Yeah, what those other guys said, but also watch your periscope depth. Once you're spotted it gets dicey, so keep the view head close to the water's surface and poke it up only momentarily. If you select the escorts you'll see their various line-of-sights so try to stay out of the inner-most ring off their bows. And like was said, shoot and scoot, go as deep as you can and creep away on silent speed.
Runing underneath one of the ships in the convoy is a good place to hide. Works really well if you destroy a merchant's rudder and/or props then linger beneath it. Destroyers will circle around but they eventually move off.
You can re-acquire the convoy again if needed.
I'm less scientific than some of these guys. I bee-line it to the T2s and T3s, shoot and run. Repeat as needed.