Some info for you.
From Timothy Mulligan's book 'Lone Wolf' it was policy to rotate crews out to spread the experience in an expanding fleet. Typically a crewman would serve 2 or 3 patrols on one boat then be sent to specialized training and back to the boats.
The book's focus is on U-515 and put it's career in comparison to the UBoat arm in general. U-515's captain managed to keep a higher level of experienced crew.
U-515 had 6 patrols. Of the NCO and seamen
19 served all 6
15 served on 3+
So roughly a third of the crew where plank owners.
Another quarter served more than typical.
The boat had 100 crew serve on it. Over 6 patrols the rotation rate would have been about 9. At the start of the 4th patrol 12 new crew were taken on.
Hope this helps.
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