Deck Gun Tactics

[cleverusername]

Does anyone have advice on how to best use the deck gun? I'm running RFB and sometimes merchants are only disabled and don't sink. I don't want to waste too many torpedoes so the deck gun is a good way to finish them off.

Problem: Nearly every ship is armed with one or two big naval guns, and several smaller machine guns. The machine guns eat up my crew and the bigger guns punch right through the hull of my sub. How do I sink a disabled enemy merchant without being sunk by the return fire?