Japanese cruiser Ibuki (1943)
Ibuki being scrapped at Sasebo, 14 March 1947
|Empire of Japan|
|Builder:||Kure Naval Arsenal, Kure|
|Laid down:||24 April 1942|
|Launched:||21 May 1943|
|Fate:||Scrapped, 22 November 1946|
|General characteristics (1944)|
|Type:||Light aircraft carrier|
|Displacement:||14,800 t (14,600 long tons)|
|Length:||200.6 m (658 ft 2 in)|
|Beam:||21.2 m (69 ft 7 in)|
|Draft:||6.31 m (20 ft 8 in)|
|Speed:||29 knots (54 km/h; 33 mph)|
|Range:||7,500 nmi (13,900 km; 8,600 mi) at 18 knots (33 km/h; 21 mph)|
|1 × Type 2, Mark 2, Model 1 air search radar|
|Aircraft carried:||27 aircraft|
The Japanese cruiser Ibuki (伊吹?) was a heavy cruiser built for the Imperial Japanese Navy (IJN) during World War II. The lead ship of her class, she was converted into a light aircraft carrier before completion. 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 meters (658 ft 2 in) overall, a beam of 20.2 meters (66 ft 3 in) and a draft of 6.04 meters (19 ft 10 in). Ibuki displaced 12,220 metric tons (12,030 long tons) at standard load and 14,828 metric tons (14,594 long tons) at (full load).
She was fitted with four Kampon geared steam turbine sets with a total of 152,000 shaft horsepower (113,000 kW), each driving a 3.9-meter (13 ft) 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 knots (65 km/h; 40 mph). Ibuki carried 2,163 metric tons (2,129 long tons) of fuel oil which gave her an estimated range of 6,300 nautical miles (11,700 km; 7,200 mi) at 18 knots (33 km/h; 21 mph).
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 29,400 yd (26,900 m). 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 14,800 meters (16,200 yd), and a maximum ceiling of 9,400 meters (30,800 ft). 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 7,500 meters (8,202 yd), and an effective ceiling of 5,500 meters (18,000 ft). 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 (24 in) Type 92 torpedo tube mounts, two on each broadside. The ship carried 24 Type 93 torpedoes, 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.
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 millimeters (3.9 in) thick and 140 millimeters (5.5 in) thick on the sides of the magazines. The outer ends of the fore and aft machinery compartments was protected by a 105-millimeter (4.1 in) transverse bulkhead. The magazines were protected by fore and aft transverse bulkheads 95–140 millimeters (3.7–5.5 in) thick.
The thickness of the armored deck ranged from 35–40 millimeters (1.4–1.6 in) on the flat and 60 millimeters (2.4 in) on the slope. The sides of the conning tower were 100 millimeters thick while its roof was 50 millimeters (2.0 in) thick. The main gun turrets had 25 millimeters (1.0 in) of armor on all sides and on the roof. The barbette armor ranged from 25 to 100 millimeters (1.0 to 3.9 in) in thickness.
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 metric tons (131 long tons) 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 3,060 metric tons (3,010 long tons) of oil, enough for 7,500 nautical miles (13,900 km; 8,600 mi) at a speed of 18 knots (33 km/h; 21 mph). The reduced power meant that Ibuki's top speed was only 29 knots (54 km/h; 33 mph).
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-meter (672 ft 7 in) flight deck was added. It had a maximum width of 23 meters (75 ft 6 in), two 13-by-11.6-meter (42.7 by 38.1 ft) 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 meters (69 ft 7 in) below the waterline. Ibuki's trials displacement increased to 14,800 metric tons (14,600 long tons) and the additional weight increased her draft to 6.31 meters (20 ft 8 in).
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-centimeter (4.7 in) 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-millimeter (3.0 in), 5.99-kilogram (13.2 lb) projectile. It had a muzzle velocity of 900–920 m/s (3,000–3,000 ft/s) that gave it a maximum range of 13,600 meters (14,900 yd), and a maximum ceiling of 9,100 meters (29,900 ft). The entire turret weighed 12,500 kilograms (27,600 lb) 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-meter (6 ft 7 in) 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.
Ibuki was ordered under the 1941 Rapid Naval Armaments Supplement Programme, and she was laid down at the Kure Naval Arsenal, Kure, on 24 April 1942; however, 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 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.
- Jentschura, Jung and Mickel, p. 87
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- Lacroix and Wells, p. 541
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