World War II Terror Weapons: How Imperial Japan Used Mini-Submarines

Sebastien Roblin

Security, Asia

And failed. 

Key point: Tokyo's mini-subs often didn't work as well as planned.

Australia was situated considerably closer to the action in the Pacific than the United States during World War II. Japanese aircraft bombed the northern city of Darwin, while ground forces advanced dangerously close in New Guinea. However, the Imperial Japanese Navy’s plans to capture nearby Port Moresby were frustrated at the Battle of the Coral Sea.

The Imperial Japanese Navy (IJN)’s next strike would target the U. S. naval base at Midway Island in June 1942. However, 8th Submarine Squadron was tapped to launch two diversionary raids using Type A Ko-hyoteki midget submarines to infiltrate harbor defenses.

Japan’s devastating Pearl Harbor attack included five Ko-Hyoteki—but not one of them succeeded in its mission. Carried atop large cruiser-submarine motherships, the two-person minisubs measured twenty-four meters long and carried two 17”-diameter torpedoes. Their lead-acid batteries afforded them only twelve hours of propulsion at slow speed. Though not intended to be suicide weapons, the Ko-hyoteki crew’s odds of escape and recovery remained extremely low.

Two cruiser-submarines sallied to ambush British ships besieging French-held Madagascar. Meanwhile, submarines I-22, I-24, I-27, and I-28 transited to Truk to load Ko-hyoteki for a southern raid, embarking a revised model with wider hulls, improved gyro-compasses, bow-mounted net-cutters on to slice through harbor nets, and accessways to allow manning while submerged.