When I started with TGW mod
Also I had the same thing about sinking ships.
Mostly I had to leave the ship lying dead into the waters but not sinkings.
Also I noticed sometimes when I had an ship leaning I stayed around the ship and after 5 or 6 hours in game , a Message popped up....''She's Going Down''
So sometimes you have to wait a long time before she finally decides sink.
Also I faslty learned how to use my torpedo's
I never fired at an greater distance then 1000 Meters
because the T1's sometimes decides to blow up in the middle of the water.
So after that I mostly fire 2 torps at an ship the 1st one I aim at here Engine compartment to make her laying dead in the water or at least to slow her down..the next one i fired under her commander tower and mostly I have the luck she sinks or doesnt struggle a long time before she decides to go down.
my average tonnage per patrol was 9,000 tons
after that I upgraded to an VII because I needed something bigger.
My tonnage rate has gone up dramaticly towards the Record of 38,000 tons (Convoy attack) I was really suprised of that score, but also In the middle of the convoy I spotted an Southampton Cruiser. worth 10,000 Tons.
my average in the VII is 16,000 per patrol.
Also maybe its adviced to go the the Navy school again m8 and do the Torpedo Practice to get weak spots of an Ship.