If you get in front of a convoy just run at silent running or sit and wait at all stop. Escorts won't even know you're there..... unless they run over you.
Then you can let them pass and pick out the juicy targets like the tankers and big cargo ships. I wait until I'm inside the convoy and them wreak havoc. The escorts will have no idea where you are until you thin the heard. Then pick them off one by one.
One trick I've used is to lie and wait under a ship that's dead in the water and not sinking. The escorts will circle and eventually give up and leave seeing they can't depth charge you.