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

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It was seven minutes before midnight on August 28, 1945, when a large unidentified object appeared on the radar screen of USS Segundo, a Balao-class submarine on patrol south of Japan. It had been 13 days since Japan’s sur­ren­der an­nounce­ment, and Segundo’s commanding of­fi­cer, Lieu­tenant Commander S.L. John­son, was on the lookout for remnants of Japan’s naval fleet. Segundo was 18 days out from Midway, and except for an en­coun­ter with a Japanese fishing boat, the patrol had been uneventful.

Soon after Segundo changed course to intercept the blip, Commander Johnson and his men realized they were on the trail of a Japanese submarine. After tracking the sub for more than four hours, Johnson tired of the cat-and-mouse game and radioed for it to stop, receiving a positive acknowledgement in reply. But as Segundo closed in, Johnson and his crew were literally in for a big surprise.

The vessel 1,900 yards off their bow was not your average Japanese submarine; it was I-401, flagship of the I-400 class known as Sen-Toku, or special submarines. At the time I-400s were the biggest submarines ever built, and they would remain so for nearly 20 years after the war. The sub Commander Johnson intercepted simply dwarfed Segundo.

Johnson and his men were about to discover that they’d happened upon one of the war’s most unusual and innovative weapon systems. Not only was I-401 bristling with topside weaponry, the sub was also designed to carry, launch and retrieve three Aichi M6A1 Seiran floatplane attack bombers. In other words, I-401 wasn’t just a major offensive weapon in a submarine fleet used to playing defense—it was actually the world’s first purpose-built underwater aircraft carrier.

Japan’s I-400 subs were just over 400 feet long and displaced 6,560 tons when submerged. Segundo was nearly 25 percent shorter and displaced less than half that tonnage. Remarkably, I-400s could travel 37,500 nautical miles at 14 knots while surfaced, equivalent to going 1½ times around the world without refueling, while Segundo could travel less than 12,000 nautical miles at 10 knots surfaced. I-400s carried between 157 and 200 officers, crew and passengers, compared to Segundo’s complement of 81 men.

Originally conceived in 1942 to attack U.S. coastal cities, the I-400 subs and their Seirans were central to an audacious, top-secret plan to stop the Allies’ Pacific advance by disguising the floatplane bombers with U.S. Army Air Forces insignia and attacking the Panama Canal. It was a desperate, Hail Mary–type mission to slow the American advance in the closing days of World War II. However, when the giant subs were finished too late in the war to be effective in stemming the Allied tide, they were reassigned to attack U.S. carrier forces at Ulithi Atoll, the launch point for a devastating air campaign against Japan in preparation for Operation Olym­pic, the planned invasion of the island nation.

But Commander Johnson and his men did not know any of this at the time because the United States was unaware that Japan had underwater aircraft carriers and knew little about its powerful attack bombers. As a result, when Johnson got a good look at I-401, he marveled at the “latest thing in Jap subs.”

After I-401 and its sister sub, I-400, surrendered in August 1945, U.S. officials were similarly staggered by their size, long-range capability and ability to carry and launch floatplane bombers. The Allies had nothing comparable in their fleet. Had the I-400s been built just six months earlier and succeeded in their mission, they could have thrown a major wrench into the Allied advance, giving Japan valuable time to regroup and rearm.

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

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