Electric torpedoes couldn't be carried on the outside of the Uboot as reserve. Unfortunately SH3-4-5 don't seem to consider this point.
The reason was the type of batteries employed. They were compact and able to withstand the high currents necessary to feed the 100HP electric motor. But they had a very high self-discharge rate, thus they needed to be periodically recharged every few days.
The torpedo tube could be moved inside the pressure hull once the single rudder of type VIIA was replaced by a pair of twin rudders for improved underwater maneuvrability. The tube passed right between the two rudders control rods.
The spare torpedo rested below the floor, between the electrical engines. The electric machinery compartment was long enough to accomodate it without stretching the boat.
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...Sinking deeper into the cold, dark oceans of life 
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