Early in the war it is possible to sneak into the convoy lanes and select some valuable targets and fire at close range. Here is a link to a post that I made a few years ago that demostrates this http://www.subsim.com/radioroom/show...retchmer+style .
As has been mentioned, you will need to dive soon after firing in order to escape successfully if you are within the convoy. If firing from outside the convoy you can go to flank speed away from the convoy and escape detection that way to set up another attack. In my current career I'm in mid 1943 where such tactics are impossible! 
Nemo
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"I'm afraid there is no disguising the fact that King's obsession with the Pacific and the Battle of Washington cost us dear in the Battle of the Atlantic".
Sir John Slessor GCB, DSO, MC, DL
AOC-in-C Coastal Command RAF
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