Type A Ko-hyoteki (甲標的甲型) Target “A” Midget Submarines and the Attack on Pearl Harbor

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”.

Individual details:

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.

The Ko-hyoteki crews were deified after the raid. Based mainly on the radio report from I-16-tou, the Japanese believed that the midget submarines penetrated Pearl Harbor and that at lease one had attacked successfully. Missing from the portrait is ENS Sakamaki, who was captured.
A photograph of Battleship Row taken from a Japanese B5N2 “Kate” during the attack. In the center, both West Virginia and Oklahoma can be seen after taking torpedo hits and beginning to list, with oil slicks forming on the water.
A tighter expansion of the previous photograph. The disturbance in the water to the left has been interpreted as the I-16-tou breaching after firing her torpedoes. The three sprays to the left of her conning tower are water being thrown up by her screw, and two torpedo wakes are visible originating from that point. Just to the right of the submarine is a small boat. The interpretation of this photograph remains controversial.
The West Lock Disaster occurred on 21MAY44, when an accidental explosion spread through amphibious assault ships loading ammunition prior to the Marianas invasion. The explosions sank six LSTs and killed 163 sailors. The accident was hushed up and remained classified until 1960.
Debris from the West Lock Disaster were quickly cleared away and dumped off the entrance to Pearl Harbor. Among the debris lie the remains of I-16-kou, broken into three sections. In places the hull is pierced and cables have been threaded through the holes so the sections could be hoisted. The bow section has empty torpedo tubes and the unique “figure 8” cable cutter fitted to the Pearl Harbor attackers.
I-18-tou was discovered by U.S. Navy divers outside of the harbor to the east of the entrance. Her torpedoes remained in their tubes, her hatch had been opened and there was no trace of her crew. Submarine contacts were reported off the harbor entrance throughout the morning and many were depth charged (the USS Ward attacked four separate contacts). I-18-tou showed damage from depth charging, perhaps she was another of Ward’s victims?
There can be no question about this one. This is the I-20-tou resting on the sea floor, the hole from Ward’s #3 4-inch gun clearly visible at the base of her sail.
This is I-22-tou. She penetrated the harbor and worked her way around to the west side of Ford Island. There she was engaged by the USS Curtiss and USS Monaghan. She fired a torpedo at each ship but missed. Her hull shows the “washboard” effect of Monaghan’s depth charges and her hull is broken from being rammed and rolled under the destroyer. She was recovered two weeks after the raid.
I-24-tou was plagued by misfortune. The last midget sub to be launched because of trouble with her gyrocompass, she ran up on a reef outside the harbor. After working free she was depth charged and her crew disoriented. She worked around Oahu to the east until she hung up on another reef. Her crew exhausted and overcome by fumes, they abandoned ship after the scuttling charge failed to ignite. ENS Kazuo Sakamaki made it to the beach to become PoW #1, CWO Kiyoshi Inagaki drowned. I-24-tou is seen after being hauled up onto the beach off Bellows Field.
The control station of I-24-tou. Behind the ship’s wheel a man has his hand on the faulty gyrocompass. Reportedly it began to work properly after it was hit firmly.
One of the items recovered from I-24-tou was a detailed map of Pearl Harbor, the entrance is at the bottom. Mooring positions and target ships are indicated. Also note that courses and turning times have been annotated to assist navigation. The Imperial Japanese Navy had spies who provided detailed observations of the harbor prior to the raid, proof such as this only fueled suspicions concerning the Japanese population on Oahu.
The I-24-tou was shipped to the mainland for use in War Bond drives. She is seen here being inspected by shipyard workers in California.