The speed difference is interesting, as the GMs ran at 750 rpm and the FMs at 720, but both were rated to produced the same output voltage and current. I wonder if the difference was really the engines, or the associated motors.

The direct-drive motors used in the Tangs were more powerful (1350 HP, total 2700 HP, 1 motor/shaft) than the high-speed motors used in the Gatos and earlier Balaos (685HP, total 2600 HP, 2 motors/shaft with reduction gear drive) but were designed for a top speed of 282 rpm when running surfaced. I can't find anything giving the top-end shaft revs for the geared motors, but somehow suspect it may have been slightly higher. I did notice that Friedman lists the top wartime surfaced speed of a Gato/Balao at 21 knots and a Tang at 20.25 knots.

After all, one of the major differences between US and German designs was that, in all but a handful of older boats (S-boats, a couple of very early fleet type designs), the engines were never directly connected to the propeller shafts, while in the German designs they were. The major advantage in this was that the engines could always run at their optimum speed, eliminating a major source of vibration and drive-train stress. It also eliminated the need for clutches, which were always one of the most troublesome parts of a direct-drive diesel system. In a diesel-electric design, the engines themselves shouldn't have much to do with the speed of the boat as long as the generator output is the same.

J.T. McDaniel