Their efficiency goes down. The dive guys will fail to keep depth levels, the torpedo men take longer to load. The radio man fails to intercept radio messages, hydrophone (who normally hears 20km out) will fail to hear things and call them out to you, your watch crew fail to see ships, this is really bad in stormy weather. Also, if the men are really low on fatigue, it takes longer to rest th m. So don't do that! Keep up with their management!
Just to add, when you see the E next to their name, their endurance has suffered because they have been overworked. This means that if they are rested their efficiency will remain low for rest of patrol.
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