Japanese submarine I-51
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.
"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.
Submarines of the Imperial Japanese Navy
The submarines were double-hulled medium sized submarines. They were derived from the (KT or K5 type, "Ro-29"-class). Several variants existed. From 1934 to 1944, the (K6 type, "Ro-33"-class) and the (K7 type, "Ro-35"-class) were built. The K6 type was equipped with a 3.25 inch (80 mm) gun and Type 95 Long Lance torpedoes. Twenty units were built: , , "Ro-35", "Ro-36, Ro-37", "Ro-38", "Ro-39", "Ro-40", "Ro-41", "Ro-42", "Ro-43", "Ro-44", , "Ro-46", "Ro-47", "Ro-48", "Ro-49", "Ro-50", "Ro-55", "Ro-56".