Design A-150 battleship

An artist's interpretation of a A-150-class battleship by Richard Allison.
Class overview
Name:	A-150
Operators:	Imperial Japanese Navy
Preceded by:	Yamato class
Planned:	2
Completed:	0
Cancelled:	2
General characteristics
Type:	Battleship
Displacement:	Approximately 70,000 long tons (78,000 short tons; 71,000 t)
Length:	263.0 m (863 ft) (est.)
Beam:	38.9 m (128 ft) (est.)
Propulsion:	Unknown
Armament:	6 × 510 mm (20.1 in)/45 caliber guns (3 × 2) "Many" 100 mm (3.9 in)/65 caliber guns
Armor:	Possibly a 460 mm (18 in) side belt; nothing more is given in sources

Design A-150, also known as the Super Yamato class, was an Imperial Japanese plan for a class of battleships. Begun in 1938–39, the design was mostly complete by 1941. However, so that a demand for other types of warships could be met, all work on Design A-150 was halted and no keels were laid. Authors William H. Garzke and Robert O. Dulin have argued that Design A-150 would have been the "most powerful battleships in history" because of the massive size of their main battery of six 510 mm (20 in) guns as well as numerous smaller caliber weapons.

Initial plans for the A-150 battleships called for eight or nine 510 mm (20.1 in) guns in double or triple turrets. The successful construction of a 480 mm (18.9 in) gun in 1920–1921 made the Japanese confident that a 510 mm (20.1 in) could be built. In addition, a top speed of 30 knots (56 km/h; 35 mph) was desired so that the class would be faster than the United States' 27 kn (31 mph; 50 km/h) North Carolina-class battleships. However, these grand specifications were curtailed when tests culminated in a ship that had a displacement of some 90,000 tons; it was felt that ships of this size would be "too large and too expensive".

Initial design studies were undertaken after the completion of plans for the Yamato class (1938–39); they focused on a ship with a displacement nearer to that of the Yamatos. As the Japanese expected that the Americans would be able to obtain the true characteristics of that class (namely the principal armament of 460 mm (18.1 in)), the use of 510 mm guns was vital to keep with Japan's policy of individual ships' superiority over their American counterparts; the A-150s were meant to counter the United States' reply to the Yamatos.

Plans were "essentially completed" sometime in 1941; however, similar to the fate of documents relating to the Yamato class, most papers and all plans relating to the class were destroyed at the end of the war, meaning that the full specifications of the ships are not known. It is known the ships would have had greater firepower than the Yamato class: a main battery of six 510 mm (20.1 inch) guns in three twin turrets and a secondary armament of "many" 100 mm (3.9 in)/65 caliber guns. The displacement was to be similar to the Yamato class, which was around 60,000–70,000 tonnes. The side armor belt was probably going to be 460 mm (18 in). This was so large that steel mills in Japan were incapable of manufacturing it; instead, "double strakes of armor plates" were going to be used, which would have been much less effective than just one single plate.