Displacement: 32,700 tons/33,223 tonnes (standard); 38,500 tons/39,116 tonnes (full load)
Length: 708ft/215.8m (oa); 660ft 4in/201.2m (pp); 669ft 9in/204.07m (wl)
Draught: 30ft/9.08m (mean)
Machinery: twenty Kampon boilers; 4-shaft Gihon turbines
Bunkerage: 1,600 tons coal, 3,400 tons oil Performance: 80,000shp = 26.7kts
Range: 5,500nm at 16kts
Protection: main belt 12in; deck 7in; barbettes 12in; casemates 1in max.; turrets face 14in; CT 14.5in
Guns: eight (4x2) 16in; twenty (20x1) 5.5in; four (4x1) 3in
Torpedo tubes: four 21 in (submerged), four 21 in (above water)
As reconstructed, 1934-6
Displacement: 39,130 tons/39,756 tonnes (standard); 46,356 tons/47,097 tonnes (full load)
Length: 734ft/224.94m (oa); 660ft 3in/201.17m (pp); 725ft 4in/221.07m (wl)
Beam: 108ft 1in/32.96m
Draught: 31ft 1in/9.49 m
Machinery: ten Kampon boilers; 4-shaft Kampon geared turbines
Bunkerage: 5,560 tons oil
Performance: 82,300shp = 25kts
Range: 8,560nm at 16kts
Protection: as before
Guns: eight (4x2) 16in; eighteen (18x1) 5.5in; eight (4x2) 5in DP; twenty 25mm
Torpedo tubes: nil
Aircraft: three, catapult one
With these, the first ships of the planned 8-8 programme (eight battleships, eight battlecruisers) and the first fully Japanese battleship design (A-102), Japan increased main gun calibre to 16in and became the first of the major naval powers to adopt such a weapon. The ships were also designed for high speed, being faster than the British Queen Elizabeths (though the latter were an earlier design). Lessons learned from wartime naval operations in Europe, particularly from the Battle of Jutland, were reflected in the new Japanese design and caused some delay in placing orders.
The new class were some 25 feet longer than Hyuga and beam was increased by 1 foot, displacement increasing by about 2,000 tons. The protective scheme included a waterline belt of 12in armour, tapered at its lower edge to 3in, above which was an 8in armoured strake. This belt protected the machinery and magazine spaces. Forward and aft of this the side armour was reduced to 8in and then to 4in. The horizontal armouring consisted of a 1in forecastle deck, increased to l.5in around the uptakes and an upper protective deck made up of three layers of plate, lin + lin + .75in, which covered the whole beam of the ship and abutted the top edge of the side armour. The lower armoured deck was made up of 2in plate on its narrow horizontal section outboard of the torpedo bulkhead, reinforced to three layers of 1in on the outer sloped section. This was carried down to the upper edge of the inner double bottom plating to meet the lower edge of the side belt, from where it was continued in the same thickness downwards and inboards to the floor of the machinery spaces. The extension of this plating through the double bottom to the skin was reduced to two layers of lin. Outboard of this armoured bulkhead were void spaces. There was a longitudinal torpedo bulkhead at a distance of 26ft 6in from the centre-line. Barbette armour was 12in and the conning tower 14.5in. The main turrets had 18in faces, 11in sides and 7.5in rears, with 9in maximum roof protection. The secondary armament casemates had 1in. Protection amounted to about 31.7 per cent of the standard displacement.
To achieve the designed speed of 26.5 knots, it was necessary to double the installed power as compared to Hyuga and to adopt oil-firing for the majority of the boilers, the latter step being as critical to the Japanese as to Britain, both having no indigenous supplies of fuel oil. But larger and more powerful boilers meant that only twenty-one boilers were required instead of the twenty-four in the previous class. Of these, fifteen were oil-fired. The main machinery was arranged on a four-shaft layout, with Gihon direct-drive turbines, developing 80,000shp.
The 16in 45cal guns were designated 3rd Year Type (1914) and fired a 2,249lb AP shell to a maximum range of 42,000 yards. They were carried in twin turrets, two forward and two aft, with Nos. 2 and 3 superfiring. The mountings were hydraulic, each twin turret weighing 1,004 tons. Elevation was 30°. Their adoption was pushed through by the Navy Minister, Admiral Kato. The secondary armament consisted of 5.5in 50cal 3rd Year Type (1914) guns, all arranged in casemate mountings. Eight of these were disposed on each beam at upper deck level under the forecastle, in two separate batteries, one of six guns forward, the other pair aft. The remaining guns were amidships abreast the forefunnel in the superstructure. These were the same as those mounted in Hyuga. Four 3in AA completed the gunnery outfit. There were four above-water tubes for 21in torpedoes and four submerged tubes. The former were on the upper deck abreast of the after funnel, the latter forward and aft of Nos. 1 and 4 barbettes.
Because these ships did not complete until 1920-1, they incorporated modifications during construction prompted by First World War experience. These almost certainly included extra horizontal armour over the magazine spaces. In 1922 they were given a distinctive curved fingernail-type smoke deflector on the forefunnel but this failed fully to solve the problem of fumes and smoke in the control positions. In 1924 further work was carried out when the forefunnel was bent back considerably and reduced to the same height as the after funnel. At the same time, a boom was fitted to the mainmast to handle floatplanes. The following year Nagato was fitted with a Heinkel catapult for experimental purposes, supported on the guns of No. 2 turret. On 25 August 1933 Nagato was taken in hand by her original builder for major reconstruction, followed by her sister on 6 September 1934 at Yokosuka. During the course of this work, the hull was given bulges which increased beam to 111 feet, these extending from the bottom plating to above the side armour. The upper bulge compartments were filled with crushing tubes, the lower ones being oil bunkers. Horizontal protection above machinery and magazines was improved by the addition of a run of lin plating on the forecastle deck and the continuing of the lower armoured deck to the uptakes with lin D steel on two layers of lin HT steel. The hull bottom in the vicinity of the original turn of the bilges was given a 1.75in strake. Improvements to the armouring increased the weight of protection by 31 per cent. The hull itself was increased in length by 26 feet to compensate the increased beam, and new machinery installed although power output was not increased. Ten Kampon boilers replaced the original steam plant, all uptakes being trunked into one funnel, and new turbines were fitted. This new machinery gave a weight saving of some 500 tons. Full oil-firing was now adopted and the speed dropped by only 1.5 knots despite full load displacement having risen to 42,784 tons. So far as the armament was concerned, the elevation of the 16in turrets was increased to 43° and that of the secondary battery to 35°. Four twin 5in DP were added, two port and starboard just abaft the pagoda tower and two more sited abreast the mainmast where they replaced the older pattern 3in AA. Twenty 25mm formed the light AA outfit in a mix of twin and single mountings. The torpedo tubes were removed to accommodate the bulges, which ran the full length of the citadel. Finally, a catapult was added to port on the shelter deck aft of No. 3 turret and a collapsible crane was fitted. Three floatplanes could be carried, initially the Nakajima E8N1 'Dave' but later replaced by the Mitsubishi F1M2 'Pete'. Nagato rejoined the fleet on 31 January 1936 and Mutsu on 30 September of that year.
During hostilities, the only modifications made were to add radar, in Nagato at least, and to augment the AA outfits. What was done to Mutsu is not clear, but by June 1944 Nagato's 25mm outfit had been increased to sixty-eight and by October to ninety-eight, in sixteen triple, ten twin and thirty single mountings. She retained all her secondary armament until early 1945 when it was landed at Yokosuka during repairs following the Leyte Gulf actions in preparation for further augmentation of the AA outfit.
Both ships participated in the Sino-Japanese conflict and at the start of the Pacific war formed the 1st Battle Squadron, Nagato being Flagship of the Combined Fleet. But she took little or no part in the opening or immediately subsequent operations, other than a sortie on 8-13 December to cover the retirement of the Pearl Harbor Attack Force, being held in reserve in home waters until the Japanese attempt to take Midway Island in May 1942. For this operation, Nagato, together with her sister and Yamato, forming the 1st Battleship Squadron, sailed with the Main Body, 1st Fleet, from Hashira Jima on 29 May but did not become involved in the carrier aircraft action and returned to the Inland Sea on 14 June after the operation was aborted following the loss of the carriers. After the Midway disaster the fleet was re-organised and the two Nagatos formed one division of the 2nd Battle Squadron. Nagato remained in the Inland Sea with the Main Force until the summer of 1943 when, on 15 June, she was assigned to the Battleship Force and sailed from Yashima two days later for Truk, where she arrived on the 23rd. After some four months of inactivity at this base, Nagato sailed for Eniwetok on 17 October and lay in readiness until the 23rd when she participated in a sortie to the south-west of Wake Island, returning on the 26th. She left Truk on 1 February 1944 for Palau and anchored there on the 4th. After only a few days she was ordered to Lingga Roads and, making frequent submarine contacts en route, arrived on the 21st. Here she joined the Second Fleet as Flagship of the 2nd Battle Squadron. On 30 March she sailed to Singapore, spending ten days in dry dock before returning to Lingga Roads. She was relieved as Flagship by Yamato on 5 May and on the 11th sailed for Tawa Tawa where she anchored three days later.
Nagato saw little further action until the Leyte operations at the end of October 1944, when US forces invaded Leyte Island. Nagato joined the two Yamatos for this operation as part of Force A, of the Northern Force, under Vice-Admiral Kurita, moving to the fleet anchorage at Brunei Bay for this purpose. Force A sailed on 22 October, escorted by eight heavy and one light cruiser plus nine destroyers, but was intercepted by US submarines which sank two of the heavy cruisers, Atago and Maya, and damaged a third, Takao. Later the Force was heavily attacked by US carrier aircraft from Task Force 38, the first being from the air groups of Intrepid and Cabot, later from Lexington and Essex. Nagato received two direct hits. One, in No. 1 port boiler room, also disabled No. 4 turret and four guns of the secondary battery as well as reducing her speed to 21 knots for an hour. The other hit was less significant. The Japanese force continued into the San Bernadino Straits and, now joined by Kongo and Haruna with their cruiser and destroyer screen, ran into part of US TF77.4, the six escort carriers of Carrier Divisions 25 and 26, together with their screen of three destroyers and four destroyer escorts off Samar. In this action two US escort carriers, two destroyers and one destroyer escort were sunk, mostly by the ships of Kongo's group, so it is not certain how much action Nagato saw. After the action was broken off by Admiral Kurita, the Japanese Force retired to home waters for repairs at Yokosuka Navy Yard, where she also underwent a minor refit. Some of the damage was never made good, in particular that abaft the bridge. Thereafter, virtually immobilised for want of fuel, and with Yokosuka yard resources diverted to more pressing work, Nagato lay unfinished at Yokosuka in only a partially operational state. On 18 July 1945 carrier planes from TF38 attacked the area, the ship receiving only two minor bomb hits and she survived in the same state, being taken over by the Americans on 20 September. Subsequently she was utilised in the Bikini Atoll atomic bomb tests, surviving Test Able on 1 July 1946 with only minor damage but was almost completely destroyed in Test Baker on the 24th and foundered on 29 July 1946.
Mutsu was serving with the 1st Battle Squadron attached to the Main Force, First Fleet, in the Inland Sea in November 1941. She was mainly employed on training duties until the spring of 1942 when she was sailed to support the Midway operation, leaving Hashira Jima with the Main Body on 29 May. But she saw no action and returned to home waters on 14 June. On 14 July she was assigned to the 2nd Battle Squadron and remained in the Inland Sea, being in refit and maintenance for eight days at Kure at the end of the month. On 9 August she was assigned to the Advance Force and sailed two days later for Truk, arriving on the 17th. Mutsu sailed on the 21st as part of the Support Force for the Fleet Train involved in the attempts to reinforce Guadalcanal, which led to the Second Battle of the Solomon Sea, (Battle of the Eastern Solomons). During this sortie she was attacked by aircraft on the 27th but without effect. After the US Task Force had been sighted south of the Solomon Islands, Mutsu's force was detached to attack it but failed to make contact, and on 2 September she returned to Truk. A week later she was incorporated into the Guadalcanal Force Main Body, but in fact lay at anchor in Truk lagoon until the end of the year. On 7 January 1943 Mutsu sailed from Truk, having been assigned to the Main Force, and returned to Japanese waters, anchoring off Yokosuka on 12 January. Here she underwent repairs and maintenance in dry dock until 6 February. At the end of the month she moved into the Inland Sea, based at Hashira Jima, and remained on training duties until on 8 June 1943, while lying in the anchorage off Hashira Jima in Hiroshima Bay, her after magazines exploded and she sank with the loss of some 1,222 men. The cause was attributed to a cordite fire. In 1970 the wreck was purchased from the Japanese government by the Fukuda Salvage Co. and on 22 July 1970 an attempt was made to raise the after section but this failed when the cables parted. Towards the end of 1971 one 16in turret was salvaged, but further attempts on the hull itself have proved abortive.