The Kaichū type submarine (海中型潜水艦, Kaichū-gata sensuikan) submarines were double-hulled medium-sized submarines of the Imperial Japanese Navy during World War II. The name was derived from the Kaigun-shiki Chū-gata Sensuikan (海軍式中型潜水艦, Navy Medium Type submarine).
Several variants existed. From 1934 to 1944, the K6 type (Ro-33 Class) and the K7 type (Senchū, Ro-35 Class) were built. They were equipped with a 76.2 mm (3.00 in) L/40 gun and four 53 cm torpedo tubes for ten type 95 Long Lance torpedoes.
Most of these submarines were destroyed in combat, suffering from Allied anti-submarine warfare measures, and only Ro-50 survived the war.
The Kaichū type submarines were divided into seven classes:
Project number S7. In 1910s, the Imperial Japanese Navy (IJN) bought a license of Schneider-Laubeuf design submarine. The IJN used the design as model and built the S Type (Schneider Type) submarine, the Ha-9 and Ha-10. The Kaichū I is the submarine which jumboized the S Type submarines.
Project number S18. The Kaichū II had an increased range compared with the Kaichū I, and the turning torpedo tubes were removed.
Project number S18. Their project number was the same as in the Kaichū II type submarine, however their performance was improved.
Project number S18A. Improved model from the Kaichū III type.
Project number S18B. They were built for the commerce raiding role. The IJN official designation of these boats was Special Purpose-Medium Type submarine (特中型潜水艦, Toku-Chū-gata sensuikan).
Project number S30. They were planned as a prototype for a mass production submarines in the wartime under the Maru 1 Programme.
Project number S44. The final design in the Kaichū series. They were equipped with a Freon air-conditioner, because the IJN took into consideration that they were to be active on the equator area too. The official IJN designation of these boats was Medium Type submarine (中型潜水艦, Chū-gata sensuikan), also called for short, Medium Type (中型, Chū-gata) or Submarine-Medium Type (潜中型, Sen-Chū-gata).
The IJN planned to build these boats under the following Naval Armaments Supplement Programmes:
However some of the boats were cancelled and their naval budgets, materials and staffs were transferred to the I-201 class submarines.