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I-121 (as I-21) in 1930.
Class overview
Name	I-121 class submarine
Builders	Kawasaki Corporation
Operators	Imperial Japanese Navy
Built	1924–1928
In commission	1927–1945
Planned	6
Completed	4
Cancelled	2
Lost	3
Retired	1
General characteristics
Type	Minelaying submarine
Displacement	1,142 long tons (1,160 t) surfaced 1,768 long tons (1,796 t) submerged
Length	85.20 m (279 ft 6 in) overall
Beam	7.52 m (24 ft 8 in)
Draft	4.42 m (14 ft 6 in)
Propulsion	2 × Rauschenbach Mk.1 diesels 2,400 bhp surfaced 1,100 shp submerged 2 shafts
Speed	14.9 knots (27.6 km/h) surfaced 6.5 knots (12.0 km/h) submerged
Range	10,500 nmi (19,400 km) at 8 knots (15 km/h) surfaced 40 nmi (74 km) at 4.5 knots (8.3 km/h) submerged
Test depth	75 m (246 ft)
Complement	51
Armament	2 × 533 mm (21 in) torpedo tubes 12 × 6th Year Type torpedoes 1 × 14 cm/40 11th Year Type naval gun 42 × naval mines

The I-121-class submarine (伊百二十一型潜水艦, I-hyaku-ni-jū-ichi-gata sensuikan) was a class of minelayer submarine in the Imperial Japanese Navy (IJN), serving from the 1920s to World War II. The IJN classed it as a Kiraisen type submarine (機雷潜型潜水艦, Kiraisen-gata sensuikan). The type name, was shortened from Kirai Fusetsu Sensuikan (機雷敷設潜水艦, Minelaying submarine).

Design and construction

The four I-121-class submarines — I-21, I-22, I-23, and I-24, renumbered I-121, I-122, I-123, and I-124, respectively, on 1 June 1938 — were the Imperial Japanese Navy's only submarine minelayers. Their design was based on that of the Imperial German Navy minelaying submarine SM U-125, a Type UE II submarine which was the largest of seven German submarines transferred to Japan as a war reparation after World War I and served in the Imperial Japanese Navy as O-6 from 1920 to 1921. Like UB-125, the Kiraisen-type submarines had two diesel engines producing a combined 2,400 horsepower (1,790 kW), could carry 42 mines, and had four torpedo tubes and a single deck gun — a 5.5-inch (140 mm) gun on the Japanese submarines in contrast to a 5.9-inch (150 mm) gun on UB-125. Compared to the German submarine, they were larger — 10 feet (3 m) longer, and displacing 220 more tons on the surface and 300 more tons submerged — and had a longer range both on the surface — 970 nautical miles (1,800 km; 1,120 mi) farther at 8 knots (15 km/h; 9.2 mph) — and submerged — 5 nautical miles (9.3 km; 5.8 mi) farther at 4.5 knots (8.3 km/h; 5.2 mph). They were 0.2 knots (0.37 km/h; 0.23 mph) slower than UB-125 both surfaced and submerged, carried two fewer torpedoes, and had could dive to only 200 feet (61 m) compared to 250 feet (76 m) for UB-125.

The Imperial Japanese Navy ordered six I-121-class submarines, of which four were completed and two were cancelled. The Kawasaki Yard at Kobe, Japan, built all four of the submarines. In mid-1940, all four submarines underwent conversion into submarine tankers. Retaining their minelaying and torpedo capabilities, they were modified so that each of them could carry 15 tons of aviation gasoline with which to refuel flying boats, allowing the flying boats to extend their range during reconnaissance and bombing missions by meeting the submarines in harbors and lagoons for more fuel.

Service

All four submarines saw front-line service during the Second Sino-Japanese War, during which they operated in northern Chinese waters, and the first half of the war in the Pacific during World War II. In the latter conflict, they laid mines and conducted anti-shipping patrols in East Asia and off Australia in the war′s opening weeks, during which I-124 was sunk. The other three submarines supported Japanese operations during the Battle of Midway and the Guadalcanal campaign, in which I-123 was lost. After service on supply runs during the New Guinea campaign, the two survivors, by then considered obsolescent, were withdrawn from combat in September 1943 and relegated to training duties in home waters, during which I-122 was sunk in the last weeks of the war. I-121 surrendered at the end of the war and was scuttled the following year.

Boats in class

References

Footnotes

1. Campbell, John Naval Weapons of World War Two ISBN 0-87021-459-4 p.191
2. ^ Hackett, Bob; Kingsepp, Sander (2018). "IJN Submarine I-121: Tabular Record of Movement". combinedfleet.com. Retrieved 4 August 2020.
3. ^ Hackett, Bob; Kingsepp, Sander (2015). "IJN Submarine I-122: Tabular Record of Movement". combinedfleet.com. Retrieved 5 August 2020.
4. ^ Hackett, Bob; Kingsepp, Sander (2015). "IJN Submarine I-123: Tabular Record of Movement". combinedfleet.com. Retrieved 6 August 2020.
5. ^ Hackett, Bob; Kingsepp, Sander (2017). "IJN Submarine I-124: Tabular Record of Movement". combinedfleet.com. Retrieved 9 August 2020.
6. ^ Boyd and Yoshida, p. 18.
7. 伊号第121潜水艦 (I-Gō Dai-121 Sensuikan). The same shall apply hereinafter.

Bibliography

• Boyd, Carl, and Akihiko Yoshida. The Japanese Submarine Force and World War II. Annapolis, Maryland: Naval Institute Press, 1995. ISBN 1-55750-015-0.
• "Rekishi Gunzō"., Gakken (Japan)
  • History of the Pacific War Vol. 17, I-Gō Submarines, January 1998, ISBN 4-05-601767-0
  • History of the Pacific War Extra, Perfect guide, The submarines of the Imperial Japanese Forces, March 2005, ISBN 4-05-603890-2
• The Maru Special, Ushio Shobō (Japan)
  • Japanese Naval Vessels No. 43, Japanese Submarines III, September 1980
  • Japanese Naval Vessels No. 132, Japanese Submarines I (New edition), February 1988
  • Japanese Naval Vessels No. 133, Japanese Submarines II (New edition), March 1988

• Submarine classes
• I-121-class submarines
• Submarines of the Imperial Japanese Navy
• World War II minelayers of Japan
• Mine warfare vessels of the Imperial Japanese Navy