Playing late war I go and park my boat at about 100m infront of the convoys estimated path and turn my sub to face the same direction that the convoy will head. This means you wont pick them up on Hydrophones until they get up pretty close. I wait for the first 2 escorts to be right above me then I rig for silent running and slowly climb up. When I reach the surface im normally in the middle of the convoy right between all the ships which cant defend themselves
and most of the time undetected. Positioning is middle/aft of the group. I set magnetics (my preferred method) and fire them so they go under the ships props and up the deck normally exploding at the engine room. I try to fire off all torpedoes at the best target/targets then I quickly dive down and escape and evade sometimes im not even detected. I normally go for 1 attack on a convoy each time unless it has a small escort or more targets of value. I return to the area where I first engaged and look for any crippled/immobilised ships I might scout further ahead in the last known direction.
I find this technique to have worked for me in a IXB up to 1944 using GWX.