Besides the midget submarines described above, Japanese designers produced a number of experimental models which did not achieve operational status. Had they done so, they would have been deployed for the final defence of Japan – as suicidal weapons.
The only model to go into production – about 14 were said to be on hand at Kure in August 1945 – was the boat known only by the codename of U-Kanamono (“U-Type Metal Fittings”). This crude weapon was strongly reminiscent of the Confederate semi-submersibles of the War Between the States: it was an awash-boat controlled by a two-man crew housed in a squat turret on a cigar-shaped hull, some 46ft (14m) long and 6.5ft (1.98m) in beam. Displacing c.15 tons (15.24 tonnes), it was powered by a single-shaft compressed-air torpedo motor, giving a maximum speed of only c.3kt (3.45mph, 5.5kmh) and a very limited action radius. Its armament consisted of one 17.7in (450mm) bow torpedo tube.
The smallest of Japan’s midgets, the two-man, 11.5 ton (11.7 tonne) Shinkai (“Sea Vibrator”), was intended for suicidal expeditions into Allied anchorages in operations somewhat resembling those of the Italian “Pig” and British “Chariot”. It was a shallow-draught (3.9ft, 1.2m) submersible armed with a detachable, magnetic, 2,000lb (907kg) warhead, to be affixed to the hull of an enemy ship. The only unit built, codenamed 9-Kanamono (“Type 9 Metal Fittings”) and completed at Ourazaki in August 1944, was powered by a 20shp electric motor giving a maximum 9kt (10.3mph, 16.6kmh) submerged. The 41ft (12.5m) craft was both unstable and plagued by mechanical failure.
Also intended for attacks on Allied anchorages was the Type C variant known as M-Kanamono (“Type M Metal Fittings”), built at Ourazaki in late 1944. Very little larger than the Type C, and with the same engines, it had no torpedo tubes and instead carried four mines. The single unit completed is said to have been equipped with tracks for crawling along the sea bottom.
The last of the experimental midgets was the Maru-Se (“SE boat”), of which one prototype was built by Kawasaki in 1944 for the Imperial Japanese Army. This craft, of which few details survive, was powered by a Walter high-test peroxide motor, a German-developed unit similar to the hydrogen- peroxide/hydrazine engine used in the experimental Kaiten II. This gave a submerged speed of c.15–20kt (17–23mph, 28–37kmh). It was to be armed with two electrically-driven torpedoes, then under development.