Japanese submarine I-51

Japanese submarine I-51 (伊号第五一潜水艦) was the lead vessel and prototype of the Kaidai-class submarines of the Imperial Japanese Navy, which served in World War II.

Background
Following World War I, the Imperial Japanese Navy General Staff began to re-consider submarine warfare as an element of fleet strategy. Before the war, the Japanese Navy regarded submarines as useful only for short-range coastal point defense. However, based on the success of the Imperial German Navy in deployment of long-range cruiser-submarines for commerce raiding Japanese strategists came to realize possibilities for using the weapon for long range reconnaissance, and in a war of attrition against an enemy fleet approaching Japan. Procurement for a large, long-range Japanese submarine was authorized in fiscal 1918 under the Eight-six fleet program, under the designation S22.

Design
Project S22 was based on the latest Royal Navy design, the British K class submarine. Japanese ties to Great Britain via the Anglo-Japanese Alliance were still strong. Project S22 was laid down at Kure Naval Arsenal on 6 April 1921, launched on 29 November 1921 and completed on 20 June 1924.

During construction, the vessel was renamed Submarine No.44 (第四四号潜水艦), however, on commissioning into the Imperial Japanese Navy, her name was formally registered as I-51.

With a displacement of 1390 tons, I-51 was the largest submarine built in Japan to date. In order to attain a design speed of 23 knots on the surface, and 15 knots submerged, the design required four diesel engines, driving four screws. To accommodate these engines, a double hull design was used, with hulls joined side-by-side forming a sideways figure "8". As completed, I-51 achieved only 18.4 knots surfaced and 8.4 knots submerged during trials, but had an unrefueled range of 20,000 nautical miles, which was considered remarkable for the time.

Operational history
I-51, despite her various technical achievements, as not regarded as a successful design, largely through problems with her Sulzer diesel engines. The vessel was never assigned to fleet service, but was retained at Kure Naval Arsenal for crew training and as a test bed for various submarine technologies.

In 1931, I-51 was fitted with an aircraft hanger housing a Yokosuka Ro-go Ko-gata floatplane, which could be raised and lowered into the water by a crane. In 1933, this was modified with the addition of an aircraft catapult, making I-51 the forerunner of the Japanese submarine aircraft carriers of World War II.

In 1932, two of her engines and associated shafts were removed, as was her main gun.

I-51 was removed from the navy list on 1 April 1940 and was scrapped in 1941.