Members of the US Navy inspecting the plane hangar of I-400.
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 Segundoclosed 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-400class 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 dwarfedSegundo.
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-401bristling with topside weaponry, the sub was also designed to carry, launch and retrieve three Aichi M6A1 Seiran floatplane attack bombers. In other words, I-401wasn’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-400subs 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.
The Smithsonian National Air and Space Museum, which has restored the last surviving Aichi M6A1 (see “Restored,” P. 58 in the print version of Aviation History, May 2008 edition), calls the I-400–class subs and their Seirans “an ingenious blend of aviation and marine technology.” In other words, it was a state-of-the-art sub with a similarly sophisticated plane designed to inflict serious damage.
The I-400s boasted a maximum speed of 18.75 knots surfaced, or 6.5 knots submerged. They could dive to a depth of 330 feet, shallower than most U.S. subs at the time, and had a draft of 23 feet—fairly deep but hardly surprising given the sub’s size.
Nevertheless, the I-400s were to submarines what the Yamato class was to battleships. They carried Type 95 torpedoes, a smaller version of the Type 93 Long Lance torpedoes, the most advanced used by any navy in the war. The oxygen-powered 95s traveled nearly three times farther than the American Mark 14s, carried more explosive punch, left virtually no wake and were the second fastest torpedoes built during the war (Type 93s were the fastest). They were launched from eight 21-inch forward torpedo tubes, four on each side (two upper and two lower). Unlike U.S. subs, I-400s had no aft torpedo tubes, which could prove a shortcoming in certain situations, but topside they were all business, with one 5.5-inch rear- facing deck gun, three triple-barrel 25mm anti-aircraft guns on top of the aircraft hangar and a single 25mm gun on the bridge.
The most innovative aspect of the I-400 subs, however, was their role as underwater aircraft carriers. Each packed three Seirans in a huge, 115-foot-long watertight hangar that projected from the bridge structure onto the deck. The hangar was so large that the conning tower had to be offset seven feet to port of centerline to accommodate it. The hangar in turn was offset two feet to starboard to compensate for its size. A massive hydraulic hangar door opened onto a 120-foot-long compressed-air catapult that launched the Seirans. A collapsible hydraulic crane lifted the planes back on board for hangar storage. It was the unusual, bulbous shape of I-401‘s hangar that especially captured the interest of Johnson and his men.
In a recent interview at his son’s home outside Tokyo, Lt. Cmdr. Nobukiyo Nambu, who captained I-401, said the I-400subs were maneuverable for their size. “I-401‘s maneuverability under the sea was no different than other subs, though it had a greater turning radius on the surface,” recalled the 97-year-old, who is surprisingly tall for a submarine captain and still maintains an erect bearing.
Born in 1911, Nambu is a living history lesson. Though he walks with a cane and is hard of hearing, he recently authored a successful book about his adventures aboard I-401. His navy career began with a scholarship to Etajima, Japan’s naval academy, attending submarine school and graduating as a member of class number 62. Nambu served as the chief torpedo officer on I-17 during the Pearl Harbor attack and later shelled Santa Barbara, Calif., in February 1942, an incident that became the basis for Steven Spielberg’s movie 1941. After the war he served in Japan’s Maritime Self Defense Force, achieving the rank of rear admiral.
Lieutenant Muneo Bando, Nambu’s chief navigator and a sometime observer aboard a Seiran, remembered I-401 as harder to navigate than a smaller sub. He said the big boat required one kilometer to stop and the crew experienced a 30-second delay in response to steering commands. But I-400s gained a reputation for riding smoothly in rough seas due to their double hull construction—essentially two large steel tubes laid side by side.
The I-400s were specifically designed as underwater aircraft carriers to support the M6A1 Seiran, designed by Aichi’s chief engineer, Toshio Ozaki, and built in the company’s Nagoya factory. The Seiran was intended to strike directly at the U.S. mainland. Unlike previous sub-based aircraft designed for reconnaissance or defensive measures, it was a purely offensive weapon built to command respect.
In the book I-400: Japan’s Secret Aircraft-Carrying Strike Submarine, Lieutenant Tadashi Funada, a test pilot who flew the first Seiran prototype, is credited with naming the aircraft. The name Seiran is composed of two Japanese words that can be translated as “storm out of a clear sky.” According to the authors, Lieutenant Funada’s hope was that the bomber would gain the key element of surprise by suddenly seeming to appear out of nowhere.
Aichi completed the first Seiran prototype in the fall of 1943, and the Imperial Japanese Navy was happy enough with the result to order production to start immediately. The original production goal of 44 aircraft was eventually reduced to 28 (including two M6A1-K trainers) due to the plane’s cost and war-driven material shortages, not to mention two major earthquakes and relentless bombing by B-29s, both of which damaged Aichi’s Seiran factory.
Former Lieutenant Atsushi Asamura, the leader of Squadron Number 1, which was responsible for the planned attack on the Panama Canal, confirmed the difficulties surrounding Seiran production. Interviewed in his Tokyo apartment, the 86-year-old former pilot said, “The Seirans that were custom built were of good quality, but as they scaled back production the quality became poor due to material shortages and difficult manufacturing conditions.” In fact, many of the Aichi employees responsible for building the Seirans were high school students.
Nevertheless, Lieutenant Asamura, who remains fit and speaks in a strong voice, recalled the Seiran as “a good performance aircraft,” confirming its reputation as streamlined and responsive, with excellent attack power. “It was a versatile plane since it was both an attack bomber and had long distance range,” Asamura said, illustrating the Seiran’s easy handling by holding his arms out like wings, then grabbing an imaginary stick. “But there was no big difference in how it handled a sea landing compared to other planes.”
Asamura also recalled that the Seiran’s liquid-cooled engine provided pilots with much better visibility than the bulkier and more common air-cooled engines in use at the time. The Atsuta 30 series 12-cylinder inverted Vee engine (Japan’s version of a German Daimler-Benz DB 601A) delivered 1,400 hp, and its liquid-cooled design meant it didn’t need as much warm-up time as an air-cooled engine, so the plane could launch faster. Given the danger subs faced on the surface, this was a distinct advantage.
The Seiran featured a metal frame construction with a riveted metal fuselage and triple-blade propeller. It required a crew of two: a pilot and an observer who sat in a tandem configuration. The observer served as radio operator and navigator, also manning the flexible rear-facing 13mm machine gun, which flipped up from a recess in the fuselage and locked into place for firing. The aircraft carried either a 551-pound bomb with its floats attached or a 1,764-pound bomb (or torpedo) without floats. The heavier ordnance meant that the pilot would have to ditch the plane upon his return, or it was a one-way suicide mission.
By necessity, the Seiran had hydraulically folding wings similar to the Grumman F6F Hellcat’s that rotated 90 degrees to ensure the aircraft fit inside its small, tubelike hangar, which was only 11 feet 6 inches in diameter. Part of the horizontal stabilizer and the tip of the vertical stabilizer also folded down to accommodate the tight fit. The plane’s floats were detachable and stored separately, as were their support pylons and spare parts.
One of the key requirements of the Seiran was that it could be rolled out on a dolly, assembled by its ground crew and launched in a very short time. Reports vary on how fast this could be accomplished. According to Commander Nambu, intensive training enabled the I-401 crew to launch three planes within 45 minutes. But Nambu also noted that given the rough handling the Seirans received during sea launches and landings, it was difficult to keep all three in good operating condition at the same time.
The Smithsonian notes the Seirans had “interesting design features built in…that ranged in engineering quality from the ingenious to the seemingly absurd.” The fact that some of the floatplane’s parts were painted with luminescent paint for night assembly certainly has to fall into the former category. Lieutenant Asamura claimed the Seiran cost “50 times more than a Zero to produce,” and though it’s not possible to confirm the exact cost, clearly they were expensive to manufacture.
Although some German and British subs had carried reconnaissance aircraft on their decks during World War I, Japan was the only nation to use submarine-launched aircraft in WWII. At the beginning of the war, it had approximately 63 oceangoing subs, 11 carrying one catapult-launched reconnaissance plane each. Eventually, Japan would expand this to a total of 41 aircraft-carrying subs.
Admiral Isoroku Yamamoto, commander in chief of the Combined Fleet and architect of the Pearl Harbor attack, gets credit for the I-400class of submarines and its Seiran bomber, though I-401‘s Commander Nambu says the actual idea for an underwater aircraft carrier probably originated from lower down in the command structure. Admiral Yamamoto’s vision in 1942 was for the underwater aircraft carriers to launch their Seiran attack bombers against U.S. coastal cities such as Washington, New York, San Francisco and Los Angeles, to deliver a Doolittle-like blow to American morale.
The original plan was to build 18 I-400– class subs, but after Yamamoto was ambushed and killed by Lockheed P-38 Lightnings in April 1943, the guiding hand behind the I-400subs was gone. Construction plans were scaled back to nine subs, due in part to steel shortages. Actual construction began on five subs but was later reduced to three, of which only two (I-400 and I-401) made it into service. A third, I-402, was converted into a fuel tanker and completed in July 1945 but never saw active duty.
Final design plans for the underwater aircraft carriers were finished by May 1942, and construction on the first sub (I-400) began at Kure’s dockyards in January 1943. I-401‘s construction quickly followed. By December 30, 1944, I-400was complete, and I-401 was completed less than two weeks later. Both subs immediately deployed for their shakedown cruises.