The I-400 class submarine was the brainchild of Admiral
Isoroku Yamamoto, Commander-in-Chief of the Japanese Combined Fleet. Shortly after the attack on Pearl Harbor in December 1941, he conceived the idea of taking the war to the United States mainland by making aerial attacks against cities along the US western and eastern seaboards using submarine-launched naval aircraft. Yamamoto submitted the resulting proposal to Fleet Headquarters on 13 January 1942. Following Yamamoto's death during an inspection tour of the Solomon Islands in April 1943, the number of aircraft-carrying submarines to be built was reduced from eighteen to nine, then five and finally three. 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. 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. The submarine included radar and radar detectors and watertight hanger for three
Aichi M6A Seiran seaplanes. The submarine had a range of 37,500 nautical miles. Construction began on January 18, 1943 and Completed on December 30, 1944 and assigned to Captain Tosho Kusaka. The three I-400 class submarines were far larger than anything ever seen before. She was the world's largest submarine. 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. The
I-400 was equipped with a snorkel, radar, radar detectors, and capacious fuel tanks that gave her a range 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 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 submarines included a unique weapon system: a watertight deck hanger capable of transporting three Aichi M6A1 Seiran floatplanes for offensive operations. These aircraft folded to fit into the 115-foot cylindrical hangar, which was slightly offset to starboard and opened forward to access the catapult. These folding floatplanes could be readied for flight in only minutes and launched by catapult.
It was planned to use the I-400 along to participate in a daring plan to attack the Panama Canal and disable its locks, or even plant to attack American cities. No book to date has tackled this incredible subject matter, and the authors debunk myths and facts of the I-400's wartime service and mission. . In June 1945 the decision was made to switch targets to hit USN anchorage at Ulithi Atoll. The plan was code-named
Arashi (storm) for the I-400 and I-401 to use its Serians on Kamikaze attacks on any carriers based there. The two subs departed Ominato on July 23, 1945. At sea, the sub suffered an electrical fire on August 5th that forced it to surface to repair the damage, but successfully reached their rendezvous point, 100 miles miles south of Ulithi on August 14th, but the I-401 was not there. The strike date was set for August 17th, but Japan surrendered on the 15th.
| Units | 18 (Planned), 3( completed) |
|---|
| 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 |
Aichi M6A1 Seiran
The Aichi M6A Seiran ("Mountain Haze") was a submarine-launched attack floatplane designed for the Imperial Japanese Navy during WWII. It was intended to operate from I-400 class submarines whose original mission was to conduct aerial attacks against the United States. From the late 1920s, the Imperial Japanese Navy had developed a doctrine of operating floatplanes from submarines to search for targets. To equip the submarine aircraft carriers, the Imperial Japanese Navy Air Service requested that Aichi design a folding attack aircraft with a range of 1,500 km (800 nautical miles) and a speed of 556 km/h (300 knots) for use aboard the I-400 submarines but the difficulties in doing so a completely new design was initiated. Aichi's final design, designated AM-24 by Aichi and given the military designation M6A1, was a two-seat, low-winged monoplane powered by a 1,400 hp Aichi AE1P Atsuta 30 engine (a licence-built copy of the Daimler-Benz DB 601 liquid-cooled V12 engine). The original specification dispensed with a traditional undercarriage but it was later decided to fit the aircraft with detachable twin floats to increase its versatility. If conditions permitted, these would allow the aircraft to alight next to the submarine, be recovered by crane and then re-used. The floats could be jettisoned in flight to increase performance or left off altogether for one-way missions. The Seiran's wings rotated 90 degrees and folded back hydraulically against the aircraft's fuselage (with the tail also folding down) and can "fold" down to fit in an 11-foot 6-inch hangar tube
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. The only one was ever recovered Seiran airplane is displayed at the National Air and Space Museum in Washington, D.C.
Writer,
Patrick Ong Tiong Han
September 26th 2010
