The Yamato class were designed in the post Washington Naval Treaty period. The treaty had been extended by the London Naval Treaty of 1930 which limited the signatories to no battleship production before 1937 - the Japanese withdrew from the Treaty at the Second London conference of 1936. Design work on the class began in 1934 and after modifications the design for a 68,000 ton vessel was accepted in March 1937. The Yamato was built at a specially prepared dock at Kure Naval Dockyards beginning on 4 November 1937 . She was launched on 8 August 1940 and commissioned on 16 December 1941 . Originally it was intended that five ships of this class would be built, but the third ship of the class, Shinano , was converted to an aircraft carrier during construction after the defeat at the Battle of Midway , the un-named "Hull Number 111" was scrapped in 1943 when roughly 30% complete, and "Hull Number 797", proposed in the 1942 5th Supplementary Program, was never ordered. Plans for a Super Yamato class, with 50.8 cm (20 inch) guns, provisionally designated as "Hull Number 798" and "Hull Number 799", were abandoned in 1942.
The class was designed to be superior to any ship that the United States was likely to produce - At the time Japanese didn't think Americans could come up with a two-ocean navy, thinking that all ships should pass the Panama-channel. The 46 cm (18.1 inch) main guns were selected over 40.6cm (16 inch) because the width of the Panama Canal would make it impracticable for the U.S. Navy to construct a battleship with the same caliber guns without severe design restrictions or an inadequate defensive arrangement. To further confuse the intelligence agencies of other countries, her main guns were officially named as 16inch Special , and civilians were never notified of the true nature of the guns. Their budgets were also scattered among various projects so that the huge total costs would not be immediately noticeable.
At the Kure Navy Yard where she was built, the construction dock was deepened, the gantry crane capacity was increased to 100 metric tonnes, and part of the dock was roofed over to prevent observation of work.
Arc welding , a relatively new procedure at that time, was used extensively during construction. The lower side-belt armor was used as a strength member of the hull structure. The undulating line of the main deck forward saved structural weight without reducing hull girder strength. Tests of models in a model basin led to the adoption of a semitransom stern and a bulbous bow , which reduced hull resistance by 8%. The ship had one single large rudder (at frame 231), which gave it a small (for a ship of that size) turning circle of 640 meters. By comparison the US Iowa class fast battleship had one of over 800 m. There was also a smaller auxiliary rudder installed (at frame 219) which was virtually useless. The steam turbine power plant was of a relatively low powered design (25 kgf / cm² (2.5 MPa ), 325 °C), and as such, their fuel usage rate was very high. This is a primary reason why they were not used during the Solomons Campaigns and other mid-war operations. There were a total
of 1147 watertight compartments in the ship.
Plans for the Super Yamato Class
During the second world war, almost all of the participating countries had plans for their own super class battleships. These are ships that for whatever reason - usually more pressing wartime programs - were designed but never built, although most were laid down. The intended superclass ships were the Montana class (U.S.), Lion class (UK), 'H' class (Germany), and Sovyetskiy Soyuz class (USSR). However, none of these ships with the possible expception of the Montana class, would have been been a match with Yamato and Musashi even if they were built. While others were planning their superclass ships, the Japanese already had built theirs. However, even further plans for the Super Yamato Class were still done. Warships number 798 and 799 were to be the first Super Yamato class battleships. These were designed in 1941 with construction to begin the following year, with completion estimated for 1946. These ships were also referred to as design A-150. The orders were never placed, so any specifications are rudimentary at best.
The A-150 Super Yamatos were planned with 20 inch main guns, have a maximum speed of 30 knots and maximum displacement of 82.000 tons. Ballistic tests were conducted on the 20-inch gun with an AP round that weighed 4,188 lb. However, no examples of this gun were found after the war. No further preparations were carried out on these ships due to the changing strategic situation after 1942 - in other words, Japanese were starting to run out of resources. With the fate that all other Yamato class ships shared, it was probably good Super Yamatos were never built.
Illustration of a possible configuration of a Super Yamato Class Battleship with 20 inch guns.
Wikipedia (Construction History), Ibiblio.org (Super Yamato), Alt-Naval (Super Yamato illustration)