Board Thread:Questions and Answers/@comment-25349773-20141127164836/@comment-9676492-20141129005603

Friendly subs definitely can take heavy damage during night battles (as you've observed), especially from depth charge and sonar-equipped enemy ships. Never seen it happen to an enemy sub, but I rarely try, since the only times when you really want to kill an enemy sub are at boss nodes, and then you're probably low on ammo and dealing with a high-armor, high-evasion boss sub.