I actually run my own system of transfers, where every patrol I transfer an average 10% (sometimes more, sometimes less) of crewmen who have served for 5 or more patrols, and I generally replace them with junior crew of the same class. For officers, I tend to be a bit more sophisticated and consider things like their rank and function more than anything else.
From what I've read, this is generally applicable for real u-boats. For instance in my WaW career, in 8 patrols I've transferred off a total of 15 crew, 4 of which were officers. (My current crew is 50).
I also play a little game on patrol where I 'rate' my crew's performance and give them 'points' for any distinctions or write little stories about their screwups. Which I also use in deciding transfers. Silly roleplay, I know, but sure adds to immersion
