You can see how much hull integeraty you had by looking at the patrol report in SHIII commander. My first patrol in a XXI ended with ALL of my sensors destroyed, so fired 3 homing fish in a spread in an act of desperation and steamed back to Bergen. Turns out I ended the patrol with only 13% hull integrity!!
Those tommies won't get me again. Got the damage from a plane which bombed me while I had my schnorkal up. Does it show up on their radar or something???