some time ago there was a hint here on the forum that I took to heart and still use today. When you are in your conning tower interior, there is a dial that shows your engine RPM's. If you are running silent and keep your RPM's below 100 (or less that 2 knots) when near an escort, you should be okay. I will get in front of the convoy, dive and go silent. When I am ready to unleash my attack I try to be in the middle of the convoy. I will come out of silent running and fire everything I have and reload if possible. When the escorts come, I crash dive and then go silent once more. The key is to keep your RPM's low. If you want to practice, the single missions are a great place to explore new tactics.
I also like to cripple 2 or 3 ships by attacking their propellers. The will come to a grinding halt, and after the escorts are finished trying to kill me and they leave, I have some nice fat merchants sitting still for me to finish off.