Building a Hozugawa Ayubune Model in 1/10 Scale – Part 2

The ayubune is not my first Japanese boat scratch build attempt. The first was the Urayasu bekabune, a boat designed for working among the seaweed nets of Tokyo Bay. But, being unaccustomed to scratch building Japanese traditional boats, I was wresting with a few construction problems and a couple errors, so I set it aside. Then, I found the ayubune on Douglas Brooks’s blog.

The ayubune is a very simple design. There is no cutwater, the side are flush, making for a very simple shape, and there few details beyond the hull and beams. This seemed to be an ideal subject to start with.

Mr. Brooks recorded 3 sizes of ayubune in Japan, a 24-shaku, 18-shaku and a 15-shaku boat. I noted that he built at least 3 of the 15-shaku boats and posted photos and notes on their construction. At a traditional 1/10 scale, the 15-shaku boat would be just about 15″ long, which seemed like a good size.

Large, 24-shaku fiberglass ayubune usee to give river tours to tourists on the Hozu river.

15-shaku ayubune on which my drawings are patterned. Photo courtesy of Douglas Brooks.

The ayubune build by Douglas Brooks in Japan. Photo courtesy of Douglas Brooks.

The plan with this model, is to build it as closely as possible to the full-sized boat. Japanese boatbuilders who model their own boats tend to build these smaller boats at 1/10 scale and use the same woods used in the construction of their full-sized boats. So, this model will be built to 1/10 scale and be made predominantly from sugi, or Japanese cedar (not to be confused with Hinoki, which is Japanese cypress).

Like the full-sized ayubune, I will cut all the mortises and fill them with wooden plugs. I tinkered with the idea of using small nails, but nobody makes miniature flat nails, so I’d have to make them myself. This seemed a bit over the top, at least for now, so I won’t actually be nailing the boat together, and the mortises will only be a visual detail.

Before doing much with the boat itself, I thought it best to get some practice cutting mortises and the try out my techniques with some new, small carving chisels I bought at a Japanese tool store in Berkeley, CA (for more on this shop, see the detail below on Kanejaku measure). These particular tools are only about $8 each and available individually or in sets. Of course, a sharpening stone is necessary for these, so at the advice of the person in the store, I bought a very simple, inexpensive, synthetic slipstone for around $12.

I’m building this boat in terms of the full-sized boat using the traditional Japanese shaku measure. And I’ll be referring to measurements at full size. My model is 1/10 scale, so everything should be scaled down accordingly, but with shaku that’s pretty easy to do.

Kanejaku measure

The traditional measure of the shaku is a unit very close to 1 foot. Technically, this is a kanejaku for traditional Japanese carpenters and boatbuilders. There is actually another shaku measure called a kujirashaku that was used by tailors, and is a little longer.

  • 1-shaku (now defined as 10/33 meters, or .9942 feet)
  • 1-sun = 1/10 shaku (about 1-3/16″)
  • 1-bu = 1/10 sun or 1/100 shaku (about 1/8″)
  • 1-rin = 1/10 bu or or 1/1000 shaku (0.012″)

It’s easiest to use an actual shaku square, which are available from Hida Tool in Berkeley, CA. They have an online shop for those outside the area. These aren’t cheap, but you’ll only ever need one. There are a few on their site, just make sure you’re getting one that is shaku and not metric/inch.

I bought a large one that is 5-sun by 10-sun and cost about $41 plus tax and shipping. However, for most things, you can probably use a small, 2.5-sun by 5-sun square, which you can get for closer to $15 plus tax and shipping. I might spring for a small one at some point, as it might be handy, but I’m happy with the larger one for now.

I first drew a line 1-sun from the edge of the plank and another 4-sun from the edge of the plank. This marked out the length of the mortises. Then, I marked off the centerlines for each mortise, 7-sun apart. Next, I marked the small end 5-bu wide and the large end 1-sun wide, finally drawing connecting lines to create the long trapezoid outlines for the mortises.

Mortises are marked out. My shaku square is visible at the top of the photo.

Cutting soft sugi requires using very sharp tools. Otherwise, the wood crushes very easily. So, this was a chance to try out my new Japanese carving chisels, and to test using an X-Acto knife and scalpel to make the cuts and see what worked best.

At this point, it was still difficult for me to decide what was easiest for cutting these mortises, for the small cuts, the Japanese chisels worked well. For the longer cuts, I ended up relying more on a sharp X-Acto. Basically, I cut the outlines of the mortises, then used the carving chisel to gouge them out.

I took a piece of cross-grained sugi, cut to length, and cut it down to the same trapezoidal shape, added some wood  glue into the mortise, and pressed the plug into place.

I clamped it into place until the glue dried, then I cut the plug loose and went to the next mortise. One thing you might notice if you look very carefully: I originally carved out the mortises in a sloping angle, which is how it’s done on the real boat. The end where the nail goes has to be deep enough to fit the head of the nail. But, I found better looking results here by carving them out evenly.

After the plugs were glued and dried, I then used the wider flat blade carving chisel to trim off the excess wood from the plug, leaving them flush with test plank.

The final result seemed good enough to carry on with the build. So, I cut paper patterns and the planks for the shiki, or the bottom of the boat.

Patterns I drew up from the plans.

One issue I ran into was with the exact shape of the bow plank, what I labeled as the omote. Traditionally, boatbuilder’s plans are drawn to the inside of the planking, but the result would have made the narrow tip, way to narrow, as it would have to be beveled to accept the hull planks. After consulting with Mr. Brooks, in the end, I had resolved to simply oversize the wood a little, to allow for any necessary beveling. This seemed to be a regular practice with Japanese boatbuilders anyway.

Next time, I’ll be cutting wood.



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