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 Carolina"s—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 was capable of speeds in excess of 26 knots.