When I'm attacking convoys I try to penetrate the lines and use the aft tubes as well. I try to fire at distances of 1500-1000m but it depends on the particular case and weather. When i see a lone merchant I usually hit it with the Deck Gun to save on torpedoes. On GWX though even merchants can be deadly so I just stick to torpedoes and pray they do their job.
When I'm going after destroyers i try to fire at small distances (less than 1000m) with fast settings to ensure they cannot avoid the torpedoes. The general rule for me is aim, fire, dive, reload, go to periscope depth, aim and fire again
