fleet boat manual
 

I refer you all to this:http://www.maritime.org/fleetsub/index.htm
It's an online reproduction of a 1946 US Navy fleet sub operations manual. I just had a wade through the pages and pages of info on the diesel electric system on a fleet boat at the end of WW2. Here's a few tidbits...

The subs ran on electric motors, even on the surface. They used the diesels to supply electricity through the generators. This means that the diesels need never be loaded or overloaded at high speeds, they just run at their ideal settings. The manual states 80/90, 80 percent of maximum load (resistance) at 90 percent power.
The reason a sub recharges faster at slow speeds is that the engineers de-couple a diesel and run that generator direct to the batteries, leaving the other diesel to supply power to all 4 elecric motors for propulsion. It can do that if the skipper doesn't want to move fast. Anyway, the manual is VERY extensive... enjoy

Piers