# Kaichū type submarine

The Kaichū type submarine (海中型潜水艦, Kaichū-gata sensuikan) submarines were double-hulled medium-sized submarines of the Imperial Japanese Navy during World War II. The name was derived from the Kaigun-shiki Chū-gata Sensuikan (海軍式中型潜水艦, Navy Medium Type submarine).
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 76.2 mm (3.00 in) 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 Ro-50 survived the war.

## Class variants

The Kaichū type submarines were divided into seven classes:
* Kaichū I (海中1型（呂一一型）, Kaichū-ichi-gata, Ro-11-class)
* Kaichū II (海中2型（呂一三型）, Kaichū-ni-gata, Ro-13-class)
* Kaichū III (海中3型（呂一六型）, Kaichū-san-gata, Ro-16-class)
* Kaichū IV (海中4型（呂二六型）, Kaichū-yon-gata, Ro-26-class)
* Toku-Chū/Kaichū V (特中型/海中5型（呂二九型）, Toku-Chū-gata/Kaichū-go-gata, Ro-29-class)
* Kaichū VI (海中6型（呂三三型）, Kaichū-roku-gata, Ro-33-class)
* Sen-Chū/Kaichū VII (潜中型/海中7型（呂三五型）, Sen-Chū-gata/Kaichū-nana-gata, Ro-35-class)

### 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 Ha-9 and Ha-10. 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 Ro-11 | 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 Ro-12 | 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 Ro-13 | 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 Ro-14 | 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 Ro-15 | 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 Ro-16 | 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 Ro-17 | 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 Ro-18 | 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 Ro-19 | 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 Ro-20 | 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 Ro-21 | 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 Ro-22 | 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 Ro-23 | 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 Ro-24 | 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 Ro-25 | 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 Ro-26 | 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 Ro-27 | 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 Ro-28 | 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 Special Purpose-Medium Type submarine (特中型潜水艦, Toku-Chū-gata sensuikan).
* Boats in class
| Boat | Builder | Laid down | Launched | Completed | Fate |
| Submarine No. 68 Ro-29 | 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 Ro-30 | 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. |
| 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 Ro-32 | 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 |
| Ro-33 | Kure Naval Arsenal | 08-08-1933 | 10-10-1934 | 07-10-1935 | Sank Australian merchant ship MV Mamutu 07-08-1942 | Sunk by HMAS Arunta and RAF patrol aircraft off Port Moresby 29-08-1942. |
| Ro-34 | Mitsubishi-Kōbe Shipyard | 25-04-1934 | 12-12-1935 | 31-05-1937 | | Sunk by USS O'Bannon and USS Strong 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 Medium Type submarine (中型潜水艦, Chū-gata sensuikan), also called for short, Medium Type (中型, Chū-gata) or Submarine-Medium Type (潜中型, Sen-Chū-gata). 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 | Ro-35 | Mitsubishi-Kōbe Shipyard | 09-10-1941 | 09-06-1942 | 25-03-1943 | | Sunk by USS Patterson near of Espiritu Santo 25-08-1943. |
| 202 | Ro-36 | Mitsubishi-Kōbe Shipyard | 07-03-1942 | 14-10-1942 | 27-05-1943 | | Sunk by USS Melvin north of Saipan 13-06-1944. |
| 203 | Ro-37 | Sasebo Naval Arsenal | 09-10-1941 | 30-06-1942 | 30-06-1943 | Damaged USS Cache 22-01-1944 | Sunk by USS Buchanan near of Santa Cruz Islands 11°47′S 164°17′E﻿ / ﻿11.783°S 164.283°E﻿ / -11.783; 164.283 22-01-1944. |
| 204 | Ro-38 | Mitsubishi-Kōbe Shipyard | 20-06-1942 | 24-12-1942 | 24-07-1943 | | Disappeared after 19-11-1943. |
| 205 | Ro-39 | Mitsubishi-Kōbe Shipyard | 08-08-1942 | 06-03-1943 | 12-09-1943 | | Sunk by USS Charrette and USS Fair east of Wotje Atoll 06°48′N 168°08′E﻿ / ﻿6.800°N 168.133°E﻿ / 6.800; 168.133 03-02-1944. |
| 206 | Ro-40 | Mitsubishi-Kōbe Shipyard | 08-08-1942 | 06-03-1943 | 28-09-1943 | | Sunk by USS Phelps north of Marshall Islands 09°50′N 166°35′E﻿ / ﻿9.833°N 166.583°E﻿ / 9.833; 166.583 16-02-1944. |
| 207 | Ro-41 | Mitsubishi-Kōbe Shipyard | 06-10-1942 | 05-05-1943 | 26-11-1943 | Sank USS Shelton 03-10-1944 | Sunk by USS Haggard south of Okinawa 22°57′N 132°19′E﻿ / ﻿22.950°N 132.317°E﻿ / 22.950; 132.317 23-03-1945. |
| 208 | Ro-42 | Sasebo Naval Arsenal | 27-04-1942 | 25-10-1942 | 31-08-1943 | | Sunk by USS Bangust northeast of Marshall Islands 10°05′N 168°22′E﻿ / ﻿10.083°N 168.367°E﻿ / 10.083; 168.367 10-06-1945. |
| 209 | Ro-43 | Mitsubishi-Kōbe Shipyard | 06-10-1942 | 05-06-1943 | 16-12-1943 | Damaged USS Renshaw 21-02-1945 | Sunk by aircraft from USS Anzio off Iwo Jima 25°07′N 140°19′E﻿ / ﻿25.117°N 140.317°E﻿ / 25.117; 140.317 27-02-1945. |
| 385 | Ro-44 | Mitsui-Tamano Shipyard | 14-02-1942 | 11-11-1942 | 13-09-1943 | | Sunk by USS Burden R. Hastings near of Enewetak 11°13′N 164°15′E﻿ / ﻿11.217°N 164.250°E﻿ / 11.217; 164.250 16-06-1944. |
| 386 | Ro-45 | Mitsubishi-Kōbe Shipyard | 20-10-1942 | 21-07-1943 | 11-01-1944 | | Sunk by USS Macdonough, USS Stephen Potter and aircraft from USS Monterey south of Truk 06°13′N 151°19′E﻿ / ﻿6.217°N 151.317°E﻿ / 6.217; 151.317 01-05-1944. |
| 387 | Ro-46 | Mitsui-Tamano Shipyard | 13-06-1942 | 23-04-1943 | 19-02-1944 | Damaged USS Cavalier 29-01-1945 | Sunk by aircraft from USS Tulagi near of Oki Daitō 24°15′N 131°16′E﻿ / ﻿24.250°N 131.267°E﻿ / 24.250; 131.267 29-04-1945. |
| 388 | Ro-47 | Mitsubishi-Kōbe Shipyard | 28-12-1942 | 20-08-1943 | 31-01-1944 | | Sunk by USS McCoy Reynolds northeast of Palau 09°19′N 136°44′E﻿ / ﻿9.317°N 136.733°E﻿ / 9.317; 136.733 26-09-1944. |
| 389 | Ro-48 | Mitsubishi-Kōbe Shipyard | 17-03-1943 | 15-10-1943 | 31-03-1944 | | (1) Sunk by USS William C. Miller northwest of Saipan 14-07-1944. (2) Sunk by USS Wyman northwest of Saipan 18-07-1944. |
| 390 | Ro-49 | Mitsui-Tamano Shipyard | 16-11-1942 | 03-08-1943 | 19-05-1944 | | Sunk by USS Hudson. |
| 391 | Ro-50 | 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 | Ro-55 | Mitsui-Tamano Shipyard | 05-08-1943 | 23-04-1944 | 30-09-1944 | | Sunk by USS Thomason west of Luzon 15°27′N 119°25′E﻿ / ﻿15.450°N 119.417°E﻿ / 15.450; 119.417 07-02-1945. |
| 640 - 644 | | | | | | | Cancelled in 1943. |
| 645 | Ro-56 (ex-Ro-75) | Mitsui-Tamano Shipyard | 02-12-1943 as Ro-75 | 05-07-1944 | 15-11-1944 | | Sunk by USS Mertz and USS Monssen near of Oki Daitō 26°09′N 130°21′E﻿ / ﻿26.150°N 130.350°E﻿ / 26.150; 130.350 09-04-1945. |
| 646 - 654 | | | | | | | Cancelled in 1943. |
| 5181 - 5223 | | | | | | | Cancelled in 1943. |

## Characteristics

| Type | Type | Kaichū I (Ro-11) | Kaichū II (Ro-13) | Kaichū III (Ro-16) | Kaichū IV (Ro-26) |
| Displacement | Surfaced | 720 long tons (732 t) | 740 long tons (752 t) | 740 long tons (752 t) | 750 long tons (762 t) |
| Displacement | Submerged | 1,000 long tons (1,016 t) | 1,003.1 long tons (1,019 t) | 997 long tons (1,013 t) | 1,080 long tons (1,097 t) |
| Length (overall) | Length (overall) | 69.19 m (227 ft 0 in) | 70.10 m (230 ft 0 in) | 70.10 m (230 ft 0 in) | 74.22 m (243 ft 6 in) |
| Beam | Beam | 6.35 m (20 ft 10 in) | 6.10 m (20 ft 0 in) | 6.12 m (20 ft 1 in) | 6.12 m (20 ft 1 in) |
| Draft | Draft | 3.43 m (11 ft 3 in) | 3.68 m (12 ft 1 in) | 3.70 m (12 ft 2 in) | 3.73 m (12 ft 3 in) |
| Power plant and shaft | Power plant and shaft | 2 × Sulzer Mk.2 diesels 2 shafts | 2 × Sulzer Mk.2 diesels 2 shafts | 2 × Sulzer Mk.2 diesels 2 shafts | 2 × Sulzer Mk.2 diesels 2 shafts |
| Power | Surfaced | 2,900 bhp | 2,900 bhp | 2,900 bhp | 2,900 bhp |
| Power | Submerged | 1,200 shp | 1,200 shp | 1,200 shp | 1,200 shp |
| Speed | Surfaced | 18.2 knots (33.7 km/h) | 16.5 knots (30.6 km/h) | 16.5 knots (30.6 km/h) | 16.0 knots (29.6 km/h) |
| Speed | Submerged | 9.1 knots (16.9 km/h) | 8.5 knots (15.7 km/h) | 8.5 knots (15.7 km/h) | 8.5 knots (15.7 km/h) |
| Range | Surfaced | 4,000 nmi (7,400 km) at 10 knots (19 km/h) | 6,000 nmi (11,000 km) at 10 knots (19 km/h) | 6,000 nmi (11,000 km) at 10 knots (19 km/h) | 6,000 nmi (11,000 km) at 10 knots (19 km/h) |
| Range | Submerged | 85 nmi (157 km) at 4 knots (7.4 km/h) | 85 nmi (157 km) at 4 knots (7.4 km/h) | 85 nmi (157 km) at 4 knots (7.4 km/h) | 85 nmi (157 km) at 4 knots (7.4 km/h) |
| Test depth | Test depth | 30 m (98 ft) | 30 m (98 ft) | 45.7 m (150 ft) | 45.7 m (150 ft) |
| Fuel | Fuel | 58.4 tons | 75 tons | 75 tons | 75 tons |
| Complement | Complement | 43 | 43 | 46 | 46 |
| Armament (initial) | Armament (initial) | • 6 × 450 mm (18 in) TTs (4 × front, 2 × turret) • 10 × Type 44 torpedoes • 1 × 76.2 mm (3.00 in) L/23.5 5th Year Type AA gun | • 6 × 450 mm (18 in) TTs (4 × front, 2 × upper deck) • 10 × Type 44 torpedoes • 1 × 76.2 mm (3.00 in) L/23.5 5th Year Type AA gun | • 6 × 450 mm (18 in) TTs (4 × front, 2 × upper deck) • 10 × Type 44 torpedoes • 1 × 76.2 mm (3.00 in) L/23.5 5th Year Type AA gun | • 4 × 533 mm (21 in) TTs (4 × front) • 8 × 6th Year Type torpedoes • 1 × 76.2 mm (3.00 in) L/23.5 5th Year Type AA gun |
| Type | Type | Kaichū V (Ro-29) | Kaichū VI (Ro-33) | Kaichū VII (Ro-35) |
| Displacement | Surfaced | 852 long tons (866 t) | 700 long tons (711 t) | 960 long tons (975 t) |
| Displacement | Submerged | 1,030 long tons (1,047 t) | 1,200 long tons (1,219 t) | 1,447 long tons (1,470 t) |
| Length (overall) | Length (overall) | 74.22 m (243 ft 6 in) | 73.00 m (239 ft 6 in) | 80.50 m (264 ft 1 in) |
| Beam | Beam | 6.12 m (20 ft 1 in) | 6.70 m (22 ft 0 in) | 7.05 m (23 ft 2 in) |
| Draft | Draft | 3.73 m (12 ft 3 in) | 3.25 m (10 ft 8 in) | 4.07 m (13 ft 4 in) |
| Power plant and shaft | Power plant and shaft | 2 × Sulzer Mk.1 diesels 2 shafts | 2 × Kampon Mk.21 Model 8 diesels 2 shafts | 2 × Kampon Mk.22 Model 10 diesels 2 shafts |
| Power | Surfaced | 1,200 bhp | 3,000 bhp | 4,200 bhp |
| Power | Submerged | 1,200 shp | 1,200 shp | 1,200 shp |
| Speed | Surfaced | 13.0 knots (24.1 km/h) | 19.0 knots (35.2 km/h) | 19.8 knots (36.7 km/h) |
| Speed | Submerged | 8.5 knots (15.7 km/h) | 8.2 knots (15.2 km/h) | 8.0 knots (14.8 km/h) |
| Range | Surfaced | 6,000 nmi (11,000 km) at 10 knots (19 km/h) | 8,000 nmi (15,000 km) at 12 knots (22 km/h) | 5,000 nmi (9,300 km) at 16 knots (30 km/h) |
| Range | Submerged | 85 nmi (157 km) at 4 knots (7.4 km/h) | 90 nmi (170 km) at 3.5 knots (6.5 km/h) | 45 nmi (83 km) at 5 knots (9.3 km/h) |
| Test depth | Test depth | 45.7 m (150 ft) | 75 m (246 ft) | 80 m (260 ft) |
| Fuel | Fuel | 143 tons | 109 tons | 218 tons |
| Complement | Complement | 44 | 61 | 61 |
| Armament (initial) | Armament (initial) | • 4 × 533 mm (21 in) TTs (4 × front) • 8 × 6th Year Type torpedoes • 1 × 120 mm (4.7 in) L/45 11th Year Type Naval gun • 1 × 6.5 mm machine gun | • 4 × 533 mm (21 in) TTs (4 × front) • 8 × Type 89 torpedoes • 1 × 76.2 mm (3.00 in) L/40 11th Year Type AA gun • 1 × 13.2 mm (0.52 in) Type 93 AA gun | • 4 × 533 mm (21 in) TTs (4 × front) • 10 × Type 95 torpedoes • 1 × 76.2 mm (3.00 in) L/40 11th Year Type AA gun • 2 × Type 96 25mm AA guns |