The { { nihongo|Ibuki-class|伊吹型|Ibuki-gata}} cruisers were the last class of heavy cruisers built for the Imperial Japanese Navy (IJN). In order to save design time, the ships were essentially repeats of the earlier . Begun during World War II, only the lead ship, , was launched, but she was in the process of being converted into a light aircraft carrier when construction was suspended in 1945. She was scrapped the following year. The unnamed second ship was scrapped less than a month after being laid down in order to clear her slipway for an aircraft carrier.
Design and description
The design of the Ibuki class was a minor improvement over the last pair of the Mogami class after those ships had been upgraded during the late 1930s. The main improvement was the replacement of the triple torpedo tube mounts in the older ships with quadruple mounts. They cost 60,000,000 yen each and had a crew of 54 officers and 822 enlisted men.
The ships had a length of overall. They had a beam of and a draft of . They displaced at standard load and at (full load).
They were fitted with four Kampon geared steam turbine sets with a total of , each driving a propeller. Steam was provided by eight Kampon Ro Gō-type three-drum water-tube boilers that operated at a pressure of and temperature of . The ships had a designed speed of . They carried of fuel oil which gave them an estimated range of at . Electrical power was supplied by three turbo generators and two diesel generators.
Armament
The main armament of the Ibuki class was intended to be ten 50-caliber 20 cm 3rd Year Type No. 2 guns mounted in twin turrets, three forward and two aft of the superstructure, numbered one through five from the bow to the stern. The first two forward turrets were on the same level, but the third turret could superfire over the first two. The guns could depress to −5° and had a maximum elevation of 55°. They fired projectiles at a muzzle velocity of . They had a maximum range of at an elevation of 45° and the ship carried 128 rounds per gun. The secondary armament was to consist of eight 40-caliber 12.7 cm Type 89 anti-aircraft (AA) guns in twin mounts. They fired projectiles at a rate between 8 and 14 rounds per minute at a muzzle velocity of ; at 45°, this provided a maximum range of , and a maximum ceiling of . The ships were also intended to be equipped with four twin 25 mm Type 96 light AA guns abreast the funnel. They fired projectiles at a muzzle velocity of ; at 50°, this provided a maximum range of , and an effective ceiling of . The maximum effective rate of fire was only between 110 and 120 rounds per minute due to the frequent need to change the fifteen-round magazines. Two twin 13.2 mm Type 93 machine gun mounts were supposed to be mounted on the bridge with 2,000 rounds per gun.
The Ibuki-class ships were intended to be armed with four rotating quadruple Type 92 torpedo tubes, two on each broadside. The ship carried 24 Type 93 torpedoes, 16 in the tubes and 8 in reserve. Quick-reloading gear was installed for every mount that allowed the reserve torpedoes to be loaded in three to five minutes in ideal conditions. The Type 93 torpedo, fueled by compressed oxygen and widely referred to in post-war literature as the "Long Lance", had three range/speed settings. It had a range of at a speed of , at , or at a speed of . Before Ibuki was launched, one proposal was made to replace the aircraft and their equipment with five quintuple Type 0 torpedo tube mounts. Two of these would be mounted on each side and the last on the centerline, but nothing was done.
Fire control, sensors and aircraft
Two Type 94 fire-control directors, one atop the bridge and the other abaft the funnel, were going to be fitted to control the main guns. They used range data received from three coincidence rangefinders. Two of these were to be installed in turrets Nos. 3 and 4 while the primary rangefinder was mounted above the bridge. A pair of Type 94 high-angle directors, one on each side of the bridge, were intended to control the Type 89 guns. Each director was fitted with a rangefinder. The 25 mm guns would have been controlled by two Type 95 directors mounted on the bridge.