied on memorized dimensions to lay out these lines when building. One of the great ironies of preserving the craft in Japan is that Japanese maritime museums have thousands of these plank drawings, but most of them can’t be used to build an accurate replica because essential information is missing.

Bansho san spiles the shape of the sheer planks much like we would in the West, using a spiling batten, which in this case is a thinly sawn flitch of cypress. We prop it in place and, using a square, mark off the location of the chine and the height of the sheer. Bansho san clamps scraps to the batten at each station, scales the heights from the drawing, and marks them on these scraps. We then make reference marks for the heights on the spiling batten itself.

The spiling batten for the sheer plank, propped in place. Photo Credit: Douglas Brooks.

Bansho san clamps scraps of wood to extend the spiling batten so he can measure the full width of the plank at each station. He then measures back from the mark on his extension 10 or 20cm to land on the spiling batten. He marks the batten and then places a symbol indicating how much to add at that mark to get the full width of the plank. Photo Credit: Douglas Brooks.

The system Bansho san uses for marking his measurements on the spiling batten. One tick on the line means he needs to add 10cm. Two ticks indicates 20cm. Photo Credit: Douglas Brooks.

For the plank angles, Bansho san uses a very interesting tool, one used by boatbuilders throughout Japan. It consists of a straight stick and a plumb bob; in Bansho san’s case, he uses his ink line as the plumb bob. At a mark one shaku from the top of the stick (for this, he doesn’t use metric, he has several wooden scales he has made, all in shaku), he measures the horizontal dimension from the string to the mark. He then scales the chine heights, sheer heights, and half breadths from his drawing, lofting them full-size on a scrap of wood. On the resulting right triangles for each station, the vertical legs are one shaku and the hypotenuses are the plank angles. He can then measure the horizontal legs of each triangle to ensure it matches the one he measured on the boat. If not, he adjusts the props holding the plank in place to change the plank angles. Every angle is remembered and recorded as this horizontal dimension, which he calls hiraki.

Bansho san uses a combination of a plumb bob and shaku stick to check the hiraki, or horizontal offset, of the spiling batten. Photo Credit: Douglas Brooks.

Here, Bansho san uses the same tool to set the rake of the transom based on a horizontal offset taken from his plank drawing. Photo Credit: Douglas Brooks.

One of the things that impresses me about Japanese boatbuilding is how design and layout have been simplified and reduced to only the most essential information. I’ve built boats over thirty feet long in Japan and never have I seen more than three stations used. Many dimensions in the boat are based on the width of the carpenter’s square. Everything seems to be tailored to efficiency, which is one reason why many of the builders I’ve worked with have been able to build hundreds of boats over the course of their careers. Bansho san began his career building only wooden boats with his father, switching to fiberglass in his late twenties after his father stopped working. Nevertheless, in a roughly ten-year period, together they built almost seventy-five boats.

Bansho san combines his ink line with his square to center the stem on the shiki or keel plank. Photo Credit: Douglas Brooks.