Japan’s Special Submarines
By Mike Bennighof, Ph.D.
July 2008

Japanese interest in aircraft-carrying submarines grew directly out of the Imperial Navy’s perceived strategic tasks. Japan’s plans for war against the United States called for submarines to wear down American fleet strength before a climactic battle somewhere in the Western Pacific. Submarines would also find and track the proigress of the approaching enemy fleet.

Submarines equipped with seaplanes would have a much better chance of performing these tasks. Experiments with seaplane-carrying submarines began in 1916, when the Royal Navy used E22 to carry a pair of Sopwith Schneider floatplanes out into the North Sea to intercept German zeppelins. After one successful trial, and one failure, the British abandoned the scheme. The seaplanes were simply mounted on deck, and when it came time to launch them, E22 submerged.

The U.S. Navy experimented with something closer to a submarine aircraft carrier in the mid-1920s, fitting S1 with a small hangar to carry an MS-1 floatplane. The aircraft would be disassembled and stowed in the hangar, and brought out and put back together for launch. After trials in 1926 the project was declared a failure, in large part because the tiny cylinder used as a hangar could only provide enough fuel for 15 minutes of flight.

Rigging Surcouf’s seaplane for launch.

The French Navy expanded on this concept with the massive submarine commerce raider Surcouf. The world’s largest submarine at the time, Surcouf mounted a turret with two eight-inch guns, 10 torpedo tubes, and carried a small seaplane in a hangar at the rear of the conning tower. Extra large fuel tanks, a large magazine for ammunition, and a special brig for up to 40 prisoners completed the outfit. Built under the 1926 program, she remains one of the world’s best-known submarines and has been the subject of many bizarre rumors.

Surcouf’s hangar arrangement inspired Japanese designers, and the Imperial Navy laid down the aircraft-carrying submarine I-5 in 1930. In place of her after deck gun, she carried two small cylindners, one with the wings of a seaplane, the other with the fuselage and floats. The aircraft would then be assembled and launched from the aft deck by means of a catapult.

Other than the seaplane arrangement, I-5 followed conventional principles, with a design derived directly from Germany’s U-142. She was the first of 47 aircraft-carrying submarines commissioned by the Imperial Navy, but the complicated aircraft assembly procedure finally led to the gear being removed in 1940 and replaced by a deck gun.

Throughout the 1930s the aircraft arrangement improved, becoming standardized as a small streamlined hangar below the conning tower, opening toward the bow where a catapult would launch the seaplane. All of these were large submarines, displacing over 2,000 tons standard (compared to 749 for the German Type VIIC u-boat, or 1,526 for an American Gato fleet submarine). Japan’s largest air-capable submarine, the giant I-400, appears in our new Leyte Gulf game.

In 1943 Japan laid down four much larger submarines, the Type AM boats. Only two would be completed, I-13 and I-14. These displaced 2,600 tons and could carry two seaplanes. However, their huge size was matched by poor underwater performance, and all of the aircraft-carrying submarines proved very vulnerable to American anti-submarine forces.

The big Type AM design provoked a suggestion from the Combined Fleet’s brilliant strategist, Admiral Isoroku Yamamoto. Could a submarine be built with the range and aircraft-carrying capability to attack the Panama Canal? American industrial capacity outstripped that of Japan to an enormous extent, but most of it lay on the East Coast. If American reinforcements had to make the lengthy journey around South America, Japan’s chances of a favorable settlement might be greatly enhanced.

I-401 with seaplane in launch position.

So began the design of the Sensuikan Toku, or special-type submarine, the world’s largest until U.S. Navy’s Polaris-carrying nuclear vessels of the early 1960s. Of the six boats begun, only three would be completed, I-400, I-401 and I-402. Twelve more were cancelled before their keels could be laid.

The 400-foot Sensuikan Toku displaced 3,510 tons standard. It had a large hangar under the conning tower, offset to port with the bridge offset to starboard to compensate; as a result the boats proved very difficult to steer. She had twin pressure hulls and a well-designed external form giving good surface stability. She had eight torpedo tubes, and mounted ten 25mm anti-aircraft guns (a single mount on the bridge, and three triple mounts on the 135-foot hangar). Dive depth was only 382 feet, and the huge submarines could not have survived a determined ASW attack for long. The very tall bridge was connected to the control room by a 25-foot tunnel; in case of emergency dive, the bridge crew would leap into this tunnel and hopefully land on a three-foot-thick cushion placed under the lower end for this purpose. She also had a snorkel to allow her to operate her diesels while under water.

Crew quarters were primitive at best; designed for a crew of 140, the boats in practice carried as many as 220 to facilitate rapid flight operations. The extra crewmen slept on top of the hangar on tatami mats when the boat was surfaced. Sanitary facilities consisted of holes cut in the deck plating above tanks that could then be empited with compressed air; frequent accidents assured that the craft stank horribly after only a day or two at sea.

Three M6A1 Seiran floatplane torpedo bombers provided the submarine’s real striking power. These had been specially designed for submarine use, to be assembled and readied for flight quickly. The original specification called for all three to be armed and ready 30 minutes after the submarine surfaced; in practice the crews managed to be ready in 45. The boats included a workshop for overhauling and testing the aircraft engines, and a magazine containing four air-launched torpedoes and 15 bombs.

I-401 and I-400 with the hangar doors open.

The first of the giant boats, I-400, completed in December 1944. She and the two Type AM boats formed Submarine Squadron One, commanded by Captain Tatsunosuke Ariizumi. I-401 and I-402 joined as they completed, forming a potential underwater carrier group of 10 airplanes after I-402 had been converted to carry fuel.

Vice Admiral Jisaburo Ozawa ordered them prepared for Operation PX. The submarine-launched planes would be used to spread germ warfare agents across American West Coast urban areas. General Yoshijiro Umezu, Chief of the Army General Staff, considered this a “crime against humanity” and ordered the operation cancelled. Instead, after some discussion of other targets, Ariizumi’s strike force was ordered to prepare for its original mission: a strike against the Panama Canal.

In March 1945 the submarines moved to Nanao Bay to practice for the strike. A full-scale model of the Gatun Locks was built in Toyama Bay and the pilots practiced torpedoing it. The squadron staff felt very confident of striking their target; wrecking the huge gates at the locks would drain Gatun Lake and render the canal unusable for many months, possibly longer.

By this point, however, so many American warships and transports had already transited the canal that destroying the passage would only inconvenience the Americans. It might delay the American invasion of Japan, but it would not deter it. On 1 July Ariizumi received new orders to bring his squadron to Ulithi, where his carefully-trained pilots would then make a suicide attack against targets of opportunity.

Bringing the monster into port.

Ariizumi raged against the new orders, pointing out that the Americans would more than likely shoot down all his planes before they got close to the anchorage, and even if they broke through, they could inflict only slight damage on the massive fleet gathered there. But the small group could make a spectacular impact in Panama. “A man does not worry about a fire he sees on the horizon,” Sixth Fleet replied, “when other flames are licking at his kimono sleeve.” Ulithi would be the target.

The submarines arrived at Truk in early August, losing I-13 along the way to an American destroyer. I-14 carried two reconnaissance floatplanes, which would scout the anchorage on the 14th. On the 17th, the six Seiran torpedo bombers from I-400 and I-401 would make their suicide runs. But on 15 August Emperor Hirohito made his declaration of Japan’s almost-unconditional surrender. Once again Ariizumi ranted against his orders, but after a council of war decided to obey. First, however, he led all three submarines out to sea where they fired off all their torpedoes and catapulted their empty aircraft into the sea. They then headed for home, surrendering to American warships off the coast of Honshu in late August.

This piece originally appeared in August 2005.

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