Japanese submarine Yu 21

Yu 21 was an Imperial Japanese Army transport submarine of the Yu 1 subclass of the Yu I type. Constructed for use during the latter stages of World War II, she served in the waters of the Japanese archipelago.

Construction
In the final two years of World War II, the Imperial Japanese Army constructed transport submarines — officially the Type 3 submergence transport vehicle and known to the Japanese Army as the Maru Yu — with which to supply its isolated island garrisons in the Pacific Ocean. Only submarines of the Yu I type were completed and saw service. The Yu I type was produced in four subclasses, each produced by a different manufacturer and differing primarily in the design of their conning towers and details of their gun armament. None carried torpedoes or had torpedo tubes. Yu 21 was a unit of the Yu 1 subclass.

The Hitachi Kasado Works (Hitachi Kasado Seisakujo) at Kudamatsu, Japan, constructed Yu 21. Records of the details of the construction of Yu 21 have not been discovered, but the earlier Yu I-type submarines were laid down and launched during the latter half of 1943 and entered service at the end of 1943 or early in 1944.

Service history
Yu 21 spent her operational career in Japanese home waters. Surviving records of the activities of Imperial Japanese Army submarines are fragmentary, and no records have been discovered describing her specific activities in support of any particular operation.

World War II ended with the cessation of hostilities on 15 August 1945. Yu 21 surrendered to the Allies later in August 1945. She subsequently either was scuttled or scrapped.