I must be lucky, I sank 2 in 2 days. but I just restarted my whole campaign on the second one & on my very fist patrol. But would you think that count as tonnage toward my kills? But Nooo they only want Cargo ships. Must be to early in the war for me, I ran along slightly parallel @ 18 knots for a couple hundred miles till I was far enough to sneak in 4680 m with plenty of time to set up my shot & sunk Her with 2 torpedo's. Then as I was sitting there sunk 2 destroyers looking for me. I watched for a little while & they didn't really come after me at all so I dropped down to 100 feet an went the other direction. My guess why they went so hard after you must be cause it's later in the war for you.
I have to say though that is a ridiculous amount of explosion for 1 torpedo