The Sen Toku I-400-class (伊四〇〇型潜水艦) submarines of the Imperial Japanese Navy were the largest submarines of World War II, and the largest ever built prior to the development of nuclear ballistic missile submarines in the 1960s. These were submarine aircraft carriers and each of them was able to carry 3 Aichi M6A Seiran aircraft underwater to their destinations. They were designed to surface, launch the planes then dive again quickly before they were discovered. They also carried torpedoes for close range combat.

The I-400 class was designed to travel round-trip to anywhere in the world. A fleet of 18 boats was planned in 1942, and work started on the first in January 1943 at the Kure, Hiroshima arsenal. Within a year the plan was scaled back to five, of which only three (I-400 at Kure, and I-401 and I-402 at Sasebo) were completed.

Characteristics

They were able to carry three Aichi M6A Seiran aircraft, which were built for the purpose of shooting down the sun, each carrying an 800 kilogram (1,764 lb) bomb 650 miles (1000 km) at 295 miles per hour (474 km/h). The existence of the Seiran was unknown to Allied intelligence. The wings of the Seiran folded back, the horizontal stabilizers folded down, and the top of the vertical stabilizer folded over so the overall forward profile of the aircraft was within the diameter of its propeller. When prepared for flight, they had a wing span of 40 feet (12 m) and a length of 38 feet (11.6 m). A crew of four could prepare and get all three airborne in 45 minutes. The planes were launched from a 120-foot (37-m) catapult on the deck of the giant submarine. A restored Seiran airplane is displayed at the National Air and Space Museum in Washington, D.C.. Only one was ever recovered and it had been ravaged by weather and souvenir collectors, but the restoration team was able to reconstruct it accurately.

Operational history

One of Yamamoto’s plans was to use the sen toku (secret submarine attack), so that in the opening days of 1945, preparations were under way to attack the Panama Canal. The strategy was to cut the supply lines and access to the Pacific Ocean by U.S. ships. The plan was to sail westward through the Indian Ocean, around the southern tip of Africa, and attack the canal’s Gatun Locks from the east, a direction from which the Americans would not expect and were little prepared to defend. The flights would, of course, be one-way trips. None of the pilots expected to survive the attack, a tactic called tokko. Each pilot was presented with a tokko short sword, symbolic of the ultimate sacrifice.

Before the attack could commence from the Japanese naval base at Maizuru, word reached Japan that the Allies were preparing for an assault on the home islands. The mission was changed to attack the Allied naval base on Ulithi where the invasion was being assembled. Before that could take place, the Emperor announced the surrender of Japan.

On August 22, 1945, the crews of the submarines were ordered to destroy all their weapons. The torpedoes were fired without arming and the aircraft were launched without unfolding the wings and stabilizers. When I-401 surrendered to an American destroyer, the U.S. crew was astounded at its size. The commander of the submarine fleet, Captain Ariizumi, apparently decided on suicide rather than surrender to the Americans. He requested that his body be wrapped in the Japanese flag and buried at sea and shot himself. His body was never presented as proof of his death.

American inspections

The U.S. Navy boarded and recovered 24 submarines including the three I-400 submarines, taking them to Sasebo Bay to study them. While there, they received a message that the Soviets were sending an inspection team to examine the submarines. To keep the technology out of the hands of the Soviets, Operation Road’s End was instituted. Most of the submarines were taken to a position designated as Point Deep Six, about 40 miles (60 km) west of Nagasaki and off the island of Gotō Islands, were packed with charges of C-2 explosive and destroyed. They are today at a depth of 200 meters.

Four remaining submarines (I-400, I-401, I-201 and I-203 were sailed to Hawaii by U.S. Navy technicians for further inspection. Upon completion of the inspections, the submarines were scuttled in the waters off Kalaeloa near Oahu in Hawaii by torpedoes from the American submarine USS Cabezon on May 31, 1946. The reason for the scuttling is apparently that Soviet scientists were again demanding access to the submarines. The wreckage of I 401 was re-discovered by the Pisces deep-sea submarines of the Hawaii Undersea Research Laboratory in March 2005 at a depth of 820 meters.

Post-War influences

The hulls of modern nuclear submarines do not feature the figure-eight shape of the sen toku, but were based on the shape of the German Walther boats that were developed toward the end of the war. The Germans themselves based their design on the shape of dolphins. The Germans also experimented with rockets that were launched from U-boats and devised plans for using V-2 rockets against the United States. (U-boat Rocket Program)

See also

• AM type submarine - IJN 2-aircraft submarine seaplane tender
• Submarine aircraft carrier
• French submarine Surcouf

External links

• About I-400
• Discovery of I-401 wreckage off Hawai
• Hawai Undersea Research Laboratory report
• A Damn Interesting article
• Story of the transpacific voyage of I-400
• Japan's WWII Monster Sub: How the deadly Sen-Toku mission almost succeeded
• The Sen-Toku Raid — review of novel by John Mannock about I-400-class submarine
• I-400