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A Complicated Expedient

The naval limitations treaties signed between the World Wars forced warship designers and strategists into a number of unusual expedients that would not have otherwise existed. Ship displacements and armament were dictated by paper limits, not by the mission foreseen for the ship, and so all sorts of warships emerged from the shipyards either too small or too large for their appointed tasks. And in some cases, completely new ship types appeared.

Three of Japan’s purpose-built seaplane carriers appear in Second World War at Sea: Strike South. They are useful ships for the Japanese player, but not nearly as capable as a small carrier would be in their place.

The first ships to take airplanes to sea were seaplane carriers, some years before the first flight-deck carriers. In 1913 the Imperial Japanese Navy converted the freighter Wakamiya Maru (a prize taken during the Russo-Japanese War) into a seaplane tender with two Farman floatplanes. She operated off the German colony of Tsingtao in August and September 1914, until striking a mine and limping back to Japan for repairs.

Wakamiya (she received warship status in 1915 and thus lost the “Maru” suffix) operated like most seaplane carriers of the period, unloading her aircraft onto the water for direct take-off under their own power. Most therefore operated from harbors or sheltered waters, but the Royal Navy and the Imperial Russian Navy launched airstrikes from the open sea by this method.

Japan operated one seaplane carrier for most of the 1920s and 1930s, but in the mid-1930s began converting several large freighters to support seaplane operations along the Chinese coast. At the same time, the “Secret Program” began to lay down a number of large auxiliaries, built to naval construction standards, that could be easily converted to light aircraft carriers during wartime.

These included the submarine support ships Takasagi, Tsurugisaki and Taigei and the passenger liners Kashiwara Maru and Izumo Maru. And also four large purpose-built seaplane carriers, Chitose, Chiyoda, Mizuho and Nisshin.


Chitose at sea.

The big seaplane carriers had a cruiser’s speed and a dual-purpose gun armament. Chitose, Chiyoda and Mizuho had four catapults and carried 24 aircraft; Nisshin carried two catapults and 20 aircraft. They had aircraft-handling, repair and service facilities just like an aircraft carrier, but still had to recover their aircraft off the water. However, both the Washington and London treaties classed seaplane carriers as auxiliaries rather than warships, and thus their tonnage did not count against Japan’s total for true aircraft carriers.

Chitose and Chiyoda, laid down in 1934 as part of the 1931 program, displaced 11,000 tons and joined they fleet in 1938. They had four 5-inch guns and their diesels could drive them at 29 knots. The almost-identical Mizuho had much less powerful (but less expensive) machinery, and could only make 22 knots. She carried two more guns than the first pair. Nisshin shared her hull design with the first three, but reverted to more powerful machinery and could make 28 knots. She had only two catapults, but was fitted to carry and lay 700 mines.

Operationally, these ships would provide reconnaissance capability, support for amphibious landings and other secondary operations to free the fleet’s carriers for more important missions. In practice, they proved far less useful than expected and within months of the outbreak of war with the United States they were being used as fast troop transports.

With seaplanes not as important as Japanese planners had assumed, the four big seaplane carriers went into the yards for a quick conversion. Large doors were cut in their sterns to allow them to launch midget submarines. Aircraft capacity was halved, as a dozen small subs took up the hangar space.

This mission also proved futile, as the Americans refused to stay in place long enough for the seaplane carriers to approach them and launch the very short-ranged little submarines. Despite great effort, the mini-submarine program had very little success outside of a few harbor strikes — and the mother ships for those operations were large submarines, not surface ships.


Nisshin off Bougainville,
firing at American scout plane.

Japan’s stunning defeat at Midway led to activation of the “Secret Program” for the seaplane carriers, the other five ships having already been taken in hand for conversion to aircraft carriers. Chitose had been seriously damaged by American aircraft during the Guadalcanal campaign while operating as a fast transport, and this delayed her reconstruction. Chiyoda began rebuilding as a flush-decked light carrier at Yokosuka Navy Yard in late 1942, and her sister began work at Sasebo Navy Yard in early 1943.

Mizuho never made it to the yards, falling victim to torpedoes from the American submarine Drum in May 1942 while off the coast of Japan. She would have required new engines, apparently the same type of destroyer boilers and turbines used to re-engine the light carriers Zuiho and Shoho when converted from submarine support ships.

Nisshin was torpedoed in September 1942, but instead of heading straight to carrier conversion was repaired and rushed into the fight for Guadalcanal, where she sailed alongside Chitose as a large, fast transport. Unlike Chitose, she remained active in this role into 1943; she was fast and with her big cranes and large hangars she could quickly unload 150mm artillery pieces, something no requisitioned freighter or other warship could accomplish. She was sunk by U.S. aircraft while making a supply run from Rabaul to the Shortland Islands in July 1943.

Under Japanese strategic thought of the early 1930s, the first two purpose-built seaplane carriers made a certain sense. Japan had not yet withdrawn from the Washington and London Naval Treaties, and the seaplane carriers represented a possible force multiplier if they could take scouting planes off carrier decks. But by the time Mizuho was laid down, the naval limitations treaties had expired and Japan had decided not to attend the second London conference. The Japanese would have bene much better off laying down fleet carriers in their place (a second Soryu in place of Mizuho and another Hiryu in place of Nisshin), and ultimately these would have been cheaper to construct than the seaplane carriers with conversion expenses factored in (though the air groups would have been much more costly).

In Strike South the seaplane carriers (Chitose, Chiyoda, Mizuho plus three merchant conversions) carry the E8N scout plane. But the big ships also have the F1M fighter, a biplane floatplane. While not a very good combat plane in other theaters, it’s adequate against the F2A Buffaloes and other weak fighters put up by the British and Dutch.

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