Japanese cruiser Ibuki (1943)

The Japanese cruiser Ibuki (伊吹) was a heavy cruiser, the lead ship of her class, built for the Imperial Japanese Navy (IJN) during World War II. Before the ship was completed, she was ordered to be converted into a light aircraft carrier that could carry 27 aircraft. The conversion was delayed and finally stopped in March 1945 in order to concentrate on building small submarines. Ibuki was scrapped in the Sasebo Naval Arsenal beginning in 1946.

Design and description
As originally designed the ship had a length of 200.6 m overall, a beam of 20.2 m and a draft of 6.04 m. Ibuki displaced 12220 t at standard load and 14828 t at (full load).

She was fitted with four Kampon geared steam turbine sets with a total of 152000 shp, each driving a 3.9 m propeller. Steam was provided by eight Kampon Ro Gō-type three-drum water-tube boilers intended to give the ship a maximum speed of 35 kn. Ibuki carried 2163 t of fuel oil which gave her an estimated range of 6300 nmi at 18 kn.

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. 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° and a maximum range of 29400 yd. The secondary armament was to consist of eight 40-caliber 12.7 cm Type 89 anti-aircraft (AA) guns in twin mounts. They had a maximum range of 14800 m, and a maximum ceiling of 9400 m. The ships were also intended to be equipped with four twin 25 mm Type 96 light AA guns abreast the funnel. They had a maximum range of 7500 m, and an effective ceiling of 5500 m. 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 61 cm Type 92 torpedo tube mounts, two on each broadside. The ship carried 24 Type 93 torpedos, commonly referred to in post-war literature as the "Long Lance", 16 in the tubes and eight 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.

Early warning would have been provided by a Type 2, Mark 2, Model 1 air search radar mounted at the top of the foremast. A Type 93 passive hydrophone system would have been fitted in the bow. The ships were designed to carry three aircraft on a platform between the funnel and the mainmast. These would have consisted of one three-seat Aichi E13A and two two-seat Yokosuka E14Y floatplanes. They would have been launched by a pair of aircraft catapults, one on each side of the aircraft platform.

Armor
The ship's waterline armoured belt ran all the way down to the double bottom. It extended from the forward to the rear magazines below the fore and aft turrets. Over the machinery spaces, it was 100 mm thick and 140 mm thick on the sides of the magazines. The outer ends of the fore and aft machinery compartments was protected by a 105 mm transverse bulkhead. The magazines were protected by fore and aft transverse bulkheads 95 - 140 mm thick.

The thickness of the armored deck ranged from 35 - 40 mm on the flat and 60 mm on the slope. The sides of the conning tower were 100 millimeters thick while its roof was 50 mm thick. The main gun turrets had 25 mm of armor on all sides and on the roof. The barbette armor ranged from 25 to 100 mm in thickness.

Conversion
Ibuki's two aft turbine sets, the four aft boilers and the two innermost propeller shafts were removed with their propellers; the exhaust uptakes for the remaining boilers were trunked together into a downward-curving funnel on the starboard side of the hull. The space made available was used for avgas tanks (133 t in capacity), additional fuel oil tanks, as well as bomb and torpedo magazines that had a capacity of 24 bombs and two torpedoes. The ship now could carry 3060 t of oil, enough for 7500 nmi at a speed of 18 kn. The reduced power meant that Ibuki's top speed was only 29 kn.

As part of the conversion, the existing superstructure was razed, a new hangar deck was built above the existing upper deck and a full-length 205 m flight deck was added. It had a maximum width of 23 m, two 13 by aircraft elevators that serviced the single hangar and a small starboard island structure. The ship was bulged to improve her stability, which increased her beam to a maximum of 21.2 m below the waterline. Ibuki's trials displacement increased to 14800 t and the additional weight increased her draft to 6.31 m.

Initially, the ship was to have a very light armament of only 22 triple 25-millimeter gun mounts, controlled by eight Type 95 fire-control directors, but this was modified in 1944 to substitute four 60-caliber 8 cm Type 98 dual-purpose guns, mounted in two twin-gun turrets, and four 28-tube launchers for 12 cm anti-aircraft rockets for six of the triple 25-millimeter gun mounts and two directors (now Type 4s). This gave the ship a total of forty-eight 25 mm guns in 16 triple mounts. The Type 98 gun fired a 76.2 mm, 5.99 kg projectile. It had a muzzle velocity of 900 - 920 m/s that gave it a maximum range of 13600 m, and a maximum ceiling of 9100 m. The entire turret weighed 12500 kg and the guns could fire at a rate of 25 rounds per minute.

At the top of the island, Ibuki was planned to have a 2 m rangefinder and a Type 21 radar. In 1944, the Type 21 radar was moved to a retractable mount in the flight deck near the bow and a Type 22 surface search and a Type 13 air-search radar were going to be installed on the island. The ship retained the Type 93 hydrophone system.

The ship's air group was designed to consist of 27 aircraft, 15 Mitsubishi A7M Reppū (Allied codename: "Sam") fighters and a dozen Aichi B7A Ryusei ("Grace") dive/torpedo bombers.

Construction
Ibuki was laid down at the Kure Naval Arsenal, Kure, on 24 April 1942, but her construction was suspended on 30 June. Construction resumed a month later to allow the ship to be launched and clear her slipway for carrier construction. She was named after Mount Ibuki on 5 April 1943 as per the IJN's naming convention for first-class cruisers. The ship was launched on 21 May and construction was suspended in July while the IJN decided what to do with her. The navy considered completing Ibuki as a high-speed replenishment oiler, but decided to convert her into a light aircraft carrier on 25 August. While plans were prepared for the conversion, she was towed to the Sasebo Naval Arsenal, Sasebo, by the submarine tender JAPANESE SUBMARINE TENDER Jingei 19–21 December.

Completion of the ship was originally scheduled for March 1945, but it was rescheduled for August because of delays. Work continued until 16 March 1945, but it was ordered to halt when she was 80% complete to concentrate on the construction of small submarines needed to defend Japan against an American invasion. She was anchored in Ebisu Bay, near Sasebo, and surrendered there on 2 September along with the rest of the Japanese military. Ibuki was scrapped in Sasebo Naval Arsenal Drydock No. 7, 22 November 1946 – 1 August 1947.