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Developement and planning of a new capital ship for Koko no Kaigun started on the sly in early 1934, after the Diet had passed the first Rearmament Bill. First drafts took off from some of the early 20s designs that had been shelved because of the Washington Naval Treaty. The 28,5-knot ones armed with eight and ten 409mm (16.1-inch) guns were immediately discarded as inferior to that of the Amagi and the cheating Kii Class,. Focus was thus shifted to the twelve-gunned 409mm design, the last approved and ordered before the WNT, and the early draft for a Battleship armed with six 460mm (18.1-inch) guns in twin turrets. At that time, the only requirement already set was a top speed of 30 knots, to allow the new battleship, if eventually built, to operate jointly with the Amagi and the two Kiis.
The 12x409mm design was the first to progress into the planning stage. By mid 1935 it called for a 52.600ton ship at standard load, 259,6m long, 35m abeam with a draft of 9,5m. 360mm belt armor, 178mm decks, 76mm bulkheads and an endurance of 7.900 nautical miles at 16knots. Secondary armament was to be made of twelve guns, either 152 (6-inch) or 155mm (6.1-inch), augmented by eight 127mm (5-inch) ones. Design for the 6x460mm-gunned ship was less advanced, but called for similar specifications except for a secondary armament of fifteen guns.
Koko's denunciation of the naval treaties changed the tables, shifting the design efforts towards the heavier gunned ships. Developement of a 460mm gun, suitably labeled 40cm/45 Type96 started in 1936 after the second Rearmament Bill passed by Morimoto's government. The Bill also approved construction of a new class of four Battleship. Following similar plans as the Japanese to give the fleet some edge over the USN and RN by building superior ships to counter numerical disatvantage designers ditched completely the 409mm-gunned design focusing solely on the 460mm one. Despite this, the Naval Staff was not satisfied with a main armament of just six 460mm guns, so a fourth twin turret was added, bringing the main battery to eight barrels. To counter the increased weight, maintaining the planned endurance and speed and thus make space for the extra main guns, the hull was lenghtened by over 20 meters and the fifth 152mm gun turret -to be placed on centerline aft, superfiring the 460mm one- was deleted entierely. Design was finalized by fall 1936, and the lead ship started construction on October 14th of the same year at Toumachi Naval Arsenal. She recieved the name Yashima a few weeks later, her sisters were to be called Suruga, Omi and Mikawa. 
Yashima's construction went on in deep secrecy -just like that of the Yamato in Japan- until her launch, on September 29th 1938, 23 months after she was laid down. Fitting out lasted for another 15 months. Yashima was commissioned as the new Koko no Kaigun Flagship on December 1st 1939, starting her trials immediately after. Despite the war in Europe was already raging since two months secretness around the newly built ship remained high. Especially, the Naval Staff accuratley avoided having any of the Kii class ships or the Amagi sitting in port or operating together with Yashima. The reason was obvious, since during her tortuous design phase the new Battleship had grown to massive dimensions.
At 281,95meters (925feet, 1inch) in overal lenght she was the longest warship ever built by any navy up to that point, dwarfing any other warship -beign Carriers, Battleships or battlecruisers- by at least 11 meters (36 feet, 1 inch). Her waterline lenght was 276,01meters (905feet, 6inches), her beam 35 meters (114 feet, 10 inches) and her draft 10,6 meters (34 feet, 9 inches) for a standard displacement of 56.425 Tons and a full load one of 62.727.
Compared to the earlier drafts, belt armor was increased to 409mm (16.1inch) of thickness, angled inwards by 10° with 270mm (10.6inch) angled bulkheads connecting the citadel ends to the main gun barbettes. Anti-torpedo bulkheads were also strenghtened to 112mm (4.4inch) while decks were left at 178mm (7inch). Steering gear was enclosed in a 200mm (7.9inch) box, with the quarterdeck also protected by 50mm (2.2inch) steel plates, a lighter 30mm (1.2inch) protection was also given to the foredeck up to the anchor capstans. Main guns were protected by 540mm (21.3inch) thick plates on face, 236mm (9.3inch) on sides and roof and 409mm (16.1inch) barbettes. Secondary guns had 50mm (2.2inch) faces and 25mm (1.inch) roofs, sides and barbettes. The conning tower was protected by 500mm (19.7inch) thick steel plates.
Yashima main gun battery consisted of eight 460mm (18.1-inch) 45 caliber guns in four twin turrets. Almost identical in specification to the Guns of the Battleship Yamato, developed during the same timeframe, they had a maximum elevation of 43° and could fire 1.360kg (2.998lbs) standard and 1.460kg (3.219lbs) armour piercing shells at a maximum range of 41.780m (45.790yards), with a rate of fire of roughly 2 shots per minute, and 100 shells carried for each gun. Secondary armament was made up by four triple 152mm (6-inch) turrets, originally intended to arm the Senjo Class Cruisers before it was decided to convert them from CL to CAs. Fitted two on each beam, abaft the fore and aft superstructure, those 60-caliber guns fired 45,6kg (100lbs) shells up to 26.890m (29.414yards) away. They had a maximum elevation of 45° and a 6-round per minute rate of fire. 165 shells were carried for each gun. The main anti-aircraft weapon was the ubiquitous 127mm (5-inch) twin gun mount, their number increased from eight to twelve and fitted at midship in raised platforms. The guns were also shielded to offer further protection to the crew from the blast of the 460mm guns firing. They could elevate up to 90° and fire 23,45kg (51,7lbs) shells up to 14.800m (16.185 yards) with an effective AA ceiling of 9.400m (30.840feet) at 75° elevation. 300 rounds were carried for each gun. Light AA armament was made up by eight triple 25mm machine guns for a total of 24 barrels.
Fire control for the main guns was provided by a 15-m rangefinder and director set fitted on top of the massive forward pagoda tower, with a backup director fitted on the aft superstructure. There were also two 4,5m rangefinder for the secondary guns fire-control located on the forward superstrucutre, and four combined HA director-rangefinder sets for the 127mm anti-aircraft fire: two on each side abaft the funnel and another two on the aft superstructure. Four searchlight and four machine-gun directors were also fitted, in addition to an 1,5m navigation rangefinder forward and an auxiliary lookout station on the aft superstrucutre. The searchlight themselves were four 110cm models fitted on platforms around the funnel, above one of the HA director sets. The ship could operate two seaplanes, Nakajima E8N, thanks to a collapsible crane and a catapult fitted at the stern. A small maintenance hangar located between the catapult itself and the aft 460mm guns could house a third dismantled seaplane.
Engine-wise, twelve oil-firing boilers powered four steame turbines generating 167.522shp, driving four three bladed propellers. Coupled with the elongated hull lines and the specifically designed bulbous bow, the engines pushed the ship with ease to the intended top speed of 30knots. Thanks to a 6.300t bunkerage, endurance was 8.100 nautical miles at a cruising speed of 16knots, roughly 200 miles more than the original design.
Complement for the Yashima was 2.363 officers and men. As flasgship this rose to 2.489.


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Interesting stuff.  Source?  or is this fiction?

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