(Imperial War Museum photo, scanned from Polmar & Carpenter's "Submarines of the Imperial Japanese Navy")
These four boats were the only Japanese submarines designed specifically for laying mines. They were based on the German U-125 obtained by Japan after the First World War. They had two mine tubes aft. In 1939 they were re-numbered I-121, I-122, I-123, and I-124, respectively. They had problems with longitudinal instability, and therefore proved unsatisfactory for laying mines. In 1940, they were modified to carry gasoline for refueling seaplanes and flying boats. I-124 was lost off Darwin to destroyer USS Edsall on 20 January 1942, and I-123 was lost 29 August 1942 to minesweeper USS Gamble near Guadalcanal. The two remaining boats were converted to training duties, but I-122 was sunk by submarine USS Skate in the Sea of Japan on 10 June, 1945.
Erminio Bagnasco's "Submarines of World War Two" credits these four boats with sinking five Allied ships of 20,009 GRT.
| Units | 4 (one survived) |
|---|---|
| Ships | I-21, I-22, I-23, I-24 |
| Year(s) Completed | 1927-1928 |
| Displacement | 1,383 tons / 1,768 tons |
| Dimensions | 279.5 ft x 24.5 ft x 14.5 ft | Machinery | 2 diesels: 2,400 hp
electric motors: 1,100 hp |
| Speed | 14.5 knots / 7 knots |
| Range | 10,500 nm @ 10 knots |
| Armament | 4x533mm TT fwd + 1x14cm/50 cal. (12 Torpedoes + 42 mines) |
| Max. Depth | 60 m (200 feet) |
| Crew | 75 officers and men |
