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 400 feet 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. The advent of guided missiles and atomic bombs transformed her from an overspecialized undersea dinosaur to a menacing strategic threat. Like Germany's Type XXI U-boat she was too late to influence World War II.
Two well-illustrated articles on these giant submarines have appeared in Maru Special, a Tokyo nautical magazine: Japanese Naval Vessels, No. 13: I-400 & I-13, 1977,2 and Japanese Navy Warships of World War II, No. 16: Submarines, 1975.3 A first-hand contemporary description by a naval constructor who helped build them is Lt. Cmdr. Shizuo Fukui, I.J.N.: The Japanese Navy at the End of World War II, 1947.4 Other good references are Erminio Bagnasco: Submarines of World War Two,5 and Commander Richard Compton-Hall, RN: Submarine Warfare: Monsters & Midgets.6
Figure 1 shows the unique asymmetrical cross section of the I-400 class boats
Note that the bridge and conning tower were offset seven feet to port of the centerline, and the huge hangar offset two feet to starboard to compensate. A Japanese quartermaster told me that he had to use a seven degree starboard helm to steer a straight course at 2 knots submerged. In conducting a torpedo attack the skipper had to take into account his larger turning circle to starboard than to port. If the offset conning tower were flooded, the diving officer would have to blow a port tank quickly to provide lateral compensation -- I wouldn't like to face this emergency.
The twin athwartships pressure hulls did not extend the entire length of her double hull; the crew compartment aft reverted to a single pressure hull, while forward the twin torpedo rooms were stacked vertically. This design permitted a sea-kindly external hull shape with good surface stability and a draft of 23 feet. Each torpedo compartment had four 21 inch tubes with ten torpedoes. The test depth of her pressure hull was 328 feet (82% of her 400 foot length).
Another of I-400's noteworthy design features was the long vertical trunk leading down from the conning tower outside the hangar to the control room. With a diving time of 56 seconds, the bridge watch leaped into spectacular action on the command Clear the Bridge! The lookouts had to jump down the conning tower hatch, then hurtle through this long tube to man their diving stations in the control room 25 feet below (Figure 3). To cushion the landing impact a three foot thick canvas hassock was positioned in the control room at the foot of the conning tower ladder. The padding was effective, but so filthy and odorous that we heaved it overboard. Afterwards an unfortunate Japanese sailor who hadn't got the word dropped down from the conning tower with his usual elan and struck the steel deck with a resounding crash. Although clearly distressed he staggered off without comment . . . no weakness would be shown in front of the Americans.
Figure 2 shows I-400's aircraft storage and catapult for her three M6A1 Seiran (Storm from a Clear Sky) torpedo-bombers.2
These specially-designed floatplanes had a length of 35 feet, a wingspread of 40 feet, a range of 654 miles, and a munitions payload of 1800 pounds. Additional fuel and bombs could be carried by jettisoning the floats on one-way missions where the pilots and planes were to be expended. The sleek Seiran bombers, built by Aichi Kokuki at Nagoya, were stowed in the hangar compartment with floats detached and wings and tails folded. A trained team could rig a floatplane for launch with fuel and armament in as short a time as seven minutes.
Accommodations for a crew of 145 were designed into the capacious twin hulls, but her complement when she was intercepted was 213. A Japanese officer told me that she'd actually carried as many as 220 men to facilitate speedy submarine and aviation operations at sea. Within 45 minutes of surfacing her skilled personnel could break out, assemble, fuel, arm and catapult all three aircraft. The I-400's great cruising range enabled her to launch her three bombers within striking distance of targets as far from Japan as San Francisco, the Panama Canal, Washington or New York. All of these missions were considered by the Tokyo naval strategists.
Below the hangar in the starboard twin hull was a special compartment equipped to conduct aircraft engine overhaul and test. An adjacent magazine stored four aircraft torpedoes, fifteen bombs, and gun ammunition; more shells were stored topside in pressure-proof, ready-use lockers handy to the guns. Each of the two engine rooms housed a pair of 1900 horsepower diesels linked through Vulcan hydraulic couplings to drive the twin propeller shafts as shown in Figure 3.
A 1200 horsepower electric motor-generator on each shaft drew electricity from her storage batteries to drive her submerged. With a clean bottom this propulsion plant gave her a top speed of 18.7 knots on the surface and 6.5 knots below snorkel depth.
Figure 4 shows I-400's primitive but effective snorkel, fitted as an afterthought abaft the periscope shears; you can see the exhaust piping branching over the hangar to reach the starboard engine room.
Also visible are the horn-shaped surface search radar, radar detector antennas, and Radio Direction Finder. Note the massive pressure proof binoculars at the forward end of the bridge covering arcs to port and starboard; these were an attempt to compensate with oversized optics for the inadequate performance of Japanese radar.
Meals for her oversize crew were prepared in a galley in the starboard hull, where large steam kettles turned out great quantities of rice. As in all long-range submarines, a four month supply of food was stowed in every cranny, including a layer of crates laid out on deck which the crew walked on until they'd eaten their way through. Supernumeraries slept on the deck wherever they could find a nook, being used to a floor and tatami mat. The oriental style heads were just holes in the decks above sanitary tanks . . . excretory inaccuracy, inevitable in a seaway, guaranteed that no one lingered there. While inspecting the giant I-boats boats Admiral Lockwood, our wartime ComSubsPac, expressed horror at these odiferous "sanitary" arrangements.
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