[Kobal2]

One thing I learnt from a youtube vid is that the destroyer screen most often searches 1) right in front of the convoy (so sitting still at 500 yards off the path is a no go - you'll be a prime target, showing a nice 90° shape to their pings) and 2) on the sides between 0.5 and 1.5 nms on both sides of the external lines.

So what you want to do is to sit tight at 3-4 nms, submerged and at a 90° angle from the convoy's path, and watch the first "wave" of the convoy, mostly DDs and small boats, sail by. When the nearest side DDs are at a 50° AoB from you, they can't ping you anymore. Being surface ships, they can't hear much of anything through their hydros, and have to rely mostly on their active sonars to "see" you.

Enter your primary target's rough coordinates in the PK so it'll keep and appear on the attack map. Now, dive deeper than the layer and move forward as fast as you dare (I've mostly played the first years of the war as of now, and by then even at ahead full, non-alerted DDs don't seem to hear me - again, surface ships have lousy ears, plus by the time you get there you'll be in their baffles anyway) untill the target is 10 to 20 degrees off your zero depending on their speed (the slower, the later you can afford to come up, fast ships mean you need to lead them more). Slow down to ahead 1/3 or full stop, and come up to periscope depth.

You'll be very close to the convoy, well behind the main DD screen and certainly nowhere expected, and can merrily blow 'em all to kingdom come. If you're the cocky type, you could even try and come up inside the convoy, with targets for both your bow and stern tubes within spitting distance. I only managed to pull this out once though, but then again I'm a mediocre skipper at best.

Just be sure not to be *too* close, especially during the day, as they WILL notice your periscope if you let it up longer than a few seconds. 1000 yards is about safe distance in daylight, nighttime is much more forgiving.

EDIT : another tactic you can use, but only with slow convoys and a fast submerged sub is wait till they've gone past you, then follow their course at flank speed submerged. Again, you'll be in the DDs' baffles, so they mostly won't notice you.
It's harder though, as 170-180° AoB shots leave very little room for mistakes, but it can be very rewarding too : succesfull stern shots seem to kill 'em dead very reliably (as opposed to the "merchant shot thrice, burning, deck awash, screws over the waterline, yet still zooming on at 10 knots" syndrome), and since the DDs will have to do a 180° turn to look for you, and since they'll mostly be ahead of the convoy too, you'll have a lot of room for your escape.