was an experimental light cruiser built during the early 1920s for the Imperial Japanese Navy (IJN) to test new concepts for reducing the hull's weight while strengthening it. Designs pioneered on Yūbari had a major impact on future Japanese warship designs. Completed in 1923, the ship was generally used as the flagship for destroyer squadrons. She spent large portions of her peacetime career in reserve or used as a training ship. The ship participated in the First Shanghai Incident in 1932 and the Second Sino-Japanese War in 1937 before World War II. During the war Yūbari was the flagship of the forces involved in the Battle of Wake Island and was then sent south to support the invasion of Rabaul in early 1942. She played a small role during the Battle of the Coral Sea as the flagship of the forces intended to invade Port Moresby, New Guinea. At the beginning of the Solomon Islands campaign, Yūbari escorted the forces that made the initial landings on the island of Guadalcanal in July. A few days after the Americans attacked the island in August, the ship participated in the Battle of Savo Island where she crippled an American heavy cruiser and a destroyer.
Yūbari spent the rest of the year on escort duties and she played a small role in the Battle of New Georgia in mid-1943 as she bombarded Allied forces a few days after they landed on the island. The ship struck a mine on the return journey and had to return to Japan for repairs that lasted for several months. After her return to the Guadalcanal area in November, she made several Tokyo Express runs to deliver reinforcements and supplies. Yūbari was damaged by several American airstrikes at Rabaul later that month and had to return again to Japan for repairs that lasted until March 1944. The ship was tasked to deliver supplies and troops to Japanese outposts in April and was sunk by an American submarine later that month.
Background and description
Construction of an experimental light cruiser was authorized by the revised 1917 8-4 Fleet Program, but construction was not approved by the Naval General Staff until October 1921 to evaluate naval architect Captain Yuzuru Hiraga's innovative design proposal. He incorporated the ship's belt and deck armor plates as structural members into the hull structure, eliminating the weight of the traditional plating and structure that backed the armor. Hiraga believed that this would allow him to design a ship that would have the combat potential of the standard light cruiser, but with a much lighter displacement. Lieutenant Commander Kikuo Fujimoto designed Yūbari, under the direction of Hiraga, to have nearly the same speed, armament and radius of action as the Sendais while displacing less than 60% of the older ships. Yūbari introduced numerous features that were adopted by subsequent heavy cruiser designs.
thumb|left|upright=1|US [[Office of Naval Intelligence recognition drawings of Yūbari made during World War II]]
The cruiser was designed to displace at normal load, but was significantly overweight for unknown reasons and actually displaced . She had an overall length of , a beam of , and a draft of , deeper than designed. Her crew numbered 328 officers and enlisted men.
Yūbaris propulsion system was derived from that of the s, enlarged by one additional propeller shaft, steam turbine and four Kampon water-tube boilers. The three turbines used steam provided by eight boilers that operated at a pressure of and a temperature of . The turbines developed a total of and were intended to give the cruiser a speed of , but the increase in draft adversely affected both her speed and range. Yūbari only reached from , rather than her intended , when the ship ran her sea trials on 5 July 1923. The ship carried a maximum of of fuel oil that gave her a range of , reduced from her designed range of , at . Hiraga trunked the forward funnel aft into the rear funnel to minimize the effects of hot exhaust gases on the bridge personnel.
Armament and fire control
The cruiser's main battery consisted of six 50-caliber 3rd Year Type guns arranged in two twin-gun turrets and two single gun mounts protected by gun shields. One mount of each type was positioned forward and aft of the superstructure in a superfiring arrangement with the single mount positioned below the twin-gun turret. A pair of rotating Type 8 twin-tube mounts for torpedoes were located amidships on the ship's centerline. Each tube was provided with a single reload torpedo. A single 40-caliber 3rd Year Type anti-aircraft (AA) gun and two Lewis machine guns on a raised platform between the torpedo mounts for anti-aircraft defense. Yūbari was fitted with six rails at the stern that could accommodate 48 No. 1 naval mines in 24 compartments in the hull.
The main guns were controlled by a Type 13 director located at the top of the tripod mast. A pair of rangefinders were located on the bridge, one on each broadside to determine the distance to the target.
Protection
thumb|upright|A half-cross section of Yūbaris hull, showing the armor thicknesses
Incorporating the ship's armor plating into the hull reinforced the longitudinal strength of the hull, allowed Hiraga to eliminate the hull plates formerly installed behind the armor and the girders that traditionally provided the necessary longitudinal strength. This cut the weight of the Yūbaris displacement devoted to the hull to 31.2 percent from the roughly 38 percent of the Sendai class and permitted Hiraga to substantially increase the amount of Yūbaris armor protection. The ship was fitted with of armor, almost exactly double the weight in the Sendai-class ships.
Yūbaris armor was designed to protect against American shells and the cruiser was equipped with an internal waterline New Vickers Non-Cemented (NVNC) armor belt that sloped outwards 10° from top to bottom. The belt was high, had a length of , and connected to the armored deck at the top and the double bottom below. Forming the top of the armored citadel, the deck had NVNC armor plates that were reinforced by plates of high-tensile steel along the centerline. The boiler uptakes were protected up to a height of by NVNC plates thick.
Shortly after the start of the Pacific War, Yūbari had a pair of improvised twin-gun mounts for 13.2 mm machine guns installed on either side of her bridge. These were replaced by Type 96 AA guns in February 1943. Six months later the aft single 14 cm mount was replaced by a triple mount for Type 96 AA guns. Yūbari also received a Type 22 surface-search radar, a Type 93 Mk.4 sonar system and a Type 93 hydrophone set. was laid down on 5 June 1922 at the Sasebo Naval Arsenal, launched on 5 March 1923 and completed, and commissioned on 31 July 1923, under the command of Captain Masao Sugiura. Completed just prior to the 1923 Great Kantō earthquake on 1 September, Yūbari was quickly pressed into service to help evacuate refugees from the Shinagawa area. Crown Prince Hirohito boarded the vessel for a tour from Yokohama to Yokosuka on 10 October. The ship was transferred to the 3rd Cruiser Division of the 1st Fleet on 1 December and Masao was replaced by Captain Yamaguchi Nobuichi. Accompanied by the light cruisers and , Yūbari cruised off the Chinese coast in March 1924. After serving as the flagship for a destroyer squadron during the annual fleet maneuvers in late 1924, she was placed in reserve on 1 December.
Yūbari made a lengthy cruise during April–June 1925 in the Western Pacific, including a visit to Australia, and observed the fleet exercises of the United States Navy west of the Hawaiian Islands. The ship was based in Japanese Taiwan in August–November and then became flagship of the 1st Destroyer Squadron of the 1st Fleet on 1 December. Twelve months later she was transferred to the 2nd Destroyer Squadron of the 2nd Fleet where her squadron exercised in Taiwanese waters during March–April 1927 and was assigned to the Combined Fleet during the Special Great Maneuvers held near the Ryukyu and Bonin Islands. Yūbari was placed in reserve on 1 December and was attached to the Imperial Japanese Naval Academy as a training ship until mid-1931. She was refitted at the Sasebo Naval Arsenal from 4 September to 23 January 1932.
During her refit, Yūbari was transferred to the 1st Fleet and was assigned as the flagship of the 1st Destroyer Squadron on 1 December 1931. The cruiser and her destroyers was ordered to Shanghai, China, on 26 January after the staged rioting had begun that prefaced the First Shanghai Incident. The squadron escorted troopships carrying Imperial Japanese Army troops past Chinese forts at the mouth of the Huangpu River in Wusong on 8 February. After the troops had disembarked, the squadron began bombarding the forts as the troops assaulted them and nearby Chinese positions in the Zhabei and Hongkou Districts through the following day.
thumb|upright=1.0|Yubari in 1937
On 15 November 1934, Captain Tadashige Daigo assumed command, and Yūbari was reassigned to the Yokosuka Guard Squadron of the Yokosuka Naval District, where she was refitted 9 July–15 November 1935. Afterwards, she was assigned to the 5th Destroyer Squadron of the 3rd Fleet and conducted patrols off of the China coast and visiting the island of Java in the Dutch East Indies in 1936. With the start of the Second Sino-Japanese War in August 1937, Yūbari assisted in the evacuation of Japanese civilians from coastal cities in southern China. On 14 September Yūbari and the destroyers and were entering the Pearl River Delta at Bocca Tigris when they encountered the elderly protected cruiser and the sloop Hai Chow (formerly the HMS Pentstemon). The Japanese ships engaged the Chinese ones and the forts defending the area with little effect and both sides retreated. As Yūbari steamed back to base, she was attacked by Northrop A-17 light bombers of the Chinese Air Force. Although the ship was not struck, near misses wounded five sailors. Subsequently, she was sent north to cover landings by the Imperial Japanese Army at Hangzhou on 20 October. The cruiser and her flotilla arrived at Kwajalein on 3 December having already received the code phrase "Climb Mount Niitaka" which set the date for the declaration of war on the United States by the Japanese as 8 December (Japanese time).
While preparing for subsequent operations, Yūbaris anti-aircraft suite was reinforced by two pairs of Type 93 machine guns, one on each side of the bridge on 3 January 1942. The 6th Destroyer Squadron then spent the next several months escorting the forces involved in the capture of Rabaul and Kavieng, New Ireland, off the coast of New Guinea on 23–24 January, followed by the Invasion of Salamaua–Lae on 8 March. This was interrupted by an unsuccessful attempt to intercept the American aircraft carrier group centered around the on 20 February. On 10 March, she was attacked by two Douglas SBD-3 Dauntless dive bombers from the Lexington, which scored two near-misses that killed a number of her anti-aircraft gun crews. She was then strafed by four Grumman F4F-3 Wildcat fighters from Lexington, which killed her executive officer and several crewmen on her bridge. The following morning she was attacked again by aircraft from . Yorktowns Dauntlesses detonated gunpowder bags near her forward twin-gun turret, causing a fire, and a Wildcat strafing run detonated gasoline drums stored in her port lifeboat. Firefighting teams found the fire hoses too short and the large fire threatened the forward torpedo mount. Captain Ban Masami ordered the torpedoes jettisoned, but the mount could not rotate due to a power failure, forcing the crew to use pulleys and ropes to dump the torpedoes manually. During this battle Yūbari evaded 67 bombs and 12 torpedoes, suffering 13 killed and 49 wounded. She was bombed again on 20 March by four USAAF B-17 Flying Fortress bombers while returning to Rabaul with four near-misses that opened three holes in her stern. She returned to Truk for emergency repairs on 25 March. These were completed, together with the addition of more splinter armor for the bridge, on 10 April.
Beginning on 29 June 1942, Yūbari took part in the Solomon Islands campaign, including landing personnel on Guadalcanal to construct an airfield. The 6th Destroyer Squadron was disbanded on 10 July and the cruiser was reassigned to No. 2 Surface Escort Force of the 4th Fleet at Truk. She was transferred to Rabaul on 17 July and began to develop problems with her center turbine around that time that limited her to on two shafts. The ship fired 96 main-gun rounds and four torpedoes during the battle. Yūbari escorted the landing force for Operation RY, the occupation of Nauru and Ocean Islands, departing Truk on 26 August and returned there on 5 September. Starting on 10 September, she began making patrols and escorted convoys between Truk and the Marshall Islands, Gilbert Islands, and Palau. She was based at Tarawa from 20 October to 20 November as a guard ship. at position , with the loss of 19 crewmen. She was stricken from the navy list on 10 June 1944.