Positive bouyancy is being a lousy, rusty tramp steamer (passenger/cargo) that takes six torpedoes before it sinks.
That happened on our last patrol, BF15, Nov. '39, windy weather. The ship was followed by a small tanker (both British) on her way East (probably Plymouth). The tanker sunk after the first hit in the bow. The steamer got more lucky – the first torpedo missed (ok, ok, I sometimes do not plot anything and just shoot, this time it didn't work). The second torpedo hit her... and just bounced off. Seas were too heavy to use magnetic pistol, so I set the depth to 2 m, impact. The third torpedo exploded and I started shadowing the ship to see it going down. But no... she listed a bit but didn't die. After 3 hours I decided to finish her with fourth torpedo, this time from the second side. Hit and blast... and nothing more. The ship was still going ahead. After 5 (five) more hours it was still going at 4 knots. That's enough, I said to myself, and... the fifth eel bounced off, despite being set very precisely. The sixth torpedo did the job and sent the rusty bucket straight to heaven in a huge blast (whole the ship went to pieces in a matter of seconds and sank soon after that).
And U-52 under Oberleun. z. S. Herbert Mann started trip back to Wilhelmshaven through Der Kanal. But noone aboard felt like a winner.
