The Japanese cruiser was a

heavy cruiser The heavy cruiser was a type of cruiser, a naval warship designed for long range and high speed, armed generally with naval guns of roughly 203 mm (8 inches) in caliber, whose design parameters were dictated by the Washington Naval Tr ...

built for the

Imperial Japanese Navy The Imperial Japanese Navy (IJN; Kyūjitai: Shinjitai: ' 'Navy of the Greater Japanese Empire', or ''Nippon Kaigun'', 'Japanese Navy') was the navy of the Empire of Japan from 1868 to 1945, when it was dissolved following Japan's surrender ...

(IJN) during

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...

. The

lead ship The lead ship, name ship, or class leader is the first of a series or class of ships all constructed according to the same general design. The term is applicable to naval ships and large civilian vessels. Large ships are very complex and may ...

of her

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...

of two ships, she was ordered to be converted into a

light aircraft carrier A light aircraft carrier, or light fleet carrier, is an aircraft carrier that is smaller than the standard carriers of a navy. The precise definition of the type varies by country; light carriers typically have a complement of aircraft only one-h ...

in 1943 before completion to help replace the aircraft carriers sunk during the

Battle of Midway The Battle of Midway was a major naval battle in the Pacific Theater of World War II that took place on 4–7 June 1942, six months after Japan's attack on Pearl Harbor and one month after the Battle of the Coral Sea. The U.S. Navy under Adm ...

in mid-1942. The conversion was delayed and finally stopped in March 1945 in order to concentrate on building small

submarine A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...

s. ''Ibuki'' was

scrapped Scrap consists of recyclable materials, usually metals, left over from product manufacturing and consumption, such as parts of vehicles, building supplies, and surplus materials. Unlike waste, scrap has monetary value, especially recovered me ...

in the

Sasebo Naval Arsenal was one of four principal naval shipyards owned and operated by the Imperial Japanese Navy. History The Sasebo Naval District was established at Sasebo, Nagasaki in 1886, as the third of the naval districts responsible for the defense of the ...

beginning in 1946.

Background

The ''Ibuki''-class cruisers were ordered in the

Rapid Naval Armaments Supplement Programme The was one of the armaments expansion plan of the Imperial Japanese Navy (IJN). Background In August 1941, the IJN started building warships for war. It extended to 293 vessels, 300,000 tons. Table of vessels See also * 1st Naval Armaments Sup ...

of November 1941, and they were slightly improved versions of the preceding after those ships had been upgraded during the late 1930s. After the heavy losses suffered in the Battle of Midway in early June 1942, the IJN reorganized its current building programs to emphasize aircraft carrier construction. ''Ibuki'', which had only been

laid down Laying the keel or laying down is the formal recognition of the start of a ship's construction. It is often marked with a ceremony attended by dignitaries from the shipbuilding company and the ultimate owners of the ship. Keel laying is one o ...

a few months earlier, had all work suspended while the IJN decided what to do with her. The navy ordered the

shipyard A shipyard, also called a dockyard or boatyard, is a place where ships are built and repaired. These can be yachts, military vessels, cruise liners or other cargo or passenger ships. Dockyards are sometimes more associated with maintenance a ...

to resume and accelerate construction the following month in order to launch her hull as soon as possible to free her

slipway A slipway, also known as boat ramp or launch or boat deployer, is a ramp on the shore by which ships or boats can be moved to and from the water. They are used for building and repairing ships and boats, and for launching and retrieving small ...

for new carriers. After she was launched in May 1943, construction was suspended again in July while the IJN decided what to do with her. The navy considered completing ''Ibuki'' as a high-speed

replenishment oiler A replenishment oiler or replenishment tanker is a naval auxiliary ship with fuel tanks and dry cargo holds which can supply both fuel and dry stores during underway replenishment (UNREP) at sea. Many countries have used replenishment oilers. The ...

, but decided to convert her into a light aircraft carrier on 25 August.

Description as a cruiser

As originally designed the ''Ibuki''s had a length of

overall Overalls, also called bib-and-brace overalls or dungarees, are a type of garment usually used as protective clothing when working. The garments are commonly referred to as a "pair of overalls" by analogy with "pair of trousers". Overalls were ...

, a

beam Beam may refer to: Streams of particles or energy *Light beam, or beam of light, a directional projection of light energy **Laser beam *Particle beam, a stream of charged or neutral particles **Charged particle beam, a spatially localized grou ...

of and a

draft Draft, The Draft, or Draught may refer to: Watercraft dimensions * Draft (hull), the distance from waterline to keel of a vessel * Draft (sail), degree of curvature in a sail * Air draft, distance from waterline to the highest point on a vessel ...

of . They displaced at

standard load The displacement or displacement tonnage of a ship is its weight. As the term indicates, it is measured indirectly, using Archimedes' principle, by first calculating the volume of water displaced by the ship, then converting that value into wei ...

and at (

full load The displacement or displacement tonnage of a ship is its weight. As the term indicates, it is measured indirectly, using Archimedes' principle, by first calculating the volume of water displaced by the ship, then converting that value into wei ...

). The ''Ibuki'' class was fitted with four

Kampon The was the externally operating division of the Ministry of the Navy of Japan responsible for the administration of naval vessel construction. From 1923 onward, it took on the role of a research institution for the research and development of n ...

geared

steam turbine A steam turbine is a machine that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Charles Parsons in 1884. Fabrication of a modern steam turbin ...

sets, each driving one

propeller shaft A drive shaft, driveshaft, driving shaft, tailshaft (Australian English), propeller shaft (prop shaft), or Cardan shaft (after Girolamo Cardano) is a component for transmitting mechanical power and torque and rotation, usually used to connect ...

using steam provided by eight Kampon Ro Gō-type

three-drum boiler Three-drum boilers are a class of water-tube boiler used to generate steam, typically to power ships. They are compact and of high evaporative power, factors that encourage this use. Other boiler designs may be more efficient, although bulkier, an ...

s. The turbines were intended to produce a total of to give the ships a speed of . They carried enough

fuel oil Fuel oil is any of various fractions obtained from the distillation of petroleum (crude oil). Such oils include distillates (the lighter fractions) and residues (the heavier fractions). Fuel oils include heavy fuel oil, marine fuel oil (MFO), bun ...

to give them an estimated range of at .

Armament and sensors

The

main battery A main battery is the primary weapon or group of weapons around which a warship is designed. As such, a main battery was historically a gun or group of guns, as in the broadsides of cannon on a ship of the line. Later, this came to be turreted ...

of the ''Ibuki'' class was intended to be ten 50-

caliber In guns, particularly firearms, caliber (or calibre; sometimes abbreviated as "cal") is the specified nominal internal diameter of the gun barrel Gauge (firearms) , bore – regardless of how or where the bore is measured and whether the f ...

20 cm (8 in) 3rd Year Type No. 2 guns mounted in twin turrets, three forward and two aft of the

superstructure A superstructure is an upward extension of an existing structure above a baseline. This term is applied to various kinds of physical structures such as buildings, bridges, or ships. Aboard ships and large boats On water craft, the superstruct ...

. The first two forward turrets were on the same level, but the third turret could

superfire Superfiring armament is a naval military building technique in which two (or more) turrets are located in a line, one behind the other, with the second turret located above ("super") the one in front so that the second turret can fire over the ...

over the first two.Lacroix & Wells, p. 542 The

secondary armament Secondary armament is a term used to refer to smaller, faster-firing weapons that were typically effective at a shorter range than the main (heavy) weapons on military systems, including battleship- and cruiser-type warships, tanks/armored ...

was to consist of eight 40-caliber 12.7 cm (5 in) Type 89

anti-aircraft Anti-aircraft warfare, counter-air or air defence forces is the battlespace response to aerial warfare, defined by NATO as "all measures designed to nullify or reduce the effectiveness of hostile air action".AAP-6 It includes surface based, ...

(AA) guns in twin mounts. The ships were also intended to be equipped with four twin 2.5 cm (1 in) Type 96 light AA guns abreast the

funnel A funnel is a tube or pipe that is wide at the top and narrow at the bottom, used for guiding liquid or powder into a small opening. Funnels are usually made of stainless steel, aluminium, glass, or plastic. The material used in its construct ...

.Lacroix & Wells, p. 825 Two twin Type 93 machine gun mounts were supposed to be mounted on the

bridge A bridge is a structure built to span a physical obstacle (such as a body of water, valley, road, or rail) without blocking the way underneath. It is constructed for the purpose of providing passage over the obstacle, which is usually somethi ...

with 2,000 rounds per gun. The ''Ibuki''-class ships were intended to be armed with four rotating quadruple Type 92

torpedo tube A torpedo tube is a cylindrical device for launching torpedoes. There are two main types of torpedo tube: underwater tubes fitted to submarines and some surface ships, and deck-mounted units (also referred to as torpedo launchers) installed aboa ...

mounts, two on each

broadside Broadside or broadsides may refer to: Naval * Broadside (naval), terminology for the side of a ship, the battery of cannon on one side of a warship, or their near simultaneous fire on naval warfare Printing and literature * Broadside (comic ...

. The ship carried 24

Type 93 torpedo The was a -diameter torpedo of the Imperial Japanese Navy (IJN), launched from surface ships. It is commonly referred to as the Long Lance by most modern English-language naval historians, a nickname given to it after the war by Samuel Eliot Mori ...

es, 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

radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...

mounted at the top of the

foremast The mast of a Sailing ship, sailing vessel is a tall spar (sailing), spar, or arrangement of spars, erected more or less vertically on the centre-line of a ship or boat. Its purposes include carrying sails, spars, and derricks, and giving necessa ...

. A Type 93 passive

hydrophone A hydrophone ( grc, ὕδωρ + φωνή, , water + sound) is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electric potenti ...

system was intended be fitted in the bow. The cruisers were designed to carry three aircraft on a platform between the funnel and the

mainmast The mast of a sailing vessel is a tall spar, or arrangement of spars, erected more or less vertically on the centre-line of a ship or boat. Its purposes include carrying sails, spars, and derricks, and giving necessary height to a navigation ligh ...

. These would have consisted of a single three-seat

Aichi E13A The Aichi E13A ( Allied reporting name: "Jake") was a long-range reconnaissance seaplane used by the Imperial Japanese Navy (IJN) from 1941 to 1945. Numerically the most important floatplane of the IJN, it could carry a crew of three and a bomblo ...

and a pair of two-seat

Yokosuka E14Y The Yokosuka E14Y ( Allied reporting name Glen) was an Imperial Japanese Navy reconnaissance seaplane transported aboard and launched from Japanese submarine aircraft carriers such as the during World War II. The Japanese Navy designation was ...

floatplane A floatplane is a type of seaplane with one or more slender floats mounted under the fuselage to provide buoyancy. By contrast, a flying boat uses its fuselage for buoyancy. Either type of seaplane may also have landing gear suitable for land, ...

s. They would have been launched by a pair of

aircraft catapult An aircraft catapult is a device used to allow aircraft to take off from a very limited amount of space, such as the deck of a vessel, but can also be installed on land-based runways in rare cases. It is now most commonly used on aircraft carrier ...

s, one on each side of the aircraft platform.

Armor

The ship's

waterline The waterline is the line where the hull of a ship meets the surface of the water. Specifically, it is also the name of a special marking, also known as an international load line, Plimsoll line and water line (positioned amidships), that indi ...

armored belt Belt armor is a layer of heavy metal vehicle armor, armor plated onto or within the outer hulls of warships, typically on battleships, battlecruisers and cruisers, and aircraft carriers. The belt armor is designed to prevent projectiles from p ...

was thick over the propulsion machinery spaces and thick on the sides of the

magazine A magazine is a periodical publication, generally published on a regular schedule (often weekly or monthly), containing a variety of content. They are generally financed by advertising, purchase price, prepaid subscriptions, or by a combinatio ...

s. The outer ends of the fore and aft machinery compartments was protected by a transverse bulkhead. The magazines were protected by fore and aft transverse bulkheads thick. The thickness of the armored deck ranged from and the sides of the

conning tower A conning tower is a raised platform on a ship or submarine, often armored, from which an officer in charge can conn the vessel, controlling movements of the ship by giving orders to those responsible for the ship's engine, rudder, lines, and gro ...

were 100 millimeters thick. The main

gun turret A gun turret (or simply turret) is a mounting platform from which weapons can be fired that affords protection, visibility and ability to turn and aim. A modern gun turret is generally a rotatable weapon mount that houses the crew or mechani ...

s were protected by 25 millimeters of armor and the

barbette Barbettes are several types of gun emplacement in terrestrial fortifications or on naval ships. In recent naval usage, a barbette is a protective circular armour support for a heavy gun turret. This evolved from earlier forms of gun protection ...

armor ranged from 25 to 100 millimeters in thickness.

Conversion

''Ibuki''s two aft turbine sets, the four aft boilers and the two innermost

s 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. These changes left her with two turbines and four boilers producing . The reduced power meant that ''Ibuki''s top speed was only . The space made available was used for

aviation gasoline Avgas (aviation gasoline, also known as aviation spirit in the UK) is an aviation fuel used in aircraft with spark-ignited internal combustion engines. ''Avgas'' is distinguished from conventional gasoline (petrol) used in motor vehicles, whi ...

tanks, additional fuel oil tanks, as well as bomb and torpedo magazines. The ship now could carry enough oil for an estimated range of at a speed of .Lacroix & Wells, p. 544 As part of the conversion, the existing superstructure was razed, a new

hangar deck A hangar is a building or structure designed to hold aircraft or spacecraft. Hangars are built of metal, wood, or concrete. The word ''hangar'' comes from Middle French ''hanghart'' ("enclosure near a house"), of Germanic origin, from Frankish ...

was built above the existing upper deck and a full-length

flight deck The flight deck of an aircraft carrier is the surface from which its aircraft take off and land, essentially a miniature airfield at sea. On smaller naval ships which do not have aviation as a primary mission, the landing area for helicopters ...

was added. It had a maximum width of , two aircraft

elevator An elevator or lift is a wire rope, cable-assisted, hydraulic cylinder-assisted, or roller-track assisted machine that vertically transports people or freight between floors, levels, or deck (building), decks of a building, watercraft, ...

s that serviced the single

hangar A hangar is a building or structure designed to hold aircraft or spacecraft. Hangars are built of metal, wood, or concrete. The word ''hangar'' comes from Middle French ''hanghart'' ("enclosure near a house"), of Germanic origin, from Frankish ...

and a small starboard

island An island (or isle) is an isolated piece of habitat that is surrounded by a dramatically different habitat, such as water. Very small islands such as emergent land features on atolls can be called islets, skerries, cays or keys. An island ...

structure. The ship was bulged to improve her stability, which increased her beam to a maximum of . ''Ibuki''s trials displacement increased to and the additional weight increased her draft to . Japanese aircraft carrier Ibuki cropped

Initially, the ship was to have a very light armament of only 22 triple 2.5-centimeter gun mounts, controlled by eight Type 95 fire-control directors, but this was modified in 1944 to substitute four 60-caliber 8 cm (3 in) Type 98

dual-purpose gun A dual-purpose gun is a naval artillery mounting designed to engage both surface and air targets. Description Second World War-era capital ships had four classes of artillery: the heavy main battery, intended to engage opposing battleships and ...

s, mounted in two twin-gun turrets, and four 28-tube launchers for 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. At the top of the island, ''Ibuki'' was planned to have a

rangefinder A rangefinder (also rangefinding telemeter, depending on the context) is a device used to measure distances to remote objects. Originally optical devices used in surveying, they soon found applications in other fields, such as photography an ...

and a Type 21 radar. In 1944, the radar was moved to a retractable mount in the flight deck near the bow and a Type 22 surface search and a Type 13 early-warning radar were going to be installed on the island. The ship retained the Type 93 hydrophone system. The ship's air group was intended to consist of 27 aircraft, 15

Mitsubishi A7M The Mitsubishi A7M ''Reppū'' (烈風, "Strong Wind") was designed as the successor to the Imperial Japanese Navy's A6M Zero, with development beginning in 1942. Performance objectives were to achieve superior speed, climb, diving, and armament o ...

Reppū ( Allied codename: "Sam")

fighters Fighter(s) or The Fighter(s) may refer to: Combat and warfare * Combatant, an individual legally entitled to engage in hostilities during an international armed conflict * Fighter aircraft, a warplane designed to destroy or damage enemy warplan ...

and a dozen

Aichi B7A The was a large and powerful carrier-borne torpedo-dive bomber produced by Aichi Kokuki for the Imperial Japanese Navy Air Service during the Second World War. Built in only small numbers and deprived of the aircraft carriers it was intended ...

Ryusei ("Grace")

dive Diving most often refers to: * Diving (sport), the sport of jumping into deep water * Underwater diving, human activity underwater for recreational or occupational purposes Diving or Dive may also refer to: Sports * Dive (American football), a ...

torpedo bomber A torpedo bomber is a military aircraft designed primarily to attack ships with aerial torpedoes. Torpedo bombers came into existence just before the First World War almost as soon as aircraft were built that were capable of carrying the weight ...

s. They would be operated by the ship's complement of 1,015 officers and men.

Construction

''Ibuki'' was

at the

Kure Naval Arsenal was one of four principal naval shipyards owned and operated by the Imperial Japanese Navy. History The Kure Naval District was established at Kure, Hiroshima in 1889, as the second of the naval districts responsible for the defense of the J ...

Kure is a port and major shipbuilding city situated on the Seto Inland Sea in Hiroshima Prefecture, Japan. With a strong industrial and naval heritage, Kure hosts the second-oldest naval dockyard in Japan and remains an important base for the Japan M ...

, on 24 April 1942; however, her construction was suspended on 30 June. Construction resumed on 30 July to allow the ship to be launched as soon as possible. She was named after

Mount Ibuki is a mountain, on the border of Maibara, Shiga Prefecture, and Ibigawa, Gifu Prefecture, Japan. It is one of the 100 Famous Japanese Mountains, and is also included on the lists of the 100 Kinki Mountains and the 50 Shiga Mountains. Mount Ibuk ...

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 until the IJN decided to convert her into a light aircraft carrier the following month. While plans were prepared for the conversion, she was towed to the

Sasebo is a core city located in Nagasaki Prefecture, Japan. It is also the second largest city in Nagasaki Prefecture, after its capital, Nagasaki. On 1 June 2019, the city had an estimated population of 247,739 and a population density of 581 persons p ...

, by the

submarine tender A submarine tender is a type of depot ship that supplies and supports submarines. Development Submarines are small compared to most oceangoing vessels, and generally do not have the ability to carry large amounts of food, fuel, torpedoes, and ...

on 19–21 December.Lacroix & Wells, pp. 540–541 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 halted 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 A dry dock (sometimes drydock or dry-dock) is a narrow basin or vessel that can be flooded to allow a load to be floated in, then drained to allow that load to come to rest on a dry platform. Dry docks are used for the construction, maintenance, ...

No. 7 from 22 November 1946 to 1 August 1947.Lacroix & Wells, p. 541

Notes

References

* * * * {{DEFAULTSORT:Ibuki (1943) Ibuki-class cruisers Ships built by Kure Naval Arsenal 1943 ships World War II cruisers of Japan Aircraft carriers of the Imperial Japanese Navy World War II aircraft carriers of Japan