The South Dakota-class battleship was a class of four battleships used by the United States Navy prior to and during World War II. They were the second class of battleships to be named after the 40th State, the first designed in the 1920s and cancelled under the terms of the Washington Treaty. They were arguably the best battleships in the US Navy at that time, being faster and better protected than the preceding North Carolina-class battleships.


There was a much debate about the requirements for the new battleships. The design board drew up a number of drafts, of which one called for a ship with nine 16 inch guns in three triple turrets, 5.9 inches thick deck armor that would have protected the ship against plunging fire out to 30,000 yards, and a top speed of at least 23 knots. The belt armor was a much more of a problem however, the 16 inch gun could penetrate 13.5 inches of plate, the thickest on a battleship in the US Navy at the time, even at 25,000 yards. To protect the ship against her own armament — a characteristic known as "balanced armor" — the main belt would have to be increased to 15.5 inches, which would have increased the weight of the vessel to prohibited levels in accordance with Washington Treaty. In order to find a solution to this problem, sloped armor was proposed. It was infeasible to use inclined armor in an external belt, as it would compromise stability of the vessel to a hazardous degree. This caused significant setbacks, complicating 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.

To minimize the setbacks of the inclined belt, it sloped outward from the keel, then back in towards the armored deck. This meant that shells fired at relatively close range would hit the upper portion of the belt at an angle, which maximized armor protection. However, the effectiveness of the upper portion of the belt was degraded at longer ranges, because plunging shells would strike it at an angle closer to the perpendicular, increasing their ability to penetrate the armor. It did reduce the area that needed to be covered by the armored deck, which saved additional weight. This enabled the upper belt to be thicker, which to an extent, removed the possibility of plunging fire being effective. Because the belt was internal, it provided the opportunity to extend it to the inner portion of the double bottom, which gave the ship better underwater protection than North Carolina-class ships. Ultimately, the complex double incline belt armor was abandoned when it became apparent that a single slanted belt could provide similar protection, and save several hundred tons of weight.

The size of the hull was also a problem. A longer hull generally equates to a higher top speed, but requires more armor to protect it. In order to keep a higher top speed on a shorter hull, higher-performance machinery is required. Since the South Dakota-class's design was much shorter than the preceding North Carolina-class's— 210 meters (680 feet) compared to 222 meters, respectively — the new ships would need improved machinery than would otherwise have been used in shorter hulls in order to retain the same speed as the longer ships. The design initially called for a top speed of at least 22.5 knots, which was deemed sufficient to keep up with opposing battleships and outrun surfaced submarines. However, in 1936, decrypted reports from the Japanese navy revealed the battleship HIJMS Nagato was capable of speeds higher than 26 knots.

In order to counter this, it was determined that a top speed of between 25.8 to 26.2 knots was possible if the power plant from USS North Carolina could be reduced in size enough to fit in the smaller hull of USS South Dakota. In order to do so the boilers were positioned directly above the turbines in the same arrangement to have been used in the 1916 Lexington-class battle cruisers. The boilers were then rearranged several times so they were staggered with the turbines, eventually ending directly alongside the turbines. The propulsion system was arranged as close together as possible, and the evaporators and distilling equipment were placed in the machinery rooms. This provided enough additional space behind the armored belt to add a second plotting room.

By this time, the design process had established that the hull was to be 203 meters long and incorporate the single internal sloped armor belt. However, in case of rejection by the General Board, naval architects produced a series of alternatives. Among these were longer, faster ships armed with 14 inch guns in triple turrets, slower ships with 14 inch guns in quad turrets, improved versions of the North Carolina-class, and a ship of 27 knots armed with nine 16 inch guns in a similar configuration to the North Carolinas.

Arguments arose, frequently over the issue of speed. The Commander in Chief refused to accept the new ship to have a speed slower than 25 knots, the Battle Force argued at least 27 knots was required to maintain homogeneity in the line of battle, and the president of the War College maintained a fast ship was optimal, but the navy would continue to operate the older battleships with a speed of only 21 knots until the 1950s and so a higher speed was not strictly required, though it would mean the class would be too slow to act as escorts for fast carrier task forces. The primary 203 meter long design was the only plan that could meet the specified requirements for speed, protection, and the nine 16 inch gun armament. By late 1937 a proposed design was agreed on, requiring only small modifications to save weight and increase the fields of fire. Berths for the crew, even the staterooms for senior officers, along with mess halls were reduced in size, and ventilation ports were completely removed. The ship would have to use artificial air circulation.

For half a century prior to laying down the Iowa-class battleships, the United States Navy had consistently advocated armor and firepower at the expense of speed. Even in adopting fast battleships such as that of the North Carolina-class, it had preferred the slower of two alternative designs. Many complicating and expensive improvements in machinery design had been used to reduce the increased power on the designs rather than make extraordinary powerful machinery (meaning much higher speed) practical. Yet the four largest battleships the US Navy produced were not much more than 33 knot versions of the 27 knot, 35,000 tonners that had preceded them. The Iowas showed no advance at all in protection over the South Dakotas. The basic armament improvement was a more powerful 16 inch gun. Ten thousand tons was a very great deal to pay for 6 knots.


USS Alabama (BB-60) anchored near the Bearing Straits in 1944.

The South Dakotas resembled the earlier North Carolina-class battleships in general appearance and in certain specifications. They carried a main battery of nine 16 inch Mk. 6 guns in three triple turrets, known as the 2-A-1 configuration that was by then common in the US Navy. USS South Dakota was built as a flagship, with an extra deck on her conning tower for extra command space, so her secondary battery had sixteen 5 inch Mk. 12 guns in eight Mk. 28 Mod 0 twin DP mounts set in eight turrets, four on either side of the superstructure. This was two turrets fewer than her sister ships who had ten twin DP mounts of twenty guns, five on either side of the ship. These turrets weighed 156,295 lb (70,894 kg) and could depress their guns to −15 degrees and elevate them to 85 degrees. The guns was supplied with 450 rounds each. The South Dakotas had a variety of anti-aircraft weapons, and the weapons mounted changed over time. Initially, the ships were designed to mount twelve 12.7 mm Browning M2HB heavy machine guns and twelve 27.9 mm guns. By March 1942, when USS South Dakota was completed, the anti-aircraft battery was modified to eight M2HBs and twenty-eight 27.9 mm guns and sixteen 20 mm Oerlikon AA guns. In September of that year, the 12.7 mm machine guns were removed and the number of 27.9 mm guns were reduced to twenty. In their place, the Oerlikon guns were increased to sixteen weapons, and sixteen Bofors AA guns were added, in four quad mounts. In February 1943, the 27.9 mm guns and one Oerlikon gun were replaced with an additional fifty-two Bofors guns, for a total of sixty-eight weapons. In December 1944, the battery was upgraded again, with seventy-two Oerlikon and seventy-two Bofors guns. In March 1945, the battery was modified for the last time, this time five Oerlikon guns were added and four Bofors guns were removed. This provided the maximum number of anti-aircraft guns, at 145 guns. The other three ships underwent similar of upgrades to the anti-aircraft armament.

The South Dakotas internal armor belt was inclined 19° from the vertical, and was 12.2 inches thick, with 7 to 8 inch thick STS plates behind the belt. This was equal to 17.3 inches of vertical belt armor, and was proof against the 2,240 lb projectile fired by the 16 inch guns of the Colorado-class battleships from a distance of 17,700 to 30,900 yards. The immune zone against the super heavy 16 inch shells fired by the South Dakotas themselves was smaller, the armor was effective only at ranges between 20,500 and 26,400 yards. The side armor extended to the bottom of the ship, and tapered from its maximum thickness of 12.2 inches down to 1 inch at the lowest portion. This feature was chosen to protect against penetration of heavy caliber gun projectiles that managed to hit the ship below the waterline. The underwater armor included four torpedo bulkheads, a multi layered system designed to absorb the energy from an underwater explosion equivalent to 700 pounds of TNT.

The final displacement of the South Dakotas was 35,000 tons standard and 44,519 tons under full load while final dimensions were 210 meters in length and 33.0 meters beam and machinery was capable of generating 130,000 shp propelling the ships at a top speed of 27 knots. In addition two Curtiss OS2U Kingfisher floatplanes in a hanger facility at the aft of the ship.


The two earlier North Carolina-class battleships had been assigned to the Fiscal Year 1937 Program, but in 1936, the General Board met to discuss the class to be allocated to the Fiscal Year 1938 Program. The General Board argued that two more North Carolina-class ships be added into the program, but Admiral William H. Standley, the Chief of Naval Operations, wanted a fresh battleship design, meaning that construction could not begin in 1938, so the ships were assigned to the Fiscal Year 1939 Program. Design work started in March 1937 and the draft for two battleships was formally approved by the Secretary of the Navy on June 23. More specific characteristics for the two ships were ironed out, and those were approved on January 4 1938. The ships were formally ordered on April 4 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 June 25 1938. The so-called "Escalator Clause" of the Second London Naval Treaty had been activated in the United States Navy so it could begin work on the following Iowa-class battleship, but Congress was willing to approve only the 35,000-ton battleships. A number of deficiencies in the preceding North Carolina-class were to be fixed in the South Dakota-class. 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-in DP mounts.

Construction began shortly before the War, with Fiscal Year 1939 appropriations. The first vessel of the class to be commissioned, USS South Dakota (BB-57), was launched on June 7 1941. Construction continued through to the summer of 1942, when the final vessel, USS Alabama (BB-60), was commissioned on August 16. 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 after World War II.

Ships in class

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