The cruisers were the last

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 ...

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 ...

s 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). In order to save design time, the ships were essentially repeats of the earlier . Begun 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 ...

, only 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 ...

, , was launched, but she was in the process of being 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 ...

when construction was suspended in 1945. She 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 ...

the following year. The unnamed second ship was scrapped less than a month after being laid down in order to clear 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 an

aircraft carrier An aircraft carrier is a warship that serves as a seagoing airbase, equipped with a full-length flight deck and facilities for carrying, arming, deploying, and recovering aircraft. Typically, it is the capital ship of a fleet, as it allows a ...

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 The is the official currency of Japan. It is the third-most traded currency in the foreign exchange market, after the United States dollar (US$) and the euro. It is also widely used as a third reserve currency after the US dollar and the e ...

each and had a crew of 54 officers and 822 enlisted men. The ships 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 ...

. They had 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 ...

). They were fitted with four Kampon 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 with a total of , each driving a propeller. Steam was provided by eight Kampon Ro Gō-type three-drum

water-tube boiler A high pressure watertube boiler (also spelled water-tube and water tube) is a type of boiler in which water circulates in tubes heated externally by the fire. Fuel is burned inside the furnace, creating hot gas which boils water in the steam-gene ...

s that operated at a pressure of and temperature of . The ships had a designed speed of . They carried of

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 ...

which gave them an estimated range of at . Electrical power was supplied by three

turbo generator A turbo generator is an electric generator connected to the shaft of a steam turbine or gas turbine for the generation of electric power. Large steam-powered turbo generators provide the majority of the world's electricity and are also used b ...

s and two

diesel generator A diesel generator (DG) (also known as a diesel Genset) is the combination of a diesel engine with an electric generator (often an alternator) to generate electrical energy. This is a specific case of engine generator. A diesel compression- ...

Armament

The main armament 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 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 ...

, 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 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 and Wells, p. 542 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.Lacroix and Wells, p. 543 The secondary armament was to consist of eight 40-caliber 12.7 cm 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. They fired projectiles at a rate between 8 and 14 rounds per minute at a

muzzle velocity Muzzle velocity is the speed of a projectile (bullet, pellet, slug, ball/shots or shell) with respect to the muzzle at the moment it leaves the end of a gun's barrel (i.e. the muzzle). Firearm muzzle velocities range from approximately to i ...

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 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 and Wells, p. 825 They fired projectiles at a

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 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 ...

s, 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, 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 Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

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 Center line, centre line or centerline may refer to: Sports * Center line, marked in red on an ice hockey rink * Centre line (football), a set of positions on an Australian rules football field * Centerline, a line that separates the service cou ...

, but nothing was done.Lacroix and Wells, p. 545

Fire control, sensors and aircraft

Two Type 94

fire-control director A fire-control system (FCS) is a number of components working together, usually a gun data computer, a director, and radar, which is designed to assist a ranged weapon system to target, track, and hit a target. It performs the same task as a hu ...

s, 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 rangefinder A coincidence rangefinder or coincidence telemeter is a type of rangefinder that uses mechanical and optical principles to allow an operator to determine the distance to a visible object. There are subtypes split-image telemeter, inverted image, ...

s. 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. Early warning would have been provided by a Type 2, Mark 2, Model 1 air search

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 would have been fitted in the bow. The ships 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 one 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 two 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 Kure Type 2

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. The ships would have carried a total of 122 powder charges for the catapults as well as four bombs for the aircraft.

Armor

The ships' armor scheme was only slightly modified from the ''Mogami''-class cruisers. Their

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 ...

armoured belt Belt armor is a layer of heavy metal 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 penetrating to t ...

extended all the way down to the

double bottom A double hull is a ship Hull (watercraft), hull design and construction method where the bottom and sides of the ship have two complete layers of watertight hull surface: one outer layer forming the normal hull of the ship, and a second inner hull ...

. It extended from the forward to the rear

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 below the fore and aft turrets and was angled inwards at the top 20° from the vertical to improve its resistance to horizontal shellfire. Over the machinery spaces, it was thick at the top and tapered to at the bottom. The outer ends of the fore and aft machinery compartments was protected by a transverse bulkhead. On the sides of the magazines, the belt was thick and tapered to 30 mm at the bottom. The magazines were protected by fore and aft transverse bulkheads thick. The steering gear and the rudder compartments had sides that consisted of plates and their ends were protected by of armor. The deck above the steering gear and rudders was thick. The thickness of the armored deck ranged from on the flat and on the slope. 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 while its roof was 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 had of armor on all sides and on the roof. 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 in thickness. The ammunition hoists for the secondary armament were protected by of armor. The funnel uptakes were provided with of armor. There was no separate anti-

torpedo bulkhead A torpedo bulkhead is a type of naval armour common on the more heavily armored warships, especially battleships and battlecruisers of the early 20th century. It is designed to keep the ship afloat even if the hull is struck underneath the belt ar ...

as that function was performed by the lower extension of the belt armor.

Ships

The two ''Ibuki''-class cruisers were ordered in November 1941 as part of the IJN's

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 ...

. Both ships were laid down without names, just as Warships No. 300 and No. 301, but the former was named ''Ibuki'' on 5 April 1943. No. 301 was ordered scrapped less than a month after she was laid down in order to clear her

for the carrier which was laid down on 1 October 1942. After her launch, the construction of ''Ibuki'' was suspended in July 1943 while her fate was discussed. A possible conversion to a fast oiler was considered until the Navy decided on 25 August to convert her to a light aircraft carrier at

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 ...

. Work on the conversion did not begin until the incomplete hull was towed to

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 ...

on 21 December. It was originally intended to complete her in March 1945, but this was extended until August. Construction was suspended on 16 March, when the ship was about 80% complete, to allow for the construction of small submarines. ''Ibuki'' was scrapped at Sasebo from 22 November 1946 to 1 August 1947.Lacroix and Wells, pp. 540–41

Notes

References

* * * * {{WWII Japanese ships Cruiser classes World War II cruisers of Japan Proposed ships Aircraft carriers of the Imperial Japanese Navy Postwar Japan