Sen-toku: Japan’s Underwater Aircraft Carriers

Sen-toku: Japan’s Underwater Aircraft Carriers

Sneaking Up On America … Again

Pearl Harbor was the first. The American West Coast was to be next for a sneak bacteriological attack. Then, bomb New York City and obliterate the Panama Canal. According to the Japanese High Command, that was all to be accomplished early in the conflict—and they were deadly serious about it. By early 1942, Japan had actually started developing the secret weapon technology required.

According to historian Sheena Pearson, Japan’s plan was to hem in American forces and surprise-attack major U.S. cities using the huge submarines capable of carrying bomber aircraft. Pearson said, “The initial plan was for the subs to travel quiet and deep, surface, launch the swift attack on the Panama Canal, recover their aircraft, then dive and head to the next target city … Admiral Yamamoto’s initial plans had bombing attacks on both coasts of the U.S. Their plan was to close the Panama Canal and create civilian panic in major coastal cities.”
Fortunately, as Japan’s unwinnable war bore down on them, it was a given by 1944 that the attacks would be of psychological benefit only. Part of that reasoning came about because of the death of Yamamoto the year before—an interesting aviation story of its own. Thus, when the operation launched in 1945, all submarine personnel were given tokko short swords, a gift representing the ultimate sacrifice. They were not expected to return.

Decades Ahead of Their Time
The Sen-toku I-400 class boats were easily the biggest, fastest and most technologically advanced submarines of their time, and they remained so until Soviet and American nuclear ballistic missile submarines became operational in the mid 1960s.
Four hundred feet long, displacing 5,900 cubic feet with a crew of 145 officers and men, it sported a rubber coating that muffled interior noise and confused enemy sonar. Each sub carried three Aichi M6A1 Seiran (“Mountain Haze”) bombers, completely unknown to Allied intelligence at the time.  Each Seiran could carry a 1,764 lb. bomb/torpedo load a range of 650 miles at 295 miles per hour. A four-man crew could ready a plane for launch in less than 17 minutes inside the sub’s cavernous hull. Launch was via an 85 ft. long, compressed air catapult on the forward deck.
The biggest risk for each sub was to get all three aircraft launched in less than 30 minutes to avoid defensive counter attacks sure to come, considering the closeness of the targets.
The subs were also equipped with eight forward torpedo tubes for short-range target attack, and they had huge fuel tanks that enabled them to travel 37,500 miles—an ability to circle the earth one and a half times. Historian Perry Moore wrote that these subs were “… fast, traveling at 23 knots on the surface or 10 knots submerged. They could submerge to 340 ft. and were powered by two 2400 hp engines.
“The I-400 series had great cruising range which enabled them to launch three bombers within striking distance of targets as far from Japan as San Francisco, Los Angeles, the Panama Canal, Washington, or New York. All of these missions were considered by the Tokyo Naval Strategists,” Moore wrote.

by David Truby

Read the article from the October 2015 issue of Flight Journal, click here.

Updated: November 27, 2017 — 4:56 PM
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