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Imperial Japan's Secret 'Underwater' Aircraft Carriers: Crazy Idea or a Game Changer?

Imperial Japan's Secret 'Underwater' Aircraft Carriers: Crazy Idea or a Game Changer?

Meet the I-400. 

On August 14, 1945, Japan’s Emperor Hirohito issued a royal proclamation. It was actually a surrender notice: all Japanese land, sea, and aerial military units were to cease hostilities immediately and comply with all Allied authorities in terms of surrender. For Sub Division One that meant running on the surface displaying a black naval flag designating a ship being surrendered. On August 18, the three remaining Sen Toku subs received word to immediately set the quickest courses for their home ports. They were not to evade Allied or U.S. Navy ships if encountered, and to acquiesce to their directives.

As soon as Colonel James Doolittle’s  B-25 raid struck Japan in April 1942, Japan sought to wreak revenge on the United States, but by 1944 devastating aerial bombings on Japan by the Americans had become all too regular.

It was not until early 1945 that the Imperial Japanese Navy (IJN) was ready to strike America even further than it already had on December 7, 1941. After considering, then ruling out San Francisco, San Diego, New York City, and Washington, D.C. as targets, the IJN chiefs settled on America’s vital Panama Canal. The plan to disable the canal––through which the United States was funneling military resources from the Atlantic to the Pacific without the long voyage around the southern tip of South America––had been the brainchild of Admiral Isoroku Yamamoto, the architect of Japan’s attack on Pearl Harbor.

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The plans called for an aerial bombing by specially designed attack planes launched from surfaced submarines. Those submarines, the Sensuika, or I-400 Series, would be the largest submarines the world would see for decades to come. Loosely translated as “Secret Attack Submarine,” Sensuikan was shortened to Sen Toku.

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In mid-1942, Yamamoto foresaw two things: how susceptible Japan would become to American aerial bombing and how Japan could reciprocate against American soil. From that he envisioned 18 huge submarines––basically underwater aircraft carriers––that could ferry attack bombers to their targets. Although Yamamoto’s plan envisioned two planes per submarine to attack America’s shoreline cities, in actuality each I-400 was designed to ferry three Aichi M6A Seiran “Mountain Haze” planes.

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Construction on the Sen Toku behemoths began on April 25, 1943, one week after Yamamoto was shot down and killed by American P-38s over Bougainville in the Solomons. But with Yamamoto’s death and the fortunes of war turning against Japan, there was nobody to champion Yamamoto’s dream.

 

Delays plagued the project from the beginning. The I-400 prototype, built at Hiroshima Bay’s Kure Navy Yard, was not commissioned until December 30, 1944, while both the I-401 and I-402 were laid down shortly thereafter at the Sasebo Navy Yard Docks at Nagasaki. Further, the naval construction facilities at Kure and Sasebo were often targeted by American planes but the damage was insignificant; the American bomber pilots had no knowledge of the giant submarines being built beneath them.

On January 8, 1945, the I-401 was commissioned and, six months later, on July 24, 1945, came the I-402. Once completed and out to sea, these vessels would become Submarine Division One.

 

Of the planned 18 submarines, however, only three would be completed: the I-400, I-401, and I-402; numbers I-404 and I-405 were still under construction when the war ended in August 1945. The remaining proposed fleet of the I-Series––403 and 406-417––was scrapped before construction could begin. In their place came two new submarines––the I-13 and I-14––that were smaller but still held the same design aspirations.

The Sen Toku were beyond comprehension of any navy but that of Japan; they were 60 percent larger than any submarine America would put forth until the nuclear submarine age. The most important part of the I-400s were the planes they sequestered within their watertight forward hangars. Japan had long mastered the art of flying piggy-backed scouting planes off of their surfaced subs.

The sea beast I-400 series measured 400 feet long with a beam of 39 feet and a draft of 23 feet. It was a double-hull configuration that the Soviets would replicate 30 years later. It operated on four diesel engines of 7,700 horsepower with two electric motors as back up. The sub’s surface tonnage was 5,223 tons; when submerged, it weighed 6,560 tons.

Prior to their deployment, the I-400s had retractable snorkels fitted. When they were submerged for extensive periods, fresh air could be taken in while poisonous diesel exhaust fumes would be expelled. The sea-roaming range was a staggering 37,500 miles without refueling. Their fastest surface speed just topped 18 knots; submerged, the speed was reduced to 6.5 knots. The deepest they could safely dive was 330 feet.

The crew, which numbered from 140 to 220 sailors per sub, had the extravagant luxury of a walk-in freezer for storing their on-board rations. Still, potable water was limited to mealtime servings only, and latrine service was less accommodating, with just one “head” per boat available.

The I-400s were well armed. From their eight forward torpedoes tubes, 20 Type-95 torpedoes could be fired. On deck, three triple-barrel 25mm guns and one 25mm single-barrel gun were mounted. An even heavier gun––a 140mm/5.5-incher––was also part of the armament.

Another unique application was a slide. Once ordered to clear the deck for a dive, sailors rushed the hatch of the conning tower. Jumping inside, they slid down a funnel onto a cushioned landing spot, thus cutting the time it took to clear the deck by more than half that of regular subs.

In appearance there was an oddity in the I-400s’ structure. From the bow’s view, the conning tower jutted off-center to the right. This was done to facilitate the 115-foot-long, 12-foot-wide, watertight aircraft hangar that housed the three attack floatplanes specially built by Aichi Aircraft Company of Nagoya just for the I-400 Series subs. Adjoining the hangar was an 80-foot-long pneumatic catapult.

In 1942 the Aichi company was put to work designing these floatplanes that became known as the M6A1 Seiran “Mountain Haze.” Each plane required a crew of two––a pilot and a gunner. The gunner faced rearward in the back seat and manned a 12.7mm Type-2 machine gun.

To save room in the subs’ hangars, the plane’s wings and tail had to be engineered to fold in along the fuselage. Instead of wheels, the Seirans came with two detachable floats. The bomb load came in varied combinations––two 551-pound bombs, one 1,764-pound bomb, or one 1,874-pound bomb. Once in flight, the Seiran could extend over 642 nautical miles/739 standard miles.

To expedite launch preparation, portions of the planes’ exteriors were coated with fluorescent paint, thus allowing the four-man teams readying the planes for flight to work in a minimum amount of light. Once the subs surfaced, a four-man deck crew could launch all three planes within 45 minutes. (After numerous delays, by July 1945, only 28 Seirans had come off the production line––16 short of the intended goal.)

The sole reason for the planes’ existence: to attack mainland America. All that was needed now was a target. Various targets were considered: West Coast cities and even Washington, D.C., were discussed. The idea of attacking the Panama Canal was also considered.

Before attacks on any of those targets became operational, however, Vice Admiral Jisaburo Ozawa set forth the I-400’s first mission. For its short existence, it was known as Operation PX, based on the achievements of Special Unit 731.

Special Unit 731 was a large, permanent-structure complex established in Harbin, Manchuria, that was opened as soon as Japan gained control on the Asian mainland. The facility was disguised as a water-treatment plant, but its real purpose was to research the causes and effects of germ/biological warfare. Their test subjects were thousands of civilian and military prisoners of war. (See the sidebar.)

By the early months of 1945, Japan was hell-bent on bringing some form of warfare upon America’s citizenry. Ozawa’s plan was to have the four subs of Sub Division One—I-400, I-401, I-13, and I-14—sail for America’s West Coast. Once in position they would launch their combined total of 10 Seirans with ceramic canister bombs filled with flea-bearing rodents infected with cholera, typhus, plague, and other pathogens designed to cause widespread illness in the United States. Previous attempts to launch such biological-warfare attacks on China had already been tried and found to be successful. The proposed target was San Diego, California.

Planning for the Panama Canal attack went forward. The birth of that plan came from two individuals with completely contradicting existences. One was a senior Japanese citizen who had once worked on the canal; he freely furnished to Japanese authorities hundreds of pages of personal notes, drafts, blueprints, and so on, that were in his possession.

The second source was less willing. He was an American soldier detained at the infamous Ofuna prisoner-of-war camp near Yokohama—a site that became well known for its excessively brutal, torturous treatment of inmates. This inmate had been stationed at the Panama Canal at the war’s outbreak. With that known to his captors, he was tortured and grilled for all he knew of the canal’s defensive positions. He informed his interrogators that as the war progressed and Japan lost more and more ground, the attentiveness of those guarding the canal had dwindled by the time he left there.