The Japanese Ko-hyoteki midget submarines were used in several theaters of the Pacific War, but their first and most famous use was during the attack of Pearl Harbor on 07DEC41. They were 80 feet in length. They were powered by a 600 horsepower (447 kW) electric motor, which could drive them at a maximum speed of 23 knots (43 km/h) or for 100 nautical miles (190 km) at a low speed. They carried a crew of two and two torpedoes, which were loaded externally from the bow.
For the Pearl Harbor raid they were carried piggy-back by five I-16 class fleet submarines and launched outside the harbor entrance. The minisubs were launched during the night before the raid, with orders to penetrate the harbor and attack. Nominally they were to rendezvous with their parent submarines after completing their missions, but the crews were under no delusions of the likelihood for successfully completing this phase and expected not to return.
There are some loose ends remaining. The Light Cruiser USS St. Lewis (CL-49) reported being missed by two torpedoes outside the harbor entrance at 1004. The Japanese fleet submarines were not positioned there so if the report is accurate, it is possible these were fired by I-16-tou. Alternatively, many believe a photograph taken of Battleship Row during the attack shows a midget sub broaching after firing her torpedoes. In either case, it is likely that I-16-tou ended up in the West Loch at the end of her mission and her wreckage was dumped off the harbor entrance in 1944.
The midget submarines are listed below by their parent subs. “I-16-tou” means “I-16’s boat”.
I-16-tou, ENS Masaharu Yokoyama and PO2c Tei Uyeda, launched at 0042. Likely penetrated Pearl Harbor, skuttled in the West Lock. Many believe a photograph taken by a Japanese aviator during the attack shows I-16-tou firing torpedoes at the USS West Virginia (BB-48) and USS Oklahoma (BB-37). Three messages were received from I-16-tou confirming a successful air attack, claiming that she had damaged U.S. warship(s), and a final message received at 0051 on 08DEC41 reporting that the submarine was unable to navigate. Her wreck was discovered in three sections in the debris field of the West Lock disaster, dumped outside the harbor during the clean-up. Torpedoes fired, scuttling charge detonated, crew unaccounted for.
I-18-tou, LTJG Shigemi Furuno and PO1c Shigenori Yokoyama, launched at 0215. Found outside of Pearl Harbor, East of the entrance, recovered by USS Current (ARS-22) on 13JUL60 from depth of 76 feet. Damaged by depth charges, abandoned by her crew, torpedoes were not fired. Currently on display at Eta Jima, Japan.
I-20-tou, ENS Akira Hiroo and PO2c Yoshio Katayama, launched at 0257. Sunk by the Destroyer USS Ward (DD-139) at 0645. The crew died in the attack, her torpedoes not fired. Found on the sea floor in 1,312 feet of water by a University of Hawaii submarine in August 2002. Declared a war grave.
I-22-tou, LT Naoji Iwasa and Petty Officer 1c Naokichi Sasaki, launched at 0116, penetrated Pearl Harbor. Fired one torpedo at the Seaplane Tender USS Curtiss (AV-4) and one torpedo at the Destroyer USS Monaghan (DD-354). I-22-tou was struck by shellfire from Curtiss at 0840, then rammed and depth-charged by Monaghan. Crew was killed in the attack. Her wreck was recovered on 21DEC41 and used as fill during construction, remains of the crew still aboard. LT Iwasa’s shoulder insignia was recovered from the wreckage confirming the identification, as he was the only full Lieutenant among the crews. The insignia is currently on display at Yasukuni.
I-24-tou, Ha-19, ENS Kazuo Sakamaki and CWO Kiyoshi Inagaki, launched at 0333. She had a faulty gyrocompass which delayed her launch. She was depth charged twice off the entrance to Pearl Harbor and ran aground. Broke free and proceeded east, then ran aground again off Bellows Field. Submarine broke free during air attack and hauled ashore by U.S. forces. Torpedoes not fired due to damage, scuttling charge failed to detonate. Inagaki killed, Sakamaki taken prisoner. Ha-19 was salvaged and went on a War Bond tour, and is currently displayed at The National Museum of the Pacific War in Fredericksburg, Texas.