Earlier submarines carried four high-speed electric motors (two per shaft), which had to be fitted with reduction gears to slow their outputs down to an appropriate speed for the shafts. This reduction gearing was very noisy, and made the submarine easier to detect with hydrophones. Eighteen late "Balao"-class submarines received low-speed double armature motors which drove the shafts directly and were much quieter, but this improvement was not universally fitted until the succeeding . The new direct drive electric motors were designed by the Bureau of Ships' electrical division under Captain Hyman G. Rickover, and were first equipped on . On all US World War II-built boats, as the diesel engines were not directly connected to the shafts, the electric motors drove the shafts all the time.