My optimal convoy attack approach is to get dead ahead of the convoy and then drive straight at them until I either see them or detect radar. Then I dive to periscope depth and go to silent running, less than 100 RPM. I aim head for the outer column, so as to avoid the lead escort. Sometimes I have to move at right angles to the convoy to get out of his way.
I try to stay more than 1500 meters from the lead escort, but have gotten away with much closer, especially in rough seas. You can never be sure what direction he's going to go in so it's always a gamble.
Once the lead escort passes by, I turn onto a track 90 degrees to the convoy's course, and adjust my speed until I get straddled by two columns.
Then I wait for them to cross my firing solution.
Early war, I suspend silent running as soon as I fire and get my crews reloading. Frequently you can get off two full salvos before the escorts find you. Then I dive full speed to 250 meters and go silent. Usually you can escape.
The later the war goes, the harder this becomes, until after 1943 it's usually not safe to attack convoys from within anymore. After that you have to be satisfied with Hail Mary shots from 5000+ meters outside the convoy.
Until you get homing torpedoes.
Then you can entice escorts to chase you and pick them off with ease.
Then return to the convoy and rip them to shreds.
Steve