The was a class of transport submarines built for the

Imperial Japanese Army The was the official ground-based armed force of the Empire of Japan from 1868 to 1945. It was controlled by the Imperial Japanese Army General Staff Office and the Ministry of the Army, both of which were nominally subordinate to the Emperor o ...

(IJA) during

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...

. The IJA called them .

Design and construction

The six-month-long

Guadalcanal campaign The Guadalcanal campaign, also known as the Battle of Guadalcanal and codenamed Operation Watchtower by American forces, was a military campaign fought between 7 August 1942 and 9 February 1943 on and around the island of Guadalcanal in th ...

came to an end in early February 1943, when the last

(IJA) forces withdrew from

Guadalcanal Guadalcanal (; indigenous name: ''Isatabu'') is the principal island in Guadalcanal Province of Solomon Islands, located in the south-western Pacific, northeast of Australia. It is the largest island in the Solomon Islands by area, and the seco ...

.Mühlthaler, p. 329. Later that month, the IJA′s 10th Army Staff Headquarters, which was responsible for all IJA-operated ships, decided it would need to develop its own transport

submarine A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...

— which it provisionally designated "transport boat" (''Yuso-tei'', abbreviated as ''Yu-tei'') — with which to supply isolated IJA island garrisons in the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

. On 5 March 1943, the 10th Army Staff Headquarters began work with the 7th Army Research Institute to design such a submarine, find suitable manufacturers and

shipyard A shipyard, also called a dockyard or boatyard, is a place where ships are built and repaired. These can be yachts, military vessels, cruise liners or other cargo or passenger ships. Dockyards are sometimes more associated with maintenance a ...

space for the construction of a fleet of them, and complete 20 of them by end of 1943, followed by an additional 400 submarines in later years. The IJA initially intended to keep its submarine program secret from the

Imperial Japanese Navy The Imperial Japanese Navy (IJN; Kyūjitai: Shinjitai: ' 'Navy of the Greater Japanese Empire', or ''Nippon Kaigun'', 'Japanese Navy') was the navy of the Empire of Japan from 1868 to 1945, when it was dissolved following Japan's surrender ...

(IJN), but the uncle of one of the

officers An officer is a person who has a position of authority in a hierarchical organization. The term derives from Old French ''oficier'' "officer, official" (early 14c., Modern French ''officier''), from Medieval Latin ''officiarius'' "an officer," fro ...

assigned to the program was an IJN vice admiral in charge of equipping IJN submarines at the

Kure Naval Arsenal was one of four principal naval shipyards owned and operated by the Imperial Japanese Navy. History The Kure Naval District was established at Kure, Hiroshima in 1889, as the second of the naval districts responsible for the defense of the J ...

Kure is a port and major shipbuilding city situated on the Seto Inland Sea in Hiroshima Prefecture, Japan. With a strong industrial and naval heritage, Kure hosts the second-oldest naval dockyard in Japan and remains an important base for the Japan M ...

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

. This soon led to cooperation between the IJA and IJN that included tours of IJN submarines for IJA officers and the sharing of IJN technical information with the IJA. The IJA staff established a requirement for a submarine capable of at least on the surface and a cargo capacity of 24 tons of

rice Rice is the seed of the grass species ''Oryza sativa'' (Asian rice) or less commonly ''Oryza glaberrima ''Oryza glaberrima'', commonly known as African rice, is one of the two domesticated rice species. It was first domesticated and grown i ...

. In early April 1943, the design for a simple "underwater transport boat" (''senko yusotei'') was submitted for approval and approved with only minor changes. The submarines had a designed diving depth of and were armed with a 37-millimeter

deck gun A deck gun is a type of naval artillery mounted on the deck of a submarine. Most submarine deck guns were open, with or without a shield; however, a few larger submarines placed these guns in a turret. The main deck gun was a dual-purpose ...

originally designed for use in

tank A tank is an armoured fighting vehicle intended as a primary offensive weapon in front-line ground combat. Tank designs are a balance of heavy firepower, strong armour, and good battlefield mobility provided by tracks and a powerful engin ...

s. They had no

torpedo tube A torpedo tube is a cylindrical device for launching torpedoes. There are two main types of torpedo tube: underwater tubes fitted to submarines and some surface ships, and deck-mounted units (also referred to as torpedo launchers) installed aboa ...

s and carried no

torpedo A modern torpedo is an underwater ranged weapon launched above or below the water surface, self-propelled towards a target, and with an explosive warhead designed to detonate either on contact with or in proximity to the target. Historically, su ...

es. As completed, they received the official designation "Type 3 submergence transport vehicle" — known unofficially to the IJA as ''Maru Yu'' — and the first production type was classified as the Yu 1 Type. With all shipyards under IJN supervision and busy to capacity fulfilling IJN requirements, the IJA turned to

locomotive A locomotive or engine is a rail transport vehicle that provides the Power (physics), motive power for a train. If a locomotive is capable of carrying a payload, it is usually rather referred to as a multiple unit, Motor coach (rail), motor ...

and

motor vehicle A motor vehicle, also known as motorized vehicle or automotive vehicle, is a self-propelled land vehicle, commonly wheeled, that does not operate on Track (rail transport), rails (such as trains or trams) and is used for the transportation of pe ...

manufacturers to build its submarines. It selected

Hitachi () is a Japanese multinational corporation, multinational Conglomerate (company), conglomerate corporation headquartered in Chiyoda, Tokyo, Japan. It is the parent company of the Hitachi Group (''Hitachi Gurūpu'') and had formed part of the Ni ...

′s Kasado Works in

Kudamatsu is a city in Yamaguchi Prefecture, Japan. , the city has an estimated population of 55,119, with 24,392 households and a population density of 616 persons per km2. The total area is 89.44 km2. The city was founded on November 3, 1939, being ...

, Japan, to build the

prototype A prototype is an early sample, model, or release of a product built to test a concept or process. It is a term used in a variety of contexts, including semantics, design, electronics, and Software prototyping, software programming. A prototyp ...

, , and the first production series. Each submarine was built in three sections — fore, middle, and aft — in steel cradles, and the cradles then were moved via

marine railway The patent slip or marine railway is an inclined plane extending from shoreline into water, featuring a "cradle" onto which a ship is first floated, and a mechanism to haul the ship, attached to the cradle, out of the water onto a slip. The m ...

s to the water's edge, where the sections were joined using

electric resistance welding Electric resistance welding (ERW) is a welding process where metal parts in contact are permanently joined by heating them with an electric current, melting the metal at the joint. Electric resistance welding is widely used, for example, in manufa ...

. When almost complete, the submarine was launched by simply moving the cradles into the water, after which final

fitting-out Fitting out, or outfitting, is the process in shipbuilding that follows the float-out/launching of a vessel and precedes sea trials. It is the period when all the remaining construction of the ship is completed and readied for delivery to her o ...

took place. Three other manufacturers also produced Yu I type submarines The

Japan Steel Works is a steel manufacturer founded in Muroran, Hokkaidō, Japan in 1907. History Japan Steel Works was set up with investment from British firms Vickers, Armstrong Whitworth and Mitsui. During World War II, they manufactured what was then the wor ...

constructed the ''Yu 1001'' series at a plant in

Hiroshima is the capital of Hiroshima Prefecture in Japan. , the city had an estimated population of 1,199,391. The gross domestic product (GDP) in Greater Hiroshima, Hiroshima Urban Employment Area, was US$61.3 billion as of 2010. Kazumi Matsui h ...

,Mühlthaler, p. 330. while the Ando Iron Works built the ''Yu 2001'' series in the

Tsukishima is a place located in Chūō, Tokyo, Japan, in the Sumida River estuary. It is a reclaimed land next to Tsukuda District. The land reclamation completed in 1892, using earth from the dredging work performed to create a shipping channel in Toky ...

district of

Tokyo Tokyo (; ja, 東京, , ), officially the Tokyo Metropolis ( ja, 東京都, label=none, ), is the capital and largest city of Japan. Formerly known as Edo, its metropolitan area () is the most populous in the world, with an estimated 37.468 ...

and the Korea Machine Factory Boat Manufacturing Works at

Inchon Incheon (; ; or Inch'ŏn; literally "kind river"), formerly Jemulpo or Chemulp'o (제물포) until the period after 1910, officially the Incheon Metropolitan City (인천광역시, 仁川廣域市), is a city located in northwestern South Kore ...

in Chosen, the Japanese name for

Korea Korea ( ko, 한국, or , ) is a peninsular region in East Asia. Since 1945, it has been divided at or near the 38th parallel, with North Korea (Democratic People's Republic of Korea) comprising its northern half and South Korea (Republic o ...

while Korea was under Japanese rule, constructed the ''Yu 3001'' series.Muehlthaler, Erich, and Bob Hackett, "Strange but True Stories: The Exceptional Maiden Voyage of Imperial Japanese Army Transport Submarine YU 3001," Nihon Kaigun, 1 February 2014 Accessed 11 May 2022
/ref>
/ref> (One source claims the ''Yu 3001'' series was built by the North Korea Machine Works in

Wonsan Wŏnsan (), previously known as Wŏnsanjin (), Port Lazarev, and Genzan (), is a port city and naval base located in Kangwŏn Province, North Korea, along the eastern side of the Korean Peninsula, on the Sea of Japan and the provincial capital. ...

, Korea.) Each manufacturer's submarines differed in detail from those built by other companies, resulting in four unofficial subclasses of the Yu I type. An improved IJA transport submarine designated the Yu II type was planned, but the prototype for it was not completed. The first submarine, , entered service in the final days of December 1943. The IJA planned to build 420 transport submarines, but completed only 38 by the end of war in August 1945.

Operations

Three submarines — , , and — deployed to the

Philippines The Philippines (; fil, Pilipinas, links=no), officially the Republic of the Philippines ( fil, Republika ng Pilipinas, links=no), * bik, Republika kan Filipinas * ceb, Republika sa Pilipinas * cbk, República de Filipinas * hil, Republ ...

in 1944, where all were lost between November 1944 and January 1945 during the Philippines campaign of 1944–1945. The rest of the IJA submarine fleet remained in Japanese home waters, operating from the main Imperial Japanese Army transport submarine base on the

Seto Inland Sea The , sometimes shortened to the Inland Sea, is the body of water separating Honshū, Shikoku, and Kyūshū, three of the four main islands of Japan. It serves as a waterway connecting the Pacific Ocean to the Sea of Japan. It connects to Osaka ...

Mishima Mishima may refer to: Places * Mishima, Fukushima, a town in Fukushima Prefecture * Mishima, Kagoshima, a village in Kagoshima Prefecture * Mishima, Niigata, a town in Niigata Prefecture * Mishima, Shizuoka, a city in Shizuoka Prefecture, Japan ...

Ehime Prefecture is a prefecture of Japan located on the island of Shikoku. Ehime Prefecture has a population of 1,342,011 (1 June 2019) and has a geographic area of 5,676 km2 (2,191 sq mi). Ehime Prefecture borders Kagawa Prefecture to the northeast, Toku ...

Shikoku is the smallest of the four main islands of Japan. It is long and between wide. It has a population of 3.8 million (, 3.1%). It is south of Honshu and northeast of Kyushu. Shikoku's ancient names include ''Iyo-no-futana-shima'' (), '' ...

. In March 1945, several began operations from Shimoda on the southern tip of the

Izu Peninsula The is a large mountainous peninsula with a deeply indented coastline to the west of Tokyo on the Pacific coast of the island of Honshu, Japan. Formerly known as Izu Province, Izu peninsula is now a part of Shizuoka Prefecture. The peninsul ...

Shizuoka Prefecture is a prefecture of Japan located in the Chūbu region of Honshu. Shizuoka Prefecture has a population of 3,637,998 and has a geographic area of . Shizuoka Prefecture borders Kanagawa Prefecture to the east, Yamanashi Prefecture to the northea ...

Honshu , historically called , is the largest and most populous island of Japan. It is located south of Hokkaidō across the Tsugaru Strait, north of Shikoku across the Inland Sea, and northeast of Kyūshū across the Kanmon Straits. The island separ ...

, and by May 1945 operations also had begun from Kuchinotsu in

Nagasaki Prefecture is a Prefectures of Japan, prefecture of Japan located on the island of Kyūshū. Nagasaki Prefecture has a population of 1,314,078 (1 June 2020) and has a geographic area of 4,130 Square kilometre, km2 (1,594 sq mi). Nagasaki Prefecture borders ...

Kyushu is the third-largest island of Japan's five main islands and the most southerly of the four largest islands ( i.e. excluding Okinawa). In the past, it has been known as , and . The historical regional name referred to Kyushu and its surroun ...

. The IJA submarine fleet made supply runs from Mishima to Ōshima Island in March 1945, from Kuchinotsu in

Tokunoshima , also known in English as is an island in the Amami archipelago of the southern Satsunan Islands of Kagoshima Prefecture, Japan. The island, in area, has a population of approximately 27,000. The island is divided into three administrative ...

in the

Amami Islands The The name ''Amami-guntō'' was standardized on February 15, 2010. Prior to that, another name, ''Amami shotō'' (奄美諸島), was also used. is an archipelago in the Satsunan Islands, which is part of the Ryukyu Islands, and is southwest o ...

in the northeastern

Ryukyu Islands The , also known as the or the , are a chain of Japanese islands that stretch southwest from Kyushu to Taiwan: the Ōsumi, Tokara, Amami, Okinawa, and Sakishima Islands (further divided into the Miyako and Yaeyama Islands), with Yonaguni ...

in July 1945, and from Shimoda to

Hachijō-jima is a volcanic Japanese island in the Philippine Sea. It is about south of the special wards of Tokyo. It is part of the Izu archipelago and within the Fuji-Hakone-Izu National Park. Its only municipality is Hachijō. On 1 March 2018, its po ...

in the

Philippine Sea The Philippine Sea is a marginal sea of the Western Pacific Ocean east of the Philippine archipelago (hence the name), the largest in the world, occupying an estimated surface area of . The Philippine Sea Plate forms the floor of the sea. Its ...

sometime during 1945. When the war ended in August 1945, the IJA was preparing to initiate a submarine supply route between Senzaki in

Yamaguchi Prefecture is a Prefectures of Japan, prefecture of Japan located in the Chūgoku region of Honshu. Yamaguchi Prefecture has a population of 1,377,631 (1 February 2018) and has a geographic area of 6,112 Square kilometre, km2 (2,359 Square mile, sq mi). Y ...

and

Pusan Busan (), officially known as is South Korea's most populous city after Seoul, with a population of over 3.4 million inhabitants. Formerly romanized as Pusan, it is the economic, cultural and educational center of southeastern South Korea, w ...

, Korea. was sunk in a U.S. air raid in the final days of the war, and the rest of the IJA submarine fleet surrendered to the

Allies An alliance is a relationship among people, groups, or states that have joined together for mutual benefit or to achieve some common purpose, whether or not explicit agreement has been worked out among them. Members of an alliance are called ...

at the end of the war. Some subsequently sank in a storm and were refloated and scrapped.Parker, p. 57. The rest presumably were either scrapped or

scuttled Scuttling is the deliberate sinking of a ship. Scuttling may be performed to dispose of an abandoned, old, or captured vessel; to prevent the vessel from becoming a navigation hazard; as an act of self-destruction to prevent the ship from being ...

Submarines

''Yu I'' type

The class consists of four unofficial subclasses resulting from manufacturing differences among the contracted builders. The manufacturer can be discerned by the appearance of the

conning tower A conning tower is a raised platform on a ship or submarine, often armored, from which an officer in charge can conn the vessel, controlling movements of the ship by giving orders to those responsible for the ship's engine, rudder, lines, and gro ...

''Yu 1''-class

Basic model of ''Yu I'' type. First boat ''Yu 1'' was prototype of the ''Yu I''-type submarines. The Hitachi-Kasado Factory built all of the ''Yu 1''-subclass boats. Their conning tower was closed. * ''Yu 1'', laid down in February 1943, completed on 31 October 1943, sunk by U.S. aircraft in

Lingayen Gulf The Lingayen Gulf is a large gulf on northwestern Luzon in the Philippines, stretching . It is framed by the provinces of Pangasinan and La Union and sits between the Zambales Mountains and the Cordillera Central. The Agno River and the Balili ...

on 2 January 1945. * ''Yu 2'', sunk by , , , and in

Ormoc Bay Ormoc Bay is a large bay in the island of Leyte in the Philippines. The bay is an extension of the Camotes Sea. The city of Ormoc lies at the head of the bay and exports rice, copra and sugar. The World War II Battle of Ormoc Bay took place fr ...

on 28 November 1944. * ''Yu 3'', attacked by U.S. aircraft and

by her crew in Lingayen Gulf on 5 January 1945, salvaged by on 18 January 1945, transported to

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

by in May 1945. * ''Yu 4'', survived war. * ''Yu 5'', survived war. * ''Yu 6'', assigned Detachment 2, Transport Submarine Group on 13 February 1945, survived war. * ''Yu 7'', assigned Detachment 2, Transport Submarine Group in November 1944, survived war. * ''Yu 8'', survived war. * ''Yu 9'', survived war. * ''Yu 10'', assigned Detachment Kuchinotsu, Transport Submarine Group on 15 May 1945, survived war. * ''Yu 11'', assigned Detachment Kuchinotsu, Transport Submarine Group on 15 May 1945, assigned Detachment Mikuriya in June 1945, survived war. * ''Yu 12'', assigned Detachment Kuchinotsu, Transport Submarine Group on 15 May 1945, survived war. * ''Yu 13'', assigned Detachment Mikuriya in June 1945, survived war. * ''Yu 14'', assigned Detachment Kuchinotsu, Transport Submarine Group on 15 May 1945, assigned Detachment Mikuriya in June 1945, survived war. * ''Yu 15'', survived war. * ''Yu 16'', survived war. * ''Yu 17'', survived war. * ''Yu 18'', survived war. * ''Yu 19'', survived war. * ''Yu 20'', survived war. * ''Yu 21'', survived war. * ''Yu 22'', survived war. * ''Yu 23'', survived war. * ''Yu 24'', survived war. * ''Yu 25'', not completed.

''Yu 1001''-class

The Japan Steel Works-Kaita Factory built all of the ''Yu 1001''-subclass submarines. They were longer and had a slightly higher

displacement Displacement may refer to: Physical sciences Mathematics and Physics *Displacement (geometry), is the difference between the final and initial position of a point trajectory (for instance, the center of mass of a moving object). The actual path ...

than the ''Yu 1'' subclass and a more powerful

diesel engine The diesel engine, named after Rudolf Diesel, is an internal combustion engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to mechanical compression; thus, the diesel engine is a so-call ...

which increased their maximum surface speed by about . They had an open-top conning tower. One source claims that it is "unlikely" that they mounted a deck gun. * ''Yu 1001'', launched on 27 March 1944, completed on 15 June 1944, assigned Detachment 2, Transport Submarine Group in November 1944, sunk by air raid at Shimoda on 12 August 1945. * ''Yu 1002'', assigned Detachment 2, Transport Submarine Group on 11 February 1945, survived war. * ''Yu 1003'', assigned Detachment 2, Transport Submarine Group on 11 February 1945, survived war. * ''Yu 1005'', assigned Detachment 2, Transport Submarine Group on 13 February 1945, survived war. * ''Yu 1006'', survived war. * ''Yu 1007'', assigned Detachment Kuchinotsu, Transport Submarine Group on 15 May 1945, assigned Detachment Mikuriya in June 1945, survived war. * ''Yu 1008'', survived war. * ''Yu 1009'', survived war. * ''Yu 1010'', survived war. * ''Yu 1011'', not completed. * ''Yu 1012'', not completed. * ''Yu 1013'', not completed. * ''Yu 1014'', not completed.

''Yu 2001''-class

The Andō Iron Works-Tsukishima Factory built all of the ''Yu 2001''-subclass submarines. First boat (''Yu-2001'') was the second prototype of the ''Yu I''-type submarines. The ''Yu 2001 subclass'' featured a "repose room" — a

deckhouse A cabin or berthing is an enclosed space generally on a ship or an aircraft. A cabin which protrudes above the level of a ship's deck may be referred to as a deckhouse. Sailing ships In sailing ships, the officers and paying passengers wou ...

built aft from the root of the conning tower — for improved crew comfort. * ''Yu 2001'', launched on 12 February 1944, survived war. * ''Yu 2002'', launched on 31 March 1945, survived war. * ''Yu 2003'', not completed. * ''Yu 2004'', not completed. * ''Yu 2005'', not completed. * ''Yu 2006'', not completed.

''Yu 3001''-class

The Chōsen Machinery-Jinsen Factory built all of the ''Yu 3001''-subclass submarines. No information is available on their design. * ''Yu 3001'', launched on 10 April 1944, completed on 2 August 1944, survived war. * ''Yu 3002'', sunk by rough weather in 1945. * ''Yu 3003'', survived war. * ''Yu 3005'', not completed. * ''Yu 3006'', not completed. * ''Yu 3007'', not completed. * ''Yu 3008'', not completed. * ''Yu 3009'', not completed. * ''Yu 3010'', not completed.

''Yu II'' type

Improved model of the ''Yu I'' type. ''Yu II'' was built by

Kampon The was the externally operating division of the Ministry of the Navy of Japan responsible for the administration of naval vessel construction. From 1923 onward, it took on the role of a research institution for the research and development of n ...

technical guidance. The IJN used the ''Ha-101''-class submarine drawings and designed this. * ''Ushio'', prototype of the ''Yu II'' type, laid down in August 1944, launched on 16 May 1945, not completed.

Footnotes

Bibliography

* * * * * *Rekishi Gunzō, History of Pacific War Extra, ''Perfect guide, The submarines of the Imperial Japanese Forces'', Gakken, Tokyo Japan, 2005, . *Rekishi Gunzō, History of Pacific War Vol.45, ''Truth histories of the Imperial Japanese Naval Vessels'', Gakken, Tokyo Japan, 2004, . *Ships of the World No.506, Kaijinsha, Tokyo Japan, 1996. *The Maru Special, Japanese Naval Vessels No.43 ''Japanese Submarines III'', Ushio Shobō, Tokyo Japan, 1980. *Atsumi Nakashima, ''Army Submarine Fleet, "The secret project !, The men challenged the deep sea"'', Shinjinbutsu Ōraisha, Tokyo Japan, 2006, . *''50 year history of the Japan Steel Works'' (first volume and second volume),

, 1968. {{WWII Japanese ships Submarine classes Submarines of the Imperial Japanese Army World War II submarines of Japan Merchant submarines