http://www.uboataces.com/uboat-type-work.shtml
The diesel engines are used to turn the electric motors so that they act as dynamos (generators) to recharge the depleted batteries. For rapid recharge, both diesel engines would be clutched onto the electric motors. For maximum range, one diesel engine would be clutched to an electric motor, which in turn acted as a generator to drive the other electric motor.
Both propellers were turning in this way with only one diesel running.
http://www.uboataces.com/uboat-type-vii.shtml
Type VIIF (milk cow?)
Although sounding good in theory, in practice the physical transfer of torpedoes at sea seldom worked. This involved two u-boats secured to each other at sea, unable to dive and practically vulnerable to enemy fire until the transfer was complete. The task was slow and laborious and the practice was quickly discontinued. Instead, these U-boats were relegated to transport duties, with U-602 delivering a load of torpedoes to their Japanese ally in Penang in 1944.
A total of four Type VIIF were commissioned and quickly relegated to transport duties.
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