Junsen type submarine

The Junsen type submarine (巡潜型潜水艦, "巡洋潜水艦") was a ship class of submarines of the Imperial Japanese Navy (IJN). There were four submarine designs of the Junsen type: J1, a modified J1, J2 and the J3.

Class variants
The Junsen type submarines were divided into four classes:
 * Junsen I (巡潜一型（伊一型）)
 * Junsen I Mod. (巡潜一型改（伊五型）)
 * Junsen II (巡潜二型（伊六型）)
 * Junsen III (巡潜三型（伊七型）).

Junsen I (I-1 class)
Four boats were built in 1923-1929. Genealogy of the large-size submarine in the IJN began with SMU U-142. Japan received six U-boats from Germany as reparations of World War I. The IJN copied SMU U-125, one of the six, producing the I-21-class minelayer submarine. The IJN could not find an optimal design of fleet submarine, so they and Kawasaki Heavy Industries sent many technical officers to the United Kingdom and Germany and got drawings of advanced submarines. The British L class became the Kaidai I, the K class became the Kaidai II and U-142 become Junsen I.

Junsen I Mod (I-5 class)
This is a type which added a floatplane to the Junsen I.

Junsen II (I-6 class)
Project number S32. This is a type which added a catapult to the Junsen I Mod. She was built in 1931 under the 1st Naval Armaments Supplement Programme (Maru 1).

Junsen III (I-7 class)
Project number S33. These boats combined the good points of the Junsen II and the Kaidai V. They were built in 1934 under the Maru 2 Programme. Junsen III became a 'typeship' for the Type-A, B and C.