The Ko-hyoteki (甲標的, "Type 'A' Target") class of Japanese midget submarines had hull numbers but no names. For simplicity, they are most often referred to by the hull number of the mother submarine. Thus, the midget carried by I-16 was known as "the I-16 midget". The midget submarine hull number began with the character "HA", but is visible only on the builder's plate inside the hull.
Fifty were built. The "A Target" name was assigned as a ruse—if their design was prematurely discovered by Japan's foes, the Japanese Navy could insist that the vessels were battle practice targets. They were also called "tubes" and other slang names.
The first two, Ha-1 and Ha-2, were used only in testing. They did not have conning towers, which were added to the later boats for stability underwater.
Ha-19 was launched by I-24 at Pearl Harbor. Most of the other fifty are unaccounted for, although three were captured in Sydney (Australia), and others in Guam, Guadalcanal, and Kiska Island, accounting for some of the other hull numbers.
The submarines were each armed with two 17.7 inch (450 mm) torpedoes in muzzle-loading tubes one above the other on the port bow. In the Pearl Harbor attack, the specially designed Type 97 torpedo was used, but problems with the oxygen flasks meant that all later attacks used a different torpedo. Some have stated that a version of the Type 91 torpedo, designed for aircraft launching, was used, but other reports indicate that the Type 97 torpedo was modified to the Type 98, otherwise known as the Type 97 special. There is no definitive information that the Type 91 was used. The Type 98 was later supplanted by the Type 02 torpedo. There was also a demolition charge which it has been suggested was large enough to enable the submarine to be used as a suicide weapon, but there is no evidence that it was ever used as one.
Each submarine had a crew of two men. A junior officer conned the boat while a petty officer manipulated valves and moved ballast to control trim and diving.
Five of these boats participated in the Pearl Harbor attack, with at least one actually making it into the harbor. Of the five used at Pearl Harbor, HA-19 was captured where it grounded on the east side of Oahu. During World War II, HA-19 was put on tour across the United States to help sell War Bonds. Now a US National Historic Landmark, HA-19 is on exhibit at the National Museum of the Pacific War.
A photograph taken from a Japanese plane during the Pearl Harbor attack appears to show a midget submarine inside the harbor firing torpedoes at Battleship Row. According to analysis conducted by the United States Naval Institute in 1999, the midget submarine may well have scored a direct hit on West Virginia. Alternatively, the submarine in the photo may the same one which fired its torpedoes at Curtiss and Monaghan. Both of those torpedoes missed and are believed to have hit a dock at Pearl City and the shore of Ford Island. This submarine was sunk by Monaghan at 0843 7 December and later recovered and used as fill during construction of a new landside pier at the Pearl Harbor submarine base.
One of the Pearl Harbor midget submarines was located by U.S. Navy divers off Keehi Lagoon east of the Pearl Harbor entrance on 13 June 1960. The submarine had been damaged by a depth charge attack and abandoned by its crew before it could fire its torpedoes. This submarine was restored and placed on display at the Imperial Japanese Naval Academy on 15 March 1962.
The midget submarine attacked by USS Ward (DD-139) at 0637 7 December was located in 400 meters of water five miles outside Pearl Harbor by a University of Hawaii research submersible on 28 August 2002. The fifth submarine used at Pearl Harbor has not been located.
On the night of 29 May 1942, five large Japanese submarines positioned themselves 56 kilometres north-east of Sydney Heads. At 3.00am the next day one of the submarines launched a reconnaissance aircraft. After circling Sydney Harbour the aircraft returned to its submarine, reporting the presence of 'battleships and cruisers' moored in the harbour. The flotilla's commanding officer decided to attack the harbour with midget submarines the next night. The next day the five submarines approached to within 11 kilometres of Sydney Heads, and at about 4.30pm they released three midget submarines which then began their approach to Sydney Harbour.
The outer-harbour defences detected the entry of the first midget submarine at about 8.00pm, but it was not identified until it became entangled in an anti-torpedo net that was suspended between George's Head and Green Point. Before HMAS Yarroma was able to open fire, the submarine's two crew members destroyed their vessel with demolition charges and killed themselves.
The second submarine entered the harbour at about 9.48pm and headed west towards the Harbour Bridge, causing a general alarm to be issued by the Naval Officer in Charge, Sydney. About 200 metres from Garden Island the submarine was fired on by the heavy cruiser USS Chicago. The submarine then fired its two torpedoes at the cruiser. One torpedo ran ashore on Garden Island but failed to explode. The other passed under the Dutch submarine K9 and struck the harbour bed beneath the depot ship HMAS Kuttabul (ship) where it exploded, killing 21 sailors (19 Royal Australian Navy and 2 Royal Navy). The submarine then slipped out of the harbour, its mission complete.
The third submarine was sighted by HMAS Yandra at the entrance to the harbour and was depth-charged. Some four hours later, having recovered, it entered the harbour but it was subsequently attacked with depth charges and sunk in Taylor Bay by vessels of the Royal Australian Navy. Both members of the submarine's crew committed suicide.
The two submarines that were recovered were identical, and their remains were used to reconstruct a complete submarine, which toured New South Wales, Victoria and South Australia before being delivered to the Australian War Memorial in Canberra in 1943, where it remains on display.
In May 1942, Type A midget submarines were also used to attack the Allied campaign in Madagascar.