South Dakota - class battleship ( 1920 ) The first " South Dakota " class was a class of six battleship s , laid down in 1920 but never completed . They would have been the last dreadnought s in the United States ' s Naval Act of 1916 to be commissioned had the Washington Naval Treaty not caused their cancellation one - third of the way through their construction . They would have been the largest , most heavily armed and armored battleships in the U . S . Navy and , designed to achieve 23 knots , represented an attempt to abandon its 21 knot standardized fleet speed and catch up with the increasing fleet speeds of its main rivals , the British Royal Navy and Imperial Japanese Navy . In this , size and secondary armament , they represented a break from the Standard - type battleship that had dominated American capital ship design for the last few ship classes , while their use of standardized bridge s , lattice mast s and other features was a continuation of this practice and the increase in the number of main guns from the preceding had long been standard U . S . naval policy . The main restriction to which they had to adhere was the ability to pass through the Panama Canal . The " South Dakota " s were authorized 4 March 1917 , but work was postponed so that the U . S . Navy could incorporate information gained from the Battle of Jutland , fought in 1916 , in this class ' s final design . Work was further postponed to give destroyers and other small fighting vessels priority as they were needed urgently to fight German U - boat s in the North Atlantic . Construction started only in 1920 . As the Washington Naval Treaty restricted both the total allowable battleship tonnage allowed the U . S . Navy , and limited individual ship size to 35000 tons , construction was halted 8 February 1922 . While the unfinished hulls ( most over 30 % completed ) were scrapped in 1923 , the armor plates already prepared were left unused in the shipyards until World War II . The 40 - and 50 - ton plates intended for " Montana " , for instance , were sent in 1941 or 42 to the Panama Canal to reinforce the defenses and locks there . The 16 " guns were transferred to the U . S . Army for use in coast artillery . Design . The design characteristics of the " South Dakotas " closely followed those of the and classes . The increase in the number of main guns was a continuation of U . S . Navy practice from the beginning of the dreadnought era . Like the " Tennessee " s and " Colorado " s , they would have been fitted with standard bridges and lattice mast s . Although Norman Friedman describes the " South Dakota " s as the ultimate development of the series of U . S . battleships that began with the , they were also a departure in size , speed and intermediate armament from the " Standard Type " that characterized the " Nevada " through classes . The main restriction imposed on them by the Navy was the ability to pass through the Panama Canal . This was a policy to which capital ship designs were to strictly adhere due to the savings in time when ships needed to travel from the Pacific Ocean to the $ 3 or vice versa . Overall dimensions . The " South Dakota " s would have been long , wide and displaced 43 , 200 tons . Propulsion . Turbo - electric propulsion , which the U . S . Navy had adopted for capital ships with the , was continued in this class . This circumvented one potential bottleneck since American companies struggled to produce the very large reduction gears necessary for such big ships . The separate turbine normally needed to steam astern could be eliminated since the direction of rotation could simply be reversed by means of the commutators on the electric motors , which ensured rapid changeover . This also allowed the ships to apply full engine power in reverse , which was impossible with traditional turbines ( the reverse turbine was rarely as large as the main turbines ) . The turbines could run at their optimum speed , without regard to propeller speed , which was economical on fuel . The machinery could also be subdivided more tightly , which could increase the ships ' ability to withstand torpedo hits . On the down side , the machinery was heavier and bulkier than geared turbines and there was the danger of high voltage to the crew . Also , despite elaborate attempts at insulation , protection against moisture damage or flooding ( from battle damage and other causes ) remained inadequate . In the " South Dakota " s , two turbo generator s ( General Electric for " Indiana " and " Montana " , Westinghouse for the others ) would have been coupled to two AC alternators of 28 , 000 KVA and 5000 volts . These would feed four electric motors , one per propeller shaft , each rated at of direct current ( DC ) . Sixteen water - tube boilers , each in their own individual compartment , would have lined the turbine rooms to provide steam for the generators . With a total of 60000 electrical horsepower ( EHP ) , top speed was expected to be 23 knots . Armament . Main guns . The " South Dakotas " were slated to carry carried twelve 16 - inch 50 caliber Mark 2 guns in four triple turrets . These fired the same shell as the Mark 1 of the " Colorado " class with a muzzle velocity of to an effective range of . at a maximum elevation of 46 degrees . The Mark 2 was not fitted on any ship ; the 16 " / 50s used later on the s were of a lighter Mark 7 design . With the cancellation of the " South Dakotas " and " Lexingtons " , the existing guns were transferred to the Army and installed in coastal defense batteries in place of the Army ' s more massive and much more expensive M1919 16 - inch Coast Gun . Secondary guns . Secondary armament would have been sixteen 53 caliber guns . Twelve of these were to be on Mark 13 mountings in unarmored casemate s ; the rest would have been placed open on the superstructure deck . This was a departure from the 51 caliber guns used in U . S . battleships since the of 1908 . The 6 " / 53 fired a 105 - pound ( 47 . 6 kg ) projectile a velocity of 3 , 000 feet per second ( 900 m / s ) to a maximum range of at a maximum elevation of 20 degrees . They were installed in s when the " South Dakota " s were canceled under the Washington Naval Treaty and would become a mainstay of U . S . Navy light cruiser s and submarine cruisers built during the 1920s . Armor . According to Siegfreid Breyer , the protective system of the " South Dakota " s was designed to be 50 percent stronger than that of . Belt armor was a consistent , not tapered . The upper armored deck , thick , rested on the top edge of the belt armor . One deck below this was a second armored belt , . Fore and aft transverse armored bulkheads of sealed off the machinery and magazines . Past these , the lower armored deck continued at a thickness of to the ends of the ship . Forward , above this , the upper armored deck continued at a thickness of . Flue gas exits were armored for one deck upwards from the upper armored deck at a thickness of Protection for the main guns was also considerable . Barbette s were above the upper armored deck and from there to the lower armored deck . Turret rings were also armored to a thickness of . Turret faces were , sides , top and rear . Conning tower armor was on the front and sides , on top . Underwater protection was more subdivided longitudinally than in the " Colorado " s but was otherwise similar , with three armored bulkheads thick from the lower armored deck to the ship ' s bottom to the armored transverse bulkheads near the ends . Ships in class . Class Notes : Construction was suspended on all ships Feb . 8 , 1922 and cancelled on Nov . 8 - 10 , 1923 due to the limitations on naval construction of the Washington Naval Treaty .