The were a pair of dreadnought battleships (lead ship and her sister ship ) built for the Imperial Japanese Navy (IJN) towards the end of World War I, although they were not completed until after the war. The last of Japan's pre-Treaty capital ships, they were the first class to carry guns, the largest afloat at the time and the first bigger than . , the lead ship of the class, frequently served as a flagship. Both ships carried supplies for the survivors of the Great Kantō earthquake in 1923. They were modernized in 1933–1936 with improvements to their armor and machinery and a rebuilt superstructure in the pagoda mast style. Nagato and Mutsu briefly participated in the Second Sino-Japanese War in 1937 and Nagato was the flagship of Admiral Isoroku Yamamoto during the attack on Pearl Harbor on 7 December 1941 that began the Pacific War.
The sisters participated in the Battle of Midway in June 1942, although they did not see any combat. Mutsu saw more active service than her sister because she was not a flagship and participated in the Battle of the Eastern Solomons in August before returning to Japan in early 1943.
One of Mutsus aft magazines detonated in June, killing 1,121 crew and visitors and destroying the ship. The IJN conducted a perfunctory investigation into the cause of her loss and concluded that it was the work of a disgruntled crewmember. They dispersed the survivors in an attempt to conceal the sinking to keep up morale in Japan. Much of the wreck was salvaged after the war and many artifacts and relics are on display in Japan.
Nagato spent most of the first two years of the war training in home waters. She was transferred to Truk in mid-1943, but did not see any combat until the Battle of the Philippine Sea in mid-1944 when she was attacked by American aircraft. Nagato did not fire her main armament against enemy vessels until the Battle of Leyte Gulf in October 1944. She was lightly damaged during the battle and returned to Japan the following month for repairs. The IJN was running out of fuel by this time and decided not to fully repair her. Nagato was converted into a floating anti-aircraft platform and assigned to coastal defense duties. After the war, the ship was a target for U.S. nuclear weapon tests during Operation Crossroads in mid-1946. She survived the first test with little damage, but was sunk by the second test.
Background The IJN considered a battle fleet of eight modern battleships and eight modern armored cruisers necessary for the defense of Japan, and the government adopted that policy in 1907.Evans & Peattie, p. 150 This was the genesis of the Eight-Eight Fleet Program, the development of a cohesive battle line of 16 capital ships less than eight years old.Stille, p. 7 Advances in naval technology like the British battleship and the battlecruiser forced the IJN to several times re-evaluate the ships that it counted as modern. By 1910, the IJN considered none of its current ships to be modern and restarted the program in 1911 with orders for the dreadnoughts and the s. By 1915, the IJN was halfway to its goal and wanted to order four more dreadnoughts, but the Diet rejected its plan, and the 1916 budget authorized only one dreadnought, later named , and two battlecruisers. Later that year, American President Woodrow Wilson announced plans for 10 additional battleships and six battlecruisers, and the following year the Diet authorized three more dreadnoughts in response, one of which would later be named .Evans & Peattie, pp. 160, 166–167
Design
Allocated project number A-102, the Nagato class was designed before Commander Yuzuru Hiraga was reassigned to the Navy Technical Department (NTD) responsible for ship design, although Hiraga is often credited with the design of these ships. In contrast to earlier designs, the Nagato class used the American "all or nothing" armor scheme that maximized the armor thickness protecting the core of the ship by eliminating armor elsewhere. The design had two armored decks of medium thickness rather than the single thicker deck used formerly. The ships also used a new type of underwater protection system that successfully resisted penetration by torpedo warheads in full-scale trials. It consisted of a deep water-tight compartment adjacent to the hull, backed by a thick torpedo bulkhead that connected to the side and deck armor plates, with a deep fuel oil tank behind it.Lengerer, pp. 4–5; Skwiot 2008, pp. 3, 8
Although the United States Navy planned to arm its with guns before the Nagato class was designed, Nagatos guns made her the first dreadnought that was launched armed with guns larger than .Lengerer, p. 4
On 12 June 1917, well before Mutsu was laid down, Hiraga proposed a revised design for the ship that reflected the lessons from the Battle of Jutland that had occurred the previous year, and incorporated advances in boiler technology. Given project number A-125, his design added an extra twin main-gun turret, using space and weight made available by the reduction of the number of boilers from 21 to 12, while the power remained the same. He reduced the secondary armament from 20 guns to 16, although they were raised in height to improve their ability to fire during heavy weather and to improve their arcs of fire. To increase the ship's protection he proposed angling the belt armor outwards to improve its resistance to horizontal fire, and increasing the thickness of the lower deck armor and the torpedo bulkhead. Hiraga also planned to add anti-torpedo bulges to improve underwater protection. He estimated that his ship would displace as much as Nagato, although it would cost about a million yen more. Hiraga's changes would have considerably delayed Mutsus completion and were rejected by the Navy Ministry.Lengerer, pp. 3, 6–7
Description The ships had a length of between perpendiculars and overall. They had a beam of and a draft of .Skwiot 2008, p. 4 The Nagato-class ships displaced at standard load and at full load.Whitley, p. 200 Their crew consisted of 1,333 officers and enlisted men as built and 1,368 in 1935.Jentschura, Jung & Mickel, p. 28 In 1944, the crew totaled around 1,734 men.Stille, p. 34
The Nagato class was equipped with a unique heptapodal (seven-legged) mast designed to maximize rigidity for range-finding purposes and survivability under shellfire. It consisted of a thick vertical leg in the center surrounded by six outer legs. The central leg was large enough to accommodate an electric elevator running between the foretop and main deck.Gardiner & Gray, p. 231 In November 1944, the tops of Nagatos mainmast and funnel were removed to improve the arcs of fire for her anti-aircraft guns.
In 1927, Mutsus bow was remodeled to reduce the amount of spray produced when steaming into a head sea. This increased her overall length by to . This proved successful and her sister's bow was rebuilt in 1930. During their 1934–1936 reconstruction, the ships' sterns were lengthened by to improve their speed and their forward superstructures were rebuilt into a pagoda mast. They were given torpedo bulges to improve their underwater protection and to compensate for the weight of the additional armor and equipment. These changes increased their overall length to , their beam to and their draft to . Their displacement increased over to at deep load.Skwiot 2008, pp. 6, 9–10, 34, 71–72
Propulsion The ships were equipped with four Gihon geared steam turbines, each of which drove one propeller. The turbines were designed to produce a total of , using steam provided by 21 Kampon water-tube boilers; 15 of these were oil-fired while the remaining half-dozen used fuel oil that was sprayed on the coal to increase its burn rate. The boilers had a working pressure of .Skwiot 2008, pp. 43–44
The ships had a stowage capacity of of coal and of oil, giving them a range of at a speed of . Nagato exceeded her designed speed of during her sea trials, reaching at and Mutsu reached the same speed with . The US Navy did not learn their actual speed capability until about 1937; previously it had believed that the ships were capable of only .
During refits in 1923–25, the fore funnel was rebuilt in a serpentine shape in an unsuccessful effort to prevent smoke interference with the bridge and fire-control systems. The rebuilt fore funnel was eliminated during the ships' 1930s reconstructions when their boilers were replaced by 10 oil-fired Kampon boilers, which had a working pressure of and temperature of .Skwiot 2008, pp. 73, 76–77 The turbines were also replaced by lighter, more modern, units and their propellers were replaced by larger ones. When the ships conducted their post-reconstruction trials, they reached a speed of with .Skwiot 2008, pp. 43, 78 Additional fuel oil was stored in the bottoms of the newly added torpedo bulges, which increased their capacity to and thus their range to at 16 knots.
Armament
The main armament of the Nagato-class ships consisted of eight 45-caliber 3rd Year Type naval guns, mounted in two pairs of twin-gun, superfiring turrets fore and aft. Numbered one through four from front to rear, the hydraulically powered turrets gave the guns an elevation range of −2 to +35 degrees. The rate of fire for the guns was around two rounds per minute. Their turrets were replaced in the mid-1930s using the turrets stored from the unfinished s. While in storage they were modified to increase their range of elevation to −3 to +43 degrees,Skwiot 2008, p. 19 which increased the gun's maximum range from .Friedman, p. 269
By World War II, the guns used Type 91 armor-piercing, capped shells. Each of these shells weighed and had a muzzle velocity of . Also available was a high-explosive shell that had a muzzle velocity of . A special Type 3 Sankaidan incendiary shrapnel shell was developed in the 1930s for anti-aircraft use.Campbell, pp. 175, 181–182
The ships' secondary armament of twenty 50-caliber 3rd Year Type naval gun was mounted in casemates on the upper sides of the hull and in the superstructure. The latter guns had a maximum elevation of +20 degrees and the former could elevate to +25 degrees which gave them ranges of and respectively.Skwiot 2008, pp. 24, 73 Each gun could fire a high-explosive projectile at a rate up to 10 rounds per minute.Campbell, p. 190 Anti-aircraft defense was provided by four 40-caliber 3rd Year Type 3-inch AA guns in single mounts. The high-angle guns had a maximum elevation of +75 degrees, and had a rate of fire of 13 to 20 rounds per minute. They fired a projectile with a muzzle velocity of to a maximum height of .Campbell, p. 198 The ships were also fitted with eight torpedo tubes, four on each broadside, two above water and two submerged. They used the 6th Year Type torpedo which had a warhead of Shimose powder. It had three settings for range and speed: at , at , or at .Skwiot 2008, p. 31
Around 1926, the four above-water torpedo tubes were removed and the ships received three additional 76-millimeter AA guns that were situated around the base of the foremast.Skwiot 2008, p. 70 They were replaced by eight 40-caliber 12.7-centimeter Type 89 dual-purpose guns in 1932,Hackett, Kingsepp & Ahlberg, Nagato fitted on both sides of the fore and aft superstructures in four twin-gun mounts.Whitley, p. 202 When firing at surface targets, the guns had a range of . Their maximum rate of fire was 14 rounds a minute, but their sustained rate of fire was around eight rounds per minute.Campbell, pp. 192–193 Two twin-gun mounts for license-built Vickers 2-pounder light AA guns were also added to the ship that same year. They had a rate of fire of 200 rounds per minute.Campbell, p. 74