By definition, a midget submarine is less than 150 tons, has a crew of no more than eight, has no on-board living accommodation, and operates in conjunction with a mother ship to provide the living accommodations and other support. The Japanese Navy built at least 800 midgets in 7 classes, but only a fraction had any noticeable impact on the war. Their intended purpose initially was to be deployed in front of enemy fleets, but their actual use would be in harbor attacks and coastal defense.
The Japanese midget subs were not named but were numbered with “Ha” numbers (e.g., Ha-19). These numbers were not displayed on the exterior and operationally the midgets were referred to according to the numbers of their mother ships. Thus, when I-24 launched Ha-19, the midget was known as “I-24tou” (designated “M24” in some texts). The “Ha” numbers were not unique either; some Type D’s were numbered Ha-101 through Ha-109.
In mid-1944, with coastal defense requirements becoming urgent, the Japanese Navy developed the Koryu Tei Gata Type D. More than just another improved version of the Type A, this was a new design. They were the largest of Japan’s midgets, displacing about 60 tons, 86 feet (26 meters) in length, with a five-man crew, featuring a more powerful diesel engine, and had improved operating endurance. Koryu’s armament consisted of two muzzle-loaded 17.7-inch torpedoes. As with the earlier types, individual boats had alpha-numeric names in the “Ha” series beginning with Ha-101.
Some 115 units had been completed when Japan capitulated in August 1945. At the end of the war, Allied Occupation forces found hundreds of midget submarines built and building in Japan, including large numbers of the “Koryu” type; nearly 500 more were under construction. Some of these submarines intended for training pilots for Kaiten type manned torpedoes, had an enlarged conning tower and two periscopes.