The Buzz

Japan Built ‘Underwater’ Aircraft Carriers to Launch a Second Pearl Harbor

Lieutenant Commander Stephen L. Johnson had a problem on his hands; a very large problem. His Balao-class submarine, the Segundo, had just picked up a large radar contact on the surface about 100 miles off Honshu, one of Japan’s home islands, heading south toward Tokyo.  World War II in the Pacific had just ended, and the ensuing cease fire was in its 14th day. The official peace documents would not be signed for several more days, on September 2, 1945, aboard the battleship USS Missouri in Tokyo Bay.

As Johnson closed on the other vessel, he realized it was a gigantic submarine, so large in fact that it first looked like a surface ship in the darkness. The Americans had nothing that size, so he realized that it had to be a Japanese submarine.

This was the first command for the lanky 29-year-old commander. He and his crew faced the largest and perhaps the most advanced submarine in the world. The Japanese I-401 was longer than a football field and had a surface displacement of 5,233 tons, more than three times the Segundo’s displacement. More troubling though was the sub’s bristling weaponry that included a 5.5-inch gun on her aft deck, three triple-barreled 25mm antiaircraft guns, a single 25mm gun mounted on the bridge, and eight large torpedo tubes in her bow.

The large sub displayed the mandatory black surrender flag, but when the Segundo edged forward, the Japanese vessel moved rapidly into the night. The movement and the continuing display of the Rising Sun flag caused concern.  Johnson’s vessel pursued the craft that eventually slowed down as dawn approached. He brought his bow torpedo tubes to bear on the craft as the two vessels settled into a Mexican standoff.

Johnson and his crew had received permission by now to sink the reluctant Japanese vessel if necessary, but he realized he had a career-boosting and perhaps a technologically promising prize in his sights. Much depended on this untried American submarine captain and his wily opponent in the seas off Japan.

Little did Johnson know that the Japanese submarine was a part of the I-400 squadron, basically underwater aircraft carriers, and that the I-401 carried Commander Tatsunosuke Ariizumi, developer of the top-secret subs initially designed to strike the U.S. homeland in a series of surprise attacks. Ariizumi was considered the “father of the I-400 series” and a loyal follower of the emperor with years of experience in the Japanese Navy, so surrender was a disgrace he could not endure.

Johnson also had to contend with Lt. Cmdr. Nobukiyo Nambu, skipper of the I-401, who traced his combat experience back to Pearl Harbor. He now commanded the world’s largest submarine designed to carry three state-of-the-art attack planes in a specially built hanger located atop the vessel. These secret Aichi M6A1 planes were initially designed for “a second Pearl Harbor” or another surprise attack, possibly even against New York City or Washington, D.C. The I-400 series submarines were themselves full of technological surprises.  They was capable of traveling around the world one and a half times without refueling, had a top surface speed of 19 knots (or nearly 22 miles per hour), and could remain on patrol for four months, twice as long as the Segundo.

Neither Nambu nor Commander Ariizumi readily accepted the emperor’s surrender statement when it was broadcast on August 15. The subsequent communiqués from Tokyo were exceptionally confusing, especially Order 114, which confirmed that peace had been declared but that all submarines were to “execute predetermined missions and attack the enemy if discovered.”

The I-400: Weapon For a Second Pearl Harbor: 

It was Admiral Isoroku Yamamoto, commander of Japan’s Combined Fleet and developer of the Pearl Harbor attack, who called for the construction of the I-400 series some three weeks after Pearl Harbor. The insightful Yamamoto, who was later killed when his aircraft was shot down by U.S. fighter planes, had toured the United States years before and had warned against a prolonged war with the highly industrialized United States.

However, once Japan was committed to war, he believed that submarine aircraft carriers dropping bombs “like rain” over major U.S. cities would surely cause the American people to “lose their will to fight.” A second surprise attack with even more to come would prove psychologically devastating to the Americans, Yamamoto believed, and perhaps would be the best way to get the Americans to sue for peace.

The Japanese had previous experience with plane-carrying submarines, but these were float planes used largely for reconnaissance. The float planes could be easily shot out of the sky by American fighters, and each submarine carried only one plane, hardly enough to prod the Americans to the negotiating table. Yamamoto always thought big, and he called for a submarine that could travel 40,000 nautical miles without refueling, or nearly four times the range of a Balao-class submarine like the Segundo.

In addition, the I-400 series submarines would carry 1,750 tons of fuel, food for four months at sea for its crew of 147 to 157 men, and two attack planes with a speed of 220 miles per hour and a range of some 600 miles. The hangar atop the sub would need to be at least 100 feet long to initially accommodate two aircraft and be strong enough to withstand the pressure of the deep and a possible attack from enemy planes when surfaced.

Yamamoto’s I-400-class submarines would displace some 6,560 tons submerged, about three times the displacement of the largest U.S. subs, and would be slightly more than 400 feet long, making them about the size of a small cruiser. Because one of the submarines was larger than a destroyer, it “wasn’t an exaggeration, then, to say that Yamamoto was asking for something akin to a small underwater aircraft carrier,” noted one observer.

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