As I've several different approaches depending upon when, where and how convoy detection and intercept occur, it's difficult to explain what I do. However, I'll explain how I do things for the 'approach of my choice'.
Where I
want to be is at least 50 klicks away, on the surface, almost dead ahead, of the convoy's projected path. When I submerge is dependent upon: 1) weather conditions and 2) current year.
If it's heavy precip and heavy fog I'll submerge about 20 klicks out (I won't be able to visually approach unless they're within 400 meters, nor will I be able to see an escort in my lap until then, so why risk that?). If Allied radar is an issue (in latter years) I'll submerge about 20 - 30 klicks away; once they've spotted a potential contact the escorts get friskier.
Otherwise I submerge at about 6 - 8 klicks away, to a depth of 15 - 18 meters, again to avoid potentially being spotted and waking up the escorts to general quarters. 15 - 18 is below periscope depth, but has two advantages: one is that my conning tower won't broach the surface if the swells are running heavy, and two is that the passive sound detection of escorts aren't able to easily detect sounds above 20 meters. And passive detection should be all I
have to worry about as long as the escorts haven't gone to a higher alert due to a recent visual or radar contact.
At 5 klicks range I want to be between 500 meters to 1,500 meters to the side of the convoy path, with my approach intercept angle being 0 - 15 degrees, running at silent. Periodically I'll pop up to periscope depth and take a few quick readings. That accomplishes several things: it lets me keep track of the escorts positions and headings, and I take bearing records of the convoy leaders, marking their positions on my map and refining their course and speed (adjusting my position as needed to maintain my approach) before dropping back down to 15 - 18 meters.
If all goes well (and I'm surprised at the frequency it does, even in late '42 and running GW1.1 and NYGM) once around 1,500 meters away from the lead escorts I'll maintain a depth of 14 meters and pop the scope up about 5 seconds every 2 minutes, using the hydrophone heavily to keep track of the approaching escorts so I can pre-position my 'scope for snapshot peeks at the escorts. It's quite nervewracking to watch a Black Swan creep by at 5 knots on a back-and-forth sweep pass by me at 400 meters!
Once the forward screen has passed I drop again to 15 - 18 meters (no reason to tempt the Fates and possibly have my conning tower spotted to due a freak swell broach, or something else just as freakish). By now the leading row of the convoy should be 3 klicks out or thereabouts, and I should be 500 - 1,000 meters to one side of their baseline course, at an approach angle of 0 - 10 degrees . . . which effectively has me sliding under and towards the middle of the convoy.
When my hydrophone checks start picking up the closest contact at short range, I drop silent running, go to ahead one third, and rise to periscope depth. I look around, adjust to pass between two of the columns, pick 2 targets of one column for my forward tubes and one for my stern (btw, I almost always drive a VIIB or VIIC after trading in my IIA (if I started that early and with that, anyway)). I don't keep the scope up; it's up for 10 - 20 seconds every minute or so, until the actual firing run.
(Before GW1.1 and NYGM I would have selected 5 targets total. But I'd rather go for the 'certain kill' and use 2 fish per ship than go hogwild and fire one per ship and risk seeing four damaged targets continue steaming frantically away at their full speed. Then again, this isn't a 'firm-and-fixed' rule of mine either. If the weather is good and the seas are calm I'll fire one per ship. The reasons for that is: 1) I'm usually less than 1,000 meters from my target, 2) have a firing angle within +/- 30 degrees, 3) have an AoB within 65 - 115 degrees and, 4) can manually target certain critical areas (ex. fuel bunker, engine room) with a high certainty of fatally damaging the ship with one well-placed shot.)
When my targets are about 1,500 meters in front of me and at an angle of 30 or so degrees relative to my bow, I do a 90 degree turn and go to 1 knot (for depth station keeping) after the turn is complete, positioning myself so my two selected targets will pass in front.
Then I wait.
I wait until the lead target passes in front, and
passes. No more than 15 - 20 degrees, then I set final speed and depth to the fish, open the tube doors, carefully aim and fire, then immediately pick up my second selected target and begin tracking.
Why wait until it passes?
The moment a torpedo explodes the jig is up; the convoy knows a U-boat is out there and,
worse, is actively tracking and firing at them! Those nice steady courses of theirs begins to alter and change as they zigzag and weave. By letting the first ship pass by, I give my
second targets more time to get closer. Closer means more accuracy, less torpedo travel time, less time for the target to evade.
Immediately after firing the first time I make a mark on the map right where my position is, then I return to tracking and firing. After all five have been fired, I drop scope, go to ahead third, go to 15 - 18 meters again and slide under the convoy to the other side, using the sounds of their screws (and, hopefully, their explosions and sinkings!) to cover and screen the sounds of my passage.
So far this has seemed to work surprisingly well for me.