[ColonelSandersLite]

U-boats are direct drive, not diesel-electric. In their case, the electric motors are also the generators. They had systems of clutching the diesels and electrics. During economy cruise, one engine would be running in direct drive mode turning the screw, while the electricity generated by the electric motor in charge mode on the same shaft would be used to turn the opposing shaft. In charge mode, the diesel on one side would be running in direct drive mode providing some power, while the opposite engine would be disconnected from the screw and just turning the electric motor in charge mode.

Compare this to a US fleet boat where any one or more of 5 engines can be generating electricity and the screws are always turned by the electric motors.



I can't answer the question about historical SOP, though I very much doubt that it was general practice to run the battery all the way down. More likely they tended to recharge when battery charge was at some specific percent of capacity, or on a time table, or perhaps just tended to run with the snorkel up continuously if the strategic situation allowed.