Appendices - Hirohito's War

APPENDIX A: SUBMARINES - THE UNDERSEA WAR

The Planned Submarine Attack on the Panama Canal: In the summer of 1943 Commander Yasuo Fujimori conceived a plan with Captain Chikao Yamamoto to launch an attack on the Panama Canal with the objective of interdicting the flow of matériel from the US industrial heartlands on its eastern seaboard. Fujimori was a graduate of the Imperial Naval Academy in 1928. After a career serving in various capacities on submarines, he had been given command of RO-60 in June 1941. Later that year, two weeks after Pearl Harbor, Fujimori’s medium size L4 Type submarine, copied from the British L52 Type, and launched in 1922, was attacked by Lieutenant David Kliewer in his Wildcat fighter. In spite of its crash dive, Fujimori’s submarine suffered punctures to its diving tanks and periscope from two 100lb bombs. Fujimori quickly concluded that RO-60 could no longer dive safely. In January 1942 Fujirmori took charge of I-121, a largely obsolete Kiraisen Type mine-laying submarine based on a German design, before being sent to the Imperial Naval College where he graduated from ‘A’ Class. He joined the Imperial Navy General Staff in May 1943. Over the next 20 months his plan to attack the Panama Canal was developed in detail.

As a result, when Japan’s four submarine-aircraft carriers, two I-400 Class submarines and two modified A2 Type submarines, arrived at Nanao Wan on 5 June 1945 they found that the Maizuru Naval Arsenal had built a full scale mock-up of the Panama Canal’s Gatun Locks, the whole ‘triple flight’ that lowers ships 86ft before they navigate the passage into the Atlantic. The four submarines, I-400, I-401, I-13, I-14, were capable of carrying an aggregate of 10 aircraft; the Aichi M6A 1 Seiren sea-planes would bomb the Gatun Locks and, by Fujimori’s estimation, put the Panama Canal out of action for six months. It was a plan made possible by the development of Japan’s unique aircraft carrying submarines. First suggested by Admiral Isoroku Yamamoto, Commander in Chief of the Japanese Combined fleet, the I-400 Type had been conceived as a means of making attacks on US cities on their eastern and western seaboards. The plan called for the building of 18 such submarines although the eventual build was scaled back to just three. I-400 was laid down at Kure Dock Yards in January 1943; I-401 followed in April. I-403 was completed just five weeks before the end of the war and never made it to sea.

At 400ft long with a 39.4ft beam and displacing 6,560 tons, the I-400 Type was by far the largest submarine built by any nation in World War II. Its complement of 144 officers and crew was more than double that of the standard US fleet submarine, the Gato Class that was 311ft long with a 27.25ft beam and displacing 857 tons. Moreover the I-400 type was three times the size of the archetypal U-Boat. The Japanese behemoth was not exceeded in size until the US development of nuclear submarines in the early 1960s.

The I-400, with its pressurized hull, built with a unique figure of eight cross section designed to be strong enough to support an on-deck aircraft hangar, carried three Aiichi float planes with hinged wings that could be made combat ready in 45 minutes. The Aiichi Seiran bomber, a plane unknown to Allied intelligence until after the war, had a top speed of 295mph, a range of 650 miles and could carry an 800kg bomb. On top of the 102ft by 11ft cylindrical hanger that was integrated with the off-center conning tower were placed three 25mm cannon for anti-aircraft defense. A massive hinged, watertight door opened onto a 120ft ramp along which the Aiichi Seiran could be catapulted by compressed air. Lieutenant Atsushi Asamura, who was responsible for planning the mooted attack on the Panama Canal reported that the Aiichi Seiran, the only surviving example of which is displayed at the Smithsonian National Air and Space Museum in Washington, recalled that it was “a good performance aircraft. It was a versatile plane since it was both an attack bomber and had long distance range.”¹

Lieutenant Commander Nobukiyo Nambu later noted, “I-401’s maneuverability under the sea was no different than other subs, though it had a greater turning radius on the surface.”² As was the case with most Japanese submarines, the I-401 had a weaker hull than US submarines, and had a dive depth of 330ft versus 400ft for the US Balao Class. Remarkably the I-400 carried enough diesel gas to circumvent the world without refueling; it could make three trips to the US western seaboard without refueling.

However by the time that the aircraft carrier-submarines had assembled for their Panama Canal mission, Okinawa had fallen. As a result it was decided that it was too late to make a meaningful impact on the US supply lines. A disappointed Lieutenant Asamura noted that “I understood the importance of the Panama mission, but the US was on our doorstep and that was more imperative.”³ The Imperial Navy’s high command then toyed with the idea of dropping rats infected with bubonic plague and other diseases on the United States, a strategy that Unit-731, the Japanese Army’s human medical experimentation group, had used to considerable effect in Manchuria. But the plan was rejected by General Yoshijiro Umezu, Chief of the Army General Staff, clearly with an eye to Japan’s impending defeat, who thought that “germ warfare against the United States would escalate to war against all humanity.”⁴ Instead it was decided to use the I-400 force to attack American aircraft carriers at Ulithi Atoll.

The war ended before this attack could be carried out but not before I-13 was attacked and crippled on the surface by Grumman TBM Avengers as she was on her way to the Japanese island garrison on Truk. The I-400 surrendered to a US destroyer whose sailors were staggered by its size. Similarly when Commander Johnson, captain of the USS Segundo, a Balao Class submarine, took the surrender of I-401 he marveled at a Japanese submarine that was 25 per cent longer and displaced twice the tonnage of his own command. At 5am on 31 August 1945 Commander Nambu surrendered two Japanese samurai swords to Lieutenant JE Balson and a US flag was hoisted on I-401. The less compliant Captain Ariizumi shot himself in his cabin. Along with the large superfast I-201 and I-203, the aircraft carrier submarines, I-401 and I-14,were taken to Hawaii for inspection before being sunk when the Soviet Union demanded access to them.

Whatever their technological sophistication the carrier-subs were not comfortable at sea; a US seaman from USS Blenny who crewed one of the carrier-submarines back to Hawaii complained that “Because of the conning tower being off center to make room for the hangar they were very lopsided and bounced like a cork in rough seas. We started out in a terrific storm and didn’t serve meals for three days because almost everyone was seasick.”⁵

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