I really dislike the way crew fatigue has been done in SH3. It's totally unrealistic. In the middle of a convoy attack, my engine room crew suddenly decide they are rather tired and want to have a lie-down....
That's why I disable crew fatigue entirely. In real life the commanding officer wouldn't have to concern himself with crew rotation. That being said, I do rotate my crew because I want everyone to gain experience. And besides being a demanding commander, I feel sorry leaving the same guys on the bridge for 2 weeks in an atlantic storm....