Submarine Propulsion System 101
There was only one German boat with more than two propulsion engines. The Type IXd being the only one with more than two, and that was a failure. This boat had the engines connected directly to the propellers through a complex shaft arrangement, this along with the general design of the engines made this system very problematic. Though these were the fastest boats for their time. They were relegated to cargo duties after having the complex propulsion system removed and replaced by the standard German system.
The way the German boats worked was they had Diesels that were directly connected to the propeller shaft, which was connected by a clutch. On the shaft was connected the Alternator/motor. When on the surface and charging the batteries one propeller was disconnected and the diesel turned at the highest stable rpm and turned the alternator. This is the side that charged ONLY the boat's batteries.
The other engine was connected to both the propeller as well as turning an alternator, which provided power for the boat's systems. The electrical load to run the systems was a fraction of that needed to charge the batteries.
When the batteries were charged, the charging diesel would be clutched to the propeller, and both power plants would be used to both keep the batteries charged, and provide the power for the boat's systems, as well as provide propulsion.
Now, when running on electrics, the diesels were clutched out from the drive line, and the alternators became propulsion motors. During an alarm, the diesels would be de-clutched pretty much at the same time they were shut down (Sh shows this accurately), after the boat was pushed to flank speed to facility a quick dive. The electrical connections were made to make the alternators act as motors.
All American fleet boats had four engines that were connected to only a generator. THEY WERE NOT CONNECTED TO THE PROPELLER SHAFTS.
The American system was MUCH different. (as was the electro boots) These boats were propelled by electric motors at all times. The diesels powered either the Boat's systems, or charged batteries, OR, provided the power for the drive motors. When the batteries were charged, the Diesels kept the batteries charged and the boat's systems were powered off the batteries (similar to how your car's battery is the source of electrical power for its systems), and provide power for the drive motors.
This propulsion is not unlike the system on a diesel electric locomotive. There is no buffer like a battery between the motor and generator, and so the rpm of the generator directly effects the speed of the motor, hence the reason as the boat slows down the rpm of the diesel slows down. Below a specific RPM the alternator is shunted out, and the motors switch to batteries, or stop.
During a dive there is no time when the motors would stop turning (sh4 inaccurately depicts this) and when charging the batteries both propellers were turning.
The type XXI, and type XXIII operated like this as well.
The S- boat however was much like the German system for boats before the XXI. These also could get assistance from the electric motors when running at flank speed, but this drained the batteries,and no charge was applied to the batteries to keep them charged since the generators were in the drive motor connection. These motors/ alternators acted as either a motor or alternator. They could not act as both at once, not electrically possible. German boats (to my knowledge) did not do this assistance and just went flank through the sheer power of the diesels.
Now, here is something most don't know. The real early boats had constant speed engines (ones that turned at 1 set rpm, usually VERY high). The boat's speed and direction was controlled by changing the pitch of the propeller blades!! But these could be backed up on the diesels and one could be pushing while one pulled, for maneuvering.