Hi again :

The electric motors were standard naval units, built to identical design by Siemens, ÄEG or Brown-Boveri, designed to work on the direct current supplied by the boat's storage batteries. They were a double-commutator type motor rated at 500hp each. This gave 280rpm at 720 amps and 228 volts, resulting in a maximum speed of 7.4 knots. This level of energy output could be supported by the boat's batteries for only a half an hour. Creep speed (Schleichfahrt), the speed at which the motors made the least noise, was generally about 90rpm, though this differed slightly from boat to boat. This gave a speed of about 2.5 knots through the water. The electrical connection of each motor could be reversed. In drive mode, it would draw power from the batteries to drive the screws. With polarity reversed, it would generate electricity, charging the batteries as it was turned by its diesel. It could also be set to spin freely, neither driving the boat nor charging the batteries. This setting was controlled by the switchboards in the motor room.
Each motor was connected by a clutch to the drive shaft to the diesel engine immediately forward. Another clutch immediately aft of each motor controlled its connection to the propeller shaft. This allowed a large variety of possible configurations of diesel engines and/or electric motors depending on the needs of the moment. For instance, the normal configuration for highspeed surface travel (AK - Außerste Kraft) was to have both diesels operating and all clutches engaged, with both electric motors spinning freely. An 'unofficial' extra burst of power, giving about a half-knot of extra speed, could be achieved in emergencies by hooking up the motors in drive mode. This was called 'Zweimal AK' (twice AK), even though there was no such notation on the engine room telegraph. This speed was used only in emergencies because it rapidly drained the batteries, reducing the CO's tactical options. Economical surface drive generally involved shutting down one of the diesels and disengaging the clutch between it and the motor on that side, driving the boat with only the other diesel. The motor on the driven side was hooked up in generator mode and used to run the opposite motor, so both shafts were powered. Experimentation in extending the range of the Type VII so that it could operate off the coast of North America showed that shutting down one engine and running the other at most economical speed gave a speed of 6 knots. The working engine would be switched every four hours to equalize wear.
When a U-boat first surfaced, the highest priority was to recharge the batteries and fill the compressed air tanks. The batteries were charged by running the diesel and motor on one side in generator mode with the clutch between the motor and the screw disengaged. This allowed the engine to turn the motor on that side at high speed, giving maximum generating performance. Air was compressed either by the electric compressor or the diesel-driven Junkers compressor. The electrical apparatus was battery-driven and was intended primarily for use underwater. The Junkers compressor could only be run when the boat was surfaced.

(From "Type VII U-Boats", Robert C. Stern)

Bye
Enrique