were installed in these boats during construction. The Government boats received a new model GM-Winton 16-248 V16. Steady development work by GM-Winton had largely corrected earlier problems and this engine proved to be fairly reliable and rugged. The three Electric Boat units received a nine-cylinder version of the Hooven-Owens-Rentschler (HOR) double-acting engine. This was based on a successful steam engine design. Having a power stroke in both directions of the piston, this engine promised nearly twice the horsepower in a size similar to a conventional in-line or V-type engine. Unfortunately, HOR encountered severe design and manufacturing difficulties converting the concept to internal combustion. They vibrated excessively due to imbalances in the combustion chambers. This broke engine mounts and caused difficulties in the drive train. Improper manufacture of the gearing resulted in broken gear teeth. Reluctant to give up on the promise of the engine, the Navy coddled the HORs along until after the United States entered the Pacific War following the attack on Pearl Harbor, when increased funding and operational needs caused these engines to be replaced with GM-Winton 16-278As during the boat's first wartime overhauls. Serious problems were encountered with the Porpoise-class' diesel-electric drive. This drove the decision to radically alter the propulsion plant. The Salmons were fitted with a so-called "composite drive". In this arrangement, two main engines in the forward engine room drove generators in the fashion set by the Porpoises. In the after engine room, two side-by-side engines were clutched to reduction gears which sat forward of the engines, with vibration-isolating hydraulic clutches. The propeller shafts led aft from each of the reduction gears, and were sited outboard of the engines. Two high-speed electric motors were mounted outboard of each shaft, connected directly to the reduction gears. For surfaced operation the engines were clutched in to the reduction gears and drove the propellers directly, with the generator engines providing additional voltage to the motors. For submerged operation, the direct drive engines were declutched from the reduction gears and the motors drove the shafts with electricity supplied by the batteries. Since World War I the US Navy had sought a 21-knot fleet submarine to maneuver with the Standard-type battleships. The first attempts, with the and the Barracuda-class "V-boats" in the 1920s, were failures due to unreliable engines. The Salmons were the first US submarine class to achieve the desired speed of 21 knots with a reliable propulsion