The Type B1 submarine (巡潜乙型潜水艦), also called I-15-class submarine (伊一五型潜水艦) was the first group of boats of the Type B cruiser submarines built for the Imperial Japanese Navy (IJN) during the 1940s. In total 20 were built, starting with JAPANESE SUBMARINE I-15, which gave the series their alternative name.

Design and description

The Type B submarines were derived from the earlier KD6 sub-class of the Kaidai-class submarine and were equipped with an aircraft to enhance their scouting ability. They displaced 2589 LT surfaced and 3654 LT submerged. The submarines were 108.7 m long, had a beam of 9.3 m and a draft of 5.1 m. They had a diving depth of 100 m. For surface running, the boats were powered by two 6200 bhp diesel engines, each driving one propeller shaft. When submerged each propeller was driven by a 1000 hp electric motor. They could reach 23.6 kn on the surface and 8 kn underwater. On the surface, the B1s had a range of 14000 nmi at 16 kn; submerged, they had a range of 96 nmi at 3 kn. The boats were armed with six internal bow 53.3 cm torpedo tubes and carried a total of 17 torpedoes. They were also armed with a single 140 mm/40 deck gun and two single mounts for 25 mm Type 96 anti-aircraft guns. In the Type Bs, the aircraft hangar was faired into the base of the conning tower. A single catapult was positioned on the forward deck. Late in the war, some of the submarines had their aircraft hangar removed, to replace it with an additional 14 cm gun. In 1944, I-36 and I-37 had their aircraft hangar and catapult removed so that they could carry four Kaiten manned torpedoes, with I-36 later being further modified to carry six.

Service

The series was rather successful, especially at the beginning of the war.

Losses

Altogether the Type B submarines (B1, B2, and B3 combined) are credited with sinking 56 merchant ships for a total of 372,730 tonnes, about 35% of all merchant shipping sunk by Japanese submarines during the war. All B1 type submarines were lost during the conflict, except for JAPANESE SUBMARINE I-36, which was scuttled off Gotō Islands by the US Navy on 1 April 1946.