You might also want to consider tracking the convoy visually. If you keep the masts visible and the hull below the horizon, they can't see you.
You could probably track them visually if the hull is visible, but that is risky.
Visually tracking the convoy also alerts you to any course changes.
The problem is that tracking a convoy and using TC can be mutually exclusive of each other.
Track them in the day time, sink them at night. Worked for the Germans
