As of late my preferred method of engaging a convoy 1942 onwards is to try and get ahead of the convoy as much as possible and get into position for a night submerged attack.I normally ignore the escorts and single out 1 or 2 important ships in the convoy and stick to my plan from then on.I will usually be no more than about 2km from the convoy flank as stated earlier,and then open fire as the target ships arrive.All my firing solutions are worked out well in advance.If I am detected I will head on a parallel course away from the convoy and hope to escape out the rear if possible,hoping the convoy's noise will dampen our own noise...hopefully.
After about 1 hr evasion I will turn around and take up the chase again and repeat the above procedure.I will even try to maintain the attack in daylight too,but the firing distance will probably be increased to about 3-4km,as the escorts have been known to spot the tip of the periscope even at 2km in daylight!
I personally DO think the game's detection chances to be quite overkill at times,making attacking convoys very difficult,more so as the war drags on.Having said that,making that successful attack exactly as you planned it is even more satisfying.
It is important to note that I have not progressed further than Aug 1942 at ANY time!...there is still lots for me to do in the game.
Reading through my 'uboat commander's handbook' by doenitz at times is very helpful,and gives a good insight as to how real captains handled difficult situations.Anyone that doesnt have this book I suggest you try to get it,I managed to get a brand new copy from Amazon,but there were only 2 or 3 in stock at the time.That was about 1 yr ago.