Japan’s Panama Canal Buster
By John Geoghegan; research assistance by Takuji Ozasayama

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By 1945 there was little doubt among the Japanese that the war was going badly. If Germany was defeated, the Allies would be on their doorstep next. The Panama Canal was a major transshipment point for war materiel essential to the Pacific theater. Closing it off would slow down if not stop the Allied advance, which would give Japan much-needed breathing room. As a result, the plan to at­tack the canal, drain Gatun Lake and block Allied shipping made strategic sense.

Japanese engineers had helped to build the canal, so Japan had construction plans to work from. Since Japanese carriers couldn’t get close enough to at­tack without being discovered, the 1st Sub­­marine Flotilla and 631st Koku-tai were selected for the task.

The four submarines were to leave Japan in June 1945 and surface 100 miles off the coast of Ecuador, where they would launch their 10 Seirans at night. The Seirans, painted to resemble U.S. Army Air Forces planes, would fly northeast over Colombia, turn west over the Caribbean, then attack from the north at dawn, torpedoing the Gatun locks. After returning to their launch point, the pilots would ditch their planes and swim to their respective subs.

Before I-400 and I-401 crews could begin training for the mission, however, the Japanese had to deal with a severe fuel shortage resulting from the Allies’ sinking their tankers. The I-400s did not have enough die­sel to complete their mission, so I-401, disguised as a frigate with a false funnel, was ordered to Manchuria to get more fuel. On April 12, shortly after departure, the sub was damaged by a mine and had to return to port for repairs, but I-400 was sent in its place and returned with the necessary fuel.

By June 4, the sister subs had arrived in Nanao Bay for battle training. There the crews practiced speeding up the assembly of the Seirans, night catapult launches, and submerging and surfacing the submarines in preparation for launches.

“The sub’s pitching and rolling made catapult launches difficult; the navigator had to time it just right,” Lieutenant Asamura remembered. “Nevertheless, compressed air made it a smoother launch than catapults that used gunpowder.” Asamura also recalled the importance of launching against the wind to make sure the Seiran got enough lift. As a result, he said, “It could be dangerous if the wind direction changed on you during a catapult launch.”

A full-scale mockup of the Gatun locks was constructed to practice Seiran torpedo runs, but training conditions proved ex­tremely difficult. The I-400s had to deal with relentless Allied bombing and strafing as well as heavily mined waters. There were not enough experienced pilots for the mission, and two Seirans were lost during training. In fact, only one pilot had the requisite torpedo experience, so it was decided the Seirans would carry a single large bomb instead of a torpedo. To ensure success, the pilots would fly their aircraft directly into the locks rather than risk inaccurate bomb drops.

Born in Osaka in 1922, Asamura now lives in Tokyo’s Nezu section in a high-rise apartment with his wife. A small, balding man, he has an interest in history and a fair understanding of English.

Asamura remembered that for the pilots, “life on a submarine was 180 degrees different than flying in the air. You couldn’t tell night from day on the sub, so I never knew what meal I should be eating.” But he also noted that though they ate canned rather than fresh food, there was enough to go around, which often wasn’t the case for the Japanese army. Pilots had no duties to perform on the sub, and he recalled that crew relations were good.

Asamura said the Panama Canal mission was an open secret among I-401’s crew. But with the U.S. already positioning an enormous armada of ships, aircraft and troop transports in the Pacific for the planned invasion of Japan, the Japa­nese navy’s high command decided the Seirans should attack U.S. carriers at Ulithi Atoll instead of the canal.

Tags: Aviation History, Flight Technology, Military Technology, Sea-Air Operations, World War II

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