From The Naval Historical Center Sight:
"In mid-1944, with coastal defense requirements becoming urgent, the Japanese Navy developed an improved version of the Type A, B & C midget submarines. Designated "Type D" and nicknamed "Koryu", the new design was somewhat larger than the earlier types, featured a more powerful diesel engine and had improved operating endurance. The "Koryu" also had a five-man crew, two more than in the Type C, but the same armament of two 45cm (17.7") diameter torpedoes.
The Type D midgets displaced about 60 tons and were 86 feet long. Propelled by a 500 horsepower electric motor, maximum submerged speed was 16 knots. On the surface, with an 150 HP diesel charging the electric batteries, speed was 8 knots and range about 1000 nautical miles. As with the earlier types, individual boats had alpha-numeric names in the "HA" series."