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 16-inch (406 mm) guns in three triple turrets, 5.9 in-thick deck armor that would have made the ship immune to plunging fire out to 30,000 yards, and a top speed of at least 23 knots.
The belt armor was a much more intractable problem; the 16-inch gun could penetrate 13.5 inches of plate, the thickest in an American battleship at the time, even at 25,000 yards.
To proof 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 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.

To minimize the drawbacks 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 fire 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 ameliorated the vulnerability to plunging fire.
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 the North Carolinas.
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 design was much shorter than the preceding North Carolinas—680 ft (210 m) compared to 729 ft (222 m), 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 late 1936, cryptanalysts deciphered radio traffic from the Japanese navy which revealed that the battleship Nagato was capable of speeds in excess of 26 knots.

To counter this, it was determined a top speed of between 25.8–26.2 kn (29.7–30.2 mph; 47.8–48.5 km/h) was possible if the power plant from North Carolina could be reduced in size enough to fit in the tighter hull of 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 battlecruisers.
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 666 ft long (203 m) between perpendiculars and incorporate the single internal sloped armor belt.
However, in case of rejecti
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 16-inch (406 mm) guns in three triple turrets, 5.9 in-thick deck armor that would have made the ship immune to plunging fire out to 30,000 yards, and a top speed of at least 23 knots. The belt armor was a much more intractable problem; the 16-inch gun could penetrate 13.5 inches of plate, the thickest in an American battleship at the time, even at 25,000 yards. To proof 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 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.
To minimize the drawbacks 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 fire 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 ameliorated the vulnerability to plunging fire. 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 the North Carolinas. 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 design was much shorter than the preceding North Carolinas—680 ft (210 m) compared to 729 ft (222 m), 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 late 1936, cryptanalysts deciphered radio traffic from the Japanese navy which revealed that the battleship Nagato was capable of speeds in excess of 26 knots.
To counter this, it was determined a top speed of between 25.8–26.2 kn (29.7–30.2 mph; 47.8–48.5 km/h) was possible if the power plant from North Carolina could be reduced in size enough to fit in the tighter hull of 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 battlecruisers. 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 666 ft long (203 m) between perpendiculars and incorporate the single internal sloped armor belt. However, in case of rejecti
South Dakota-class battleship (1939)