Appendix B of the GWX manual has a great section about the finer points of attacking a convoy.
I used to be a bit scared of the convoys, and especially from within, but in this career I've been able to infiltrate the destroyer screens with relative ease coming from an oblique angle in front of the convoy. I go in surfaced with decks awash, always at night and never with "visibility unlimited". Once past the destroyers, I go to PD and I pick one or two juicy targets in the middle of the convoy. After my torpedoes hit, I go to flank speed for a few seconds and get down to 100 to 120 m depth as fast as I can. Usually at this point the DD's have trouble hearing you in the midst of all the commotion and you can get most of the way down before cutting your engines to 2 knots and going to silent running. I slip out of the back of the convoy, evading if I have to, and by the time the DD's give up searching, if you've been quiet enough you're really not that far behind the convoy. Make an end run at full speed just out of visual range of the escorts, and repeat.
I've also had some luck attacking parallel to ships between columns. Something like this:
(Crude image, but you get the point)