Scapa Flow Question

Okay. This is for Scapa Flow or any harbour really. So I was playing one of my careers last night and was on my way back around the north end of Britain. When I decided to look at the capital ship dates and where they were moored at. And realized the Hood was in Scap Flow!!! So I timed it and go to Scapa at night and snuck in on the surface (This is October 1939) then I found it got about 5km away, set my 4 torps to run on surface (because of sub nets) and fired (from a perfect 90* angle) and took off. Only problem was it didnt sink, If it would have been in deeper water I think it would have. I saw life boats deployed but no ship sunk message. My question is this. In Scapa Flow (Or any shallow harbour for that matter) do I need to hit them with enough torpedoes to destroy their hull "hit points" in GWX?? Or can I "sink" a battle ship there and get credit??

What happened is the hood just listed to the side all four eels hit on heavily. Im just mad because I spent all that time sneaking in without a hitch, fired four perfect angle impacts, which all hit. (they were spread to hit different parts of the ship) and got away with out having a single trashcan dropped on me. All to find out that I didnt in fact sink a single battle ship