Design A-150 battleship
The Super Yamato-class (超大和型戦艦) of battleships were designed to have even greater fire power and size than the Yamato-class, which were the largest battleships ever built.
The design called for six 508 mm (20-inch) guns in three twin turrets on a normal displacement of 70,000 tons. Maximum speed was to be about 27 knots, the same as the Yamato but less than contemporary foreign battleships, as well as most Japanese aircraft carriers. The 20-inch would have been the largest caliber and most powerful naval gun in the world, firing armor-piercing shells weighing over 4,000 pounds. The 460 mm (18.1-inch) gun of the Yamato fired shells weighing up to 3,200 pounds. The 406 mm (16-inch guns) of the American Iowa-class fired an armor-piercing shell weighing 2,700 pounds.
Two ships were planned under the 1942 Program, which were provisionally designated as "Hull Number 798" and "Hull Number 799". No Project Number was assigned to even a rough design and no orders were ever placed. After the Battle of Midway construction of Japanese warships other than carriers, destroyers and submarines were reduced to the lowest priority. The Super Yamato design does not seem to have progressed as far as that of the Super Type A Cruiser which was also projected under the 1942 Program but never ordered.
The Japanese had found that recoil and blast from the Yamato's 460 mm gun was significantly greater than the stresses of a 16-inch gun, requiring much heavier mountings, enclosed mounts for the secondary guns and special hangars for the ship's boats and aircraft. Blast from the 20-inch gun would probably have been at least twice that of a 16-inch gun. No seaman could even be on deck or be instantly vaporized by the shockwave.. The reduction in the number of guns from nine to six would have resulted in turrets and mounts similar in weight to those of the Yamato but the rate of fire would probably have been lower.
Because most Japanese harbors were relatively shallow, the Yamato was very wide in order to reduce her draft. This increased her buoyancy and thus her ability to survive underwater damage, but also lowered her speed. The Super Yamato would presumably have been even wider because of the extra weight and would have had either more powerful machinery or a lower speed.
Competition
The Super Yamato followed the Japanese pattern of attempting to build ships that were individually more powerful than foreign counterparts in order to offset the superior numbers that the United States, with its far superior shipbuilding capacity, could produce.
Japan also mistakenly assumed that the United States would not build battleships larger than the Iowas because such ships were too big for the Panama Canal, as the US broke that precedent with the proposed Montana-class battleships.
If Japan, the United States and Great Britain had completed all their planned battleships during the war, Japan would have had only four Yamatos and two Super Yamatos against 17 new American and nine new British battleships (the King George V and Lion-class battleships). As it was, the United States built ten battleships (the North Carolina-, South Dakota-, and Iowa-class battleships), while the Japanese completed just two.
American and British battleships also had the technological advantages of superior gunnery, radar and speed, the former giving an improved chance of inflicting damage while the latter improved deployment.
References
- Hansgeorg Jentshura, Dieter Jung and Peter Mickel, Warships of the Imperial Japanese Navy 1869–1945, Arms & Armour Press, 1977.