according to UBoat.net:
Most successful boat of the war was U-48, she had completed 12 patrols for a total of about 326,000 tons... this averages out to about 27,000 tons per patrol
however those results span most of the war and the results are not typical.
There were many many boats which never fired a single shot in aggression
I would say a good average per patrol to go by... with all difficulty settings checked would be about 10-12,000 tons per patrol.
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