Although it proved successful with improved equipment beginning with the "Tambor" class of 1940, the diesel-electric drive was troublesome at first. In this arrangement, the boat's four main diesel engines drove only electric generators, which supplied power to high-speed electric motors geared to the propeller shafts. The engines themselves were not connected to the propeller shafts. For submerged propulsion, massive storage batteries supplied electricity to the motors. Problems arose with flashover and arcing in the main electric motors. There was also a loss of in transmission through the electrical system. The Winton Model 16-201A 16-cylinder diesels also proved problematic, and were eventually replaced with 12-278As.