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.

Subscribe Today

• Did you enjoy our article?
• Read more in Aviation History magazine
• Subscribe online and save nearly 40%!!!
 

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-400 subs 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.

Help HistoryNet by bookmarking to