Japanese submarine Ro-21

The submarines of the Kaichu III sub-class were a slightly improved version of the preceding Kaichu II subclass, the man difference being an increase in diving depth from 30 to 45.7 meters (98 to 150 ft).[1] They displaced 752 tonnes (740 long tons) surfaced and 1,013 tonnes (997 long tons) submerged. The submarines were 70.10 meters (230 ft 0 in) long and had a beam of 6.12 meters (20 ft 1 in) and a draft of 3.70 meters (12 ft 2 in).

For surface running, the submarines were powered by two 1,450-brake-horsepower (1,081 kW) Sulzer Mark II diesel engines, each driving one propeller shaft. When submerged each propeller was driven by a 600-horsepower (447 kW) electric motor. They could reach 16.5 knots (31 km/h; 19 mph) on the surface and 8.5 knots (15.7 km/h; 9.8 mph) underwater. On the surface, they had a range of 6,000 nautical miles (11,000 km; 6,900 mi) at 10 knots (19 km/h; 12 mph); submerged, they had a range of 85 nmi (157 km; 98 mi) at 4 knots (7.4 km/h; 4.6 mph).

The submarines were armed with six 450 mm (18 in) torpedo tubes, four internal tubes in the bow and two external tubes mounted on the upper deck, and carried a total of ten Type 44 torpedoes. They were also armed with a single 76.2 mm (3.00 in) deck gun mounted aft of the conning tower.[1]

Ro-21 was laid down as Submarine No. 39 on 28 July 1919 by the Yokosuka Naval Arsenal at Yokosuka, Japan.[2] Launched on 26 October 1920,[2] she was completed and commissioned on 1 February 1922.[2]

Upon commissioning, Submarine No. 39 was attached to the Yokosuka Naval District, to which she remained attached throughout her career.[2] On 6 February 1922, she was assigned to Submarine Division 5 and to the Ominato Defense Division,[2] and on 1 November 1924 she was renamed Ro-21.[2] On 1 November 1925, Submarine Division 5 was reassigned to the Yokosuka Defense Division,[2] then directly to the Yokosuka Naval District on 1 December 1926, and then to the Yokosuka Defense Division again on 10 January 1927.[2]

Ro-21 was stricken from the Navy list on 1 April 1934.[2] Her engines were removed at Uraga Shipyard, and on 6 July 1935 she was sold to the Fisheries Association of Miura District in Kanagawa Prefecture for use as an artificial reef. She subsequently was scuttled to serve as a breeding ground for fish and fishing reef.

• "Rekishi Gunzō"., History of Pacific War Vol.17 I-Gō Submarines, Gakken (Japan), January 1998, ISBN 4-05-601767-0
• Rekishi Gunzō, History of Pacific War Extra, "Perfect guide, The submarines of the Imperial Japanese Forces", Gakken (Japan), March 2005, ISBN 4-05-603890-2
• 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