I play using GWX 2.0. I play realism at 64%. I keep the no external camera off and the no event camera off. (cause I like to see the fireworks
) I play with all the limits (fuel, O2, etc.) I use realistic reload times, realistic sensors and I play with dud torps. I don't use realistic vulnerability or realistic sinking time.
I realize these tactics may not work in RL, but I"m playing a game and it has rules, and I play to win within those rules.
When I'm going after a convoy, I've found its good to eliminate the escorts when possible. I go at flank speed to the convoy to get the escort's attention. If that doesn't do it, popping a fish up the tail pipe of one of the merchants their gaurding usually does.
Then, remaining at periscope depth, I follow a tactic similar to the one you describe in one. Find one of them headed straight at you, turn to face him, go to silent running, target, open tubes, wait till he's within 400-600 meters and use a steam fish set to fast with a magnetic detonator, depth set to just at his keel depth. One fish is usually good to take out any single warship.
I'll admit the bigest patrol I've had was 60,213 tons. On that patrol I sank 4 warships, and 13 merchant ships. Some of those merchants went down becuase of my deck gun. I'd wound the merchant, then pop up and finish them off with the deck gun.