[msxyz]

You're lucky that Silent Hunter doesn't correctly reproduce all the problems to torpedoes suffered during the first year of the campaign.

The BdU was so frustrated (and refused any small talk about the incompetence of its officers) that wrote several recorded letters lamenting that the tonnage sunk might have been twice as much without all the duds.

Back to the topic, you can try using the flak gun. Probably it's too late for this warining, but saving and reloading might affect adversely your chances of sinking it (sometimes heavily damaged ships sink after minutes or even hours especially in rough seas. It might be a randomized event) Only basic damage information is saved by SH (a known limit since SH3)