Tomorrow the man who is making our sail is coming to the island. I learned that he is soaking the cotton in real shark liver oil as a preservative. I also just recently heard someone claim that in the old days sails were soaked in pigs blood as a preservative, but I am not sure about that. The traditional sails here were a tanbark kind of color, and I guess it was usually shark liver oil. I will try to confirm all this tomorrow.
I also learned from Shimojo san yesterday that the small mast that was sometimes stepped in the bow of the boat was about 2/3rds the height of the mainmast, which is 2/3rds the length of the boat. He said that the sail shape of the jib was just a reduced version of the mainsail, and he said this auxiliary sail was only flown in light winds. So it clearly wasn't really like a jib; that is, slotting the main to increase its efficiency.
As for the tempura oil, I estimate about five gallons have been used on the boat inside and out. The cedar just keeps soaking it up, and the pine, despite how hard a wood it is, is an amazing sponge. At any time the boat could be toweled completely dry to the touch, and there is no smell. Remember though, in Japan small boats even today are pulled out of the water at the end of each day. I don't think that this would be a finish that could be left immersed for long periods.