FWIW, these are some of things I do to reacquire a convoy after I have attacked...
As Capt Vlad said, patience is the key. Rushing back into the attack when there are alerted escorts will just get you killed! Without radar, reacquiring the convoy can be hit and miss (just like it really was), but with some educated guesswork you should be able to find it again.
The increased visibility range with the 1.4 patch makes the 'end around' a little easier - you can maintain sight of the convoy from a greater stand-off distance.
Before you attack take note of the base course and speed the convoy is travelling on - it gives a good first reference point for when you resume your search. The convoy may have changed course but should still be heading in the same general direction towards its original destination.
Listen on the hydrophones to find out what bearing(s) the convoy is on - this will also give you an indication of which direction is is going.
When you have successfully evaded/eluded any escorts, listen on hydrophones and/or take a cautious look with the 'scope and see which direction the escort is heading, as that heading will likely lead you straight back to the convoy.
Be careful with this though as the escorts can be cunning (I use TM mod, don't know about stock) and lie doggo near your last known position - you can't hear them on the hydrophones. If you surface without first doing the proper checks you are likely to get a face full of shells, small arms and depth charges!!
I've found that if there are multiple escorts, one seems to remain behind or position itself between your last suspected position and the convoy to keep you away, whilst the remainder return to the convoy. If that is the case try and get the bearing the ones returning are travelling on as this will likely lead you back to the convoy.
Good luck and good hunting!!
Cheers