I-400 Class SSG
Note: drawing contains some parts and lower hull details taken from Shipbucket.com)

Ship Characteristics

Dimensions Length 400 ft
Beam 39.4 ft
Draft 23.0 ft
Displacement (Surfaced) 5,223 tons
Displacement (Submerged 6,560 tons
Performance Speed (surfaced) 19.0 knots
Speed (submerged) 6.5 knots
Armament Torpedoes 8 21 inch.
Guns 15 25mm
Missiles 5 Ohka


The Sen Toku I-400-class Imperial Japanese Navy submarines were originally intended as submarine aircraft carriers able to carry three Aichi M6A Seiran aircraft underwater to their destinations. They were designed to surface, launch the planes then dive again quickly before they were discovered. They also carried torpedoes for close-range combat. Each submarine had four 2,250 hp engines and carried enough fuel to go around the world one-and-a-half times—more than enough to reach the United States travelling east or west. The cross-section of its pressure hull had a unique figure-of-eight shape which afforded the necessary strength and stability to handle the weight of a large on-deck aircraft hangar.

To allow stowageof three aircraft along the vessel's centreline, the conning tower was offset to port. Located approximately amidships on the top deck was a cylindrical watertight aircraft hangar, 102 ft long and 11 ft in diameter. The outer access door could be opened hydraulically from within or manually from the outside by turning a large hand-wheel connected to a rack and spur gear. The door was made waterproof with a 2.0 in rubber gasket. Sited atop the hangar were three water-proofed Type 96 triple-mount 25 mm (1.0 in) machine guns for AA defence, two abaft and one forward the conning tower. A single 25 mm (1.0 in) gun on a pedestal mount was also located just abaft the bridge. One Type 11, 140 mm (5.5 in) deck gun was positioned aft of the hangar. A special anechoic coating made from a mixture of gum, asbestos, and adhesives was applied to the hulls from the waterline to the bilge keel. This was intended to absorb or diffuse enemy sonar pulses and dampen reverberations from the boat's internal machinery, making detection while submerged more difficult. the submarines were also fitted with a snorkel allowing the boat to run its diesel engines and recharge its batteries while remaining at periscope depth.

The original plan was to build 18 submarines of this class but financial and other shortages caused the program to be reduced to five boats. These were completed between 1945 and 1947. They were not considered successful since the M6A aircraft were obsolete before they were put into service. The chance of the submarines launching an attack was considered remote and the payload carried by the aircraft was inconsequential.

In 1952, the Japanese initiated their first nuclear weapon and began a search for the best delivery system for the new weapon. Their first choice was the Ohka, a manned anti-ship missile intended for release from aircraft or fired from shore batteries. The I-400 class submarines were seen as a launch platform for the nuclear-tipped Ohka and were converted to the new role. The 5.5 inch gun and single 25mm weapons were removed and replaced by two additional 25mm triple mounts. The hangars were converted to carry a total of five nuclear Ohkas that would be launched from the catapult. The I-400 re-entered service in 1954 and carried out her first deterrent cruise later that year. By late 1955, all five I-400 class boats were back in service and one or two were stationed off the U.S. West Coast from that year onwards.

The I-400 class were not ideal for the role since their hangars were designed for the M6A and did not use space efficiently. Later SSGs and SSGNs had new hangars that offered greater missile capacity. By the late 1960s, the original five I-400s were considered too vulnerable for West Coast patrols and were relegated to anti Triple Alliance patrols off the coast of China. They were finally withdrawn from service in the late 1970s.

Class Members

Number Ordered Laid Down Launched Commissioned Fate
I-400 1943 1943 1944 1945 Decommissioned and scrapped 1975
I-401 1943 1943 1944 1945 Decommissioned and scrapped 1976
I-402 1943 1944 1945 1945 Decommissioned and scrapped 1977
I-403 1943 1944 1945 1946 Decommissioned and scrapped 1978
I-404 1943 1945 1946 1947 Decommissioned and scrapped 1978
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