I-121-class submarine

The I-121-class submarine (伊百二十一型潜水艦) 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). The type name, was shortened from Kirai Fusetsu Sensuikan (機雷敷設潜水艦).

Design and construction
The four I-121-class submarines — I-21, I-22, I-23, and I-24, renumbered JAPANESE SUBMARINE I-121, JAPANESE SUBMARINE I-122,  JAPANESE SUBMARINE I-123, and JAPANESE SUBMARINE 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 SMU 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 hp, could carry 42 mines, and had four torpedo tubes and a single deck gun — a 5.5 in gun on the Japanese submarines in contrast to a 5.9 in gun on UB-125. Compared to the German submarine, they were larger — 10 ft longer, and displacing 220 more tons on the surface and 300 more tons submerged — and had a longer range both on the surface — 970 nmi farther at 8 kn — and submerged — 5 nmi farther at 4.5 kn. They were 0.2 kn slower than UB-125 both surfaced and submerged, carried two fewer torpedoes, and had could dive to only 200 ft compared to 250 ft 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.