The South Dakota class was a group of four fast battleships built by the United States Navy. They were the second class of battleships to be named after the 40th state; the first were designed in the 1920s and canceled under the terms of the Washington Naval Treaty. Four ships comprised the class: , , , and . They were designed to the same treaty standard displacement limit of as the preceding and had the same main battery of nine 16"/45 caliber Mark 6 guns in three-gun turrets, but were more compact and better protected. The ships can be visually distinguished from the earlier vessels by their single funnel, compared to twin funnels in the North Carolinas.
Construction began shortly before World War II, with Fiscal Year (FY) 1939 appropriations. Commissioning through the summer of 1942, the four ships served in both the Atlantic, ready to intercept possible German capital ship sorties, and the Pacific, in carrier groups and shore bombardments. All four ships were retired shortly after World War II; South Dakota and Indiana were scrapped in the 1960s, Massachusetts and Alabama were retained as museum ships.
Development
Background
The preceding two s had been assigned to the FY1937 building program, and in 1936, the General Board met to discuss the two battleships to be allocated to FY1938. The General Board argued for two more North Carolinas, but Admiral William H. Standley, the Chief of Naval Operations, wanted the ships to be of a new design. That meant construction could not begin by 1938, so the ships were assigned to FY1939. Design work started in March 1937 and the draft for two battleships was formally approved by the Secretary of the Navy on 23 June. More specific characteristics for the two ships were ironed out, and those were approved on 4 January 1938. The ships were formally ordered on 4 April 1938.
Due to the deteriorating international situations in Europe and Asia, Congress authorized a further two battleships of the new design, for a total of four, under the Deficiency Authorization of 25 June 1938. The "Escalator Clause" of the Second London Naval Treaty had been activated in the U.S. Navy so it could begin work on the follow-on s, but Congress was willing to approve only the 35,000-ton battleships. A number of deficiencies in the preceding North Carolinas were to be fixed in the South Dakotas; these included insufficient underwater protection and turbine engines not of the most recent technology. The North Carolinas also did not have sufficient space to act as fleet flagships, so the lead ship of the new class was designed with an extra deck on the conning tower specifically for this purpose, although the increase in space and weight from this necessitated removal of two twin 5-inch dual-purpose (DP) gun mounts.
Design
There was a great deal of debate on the requirements for the new battleships. The design board drew up a number of proposals; one called for a ship with nine guns in three triple turrets, deck armor that would have made the ship immune to plunging fire out to , and a top speed of at least . The belt armor was a much more intractable problem; the 16-inch gun could penetrate of plate, the thickest in an American battleship at the time, even at . To proof the ship against her own armament—a characteristic known as "balanced armor"—the main belt would have to be increased to , which would have increased the weight of the vessel to prohibitive levels. To mitigate this problem, sloped armor was proposed; it was infeasible to use inclined armor in an external belt, because it would compromise stability to a dangerous degree. Instead, an internal armor belt was constructed behind unarmored hull plates. This had serious drawbacks, however; it complicated the construction process, and if the armored belt was damaged, the external plating would have to be cut away first before the belt could be repaired.