Surface Convoy Attack

[Schnee]

Ok -- now back to the Atlantic after playing a full campaign in a fleet boat in SH4 -- time to return to my roots.
Question is -- I have always been cautious in making convoy attacks submerged, now its time to be more agressive.

In anyone's opinion, what is the latest you could make a successful surface convoy attack? By that I mean 1-2 kills and then escaping unscathed without being picked off by destroyer gunfire?

By which time did escort radar become advanced enough to make a surface attack untenable.