Japanese Cruiser Ibuki

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Ibuki was the last named heavy cruiser begun by the Imperial Japanese Navy. She was ordered under the 1941 Program as a slightly modified version of the Tone class, with eight 8-inch guns mounted forward & a flight deck with catapults for several reconnaissance floatplanes aft. Before she was laid down in early 1942 the design was altered to an improved Mogami design.

Ibuki was to have a similar armament to Mogami, with ten 8-inch guns mounted in five twin turrets, three turrets forward & two aft. She was to have a secondary armament of eight 5-inch dual-purpose guns, sixteen 24-inch torpedo tubes and two catapults for three floatplanes. She was to have a maximum speed of 33 knots. The Mogamis had been extensively but poorly welded & were unstable because too much armor & armament were placed on too small a displacement. Because Ibuki was ordered after Japan had withdrawn from the Washington Naval Treaty and was not hindered by the 10,000-ton displacement limitation for cruisers, she was designed to weigh 12,200 tons and make a good balance of armament, armor, speed & seaworthiness.

Ibuki's construction was slowed after the Battle of Midway to give priority to aircraft carrier construction. Construction of a second Ibuki-class cruiser, designated No. 301 but never given a name, was begun on June 1, 1942, but suspended a month later. No. 301 was scrapped in 1943 but Ibuki was launched at Kure Dockyard on May 21, 1943. Construction was suspended a month later before Ibuki's armament was installed.

[edit] Conversion to Aircraft Carrier

The Japanese Navy considered completing Ibuki as a fleet oiler for underway replenishment, a capability that the Navy desperately needed. However, both the United States & German Navies had begun conversions of incomplete cruisers to light fleet aircraft carriers. The speed & hull armor of cruisers was similar to that required for fleet carriers, though cruiser conversions could operate fewer aircraft than purpose-built carriers. Since Ibuki's hull was already complete & Japan was desperate for carriers, she was towed to Sasebo in November 1943 & her conversion begun.

Ibuki was built with a full-length flight deck & a small starboard island structure. She was to have a very light armament of only four 3-inch & 48 25mm anti-aircraft guns. Even so, she would only have been able to operate 27 aircraft, less than half that of most Japanese fleet carriers. Having lost so many ships to American submarines, Ibuki was to have stern rails for up to 30 depth charges. Her maximum speed was reduced to 29 knots, which was still enough for fleet operations. A radar set was to be fitted & launchers for 120mm anti-aircraft rockets were later included.

By late 1944 the Japanese Navy was so short of advanced aircraft, trained aircrewmen & aviation fuel that the shortage of carriers had become irrelevant. However, work continued until March 1945, when shipyard damage & material shortage from American bombing & submarine attacks made construction impossible. When Ibuki was surrendered to occupying forces in September she was 80% complete. She was scrapped at Sasebo in 1947.

Ibuki compares favorably in many respects to her American counterparts, both as a cruiser & as a carrier. As a heavy cruiser she was not quite as well armed or armored as the Baltimore class cruisers, but she was slightly smaller & was fast & as seaworthy. The Independence class carriers, which were converted from incomplete Cleveland class cruisers, were slightly smaller than Ibuki but could operate 45 aircraft & were slightly faster. However, the American light carriers were extremely cramped with few aircraft maintenance facilities & were very vulnerable to damage. Because of Japan's long experience with carriers, Ibuki probably would also have compared favorably with the German cruiser Seydlitz, which was also scheduled for conversion to a carrier but never finished.