I-400 CLASS SEAPLANE CARRIER SUBMARINE
At the beginning of World War II, the Japanese had 11 submarines capable of carrying submarine-borne aircraft; by mid-1945, that number had increased to 27. Among those was a great secret project — a giant submarine that was kept so well hidden that the United States did not discover it until after the Japanese had surrendered. Described as the I-400 class, it was an undersea aircraft carrier with hangar space for three aircraft. It was 400 feet long, displaced 3,900 short tons on the surface, and was capable of cruising 37,500 miles without refueling. Originally 18 of the subs were planned, but because of material shortages that number was later decreased to five. By 1945, three of the I-400-class subs had been nearly finished. Of the three completed — I-400, I-401 and I-402 — two were outfitted as aircraft carriers and one was turned into a supply ship.
The Japanese planned to use their I-400-class submarines and their aircraft for a raid on the Panama Canal. The normal scouting planes would be of no use, so a light submarine-bomber was needed. The Japanese navy asked the Aichi Aircraft Company to provide them with a suitable design. One of the requirements was that the aircraft could be catapult-launched without landing gear, in order to reduce weight and allow for a larger bombload and fuel supply. After the raid had been carried out, the aircraft would return to the submarine and ditch close by, and then the crew would be recovered.
The resulting design was the Aichi M6A1 Sieran (“mountain haze”), the only submarine-based aircraft ever made with offense as its primary objective. Two versions were built, the M6A1 and the M6A1-K. Each was powered by a 1,400-hp Aichi AE1P Atsuta 30, 12-cylinder, inverted-vee, liquid-cooled engine, and both had detachable floats. The aircraft was designed by Norio Ozaki, Yasushiro Ozawa and Morishige Mori. The folding configuration that made it possible to stow the aircraft in a watertight hangar demanded a complex design.
Last Ditch Effort
By the time I-400, I-401 and I-402 were ready for sea, Japan’s wartime situation was desperate. Japan desperately needed to make the attack on the Panama Canal to stem the flow of American war materiel to the Pacific and to boost flagging morale at home.
Japanese plans to attack the Gaton locks on the Panama Canal continued throughout 1945 despite interference from frequent American air raids. In July 1945, the submarine flotilla was brought together for the first time. It consisted of I-400, I-401, I-13 and I-14. The task force was equipped with 10 aircraft, and although the two smaller subs did not have the fuel capacity for the round trip to Panama, they were to refuel from the two larger subs.
The subs were provisioned for a four-month cruise, but time was running out. En route to South America, they were diverted to attack Ulithi Atoll, where some American carriers were anchored. On July 16, 1945, the task force was attacked by carrier aircraft, and I-13 was sunk. The other boats did not press home their attack on Ulithi and were still at sea when the war ended on August 14. Not one of the giant submarines saw action.
Type 22 Radar on Submarines
In wide use by mid-to late 1944, Type 22 radar, while not designed for gunnery control, provided moderately accurate data for this purpose. Type 22 was the type of radar that was most widely employed aboard ships and subs.
The only one of the subs listed that I am 100% certain carried radar is the I-58, the others shown almost certainly did. It is likely that other subs had Type 22 radar installed late in the war. The dates of installation for most of these ships are as of commissioning. Only I-54 and I-56 would have had it installed later in the subs named below.
| 1944 | 7 September | I-58 |
| 16 December | I-13 | |
| 30 December | I-400 | |
| late 1944 | I-54, I-56 | |
| 1945 | 8 January | I-401 |
| 14 March | I-15 | |
| 24 July | I-402 |
Ships: I 400, I 401, I 402
Completed: 1944-45
Decommissioned: 1945
Fate: Scuttled in the Pacific by the US Navy, 1946
General characteristics
Displacement: 5,223 tons / 6,560 tons
Dimensions: 122 × 12.0 × 7.0 m
(400.3 × 39.3 × 23 ft)
Surface propulsion: 4 diesels: 7,700 hp (5.7 MW)
Submerged propulsion: Electric motors: 2,400 hp (1.8 MW)
Surface speed: 18.75 knots (35 km/h)
Submerged speed: 6.5 knots (12 km/h)
Maximum depth: 100 m (330 ft)
Range: 37,500 nautical miles at 14 knots
(69,500 km at 26 km/h)
Complement: 144
Armament:
• 3 Aichi M6A1 Seiran sea-planes
• 8 × 533 mm forward torpedo tubes
• 1 × 140 mm (5.51 in) 40 caliber gun
• 3 × 25 mm 3-barrel machine gun
• 1 × 25 mm machine gun
