Torpedo tube destroyed!

[siber]

Nov 1940.
U-99 is ordered to patrol off the Rockall banks, presumably to intercept convoys heading into and out of the Irish sea. Soon, a convoy comes steaming towards us and U-99 positions itself in an ideal place to attack.

However, even though submerged, engines stopped and under orders for silent running, the convoy front-escort vessel somehow detects us and races in with depth charges. Quickly U-99 crash dives but is caught by a depth charge on the way down. Off with silent running, stabilise at 90m and contain flooding. We continue to get depth charged until the flooding is stopped, the water pumped out and the ship set back to silent running.

I have two questions: Firstly, how on earth was I detected? The escort (Black Swan I think) passed within 500m or so, but we were COMPLETELY silent. The swan was motoring at 10kts+!

Secondly, in the attack and subsequently after repairs, torpedo tube #4 was destroyed. Does that mean that I cannot now use the torpedo loaded within it? Or, can I unload the fish and then put it into another tube? If so, how do I do this?

Thanks in anticipation of helpful responses!