Imperial Japanese Navy submarines originated with the purchase of five Holland type submarines from the United States in 1904. Japanese submarine forces progressively built up strength and expertise, becoming by the beginning of World War II one of the world's most varied and powerful submarine fleets.
- 1 Origins
- 2 World War II
- 2.1 Submarine aircraft carriers
- 2.2 Yanagi missions
- 2.3 1st Class submarines
- 2.3.1 Type KD1 (I-51)
- 2.3.2 Type KD2 (I-152)
- 2.3.3 Type KD3a (4 units)
- 2.3.4 Type KD3b (3 units)
- 2.3.5 Type KD4 (3 units)
- 2.3.6 Type KD5 (3 units)
- 2.3.7 Type KD6a (6 units)
- 2.3.8 Type KD6b (2 units)
- 2.3.9 Type KD7 (10 units)
- 2.3.10 Type J1 (I-1, I-2, I-3, I-4)
- 2.3.11 Type J1 Mod. (I-5)
- 2.3.12 Type J2 (I-6)
- 2.3.13 Type J3 (I-7, I-8)
- 2.3.14 Type A1 (I-9, I-10, I-11)
- 2.3.15 Type A2 (I-12)
- 2.3.16 Type AM (I-13, I-14)
- 2.3.17 Type B1 (20 units)
- 2.3.18 Type B2 (6 units)
- 2.3.19 Type B3 (I-54, I-56, I-58)
- 2.3.20 Type C1 (5 units)
- 2.3.21 Type C2 (I-46, I-47, I-48)
- 2.3.22 Type C3 (I-52, I-53, I-55)
- 2.3.23 Type D1 (11 units)
- 2.3.24 Type D2 (I-373)
- 2.3.25 Kiraisen Type (I-121, I-122, I-123, I-124)
- 2.3.26 Sen-Ho Type (I-351)
- 2.3.27 Sentoku Type (I-400, I-401, I-402)
- 2.3.28 Sentaka Type (3 units)
- 2.4 2nd Class submarines
- 2.5 3rd Class submarines
- 3 References
- 4 External links
The Imperial Japanese Navy (IJN) acquired its first submarines during the Russo-Japanese War on 12 December 1904 where they arrived in sections at the Yokahama dockyards. The vessels were purchased from the relatively new American company, Electric Boat, and were fully assembled and ready for combat operations by August 1905. However, hostilities with Russia were nearing its end by that date, and no submarines saw action during the war.
The submarines that Electric Boat sold to Japan were based on the Holland designs, known as Holland Type VIIs similar to the American Plunger-class submarines. The five imported Hollands were originally built at Fore River Ship and Engine Company in Quincy, Massachusetts under Busch's direction for the Electric Boat Company back in August–October 1904. They were shipped by freighter from Seattle, Washington in "knocked-down" kit form to Japan, and then reassembled by Arthur Leopold Busch at the Yokosuka Naval Arsenal, which was then Japan's largest naval shipyard, to become Hulls No. 1 through 5 and were designed Type 1 submarines by the Japanese Navy.
Frank Taylor Cable, an electrician who was working for Isaac Rice's Electro-Dynamic and Storage Companies along with Rice's Electric Boat, arrived some six months after Busch, training the IJN in the operation of the newly introduced vessels.
In 1904 Kawasaki Dockyard Company purchased plans for a modified version directly from Holland, and built two boats (Hulls No. 6 and 7), with the help of two American engineers, Chase and Herbert, who had been assistants to Holland. The Kawasaki-type submarines displaced 63 or 95 tons when submerged, and measured 73 or 84 feet in overall length, respectively. both vessels measured 7' at the beam. This contrasted with the original five imported Hollands-type submatines which had arrived that same year, at over 100 tons submerged, 67 foor overall length and 11 foot beams. The Kawasaki Type #6 and #7 submarines had gained extra speed and reduced fuel consumption by 1/4. However both boats could launch only one 18" torpedo, and each was manned by 14 sailors, whereas the imported Holland-type submarines could fire two torpedoes and could be operated by 13 sailors. This new type was designed the Japanese Type 6 submarine by the Japanese Navy, and was used primarily for test purposes.
The Kaigun Holland #6 was launched at Kobe on 28 September 1905 and was completed six months later at Kure as the first submarine built in Japan. It sank during a training dive in Hiroshima Bay on 15 April 1910. Although the water was only 58 feet deep, there were no provisions at all for the crew to escape while submerged. The commanding officer, Lieutenant Tsutomu Sakuma, patiently wrote a description of his sailor's efforts to bring the boat back to the surface as their oxygen supply ran out. All of the sailors were later found dead at their duty stations when this submarine was raised the following day. The sailors were regarded as heroes for their calm performance of their duties until death, and this submarine has been preserved as a memorial in Kure, Japan.
Although the capabilities of these first submarines were never tested in combat during the Russo-Japanese War, the first submarine squadron was soon formed at Kure Naval Base in the Inland Sea. Following the war, the Japanese government followed submarine developments by the Royal Navy with interest, and purchased two British C class submarines directly from Vickers, with an additional three built from kits by the Kure Naval Arsenal. These became respectively the Japanese Ha-1 class and Ha-3 class submarines. An additional two vessels, forming the Ha-7 class were later built by the Kure Naval Arsenal.
In 1909, the first submarine tender, Karasaki, was commissioned. Japan, along with the rest of the Allies, drew heavily upon Germany's Guerre de Course (commerce raiding) operations during the First World War, and their submarine successes reinforced Japan's willingness to develop this weapon, resulting in eighteen ocean-going submarines being included in its 1917 expansion program. At the end of World War I, Japan received nine German submarines as reparations, which allowed her and the other Allies to accelerate their technological developments during the interwar period.
World War II
Imperial Japanese Navy submarines formed by far the most varied fleet of submarines of World War II, including manned torpedoes (Kaiten), midget submarines (Ko-hyoteki, Kairyu), medium-range submarines, purpose-built supply submarines (many used by the Imperial Japanese Army, see Type 3), fleet submarines (many of which carried an aircraft), submarines with the highest submerged speeds of the conflict (Sentaka I-200), and submarines able to carry multiple bombers (World War II's largest submarine, the Sentoku I-400). They were also equipped with the most advanced torpedo of the conflict, the oxygen-fuelled Type 95, sometimes confused with the type 93 Long Lance torpedo. A plane launched from one such fleet submarine, I-25, conducted what remains the only aerial bombing attack on the continental United States, when Warrant Flying Officer Nobuo Fujita piloting a Yokosuka E14Y scouting plane dropped four 168-pound bombs in an attempt to start forest fires outside the town of Brookings, Oregon, on September 9, 1942. In February 1942, the submarine I-17 fired a number of shells from her deck gun at the Elwood Oil Fields near Santa Barbara, California. None of the shells caused any serious damage.
Overall, despite their technical innovation, Japanese submarines were built in small numbers and had less effect on the war than those of the other major navies. The IJN pursued the doctrine of guerre d'escadre (fleet vs fleet warfare), and consequently submarines were often used in offensive roles against warships, which were fast, maneuverable and well-defended compared to merchant ships. During the Battle of Midway, I-168 administered the coup de grace to USS Yorktown (CV-5), as well as sinking the destroyer USS Hammann (DD-412). Later in 1942, Japanese submarine I-19 sank the fleet carrier USS Wasp (CV-7), damaged the battleship USS North Carolina (BB-55), and damaged the destroyer USS O'Brien (DD-415) (which sank later on 19 October 1942) with a single salvo of torpedoes. However, as fuel oil diminished and air superiority was lost, Imperial submarines were no longer able to continue with such successes. Once the United States was able to increase its production of destroyers and destroyer escorts, as well as bringing over highly effective anti-submarine techniques learned during the Battle of the Atlantic, they continually took more and more of a toll on Imperial Japanese submarines, which also tended to be slower and not as deeply-diving as their Kriegsmarine counterparts. The Imperial Japanese Navy's doctrine of fleet warfare (guerre d'escadre) resulted in its submarines seldom posing a threat to allied merchant convoys and shipping lanes to the degree that the Kriegsmarine's U-boats did as they pursued commerce raiding against Allied and neutral merchant ships.
During the last two years of the War in the Pacific, the IJN submarines instead were often used to transport supplies to isolated island garrisons—ones deliberately bypassed by the Americans and the Australians. During the war, IJN submarines did sink about 1 million tons (GRT) of merchant shipping (184 ships) in the Pacific; by contrast U.S. Navy submarines sank 5.2 million tons (1314 ships) in the same period, while U-boats of the German Navy, the IJN's Axis partner, sank 14.3 million tons (2,840 ships) in the Atlantic and other oceans.
Early models of IJN submarines were not very maneuverable under water, could not dive very deeply, and lacked radar. (Later in the war units that were fitted with radar were in some instances sunk due to the ability of American radar sets to detect their emissions. For example, the USS Batfish sank three such IJN submarines near Japan in just four days). After the end of the conflict, several of Japan's most innovative and advanced submarines were sent to Hawaii for inspection in "Operation Road's End" (I-400, I-401, I-201, and I-203) before being scuttled by the U.S. Navy in 1946 when the Soviet Union demanded to have access to the I.J.N. submarines, also.
Submarine aircraft carriers
The Japanese applied the concept of the "submarine aircraft carrier" extensively, starting with the J3 type of 1937-38. Altogether 41 submarines were built with the capability to carry seaplanes. Most IJN submarine aircraft carriers could carry only one aircraft, but I-14 had hangar space for two, and the giant I-400 class three.
These were missions enabled under the Axis Powers' Tripartite Pact to provide for an exchange of strategic materials and manufactured goods between Germany, Italy and Japan. Initially, cargo ships made the exchanges, but when this was no longer possible, submarines were used.
Only six submarines attempted this trans-oceanic voyage during World War II: I-30 (April 1942), I-8 (June 1943), I-34 (October 1943), I-29 (November 1943), and German submarines U-511 (August 1943) and U-864 (December 1944). Of these, I-30 was partially successful but was later sunk by a mine, I-8 completed her mission, I-34 was sunk by British submarine Taurus, and I-29 by the United States submarine, Sawfish (assisted by Ultra intelligence). I-52 made the final attempt.
1st Class submarines
This class includes the largest of Japanese submarines, characterized by great size and range.
Type KD1 (I-51)
Type KD2 (I-152)
The Type KD2 (I-152 class) submarine was based on the U-139 and the British K class submarine.
- I-152 - scrapped 1948.
Type KD3a (4 units)
The Type KD3a (I-153 class) submarines (I-53/I-153, I-54/I-154, I-55/I-155, I-158) were similar to the Type KD1 and KD2 but with strengthened hulls.
- I-53/I-153 - sank the SS Moesi in the Bali Strait February 27, 1942 and the SS City of Manchester off Java February 28, 1942. The I-153 herself was scuttled off Iyo Nada in May 1946.
- I-54/I-154 - scuttled in the Iyo Nada in May 1946.
- I-55/I-155 - sank the SS Van Cloon off Java February 7, 1942 and the RMS Derrymore in the Java Sea February 13, 1942. The I-155 herself surrendered in September 1945 and was scuttled in the Iyo Nada in May 1946.
- I-58/I-158 - sank the SS Langkoeas in the Java Sea January 3, 1942, the SS Camphuys off Bawean Island January 9, 1942, and the SS Boeroe S of the Sunda Strait February 25, 1942. The I-158 herself surrendered September 2, 1945 and was scuttled off the Goto Islands on April 1, 1946.
Type KD3b (3 units)
The Type KD3b (I-159 class) submarines (I-59/I-159, I-60, I-63) were similar to the Type KD3a but were 16 inches longer and had a different bow shape.
- I-59/I-159 - sank the SS Rooseboom off Sumatra March 1, 1942. The I-159 surrendered September 2, 1945 and was scuttled off the Goto Islands on April 1, 1946.
- I-60 - rammed I-63 off Mizunoko Light February 2, 1939. The I-60 herself was sunk off Kratakoa Island January 17, 1942 by HMS Jupiter.
- I-63 - rammed by I-60 off Mizunoko Light February 2, 1939.
Type KD4 (3 units)
The Type KD4 (I-61/I-162 class) submarines (I-61, I-62/I-162, I-64/I-164) were slightly smaller and had four torpedo tubes, but were otherwise similar to the Type KD3.
- I-64/I-164 - sunk off Cape Ashizuri May 17, 1942 by USS Triton.
Type KD5 (3 units)
The Type KD5 (I-165 class) submarines (I-65/I-165, I-66/I-166, I-67) were similar to the Type KD4 but had an improved operating depth.
- I-65/I-165 - depth-charged off Saipan June 27, 1945 by aircraft from VPB-142.
- I-66/I-166 - sunk off One Fathom Bank July 17, 1944 by HMS Telemachus.
Type KD6a (6 units)
Type KD6b (2 units)
The Type KD6b (I-174 class) submarines (I-74/I-174, (I-75/I-175) were similar to the KD6a but were one foot longer and 25 tons heavier.
- I-174 - sunk off Truk April 12, 1944 by aircraft from VB-108.
- I-175 - sunk off Wotje Atoll February 17, 1944 by USS Nicholas.
Type KD7 (10 units)
The Type KD7 (I-176 class) submarines (I-176, I-177, I-178, I-179, I-180, I-181, I-182, I-183, I-184, I-185) were similar to the KD6 but with the torpedo tubes moved forward and a slightly improved operating depth.
- I-176 sank USS Corvina (SS-226) patrolling off Truk on November 16, 1943, the only known Japanese submarine success against a US submarine - Snook was a probable second victim by Japanese submarines. I-176 was lost a year later.
Type J1 (I-1, I-2, I-3, I-4)
- I-1 - sank the SS Siantar off Western Australia March 3, 1942. The I-1 herself is attacked by HMNZS Kiwi and runs aground on Fish Reef January 29, 1943; valuable codes and code books from the wreck are salvaged by Allied forces.
- I-2 - sank the HMS Nam Yong off Christmas Island February 28, 1942 and the SS Parigi in the Indian Ocean March 1, 1942. The I-2 herself is sunk in the Bismarck Sea April 7, 1944 by USS Saufley.
- I-3 - sunk December 9, 1942 near Kamimbo Bay by PT-59.
- I-4 - sank the Høegh Merchant off Oahu December 14, 1941, the Ban Ho Guan off Bali February 28, 1942 and the USS Washingtonian off Eight Degree Channel April 6, 1942. The I-4 herself is sunk in St. George's Channel December 21, 1942 by USS Seadragon.
Type J1 Mod. (I-5)
The Type J1 Modified (I-5 class) submarine was similar to the Type J1, but with facilities for one aircraft.
- I-5 - possibly sunk July 19, 1944 off Guam by USS Wyman.
Type J2 (I-6)
The Type J2 (I-6 class) submarine was similar to the I-5 class, but with a catapult for aircraft.
- I-6 - sank the Clan Ross in the Arabian Sea April 2, 1942 and the Bahadur in the Arabian Sea April 7, 1942. The I-6 herself is rammed and sunk June 16, 1944 off Hachijo-shima by the Toyokawa Maru.
Type J3 (I-7, I-8)
The Type J3 (I-7 class) submarines (I-7, I-8) combined the benefits of the Type J2 and the Kaidai V (KD5). This type later led to the Type A, Type B, and Type C submarines.
- I-7 - sank the Merkus off Cocos Island March 4, 1942, the Glenshiel in the Indian Ocean April 3, 1942 and the USS Arcata off Unalaska July 14, 1942. The I-7 herself was damaged by gunfire off Kiska June 22, 1943 from USS Monaghan and ran aground on the Twin Rocks. The I-7 was scuttled June 23.
- I-8 - sunk off Okinawa March 31, 1945 by USS Morrison.
Type A1 (I-9, I-10, I-11)
The Type A1 (I-9 class) submarines were large seaplane-carrying submarines, with communication facilities to allow them to operate as command ships for groups of submarines. The type was also equipped with a hangar for one aircraft.
- I-9 - sunk June 14, 1943 off Kiska by USS Frazier.
- I-10 - sunk July 4, 1944 off Saipan by USS David W. Taylor and USS Riddle.
- I-11 - disappeared January 11, 1944 off Ellice Island. The I-11 possibly struck a mine.
Type A2 (I-12)
The Type A2 (I-12 class) submarine was similar to the Type A1, but with less powerful engines, giving the type slower surface speed but a longer range.
Type AM (I-13, I-14)
The Type AM (A Modified) (I-13 class) submarines was a large seaplane-carrying submarine, with hangar space for two aircraft. These giant submarines were originally of the A2 type, but their design was revised after construction started to carry a second aircraft. The seaplanes were to be the Aichi M6A1 bomber carrying 800 kg bombs.
The range and speed of these submarines was remarkable (21,000 nmi at 16 knots), but their underwater performance was compromised, making them easy targets. I-13 was sunk on 16 July 1945 by the destroyer escort USS Lawrence C. Taylor and aircraft action from escort carrier USS Anzio about 550 miles (890 km) east of Yokosuka. I-14 surrendered at sea at the end of the war, and was later scrapped off the coast of Oahu at a depth of 2600 feet perhaps to prevent Russia from obtaining the technology. The wreck was recently found.
Type B1 (20 units)
Type B1 (I-15 class) submarines (I-15, I-17, I-19, I-21, I-23, I-25, I-26, I-27, I-28, I-29, I-30, I-31, I-32, I-33, I-34, I-35, I-36, I-37, I-38, I-39) were the most numerous type of submarines of the Imperial Japanese Navy during World War II. In total 20 were made, starting with I-15, the class ship. These were fast, very long ranged, and carried a single Yokosuka E14Y seaplane, located in a hangar in front of the conning tower, launched by a catapult.
The series was rather successful, especially at the beginning of the war. I-26, in 1942, crippled the aircraft carrier USS Saratoga. I-19, on 15 September 1942, fired six torpedoes at aircraft carrier USS Wasp, two of which hit the carrier and sank her, the remainder damaging the battleship USS North Carolina and the destroyer USS O'Brien (which sank later); I-25 conducted the only aerial bombing to occur on the continental United States during World War II. On 9 September 1942, I-25 launched its reconnaissance plane, a Yokosuka E14Y code named Glen which proceeded to drop four (4) 168 pound bombs in a forest near present day Brookings, Oregon. Several of these ships also undertook "Yanagi" missions to Europe (I-30, I-34, I-29).
Type B2 (6 units)
The Type B2 (I-40 class) submarines (I-40, I-41, I-42, I-43, I-44, I-45) were externally similar to the Type B1, but with a high-tensile strength steel hull and diesel engines of a simpler design.
Type B3 (I-54, I-56, I-58)
Eighteen of the twenty-one Type B3 (I-54 class) submarines were cancelled in 1943 in favor of the Type E submarine, leaving the I-54, I-56, and I-58.
- I-54 - sank October 28, 1944 by USS Richard M. Rowell.
- I-56 - possibly sank the USS Snook  sometime after April 8, 1945. The I-56 was herself later sunk April 18, 1945 by USS Collett.
- I-58 - sank the USS Indianapolis July 30, 1945. The I-58 surrendered on September 2, 1945, and scuttled off the Goto Islands on April 1, 1946.
Type C1 (5 units)
The Type C1 (I-16 class) submarine (I-16, I-18, I-20, I-22, I-24) was based on the Junsen type submarine and developed from the Type KD6. This type, like the other Type C submarines, was utilized as mother ships for the Kō-hyōteki midget submarines and the Kaiten suicide torpedoes.
Type C2 (I-46, I-47, I-48)
The Type C2 (I-46 class) submarines were nearly identical to the Type C1 with the exception that the Type C2 lacked the capability to carry the midget submarines.
- I-46 - possibly sank by the USS Gridley and USS Helm on October 28, 1944.
- I-47 - sank the USS Mississinewa on November 20, 1944. The I-47 surrendered on September 2, 1945 and was scuttled off the Goto Islands on April 1, 1946.
- I-48 - sunk January 23, 1945 by USS Conklin.
Type C3 (I-52, I-53, I-55)
The Type C3 submarines (I-52 class) were submarines of the Imperial Japanese Navy, designed and built by Mitsubishi Corporation, between 1943 and 1944, as cargo carriers. They were quite long and carried a crew of up to 94 officers and men. They also had a long cruising range at a speed of 12 knots (22 km/h).
The Japanese constructed only three of these during World War II (I-52, I-53 and I-55), although twenty were planned. They were among the largest submarines ever built to date, and were known as the most advanced submarines of the period. One of them, I-52, was selected for a Yanagi (exchange) mission to Germany. She was sunk on 24 June 1944 by aircraft from USS Bogue 800 miles (1,300 km) southwest of the Azores. Her cargo consisted of rubber, gold, quinine, and Japanese engineers to Germany.
Type D1 (11 units)
The Type D1 (I-361 class, I-372 class) submarines (I-361, I-362, I-363, I-364, I-365, I-366, I-367, I-368, I-369, I-370, I-371, I-372) were based on the U-155. This type was designed as transport submarines with torpedoes for self-defense.
Type D2 (I-373)
The Type D2 (I-373 class) submarine was designed as a tanker submarine based on the Type D1 but with no torpedoes.
- I-373 - sunk August 14, 1945 by USS Spikefish. The I-373 was the last Japanese submarine sunk in World War II.
Kiraisen Type (I-121, I-122, I-123, I-124)
- I-121 - surrendered in September 1945 and scuttled in Wakasa Bay on April 30, 1946.
- I-122 - sunk in the Sea of Japan June 10, 1945 by USS Skate.
- I-123 - sunk off Savo Island August 29, 1942 by USS Gamble.
- I-124 - sunk off Darwin January 20, 1942 by HMAS Deloraine. The I-124 is the first IJN warship sunk by the Royal Australian Navy.
Sen-Ho Type (I-351)
The Sen-Ho (潜補, Submarine-Tanker) I-351 class was a tanker/transport submarine.
- I-351 - sunk July 14, 1945 in the South China Sea by USS Bluefish.
Sentoku Type (I-400, I-401, I-402)
The Sentoku (伊四〇〇型潜水艦) I-400 class displaced 5223 tons surfaced and measured 400 ft 3 in (122m) overall. They had a figure-eight hull shape for additional strength to handle the on-deck hangar for housing the three Seiran aircraft. In addition, they had four anti-aircraft guns and a large deck cannon as well as eight torpedo tubes from which they could fire the 21 inch (53 cm) Type 95 torpedo. Three of the Sen Toku were built (I-400, I-401, and I-402). Each had four 1825 horsepower (1360 kW) engines and range 37,500 nm at 14 knots (26 km/h).
The submarines were also able to carry three Aichi M6A Sei ran aircraft, each carrying an 800 kilogram (1764 lb) bomb 550 nautical miles (1,020 km) at 360 miles per hour (580 km/h). To fit the aircraft in the hangar the wings of the aircraft were folded back, the horizontal stabilizers folded down, and the top of the vertical stabilizer folded over so the overall profile of the aircraft was within the diameter of its propeller. A crew of four could prepare and get all three airborne in 45 minutes launching them with a 120 foot (37 m) catapult on the fore deck of the giant submarine.
- I-400 - sunk as a target off Pearl Harbor June 4, 1946 by USS Trumpetfish.
- I-401 - sunk as a target off Pearl Harbor May 31, 1946 by USS Cabezon. The wreck of the I-401 was found in 2005.
- I-402 - scuttled with Ha-201 16 nmi off Kinai Island April 1, 1946.
Sentaka Type (3 units)
The Sentaka Type (潜高, High-speed submarine) I-201 class submarines were modern design, and known as Sentaka (From Sen, abbreviation of Sensuikan, "Submarine", and Taka, abbreviation of Kōsoku, "High speed"). Three were built, I-201, I-202, and I-203 (I-204 to I-208 were not completed).
They displaced 1070 tonnes, had a test depth of 360 feet (110 m), and were armed with four torpedo tubes and 25 mm guns in retractable mounts to maintain streamlining. These submarines were designed for mass production. They were high-performance boats, with streamlined all-welded hulls and a high battery capacity supplying two 2,500 hp (1,900 kW) motors, which had nearly double the horsepower of the German-designed MAN diesels. The submerged speed was 19 knots (35 km/h), more than double that achieved by contemporary American designs. They were equipped with a snorkel, allowing for underwater diesel operation while recharging batteries.
- I-201 - sunk as a target off Pearl Harbor May 23, 1946 by USS Queenfish. The wreck of the I-201 was found in 2009 along with the I-14.
- I-202 - scuttled 13 nmi off Kongo Point April 5, 1946.
- I-203 - sunk as a target off Hawaii May 21, 1946 by USS Caiman.
2nd Class submarines
These submarines included medium-sized medium-ranged units of the Imperial Japanese Navy.
Kaichu Type (20 units)
The Kaichus were double-hulled medium sized submarines. They were derived from the Kaitokuchu type submarine (KT). Several variants existed. From 1934 to 1944, the K6 type (Ro-33 Class) and the K7 type (Ro-35 Class) were built. The K6 type was equipped with a 3.25 inch (80 mm) gun and Type 95 Long Lance torpedoes. Twenty units were built: Ro-33, Ro-34, Ro-35, Ro-36, Ro-37, Ro-38, Ro-39, Ro-40, Ro-41, Ro-42, Ro-43, Ro-44, Ro-45, Ro-46, Ro-47, Ro-48, Ro-49, Ro-50, Ro-55, Ro-56.
Type L1 (Ro-51, Ro-52)
The Type L1 (Ro-51 class) submarines were British L class submarines built under license by Mitsubishi.
Type L2 (Ro-53, Ro-54, Ro-55, Ro-56)
The Type L2 (Ro-53 class) submarines were similar to the Type L1 but with no torpedo tubes and a change in the battery groups.
Type L3 (Ro-57, Ro-58, Ro-59)
The Type L3 (Ro-57 class) submarines were copies of the British submarine L9.
Type L4 (9 units)
The Type L4 (Ro-60 class) submarines (Ro-60, Ro-61, Ro-62, Ro-63, Ro-64, Ro-65, Ro-66, Ro-67, Ro-68) were copies of the British submarine L52.
Ko Type (18 units)
The Ko Type (Small Type or Submarine-Small Type) were medium sized submarines for use as point-defense submarines. Eighteen units were built: Ro-100, Ro-101, Ro-102, Ro-103, Ro-104, Ro-105, Ro-106, Ro-107, Ro-108, Ro-109, Ro-110, Ro-111, Ro-112, Ro-113, Ro-114, Ro-115, Ro-116, Ro-117.
Sen'yu-Ko Type (12 units)
The Sen'yu-Ko Type (Submarine Transport-Small Type) were transport submarines. Several of this type were converted to tankers or to mother ships for the midget submarines.
Sentaka-Ko Type (10 units)
The Sentaka-Ko Type (Submarine High speed-Small type) were small-size speed submarines commissioned for Operation Downfall. Of the 79 boats planned, only 9 were completed.
3rd Class submarines
This class includes the smallest of the Japanese submarines, from midget submarines to manned torpedoes often used for suicide attacks.
Ko-Hyoteki Type (50 units)
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 numbers beginning with the character "HA", which can only be seen on a builder's plate inside the hull. Fifty Ko-hyoteki 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.
Kairyu Type (250 units)
The Kairyu (海龍, Sea Dragon) was a class of midget submarines designed in 1943-1944, and produced from the beginning of 1945. These submarines were meant to meet the invading American Naval forces upon their anticipated approach of Tokyo.
Over 760 of these submarines were planned, and by August 1945, 250 had been manufactured, most of them at the Yokosuka shipyard.
These submarines had a two-man crew and were fitted with an internal warhead for suicide missions.
Kaiten Type (400 units)
The Kaiten (Japanese:回天) was a torpedo modified as a suicide weapon, and used by the Imperial Japanese Navy in the final stages of the Second World War. Kaiten means "return to the sky"; however, it is commonly translated as "turn toward heaven". 
Early designs allowed for the pilot to escape after the final acceleration towards the target, although whether this could have been done successfully is doubtful. There is no record of any pilot attempting to escape or intending to do so, and this provision was dropped from later production kaitens.  
Six models were designed, the types 1, 2, 4, 5 and 6 were based on the Long Lance type 93 torpedo (24 inch oxygen/kerosene), and the Type 10, based on the Type 92 torpedo (21 inch electric). Types 2, 4, 5, 6 and 10 were only manufactured as prototypes and never used in combat. 
- Jentschura p. 160
- Baldwin pp.92-110
- Boyne p. 127, 254
- Blair, Silent Victory, p.878
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