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Between the world wars several nations experimented with  aircraft-carrying submarines-France, Great Britain, Japan and the United States, while the Soviet Union considered the concept.   Floatplanes,  it was believed,  could provide long range reconnaissance for the submarines, which had severely restricted search capabilities, especially prior to the installation of radar in submarines.

At the beginning of the war in Europe (1939), only two navies had operational aircraft-carrying submarines: the one-of-a-kind French SURCOUF and several Japanese I-boats (the term for long-range submarines). The Japanese used submarine-launched floatplanes extensive! y in 1941-1942 to scout out enemy anchorag-es, often in cooperation with fleet and midget submarine attacks. And, in August 1942 the submarine I-25 launched a two-man, E14Y Glen floatplane on two nighttime incendiary raids over Oregon forests.

The 1-25 was one of 20 large scouting submarines of the B1 type; each of these long-range boats, displacing 2,198 tons standard surface displacement, could carry a single floatplane. There was an aircraft hangar and catapult forward of the bridge (with a 5.5-inch gun and lighter weapons aft). Several larger scouting submarines with a single aircraft capacity were built during the war. Two larger, AM-type submarines of 2,620 tons standard surface displacement were completed in 1944-1945; these could each accommodate two floatplanes. But all of these boats would be dwarfed by the 1-400 class. 1

In 1942 the Japanese Navy initiated the 1-400 submarine class SEN-TOKU or Special Submarines with the designation STo. They were intended specifically for the bombing of Washington, DC and New York City. But while the first units of the class were still under construction, that plan was discarded because of the direction the Pacific War was taking and it was intended to use the I-400s against the Panama Canal, to halt further U .S. rein-forcements to the Pacific area. 2

The 1-400 was the largest undersea craft ever built prior to nuclear propelled submarines. The original design of 1942 provided for a surface displacement of 4,550 tons with a hangar for two seaplanes. However, the design was enlarged to handle three floatplanes plus parts for a fourth, which could be assembled on board. The deck structure was to be similar to the AM design, with a catapult forward of the hangar, having a 85Vafoot track slightly offset to starboard. The aircraft could be pre-warmed in the hangar while the submarine was submerged through a system circulating heavy lubricating oil. The submarines, of course, would have to surface to launch their aircraft. For recovery there was a collapsible crane fitted forward.

The submarine design was unusual with a modified figure-8 configuration forward, evolving into a horizontal figure-8 amidships. This permitted the submarine to have two forward torpedo rooms, one above the other, while accommodating four diesel engines, paired side-by-side amidships. They carried sufficient diesel fuel to cruise farther than any other non-nuclear submarine ever built, and could embark supplies for a 90 day mission (see table}. In addition to carrying avgas for the aircraft, the submarine’s magazines could hold four aircraft torpedoes, three 1,760 pound bombs, and twelve 550 pound bombs.

The massive conning tower, offset to port above the aircraft hangar, mounted a snorkel, a Mark 2 radar (not a particularly reliable set although it was said to be able to detect aircraft under ideal conditions at ranges of 60 miles}, and elementary radar warning equipment. Internally the submarines were fitted with accommodations and communications to serve as squadron flagships.

A streamlined, low-wing floatplane was developed specifically for operation from the 1-400 submarines. This was the Aichi M6A 1 Seiran (Mountain Haze}, a two place, high speed aircraft with an unusual twin float configuration. The floats could be jettisoned intlight for possible suicide attacks against the Panama Canal locks or Allied warships. The aircraft was developed in total secret, as were the submarines, and hence there was no Allied code name for the aircraft. With a top speed of 295 mph at 17,000 feet, the M6Al could carry bombs or an aerial torpedo.

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The Japanese Navy planned to build 18 submarines ofthe 1-400 class with construction given a high priority. The lead ship was laid down on 18 January 1943. Changing priorities and produc-tion limitations led to only five units being laid down, of which four were launched in 1944, the 1-400, 1-401, 1-402, and 1-404. Three would be completed with work on the 1-404 being halted in March 1945 when 90 percent complete (she was later sunk at Kure by U.S. carrier aircraft).

The 1-400 was completed in December 1944 and the 1-401 the following month. Along with the aircraft-carrying 1-13 and 1-141 (AM type), they were to form Submarine Squadron 1 under Captain Tatsunoke Ariizumi. But the underwater monsters could not be sent on a mission because of delays in producing their M6A1 aircraft, caused by U.S. bombing of the Aichi aircraft factory in Nagoya. And pilot training further delayed operations. The 901st Air Group was formed specifically to train 1-400 pilots and crewmen.

Even then the 1-400 and 1-401 had to sail to Dairen, Manchuria in mid-April to take on fuel, which was desperately short in the home islands. (‘The 1-402 was modified before completion to a tanker configuration to carry fuel from the East Indies to Japan, but the war ended before she undertook a tanker mission.)

After fueling, while enroute to the Inland Sea for further pilot training, the 1-401 hit a magnetic mine laid by a B-29 bomber and had to put into Kure for repairs. Finally, flight operations began. One plane crashed at sea and another into a mountain. Practice reduced the time to unfold a Seiran ‘s wings and ready the plane for flight, in darkness, to less than seven minutes. The crews practiced until a submarine could surface, prepare the three aircraft, and launch them in 45 minutes. Although this was a long time for the submarine to be exposed, even at night, it was a remarkable achievement. (‘The submarines could dive to periscope depth in just under a minute.)

Drills were conducted against models of the Panama Canal locks. The four submarines of SubRon-1 were to carry a total of ten aircraft for the strike, six carrying a torpedo and four a 1, 760 pound bomb. The war was moving too rapidly toward Japan for even the Panama Canal attack to be undertaken. Instead, the 1-400 and 1-401 would attack the U.S. anchorage at Ulithi Atoll in the western Caroline Islands, launching a six plane strike.

The 1-400 and 1-401 sortied from the Inland Sea on 26 July 1945 with Captain Ariizumi riding the 1-401. The raid on Ulithi was planned for 17 August, the planes to be catapulted aloft in the predawn darkness.

The war in the Pacific ended on 15 August. The surrender order was received aboard the at-sea submarines in reverent silence. On 20 August, Ariizumi was ordered to destroy all offensive weapons, raise the black flag of surrender, and return to port. The 1-400 and 1-401 fired all of their torpedoes; the 1-400 pushed her planes over the side, and the 1-401 catapulted her three planes, sans pilots, into the sea. Ariizumi considered scuttling his flagship, the 1-401, but decided that he would not sacrifice his crew. Instead, as the 1-400 and 1-401 approached Yokosuka, he placed his pistol against his temple and took his own life. The tanlcer submarine I-402 returned to Kure and was then moved to Sasebo.

The giants of the 1-400 class had never fired a weapon in anger. At Yokosuka the 1-400 and 1-401 were minutely examined by U.S. intelligence officers and submariners. A distinguished British historian noted:

“The American officer first on board the 1-400, Joe Vasey Oater Admiral), found the monster submarine “incredibly filthy, with a layer of grease and leftover food on the decks … the stench was almost unbearable, particularly near the (oriental style) heads, where one of our party lost his breakfast as he was hovering over the sanitary tank opening. That well-known paper product was conspicuous by its absence …but despite the unhygienic conditions the physical appearance of the crew was remarkably good . Everyone seemed to be lean and alert. “3

The 1-402 was the first to be sunk, being scuttled off Gato Island on 1 April 1946. She was first used as a target for U.S. destroyer gunfire. A total of 24 Japanese submarines were sunk that day (and nine more on 5 April).

The 1-400, 1-401, and slightly smaller 1-14 as well as several smaller submarines were sailed to Guam and then to Pearl Harbor by U.S. crews for further examination. They too were then scuttled in 1946 ending a remarkable era in submarine developent.

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