# Ro-100-class submarine

The Ro-100-class submarine (呂百型潜水艦, Ro-hyaku-gata Sensuikan) was a group of medium-sized coastal submarines built for the Imperial Japanese Navy (IJN) during World War II. The IJN official designation for this class was Ko type submarine (小型潜水艦, Ko-gata Sensuikan, "Small type Submarine") or Senshō type submarine (潜小型潜水艦, Sen-Shō-gata sensuikan, "Submarine-Small Type"). They are also known as Type KS submarine. The type name was shortened to Sensuikan Ko-gata (潜水艦小型, Submarine-Small Type).

## Design and description

In 1940, the IJN designed a point-defence coastal submarine because they wanted to save their larger submarines for fleet battles. The Ro-100 class was derived from the preceding Kaichū type. They displaced 611 tonnes (601 long tons) surfaced and 795 tonnes (782 long tons) submerged. The submarines were 60.9 meters (199 ft 10 in) long, had a beam of 6 meters (19 ft 8 in) and a draft of 3.51 meters (11 ft 6 in). They had a double hull and a diving depth of 75 meters (246 ft).
For surface running, the boats were powered by two 500-brake-horsepower (373 kW) diesel engines, each driving one propeller shaft. When submerged each propeller was driven by a 380-horsepower (283 kW) electric motor. They could reach 14.2 knots (26.3 km/h; 16.3 mph) on the surface and 8 knots (15 km/h; 9.2 mph) underwater. On the surface, the Ro-100s had a range of 3,500 nautical miles (6,500 km; 4,000 mi) at 12 knots (22 km/h; 14 mph); submerged, they had a range of 60 nmi (110 km; 69 mi) at 3 knots (5.6 km/h; 3.5 mph).
The boats were armed with four internal bow 53.3 cm (21.0 in) torpedo tubes and carried a total of eight torpedoes. They were also armed with two single mounts for 25 mm (1 in) Type 96 anti-aircraft guns or a single 76.2 mm (3.00 in) L/40 AA gun.

## Boats

18 boats were built in 1941-1944 under the Maru Rin Programme (Boat #210-218) and the Maru Kyū Programme (Boat #400-408).
| Boat # | Name | Builder | Laid down | Launched | Completed | Results | Fate |
| 210 | Ro-100 | Kure Naval Arsenal | 1941-06-30 | 1941-06-12 | 1942-08-23 | | Sunk by naval mine off Bougainville Island 1943-11-25. |
| 211 | Ro-101 | Kawasaki-Kōbe Shipyard | 1941-09-30 | 1942-04-17 | 1942-10-31 | | Sunk by USS Saufley and PBY southeast of San Cristobal 1943-09-15. |
| 212 | Ro-102 | Kawasaki-Kōbe Shipyard | 1941-09-30 | 1942-04-17 | 1942-11-17 | | Disappeared south of Rabi after 1943-05-09. |
| 213 | Ro-103 | Kure Naval Arsenal | 1941-06-30 | 1941-12-06 | 1942-10-21 | Sank USS Aludra 1943-06-23 Sank USS Deimos 1943-06-23 | Disappeared in the Solomon Islands after 1943-07-28, possibly sunk by a mine or a PT boat. |
| 214 | Ro-104 | Kawasaki-Kōbe Shipyard | 1941-11-19 | 1942-07-11 | 1943-02-25 | | Sunk by USS England north of the Admiralty Islands 1944-05-23. |
| 215 | Ro-105 | Kawasaki-Kōbe Shipyard | 1941-11-19 | 1942-07-11 | 1943-03-05 | | Sunk by USS England north of the Admiralty Islands 1944-05-31. |
| 216 | Ro-106 | Kure Naval Arsenal | 1941-12-17 | 1942-05-30 | 1942-12-26 | Sank USS LST-342 1943-07-11 | Sunk by USS England north of the Admiralty Islands 1944-05-22. |
| 217 | Ro-107 | Kure Naval Arsenal | 1941-12-17 | 1942-05-30 | 1942-12-26 | | Sunk by USS Taylor east of Kolombangara 1943-07-21. |
| 218 | Ro-108 | Kawasaki-Kōbe Shipyard | 1942-04-20 | 1942-10-26 | 1943-04-20 | Sank USS Henley 1943-10-03 | Sunk by USS England north of the Admiralty Islands 1944-05-26. |
| 400 | Ro-109 | Kawasaki-Kōbe Shipyard | 1942-04-20 | 1942-10-26 | 1943-04-29 | | Sunk by USS Horace A. Bass south of Okinawa Island 1945-04-25. |
| 401 | Ro-110 | Kawasaki-Kōbe Shipyard | 1942-08-20 | 1943-01-26 | 1943-07-06 | | Sunk by HMAS Launceston, HMAS Ipswich and HMIS Jumna in the Bay of Bengal 1944-02-12. |
| 402 | Ro-111 | Kawasaki-Kōbe Shipyard | 1942-08-20 | 1943-01-26 | 1943-07-10 | Sank RMS Peshawar 1943-12-23 | Sunk by USS Taylor north of the Admiralty Islands 1944-06-10. |
| 403 | Ro-112 | Kawasaki-Senshū Shipyard | 1942-06-20 | 1943-03-25 | 1943-09-14 | | Sunk by USS Batfish in the Luzon Strait 1945-02-11. |
| 404 | Ro-113 | Kawasaki-Senshū Shipyard | 1942-07-11 | 1943-04-24 | 1943-10-12 | | Sunk by USS Batfish in the Luzon Strait 1945-02-12. |
| 405 | Ro-114 | Kawasaki-Senshū Shipyard | 1942-10-12 | 1943-06-19 | 1943-11-20 | | Sunk by USS Melvin and USS Wadleigh east of Saipan 1944-06-17. |
| 406 | Ro-115 | Kawasaki-Senshū Shipyard | 1942-10-12 | 1943-06-19 | 1943-11-30 | | Sunk by USS Jenkins, USS O'Bannon and USS Bell west of Mindoro 1945-02-01. Holmes contradicts the identification, indicating that Ro-115 acknowledged orders to proceed to Manila two days after the supposed date of its sinking, and was more likely to have been sunk by USS Batfish on 1945-02-10. |
| 407 | Ro-116 | Kawasaki-Senshū Shipyard | 1943-01-16 | 1943-09-13 | 1944-01-21 | | Sunk by USS England north of the Admiralty Islands 1944-05-24. |
| 408 | Ro-117 | Kawasaki-Senshū Shipyard | 1943-01-16 | 1943-09-13 | 1944-01-31 | | Sunk by USN patrol bomber southeast of Saipan 1944-05-24. |

## Footnotes

* ↑ The 小 read as "Shō" and the 潜小 read as "Sen-Shō", but the 小型 read as "Ko-gata" and the 潜小型 read as "Sen-Shō-gata" in Japanese.
* ↑ Carpenter & Dorr, p. 123
* ↑ Chesneau, p. 204
* ↑ Bagnasco, p. 193
* ↑ 呂号第100潜水艦 (Ro-Gō Dai-100 Sensuikan). The same shall apply hereinafter.
* ↑ Although her loss is credited to PT-150 and PT-152 off Lae, New Guinea on 13/14 May 1943, the submarine was later confirmed as I-6, which survived the encounter.
* ↑ Holmes, W. J., 1966, Undersea Victory: The Influence of Submarine Operations on the War in the Pacific, pg. 428-429
* ↑ Which sub was it? http://www.ussbatfish.com/whichsub.html Archived 5 August 2012 at the Wayback Machine