Similar experience with me a long time ago (gwx but also with MFM included). 6- 10 torpedoes and this one ship wouldn't sink. Was finally able to finish off with deck gunfire, and that took a while as well.
Perhaps aiming for keel shots or magnetic explosions/deckgun hits near it would be more effective.
I recommend adding thedarkwraith's
Unit damage from smoke and/or fires mod and see if that helps in making it any easier.
http://www.subsim.com/radioroom/show...33&postcount=1
i believe in stock game + gwx the ships don't take hp loss from fires.