The Imperial Japanese Navy (IJN) submarines were 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 IJN called this type of submarine . The type name was shortened to . They 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 I-400 class was designed with the range to travel anywhere in the world and return. A fleet of 18 boats was planned in 1942, and work started on the first in January 1943 at the Kure, Hiroshima arsenal. Within a year the plan was scaled back to five, of which only three (I-400 at Kure, and and I-402 at Sasebo) were completed.
Origins
The I-400 class was the brainchild of Admiral Isoroku Yamamoto, Commander-in-Chief of the Japanese Combined Fleet. 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. He commissioned Captain Kameto Kuroshima to make a feasibility study.
Yamamoto submitted the resulting proposal to Fleet Headquarters on 13 January 1942. It called for 18 large submarines capable of making three round-trips to the west coast of the United States without refueling or one round-trip to any point on the globe. They also had to be able to store and launch at least two attack aircraft armed with one torpedo or bomb. By 17 March, general design plans for the submarines were finalized. Construction of I-400 commenced at Kure Dock Yards on 18 January 1943, and four more boats followed: I-401 (April 1943) and I-402 (Oct 1943) at Sasebo; I-403 (Sept 1943) at Kobe and I-404 (February 1944) at Kure. Only three were completed.
Following Yamamoto's death in April 1943, the number of aircraft-carrying submarines to be built was reduced from eighteen to nine, then five and finally three. Only I-400 and I-401 actually entered service; I-402 was completed on 24 July 1945, five weeks before the end of the war, but never made it to sea.
Design features and equipment
Each submarine had four 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 long overall, they displaced , 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, long and 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 rubber gasket.
Situated atop the hangar were three waterproofed Type 96 triple-mount autocannon for AA defence, two aft and one forward of the conning tower. A single autocannon on a pedestal mount was also located just aft the bridge. One Type 11, deck gun was positioned aft of the hangar. It had a range of .
Eight torpedo tubes were mounted in the bow, four above and four below. There were no aft tubes.
Stowed in an open recessed compartment on the forward port side, just below top deck, was a collapsible crane used to retrieve the submarine's Seiran floatplanes. The crane had an electrically operated hoist and was capable of lifting approximately . It was raised mechanically to a height of via a motor inside the boat. The boom extended out to a length of .
A special trim system was fitted to the boats, allowing them to loiter submerged and stationary while awaiting the return of their aircraft. However, operation of this system was noisy and its usefulness was in doubt.
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.