The Imperial Japanese Navy (IJN) 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.
At , the three I-400s were the largest submarines of World War II, with the final completed submarine being finished roughly a month before the end of the war. They remained the largest submarines ever built until the construction of nuclear ballistic missile submarines in the 1960s. The official designation was the , "Sentoku" being the abbreviation of .
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 ( at Kure, and and 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 making raids on the United States mainland by launching 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.Sakaida, p. 15.
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.Sakaida, p. 16.
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 traveling 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 centerline, 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.Sakaida, p.74.
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 .Sakaida, p.100-101.
Eight torpedo tubes were mounted in the bow, four above and four below. There were no aft tubes.Sakaida, p. 17.
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 .Sakaida, p. 81.
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.Layman and McLaughlin, p. 178–179.
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 fusing system of the mines. 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.Sakaida, p. 73.
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 . The boats were also equipped with Mark 2 Model 2 air/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.Sakaida, p. 104-107.
The submarines were equipped with two periscopes of German manufacture, about long, one for use during daylight and the other at night.Sakaida, p. 104.
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 coating was apparently based on German research, though completely different in composition from German anechoic tiles such as Alberich or Tarnmatte.Boyd, Carl, and Yoshida, Akihiko, The Japanese Submarine Force and World War II, BlueJacket Books (2002), , pp. 27, 29 This was intended to absorb or diffuse enemy sonar pulses and dampen reverberations from the boat's internal machinery, theoretically making detection while submerged more difficult, though its effectiveness was never conclusively established.Sakaida, p. 92.Sakaida, p. 126.
In May 1945, I-401 was fitted with a German-supplied snorkel, a hydraulically raised air intake device allowing the boat to run its diesel engines and recharge its batteries while remaining at periscope depth. This retrofit occurred while the boat was laid up at Kure for repairs after being damaged by an American mine in April.Orita, p. 317
I-402 was completed shortly before the war ended, but had been converted during building to a tanker and was never equipped with aircraft.Hashimoto, p. 213
Characteristics The I-400-class subs were unwieldy and relatively difficult to maneuver while surfaced owing to their small rudders.Paine, Thomas O., I Was A Yank On A Japanese Sub, U.S. Naval Institute Proceedings, Volume 112, Number 9, Issue 1003 (September 1986), p. 73-78 The large superstructure also caused the sub to veer off course during any strong wind. The maximum safe diving depth of the I-400-class submarine was only 82% of its overall length, which presented problems if the submarine dived at too steep an angle in an emergency. Because of their large aircraft hangars and conning tower, all I-400-class boats had significant visual and radar signatures on the surface, and could be detected by aircraft relatively easily. Dive time was 56 seconds, nearly double that of U.S. fleet subs, which made the boats easier to destroy from the air when caught on the surface.
When submerged and traveling at a slow speed of two knots, the offset superstructure forced the helmsman to steer seven degrees starboard in order to steer a straight course. When conducting a torpedo attack the captain had to take into account his larger turning circle to starboard than to port, again because of the offset design. Like other Japanese submarines, crew members in I-400 subs had no air conditioning to control temperatures in tropical waters and no flush toilets. Lack of cold storage greatly limited the crew's diet, while inadequate sleeping quarters forced some of the crew to sleep on the decks or in passageways.
Aircraft
The hangar of the I-400s was originally designed to hold two aircraft. In 1943, however, Commander Yasuo Fujimori, Submarine Staff Officer of the Naval General Staff, requested it be enlarged. This was deemed feasible and, as remodelled, I-400s could stow up to three Aichi M6A Seiran floatplanes.Sakaida, p. 36
The Seiran was specifically designed for use aboard the submarines and could carry an bomb, with a range of at . 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. When deployed for flight, the aircraft had a wingspan of and a length of . A crew of four could prepare and launch all three in 30 minutes (or 15 minutes if the planes' pontoons were not first attached, which would make recovery impossible).Air International October 1989, p. 187. 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.Francillon, p. 292
The Seirans were launched from a Type 4 No. 2 Model 10 compressed-air catapult on the forward deck of the submarine. Underneath the catapult track were four high-pressure air flasks connected in parallel to a piston. The aircraft, mounted atop collapsible carriages via catapult attachment points along their fuselages, would be slung 70–75 feet along the track, though the piston itself only moved between eight and ten feet during operation.Sakaida, p.134
Two sets of pontoons for the Seirans were stored in special watertight compartments located just below the main deck on either side of the catapult track. From there they could be quickly slid forward on ramps and attached to the plane's wings. A third set of pontoons and additional spares were kept inside the hangar.Sakaida, p. 82
The aircraft were to be launched by catapult, and fly their missions. The launching submarine was to submerge and stay in place to allow the aircraft to navigate back to the area by dead reckoning, where it would land on the water with its floats, and be hoisted back aboard by crane. Overall the system was the same as used by Japanese Navy cruisers and light cruisers when launching their reconnaissance floatplanes (like the Aichi E13A), only with aircraft specially designed for use with the I-400 class submarine, and with the added complexity of having to locate a submerged and hidden vessel on return from the mission.
Although this was the typical mode of operation, in cases where fast launching and recovery was essential for escape (see below), the floatplanes could be launched without their floats, and ditched upon landing, saving the time spent recovering and re-hangaring the aircraft, which was a complex and lengthy procedure. This had the added benefit of eliminating the weight and considerable drag of the large and bulky floats, which in turn increased the speed and range of the aircraft, but made any recovery of the aircraft after completing the mission impossible. (For a similar defensive measure involving catapult-launched disposable aircraft used by Allied convoy groups in the Battle of the Atlantic, see CAM ship.) In extreme circumstances, the aircraft could theoretically be launched and abandoned altogether while the submarine retreated, leaving the crews to fly their missions with no hope of return, perhaps as kamikazes.
Operational history