The I-400 class were submarine aircraft carriers able to carry three Aichi M6A Seiran aircraft underwater to their destinations.

Submarines, one of the key weapons during World War II. Did you know that the Japanese developed a submarine aircraft carrier during World War II? Let us introduce you to the I-400 class submarine, the largest submarines of World War II and remained the largest ever built until the construction of nuclear ballistic missile submarines in the 1960s.

The I-400 class were submarine aircraft carriers able to carry three Aichi M6A Seiran aircraft underwater to their destinations. They were designed to surface, launch their planes, then quickly dive again before they were discovered. They also carried torpedoes for close-range combat. The purpose of Japan when creating the I-400 was to launch a surprise attack directly on the US mainland. Although they did not participate in any naval battles, the technology and design of these boats startled the Allied armies when they discovered them.

I-400 was the brainchild of Admiral Isoroku Yamamoto, commander of the Imperial Japanese fleet. During the war, Yamamoto had always worried about America as a giant but had not been awakened. Shortly after the attack on Pearl Harbor, he conceived the idea of taking the war to the United States mainland by making aerial attacks against cities along the U.S. western and eastern seaboards using submarine-launched naval aircraft.

These freakish subs were specifically designed to travel unseen to their destination and carry out hit and run raids. Being submarines, they were also equipped with traditional torpedoes for close range sea surface combat. The I-400 submarine aircraft carriers were also designed to have a large combat range and in theory hit anywhere in the world.

During proposal stages in 1942, 18 were planned for construction. Work on the fleet began in 1944 but commissioned numbers were stripped back to 5 when Yamamoto was killed. Only 3 were ever completed. The first ones came off the production line, well dock,  in January of 1943 and was christened the I-400. Through April 1943 to February 1944 four more were planned with only three ever completed. Of these only two, I-400 and I-401 ever entered active service.

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. Measuring more than 120 m long overall, they displaced 5,900 tons, more than double their typical American contemporaries. 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 stowage of 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, 31 m long and 3.5 m 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 51-millimetre-thick rubber gasket.

Situated atop the hangar were three waterproofed Type 96 triple-mount 25 mm autocannon for anti aircraft defence, two aft and one forward of the conning tower. A single 25 mm autocannon on a pedestal mount was also located just aft the bridge. One Type 11, 140 mm deck gun was positioned aft of the hangar. It had a range of 15 km. Eight torpedo tubes were mounted in the bow, four above and four below. There were no aft tubes.

Strung along the submarine’s gunwales were two parallel sets of demagnetization cables, running from the stern to the bow planes. They were meant to protect against magnetic mines, by nullifying the magnetic field which normally triggers the mines fusing system. A similar demagnetizing system was carried on many Japanese surface ships during the first part of the war, until they were later removed during refitting.

Electronics on board the I-400s included a Mark 3 Model 1 air search radar equipped with two separate antennas. This unit was capable of detecting aircraft out to a range of 80 km. The boats were also equipped with Mark 2 Model 2 surface radar sets with distinctive horn-shaped antennas. Each boat carried an E27 radar warning receiver, connected to both a trainable dipole antenna and a fixed non-directional antenna made up of a wire mesh basket and two metal rods.

The Seiran was specifically designed for use aboard the submarines and could carry an 800 kg. To fit inside the narrow confines of the hangar, the floats were removed and stowed, the wings rotated 90 degrees and folded backward hydraulically against the fuselage, the horizontal stabilizers folded down and the top of the vertical stabilizer folded over so the overall forward profile of the aircraft was within the diameter of its propeller. As the Seiran would normally be launched at night, parts and areas of the plane were coated with luminescent paint to ease assembly in the dark.

When the Americans invaded Japan, Yamamoto’s plan was aborted. Instead, the I-400 and I-401 ships began training for a mission into the Panama Canal in an effort to intercept American ships from the Atlantic to the Pacific. Japan believed that this move would slow the pace of America. However, by June 1945 this plan was also rejected.

When the occupation of Japan began, American officials were surprised by the Japanese products. Of course, the US counterpart, the Soviet Union, also wanted to study these I-400 boats under an agreement signed by the parties before the end of the war. However, the US did not deliver the boats because they considered the Soviet Union as a potential enemy, and the navy eventually sank these two submarines off the coast of Oahu, claiming they no longer had any information about the submarine’s location. The I-400 sank on March 31, 1946, and the I-401 sank on June 4, 1946.

It is believed that the I-400 inspired the US to build later cruise missile submarines. It has been revealed that the USS Grayback – the first US active combat submarine to carry the Regulus II nuclear-capable missile – had been built on the basis of the I-400.

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