I use an interesting technique:
Upon discovering a convoy (preferable in front of) I set all my torpedos and open all my torpedo doors. The sound of this usually draws the attention of the escourts. If I have to, I Flank Speed so they hear me.
When you see an escourt coming towards you, turn towards her and put her at 0 or 180 degrees. - Click on "Ship:" this will give you the optimal torpedo depth for magnetic setting. When the escort is at 425 meters and closing.. Let her have it. I destroy the pursuing escort...
Then all the other retards rush my way in attempt to kill me. Having atleast 4 front torpedo tubes.. I quickly kill 4 more escourts (all of them). [Set steam torps to "Fast"! ]
Putting the enemy at dead 0 or 180 degrees makes manual settings easy.
Once the escourts are down.. I move to a safer distance (quickly). Set it up to where the convoy broad side to your 0 degrees... surface and man the deck gun and flak gun(s). This does bring their broad side guns to bear, but their gun crew stink. This makes your boat harder to hit windage wise. Then I destroy the convoy with the deck gun as crews reload my torpedos. By the time enemy reinforcements arrive... my tubes are hot and I submerge to periscope depth to unlease hell on the fleeing surviors. As the entire convoy is sinking in flames... I "Run Silent, Run Deep!" back to base.
I average 62,000 to 100,000 tons a patrol. I don't waste torpedoes on 2,000 ton ships. A well placed impact torpedo to the bulkhead will bring down a C-2, C-3 or equivilant in one shot.
This tactic works on vanilla SH3 and GWX3.0 ... but is more hazardous in GWXs due to the armed effency of the merchants. DO NOT GET TOO CLOSE to a flock of armed merchants.. combined they are like a darn cruiser.
In vanilla SH3.. I simply weave in and around the convoy and point blank range.. submerged first then surfaced with deck gun. The broken-arrow range causes the surronding escourts to fire at me.. yet hitting their own ships. If done properly.. the armed merchants will hit each other. In 1.4b I have cause the merchants to turn on each other in retalliation and finish each other off. Especially in the choas of night.
Not bad for an Army guy!