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    I-400 class submarine - Submarine Aircraft Carrier

    The Sen Toku I-400 class submarines of the Imperial Japanese Navy were the largest submarines of World War II, the largest non-nuclear submarines ever constructed, and the largest in the world until the development of nuclear ballistic missile submarines in the 1960s. These were submarine aircraft carriers and each of them was able to carry 3 Aichi M6A Seiran aircraft underwater to their destinations. They also carried torpedoes for close range combat and were designed to surface, launch the planes then dive again quickly before they were discovered.

    I-400, with its long plane hangar and forward catapult

    Although the U.S. Navy remained discreet about it, the Japanese were ahead of the Allies in many aspects of submarine development and underwater weapons. During the Second World War, the Japanese had 30 different classes of submarines — from the one-man suicide torpedoes to the giant I-400 class of aircraft carriers, and used the world's most effective torpedoes, the Type 95.

    I-400's aircraft storage and catapult for her three M6A1 Seiran (Storm from a Clear Sky) torpedo-bombers

    While Japan built many submarines that were larger than those of other Navies, the three Sen Toku boats were far larger than anything ever seen before.  Some 60% larger than the largest contemporary American submarine, USS Argonaut, they had more than twice her range.

    Side view of I-401

    In many ways H.I.J.M.S. I-400 was decades ahead of her time.  She was the world's largest submarine, with a length of 120 m, and a surfaced displacement of 3,530 tons.  Above her main deck rose a 115 foot long, 12 foot diameter, hangar housing three torpedo-bombers.  These floatplanes were rolled out through a massive hydraulic door onto an 85 foot pneumatic catapult, where they were rigged for flight, fueled, armed, launched, and, after landing alongside, lifted back aboard with a powerful hydraulic crane.  The I-400 was equipped with a snorkel, radar, radar detectors, and capacious fuel tanks that gave her a range of 37,500 miles: one and a half times around the world.  She was armed with eight torpedo tubes, a 5.5 inch 50 caliber deck gun, a bridge 25mm antiaircraft gun, and three triple 25mm A/A mounts atop her hangar.

    Unique asymmetrical cross section of the I-400 class boats

    The most unusual feature was that they each carried three floatplane bombers (and parts for a fourth), a feat never achieved by any other class of submarine.  These aircraft folded to fit into the 115-foot cylindrical hangar, which was slightly offset to starboard and opened forward to access the catapult.  The huge double hull was formed of parallel cylindrical hulls so that it had a peculiar lazy-eight cross section, and may have inspired the Soviet Typhoon-class built some 40 years later.

    Members of the US Navy inspecting the plane hangar of I-400

    Aichi M6A1 Seiran

    The aircraft were the Aichi M6A1 Seiran, also carried by the Type AM submarines.  Each of these monoplanes could carry one aerial torpedo or a bomb weighing up to 800kg.  Powered by the 1,400hp Atsuta 32 engine they had a top speed of 295mph and were credited with a range of 642 nautical miles.  The Sen Toku submarines carried four aerial torpedoes, three 800kg bombs, and twelve 250kg bombs to arm these aircraft.  These aircraft had their assembly points coated with fluorescent paint to ease assembly in the dark, so four trained men could prepare an aircraft for launch in seven minutes.  All three aircraft could be prepared, armed, and launched in 45 minutes.

    Aichi M6A Seiran

    A restored Seiran airplane is displayed at the National Air and Space Museum in Washington, D.C. Only one was ever recovered and it had been ravaged by weather and souvenir collectors, but the restoration team was able to reconstruct it accurately.

    Operational history

    For their first mission Vice Admiral Jisaburo Ozawa, Vice Chief of the Navy General Staff, selected Operation PX, a top secret plan to use SubRon One's ten aircraft to unleash bacteriological warfare on populous areas of the American west coast and Pacific Islands.  Infected rats and insects would be dispersed to spread bubonic plague, cholera, dengue fever, typhus and other plagues.  General Ishii's infamous medical laboratory at Harbin, Manchuria, had developed the virulent germ warfare agents and confirmed their lethality by infecting helpless Chinese and Caucasian prisoners.

    US Navy personnel inspecting the gun of I-400

    On March 26th, 1945, this sinister mission was canceled by General Yoshijiro Umezu, Chief of the Army General Staff, who declared that "Germ warfare against the United States would escalate to war against all humanity." As an alternative the staff considered bombing San Francisco, Panama, Washington or New York, and decided to launch a surprise air strike against the Panama Canal's Gatun Locks.  Destroying these locks would empty Gatun Lake and block the passage of shipping for months.

    Before the attack could commence from the Japanese naval base at Maizuru, word reached Japan that the Allies were preparing for an assault on the home islands. The mission was changed to attack the Allied naval base on Ulithi where the invasion was being assembled. Before that could take place, the Emperor announced the surrender of Japan.

    I-400 beside submarine tender USS Proteus after the war

    I-400 and I-401 therefore returned to Japan and were surrendered to the Allies.  After the war, these two were taken to the United States, examined, and finally scuttled in the Pacific in 1946. While there, they received a message that the Soviets were sending an inspection team to examine the submarines. To keep the technology out of the hands of the Soviets, Operation Road’s End was instituted. Most of the submarines were taken to a position designated as Point Deep Six, about 60 km west from Nagasaki and off the island of Goto-Rettō, were packed with charges of C-2 explosive and destroyed. They are today at a depth of 200 meters.  I-402 was converted to carry precious fuel to Japan from the East Indies, but never performed such a mission.  She was scuttled off Goto Island in 1946.  Construction of two further boats of this design, I-404 and I-405, was stopped before completion, although I-404 was 90% complete.  A further 13 boats were canceled before construction started.

    Four remaining submarines (I 400, I 401, I 201 and I 203 which achieved speeds double those of American submarines), were sailed to Hawaii by U.S. Navy technicians for further inspection. Upon completion of the inspections, the submarines were scuttled in the waters off Kalaeloa near Oahu in Hawaii by torpedoes from the American submarine USS Cabezon on May 31, 1946. The reason for the scuttling is apparently that Russian scientists were again demanding access to the submarines. The wreckage of I 401 was re-discovered by the Pisces submarines deep-sea submarines of the Hawaii Undersea Research Laboratory in March 2005 at a depth of 820 meters.

    Units 3 (all survived)
    Ships I-400, I-401, and I-402
    Year(s) Completed 1944-1945
    Displacement 5,223 tons / 6,560 tons
    Dimensions 400.3 ft x 39.3 ft x 23 ft
    Machinery 4 diesels: 7,700 hp
    electric motors: 2,400 hp
    Speed 18.75 knots / 6.5 knots
    Range 37,500 nm @ 14 knots
    Armament 8x533mm TT fwd + 1x14cm/50 cal. 20 torpedoes.
    Max. Depth 100 m
    Crew 144 officers and men

    Your Comments:

    1. sentoku says:

      i-400 class submarines would have been more of a threat than you know. especially, if the germans helped the japanese with equipment for them. with german radar and other stuff, the i-400 class submarines would have been a serious threat.

      Posted on June 25, 2007 at 8:46

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