The Japanese Army Aircraft Carriers

Yes, you read that correctly. During the Second World War, the Imperial Japanese Army built and operated its own aircraft carriers.

Japanese Army aircraft carrier Akitsu Maru
photo from WikiCommons

In the years leading up to the Second World War, the Japanese government became dominated by militarists who sought to use a policy of conquest to build up a Japan-dominated “Co-Prosperity Sphere” in Asia. Within the military, however, the Japanese Imperial Army and Navy found that they couldn’t get along with each other. Strategically, they had entirely different ideas about Japan’s expansion and how it should be accomplished. The Army wanted to make a massive thrust up through China and into Russia to capture the untapped mineral resources in Siberia. Not coincidentally, this would involve a series of land campaigns in which the Army would play the central role. The Navy, on the other hand, favored a move into southeast Asia towards Australia, which would capture the oil and rubber areas in Indochina and the East Indies. Not coincidentally, this series of island conquests would leave the Navy as the key player.

As a result, both branches simply began preparing for the kind of war it wanted to fight and viewed the other merely as a rival for resources and support. In the course of the war, the Japanese Navy therefore recruited and trained its own ground troops; in places like New Guinea and Iwo Jima where both branches had forces, they operated independently of each other, with their own separate chains of command. Meanwhile the Japanese Army would go so far as to build and operate its own aircraft carriers.

By the end of 1939, the decision was made to seize the oil fields and rubber plantations of the Dutch East Indies, French Indochina, and the British colonies at Singapore and Malaysia. This would require the Japanese Army to carry out large-scale amphibious troop landings. In response, the Army developed and produced a new type of ship—the “assault ship”. These would carry around 25 small landing craft known as Daihatsu, which could be launched from a mother ship to carry troops ashore.

The invading troops would also need air support, and since the Japanese Army did not trust the Imperial Navy to provide this cover, the Generals decided to produce their own aircraft carriers. In 1941 the Army requisitioned two civilian cruise ships, of around 12,000 tons, that had just begun construction and modified them for military use. Not only would they carry 27 Daihatsu landing craft inside their hulls (deployed through a door in the hull), but they would be fitted with a flat flight deck that ran the length of the ship and a hangar area with elevator. They would be christened Akitsu Maru and Nigitsu Maru, and they officially belonged to the Imperial Japanese Army, not the Navy. The sailors were all civilian merchantmen, with Army personnel making up the air crews and the anti-aircraft gunners.

At a length of 471 feet, the two “assault carriers” each had room for 8 operational aircraft, or 20 non-operational planes that were secured for ferrying. Because the flight deck was short and initially had no arresting gear (deck wires were added later), it was possible for standard Army planes like the Hayabusa “Oscar” to take off from these carriers, but they could not land. So the aircraft that most often operated from the Akitsu Maru and Nigitsu Maru were Kokusai Ki-76 utility planes, chosen for their small size and low landing speed. Normally used for artillery spotting and ferrying officers around, the Ki-76s were modified to carry machine guns and small bombs to hit targets on the landing beaches.

Neither ship survived for very long, however. In the aftermath of Pearl Harbor, US submarines began wreaking havoc on Japanese shipping. In the last two years of the war, American submarines sunk 4.9 million tons of Japanese shipping, including 1,178 merchant cargo ships and 214 Japanese Navy combat ships, including 8 aircraft carriers, 1 battleship, and 11 heavy cruisers—about 60% of all Japanese ships lost during the war. By 1944, Japan’s shipping losses were twice that of her construction. And both the Akitsu Maru and Nigitsu Maru were sunk in 1944 by American submarines.

Now, the Japanese Army needed a way to deal with the deadly threat that US subs posed to its troop transports, and decided upon a new group of small aircraft carriers to serve as anti-submarine warfare platforms. Confiscating three partially-built Type 2TL tanker ships, the Army added a 350-foot flight deck. There was no hangar for the ship’s 8 Ki-76 aircraft—instead they were stored out on the open deck. Only one of the carriers, the Yamashio Maru, was finished before the end of the war. She entered service in January 1945 and was sunk by a submarine less than a month later.

Another carrier, the Kumano Maru, was built on a standard cargo ship hull and launched in March 1945. She had been hastily designed: her flight deck was only 360 feet, and had no arresting gear. The Kumano Maru saw no action in her short wartime experience. After the Japanese surrender, her flight deck was removed and she served as an ordinary cargo ship until being scrapped in 1948.

The most significant innovation in the Japanese Army’s aircraft carriers, however, came in the aircraft which they eventually deployed—the first combat helicopters.

Kayaba Ka-1 autogyro

In the 1930s, the US was doing a lot of research into rotary-wing helicopters, and Japan followed their progress with interest. In 1939, the Imperial Army was able to obtain an American autogyro (a type of helicopter that uses an engine for thrust and an unpowered rotor for lift) known as the Kellett KD-1A. The Japanese smashed it up in an accident shortly later, but managed to produce their own design based on it. The Japanese Army took a serious interest in the autogyro, designating it the Kayaba Ka-1 and beginning production in May 1941. A number of them were sent to the Philippines where they were used for artillery spotting.
When American submarines began devastating Japanese shipping, the Army saw the advantages of placing Ka-1 (and later larger versions designated Ka-2) onto their small carriers, equipped with depth charges as anti-submarine assets, and the short-takeoff craft were perfectly suited for the tiny flight decks. So the Ki-76 fixed-wing aircraft were replaced with Ka-1 and Ka-2 autogyros, and the Japanese Army ships became the first “helicopter carriers”. (The Akitsu Maru was carrying a full complement of eight Ka-2s when she was torpedoed and sunk.) Around 100 autogyros were produced during the war, but only about 30 ever actually reached the field. Because they were produced in such small numbers and never saw any significant action, the Americans were completely unaware of the existence of these aircraft until after the surrender.


7 thoughts on “The Japanese Army Aircraft Carriers”

  1. Fascinating stuff I never knew.

    Reading up on the Kayaba autogyro — btw, I’ve owned at least one motorcycle that used Kayaba (now branded KYB) suspension, a product of their postwar diversification — I see it was powered by the same Jacobs 300hp radial engine used in the Cessna 195 like my dad owned in the late ’60s. I guess they were procured prewar.

    Speaking of KYB, rotorcraft, and Cessna, here’s this:

    1. It is an interesting story. Technologically, the Japanese were very advanced. But unfortunately for them they never had the industrial capacity to put their technological advances into use.

      The nazis had copters too but never deployed them operationally.

      1. That kind of extreme interservice rivalry doesn’t exactly make for efficient use of resources, either.

        Did the U-boat towed observation gyrogliders see service?

  2. Interesting that the Japanese, in a manner of speaking, had their own U-boat war, which, unlike the Allies, they pretty much lost.

    In an alternative history, the U.S. has no taste for all-out war, and simply wages submarine warfare against Japan, isolating it ever more and more, until it collapses almost by itself.

    1. In a way the Japanese did the US a favor at Pearl Harbor. By eliminating all the obsolete battleships, the Japanese forced us to depend on the only ships we had left–the carriers and the submarines–which were exactly what was needed to win the war.

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