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.
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.