In the 1930s H.O.R. developed a double-acting two-stroke diesel engine, initially based on the German cruiser "Leipzig"'s MAN engines but with eight cylinders instead of seven, expanded to nine cylinders in the final submarine version. The double-acting design produced more power from a physically smaller engine than conventional designs. However, H.O.R.'s double-acting engines, particularly those of USS "Pompano", gained notoriety for their unreliability in the submarine force, where they were nicknamed "whores". Owing to the limited space available within the submarines, either opposed-piston or, in this case, double-acting engines were favored for being more compact. An inherent problem with double-acting cylinders, owing to the piston rod reducing the piston area on one side, is an imbalance in the force on each side of the piston. The H.O.R. engines were plagued by vibration and other problems as a result. This in turn overstressed the drive train and caused the gears (which themselves had been incorrectly manufactured) to shed teeth, create torsional vibration, and frequently rendered the engine and gear train inoperable. As an example of the problems caused by the unreliability of the H.O.R. engines, Captain Charles Herbert Andrews of the USS "Gurnard" recalled concerning a war patrol in support of Operation Torch, "I only used three, saving the fourth for a spare. When two of them broke down in the Bay of Biscay, I cut the patrol short and limped back to Scotland."