Modern diesel-electric boats are always electrically powered. The diesels are connected to generators; the screw(s) run off the electric motor(s). Switching from generator to battery power essentially just involves flipping a switch.

Older boats used dual purpose motor/generators, and with those you\'d have to unclutch the diesels from the shafts when switching to batteries. \"Submerged\" notwithstanding, you shut down the diesels *before* closing the main induction and exhaust, not after. (Unless you thought the best way to kill the diesels was to shut off the air supply, probably suffocating the crew in the process.)

J.T. McDaniel