The submarines were double-hulled medium-sized submarines of the Imperial Japanese Navy during World War II. The name was derived from the .

Several variants existed. From 1934 to 1944, the K6 type (Ro-33 Class) and the K7 type (Senchū, Ro-35 Class) were built. They were equipped with a L/40 gun and four 53 cm torpedo tubes for ten type 95 Long Lance torpedoes.

Most of these submarines were destroyed in combat, suffering from Allied anti-submarine warfare measures, and only survived the war.

Class variants The Kaichū type submarines were divided into seven classes:

Kaichū I (Ro-11 class)

Project number S7. In 1910s, the Imperial Japanese Navy (IJN) bought a license of Schneider-Laubeuf design submarine. The IJN used the design as model and built the S Type (Schneider Type) submarine, the and . The Kaichū I is the submarine which jumboized the S Type submarines. Boats in class Boat Builder Laid down Launched Completed Fate Submarine No. 19. The same shall apply hereinafter. Kure Naval Arsenal 25-04-1917 25-10-1917 31-07-1919 as Submarine No. 19 Renamed Ro-11 01-11-1924. Decommissioned 01-04-1932. Submarine No. 20 Kure Naval Arsenal 25-04-1917 01-12-1917 18-09-1919 as Submarine No. 20 Renamed Ro-12 01-11-1924. Decommissioned 01-04-1932.

Kaichū II (Ro-13 class)

Project number S18. The Kaichū II had an increased range compared with the Kaichū I, and the turning torpedo tubes were removed. Boats in class Boat Builder Laid down Launched Completed Fate Submarine No. 23 Kure Naval Arsenal 14-09-1918 26-08-1919 30-09-1920 as Submarine No. 23 Renamed Ro-13 01-11-1924. Decommissioned 01-04-1932. Submarine No. 22 Kure Naval Arsenal 14-09-1918 31-03-1919 17-02-1921 as Submarine No. 22 Renamed Ro-14 01-11-1924. Decommissioned 01-09-1933. Submarine No. 24 Kure Naval Arsenal 12-06-1920 14-10-1920 30-06-1921 as Submarine No. 24 Renamed Ro-15 01-11-1924. Decommissioned 01-09-1933.

Kaichū III (Ro-16 class)

Project number S18. Their project number was the same as in the Kaichū II type submarine, however their performance was improved.

Boats in class Boat Builder Laid down Launched Completed Fate Submarine No. 37 Kure Naval Arsenal 18-11-1920 22-04-1921 29-04-1922 as Submarine No. 37 Renamed Ro-16 01-11-1924. Decommissioned 01-09-1933. Submarine No. 34 Kure Naval Arsenal 24-09-1920 24-02-1921 20-10-1921 as Submarine No. 34 Renamed Ro-17 01-11-1924. Decommissioned 01-04-1936. Submarine No. 35 Kure Naval Arsenal 20-10-1920 25-03-1921 15-12-1921 as Submarine No. 35 Renamed Ro-18 01-11-1924. Decommissioned 01-04-1936. Submarine No. 36 Kure Naval Arsenal 09-09-1920 28-12-1920 15-03-1922 as Submarine No. 36 Renamed Ro-19 01-11-1924. Decommissioned 01-04-1936. Submarine No. 38 Yokosuka Naval Arsenal 28-07-1919 26-10-1920 01-02-1922 as Submarine No. 38 Renamed Ro-20 01-11-1924. Decommissioned 01-04-1934. Submarine No. 39 Yokosuka Naval Arsenal 28-07-1919 26-10-1920 01-02-1922 as Submarine No. 39 Renamed Ro-21 01-11-1924. Decommissioned 01-04-1934. Submarine No. 40 Yokosuka Naval Arsenal 22-01-1921 15-10-1921 10-10-1922 as Submarine No. 40 Renamed Ro-22 01-11-1924. Decommissioned 01-04-1934. Submarine No. 41 Yokosuka Naval Arsenal 22-01-1921 15-10-1921 28-04-1923 as Submarine No. 41 Renamed Ro-23 01-11-1924. Decommissioned 01-04-1935. Submarine No. 42 Sasebo Naval Arsenal 21-04-1919 08-12-1919 30-11-1920 as Submarine No. 42 Renamed Ro-24 01-11-1924. Decommissioned 01-04-1935. Submarine No. 43 Sasebo Naval Arsenal 19-02-1920 17-07-1920 25-10-1921 as Submarine No. 43 Renamed Ro-25 01-11-1924. Decommissioned 01-04-1936.

Kaichū IV (Ro-26 class)

Project number S18A. Improved model from the Kaichū III type. Boats in class Boat Builder Laid down Launched Completed Fate Submarine No. 45 Sasebo Naval Arsenal 10-03-1921 18-10-1921 25-01-1923 as Submarine No. 45 Renamed Ro-26 01-11-1924. Decommissioned 01-04-1940. Scrapped 1948. Her anchor is displaying at Yokosuka Naval Base. Submarine No. 58 Yokosuka Naval Arsenal 16-07-1921 22-07-1922 31-07-1924 as Submarine No. 58 Renamed Ro-27 01-11-1924. Decommissioned 01-04-1936. Scrapped 1948. Submarine No. 62 Sasebo Naval Arsenal 10-11-1921 13-04-1922 30-11-1923 as Submarine No. 62 Renamed Ro-28 01-11-1924. Decommissioned 01-04-1940. Scrapped 1948.

Kaichū V (Toku-Chū, Ro-29 class)

Project number S18B. They were built for the commerce raiding role. The IJN official designation of these boats was . Boats in class Boat Builder Laid down Launched Completed Fate Submarine No. 68 Kawasaki-Kōbe Shipyard 02-06-1921 05-12-1922 15-09-1923 as Submarine No. 68 Renamed Ro-29 01-11-1924. Decommissioned 01-04-1936. Submarine No. 69 Kawasaki-Kōbe Shipyard 27-06-1921 18-01-1923 29-04-1924 as Submarine No. 69 Renamed Ro-30 01-11-1924. Decommissioned 01-04-1942. Scrapped 1945. Submarine No. 70 Kawasaki-Kōbe Shipyard 25-09-1921 15-02-1923 as Submarine No. 70 Lost in an accident off Awaji Island 21-08-1923. Salvaged and scrapped October 1923. Her materials were used for Ro-31. Kawasaki-Kōbe Shipyard 20-12-1924 25-09-1926 10-05-1927 Decommissioned 25-05-1945. Scuttled off Sasebo 05-04-1946. Submarine No. 71 Kawasaki-Kōbe Shipyard 24-10-1921 19-03-1923 21-05-1924 as Submarine No. 71 Renamed Ro-32 01-11-1924. Decommissioned 01-04-1942. Scrapped 1945.

Kaichū VI (Ro-33 class)

Project number S30. They were planned as a prototype for a mass production submarines in the wartime under the Maru 1 Programme. Boats in class Boat Builder Laid down Launched Completed Results Fate Kure Naval Arsenal 08-08-1933 10-10-1934 07-10-1935 Sank Australian merchant ship 07-08-1942 Sunk by and RAF patrol aircraft off Port Moresby 29-08-1942. Mitsubishi-Kōbe Shipyard 25-04-1934 12-12-1935 31-05-1937 Sunk by and near Russell Islands 06-04-1943.

Kaichū VII (Sen-Chū, Ro-35 class)

Project number S44. The final design in the Kaichū series. They were equipped with a Freon air-conditioner, because the IJN took into consideration that they were to be active on the equator area too. The official IJN designation of these boats was , also called for short, or . The IJN planned to build these boats under the following Naval Armaments Supplement Programmes: 9 boats in the Maru Rin Programme (Boat # 201 - 209) 12 boats in the Maru Kyū Programme (Boat # 385 - 396) 15 boats in the Maru Tui Programme (Boat # 640 - 654) 43 boats in the Kai-Maru 5 Programme (Boat # 5181 - 5223)

However some of the boats were cancelled and their naval budgets, materials and staffs were transferred to the I-201 class submarines.

Boats in class Boat No. Boat Builder Laid down Launched Completed Results Fate 201 Mitsubishi-Kōbe Shipyard 09-10-1941 09-06-1942 25-03-1943 Sunk by near of Espiritu Santo 25-08-1943. 202 Mitsubishi-Kōbe Shipyard 07-03-1942 14-10-1942 27-05-1943 Sunk by north of Saipan 13-06-1944. 203 Sasebo Naval Arsenal 09-10-1941 30-06-1942 30-06-1943 Damaged 22-01-1944 Sunk by near of Santa Cruz Islands 22-01-1944. 204 Mitsubishi-Kōbe Shipyard 20-06-1942 24-12-1942 24-07-1943 Disappeared after 19-11-1943. 205 Mitsubishi-Kōbe Shipyard 08-08-1942 06-03-1943 12-09-1943 Sunk by and east of Wotje Atoll 03-02-1944. 206 Mitsubishi-Kōbe Shipyard 08-08-1942 06-03-1943 28-09-1943 Sunk by north of Marshall Islands 16-02-1944. 207 Mitsubishi-Kōbe Shipyard 06-10-1942 05-05-1943 26-11-1943 Sank 03-10-1944 Sunk by south of Okinawa 23-03-1945. 208 Sasebo Naval Arsenal 27-04-1942 25-10-1942 31-08-1943 Sunk by northeast of Marshall Islands 10-06-1945. 209 Mitsubishi-Kōbe Shipyard 06-10-1942 05-06-1943 16-12-1943 Damaged 21-02-1945 Sunk by aircraft from off Iwo Jima 27-02-1945. 385 Mitsui-Tamano Shipyard 14-02-1942 11-11-1942 13-09-1943 Sunk by near of Enewetak 16-06-1944. 386 Mitsubishi-Kōbe Shipyard 20-10-1942 21-07-1943 11-01-1944 Sunk by , and aircraft from south of Truk 01-05-1944. 387 Mitsui-Tamano Shipyard 13-06-1942 23-04-1943 19-02-1944 Damaged 29-01-1945 Sunk by aircraft from near of Oki Daitō 29-04-1945. 388 Mitsubishi-Kōbe Shipyard 28-12-1942 20-08-1943 31-01-1944 Sunk by northeast of Palau 26-09-1944. 389 Mitsubishi-Kōbe Shipyard 17-03-1943 15-10-1943 31-03-1944 (1) Sunk by northwest of Saipan 14-07-1944.(2) Sunk by northwest of Saipan 18-07-1944. 390 Mitsui-Tamano Shipyard 16-11-1942 03-08-1943 19-05-1944 Sunk by . 391 Mitsui-Tamano Shipyard 18-02-1943 27-11-1943 31-07-1944 Sank USS LST-577 11-02-1945 Decommissioned 30-11-1945. Sunk as target off Gotō Islands 01-04-1946. 392 - 395 Cancelled in 1943. 396 Mitsui-Tamano Shipyard 05-08-1943 23-04-1944 30-09-1944 Sunk by west of Luzon 07-02-1945. 640 - 644 Cancelled in 1943. 645 (ex-Ro-75) Mitsui-Tamano Shipyard 02-12-1943 as Ro-75 05-07-1944 15-11-1944 Sunk by and near of Oki Daitō 09-04-1945. 646 - 654 Cancelled in 1943. 5181 - 5223 Cancelled in 1943.

Characteristics Type Kaichū I (Ro-11) Kaichū II (Ro-13) Kaichū III (Ro-16) Kaichū IV (Ro-26)Displacement Surfaced Submerged Length (overall) Beam Draft Power plant and shaft 2 × Sulzer Mk.2 diesels2 shafts 2 × Sulzer Mk.2 diesels2 shafts 2 × Sulzer Mk.2 diesels2 shafts 2 × Sulzer Mk.2 diesels2 shaftsPower Surfaced 2,900 bhp 2,900 bhp 2,900 bhp 2,900 bhp Submerged 1,200 shp 1,200 shp 1,200 shp 1,200 shpSpeed Surfaced Submerged Range Surfaced at at at at Submerged at at at at Test depth Fuel 58.4 tons 75 tons 75 tons 75 tons Complement 43 43 46 46 Armament (initial) • 6 × TTs(4 × front, 2 × turret)• 10 × Type 44 torpedoes• 1 × L/23.5 5th Year Type AA gun • 6 × TTs(4 × front, 2 × upper deck)• 10 × Type 44 torpedoes• 1 × L/23.5 5th Year Type AA gun • 6 × TTs(4 × front, 2 × upper deck)• 10 × Type 44 torpedoes• 1 × L/23.5 5th Year Type AA gun • 4 × TTs(4 × front)• 8 × 6th Year Type torpedoes• 1 × L/23.5 5th Year Type AA gun

Type Kaichū V (Ro-29) Kaichū VI (Ro-33) Kaichū VII (Ro-35)Displacement Surfaced Submerged Length (overall) Beam Draft Power plant and shaft 2 × Sulzer Mk.1 diesels2 shafts 2 × Kampon Mk.21 Model 8 diesels2 shafts 2 × Kampon Mk.22 Model 10 diesels2 shaftsPower Surfaced 1,200 bhp 3,000 bhp 4,200 bhp Submerged 1,200 shp 1,200 shp 1,200 shpSpeed Surfaced Submerged Range Surfaced at at at Submerged at at at Test depth Fuel 143 tons 109 tons 218 tons Complement 44 61 61 Armament (initial) • 4 × TTs(4 × front)• 8 × 6th Year Type torpedoes• 1 × L/45 11th Year Type Naval gun• 1 × 6.5 mm machine gun • 4 × TTs(4 × front)• 8 × Type 89 torpedoes• 1 × L/40 11th Year Type AA gun• 1 × Type 93 AA gun • 4 × TTs(4 × front)• 10 × Type 95 torpedoes• 1 × L/40 11th Year Type AA gun• 2 × Type 96 25mm AA guns

Bibliography , History of Pacific War Vol.17 I-Gō Submarines, Gakken (Japan), January 1998, Rekishi Gunzō, History of Pacific War Extra, "Perfect guide, The submarines of the Imperial Japanese Forces", Gakken (Japan), March 2005, The Maru Special, Japanese Naval Vessels No.43 Japanese Submarines III, Ushio Shobō (Japan), September 1980, Book code 68343-44 The Maru Special, Japanese Naval Vessels No.132 Japanese Submarines I "Revised edition", Ushio Shobō (Japan), February 1988, Book code 68344-36 The Maru Special, Japanese Naval Vessels No.133 Japanese Submarines II "Revised edition", Ushio Shobō (Japan), March 1988, Book code 68344-37 The Maru Special, Japanese Naval Vessels No.135 Japanese Submarines IV, Ushio Shobō (Japan), May 1988, Book code 68344-39

References