The context for this question really drives the type of information I'm looking for:
A friend is writing a story in which two of her characters have to cross the tsugaru strait; it's about 1185-1189AD. We're having trouble finding the kind of information that would help her write this scene realistically: Who had boats, what were the boats like, how were they operated. What we're finding instead is a lot of "yep, they had boats" and "yep, people crossed the strait" - big picture economic or historical information, rather than the smaller details.
Could a small boat - one that could be manned by one or two people - cross the strait, assuming that one of the people knew what they were doing? And what do we know about smaller boats at that time, in terms of their construction and operation?
I feel like this information must be out there, but we just can't hit upon the right keywords that gets us there, instead of ... like, Japanese naval battles. (Even if you don't know Japan, if you know this kind of info for another region, what kinds of keywords might help us?)
My friend can read Japanese, so Japanese sources are fine.
I assume this is after the Genpei War? They probably wouldn't have. Firstly, the Tsugaru Straits would have been part of the territory of the Northern Fujiwara or perhaps a quasi-independent Emishi lordship that was a vassal of the Fujiwara. In 1185-89, the victorious Minamoto turned their forces north and the area would be involved in a war to subjugate (and often wipe out) the Northern Fujiwara and Emishi. Even after they were defeated, they continued partisan activities against the Minamoto. It would not be a fun place to be.
If they were in more peaceful times, they still would not have crossed at the Tsugaru Straits. They would have departed from Dewa, perhaps the Mogami river if you want to be particularly poetic. It was a common trade river and often a center of shipping, but also noted for its beauty. They would have boarded large boats, probably laden with several goods to trade with the Ainu. (There would have been no significant Japanese population in Hokkaido yet). Rice was especially popular. This large ship would have sailed north but could have equally sailed south or anywhere else. It would be a boat meant to traverse the ocean. The boat would have sailed up the coast of Hokkaido, trading with local tribes, then returned back to base. If you're looking at examples, you can look at Dan no Ura or other naval battles. Remove the shields, the central tower, and you have basically the same design as a Japanese merchant vessel.
As to whether it was technically possible, they could have traveled across in a small rowboat if they were determined. The straits are twelve miles apart at their narrowest. That's a journey of a few hours in a rowboat (probably about three or four, down to two if they're really strong and booking it). Period rowboats look fairly modern, with benches and row handles and oars. The main difference is they were flatter and more square than modern ones. Any good picture of Dan no Ura will show you small rowboats involved in boarding actions. These were wider and flatter still to give samurai more room to fight and a more stable platform to fire, but they should give you some idea.
In the 10th century, both sides of the Tsugaru strait belonged to the Satsumon culture. After conquered by Kiyohara no Sadahira in the 1070 Enkyū Ezo War, the Satsumon culture in Honshu was gradually assimilated into Yamato culture. In 1094, Fujiwara no Kiyohira built the Oku no Daidō route from Shirakawa to Sotogahama, and imported eagle feathers and sealskins brought by Satsumon people from Hokkaido. Nitta No. 1 site in Aomori is considered the ruins of the Fujiwara's base for the trade. The earliest record of Yamato people crossing the strait comes from 1216, when the Kamakura shogunate exiled about 50 criminals to Hokkaido.
Archaeological evidence found in Hokkaido suggests that Satsumon people used what was similar to the Ainu's itaomacip/itaomachip boats for inter-island navigation. Itaomacip are a type of dugout canoe extended with planks by rope bindings. There are plenty of records about how the Ainu built and operated them in the Edo period. They were manned by two to three people for coastal fishing, and by four to nine to set out for the open sea. They also had simple dugouts.
Yamato fishermen also used simple dugouts as well as those with some extensions. It is hard to reconstruct the whole pictures because when they went out of use, they were usually reused as a well or other wooden objects. A 12th-century dugout unearthed from Kurakawa D site, Himi is found to have been carved with an adze, a chisel and an axe, and repaired with iron clamps. Until last century, dugouts were still used by fishermen in the Oga peninsula. Mudamahagi boats, traditonally used for coastal fishing around the Tsugaru strait, are said to retain some structural features of medieval extended canoes.
Written records from the Edo period tell a lot about how to cross the strait in a sailboat. With the Tsugaru current always flowing east, travelers waited for an east wind for several days. When they sailed from Minmaya to Matsumae, they headed the boat toward Matsumae Kojima island. Otherwise, they difted east to Hakodate, but were rarely wrecked if they chose a right day.
There are also a lot of records about a small boat crossing the strait. In an example from 1815, a worker from Shimofuro fell in love with a prostitute in Hakodate, and took her out of her brothel to his home in a dugout.
Ancient Trade and the Means of Marine Transport in Hokkaido
“Jun-Kozo-Sen” and “Ita-Oma-Chip” of the Ainu Race
A Dugout Canoe Unearthed from Kurakawa D site in Himi, Toyama
The Transition of Hull Structure of the Traditional Wooden Fishing Boat in North Japan