The best way to follow a convoy is called shadowing. Meaning you are staying far enough away that they can't see you, but close enough that you can still barely see them.
This is my normal SOP when getting ready to attack a convoy.
#1: I have detected a large group on my hydrophones. I set my course to where the grpup is in and around 0(zero).
#2: I go to the nav map and mark the bearings in which the convoy takes. IE from 345 to 15. I put an X where my sub is, at 345 and at 15. Then I take the range tool ( the one the makes the circle) and draw out 10,000yds(5nms) as this is the range of detection give or take. Then I take the protractor and draw an angle throw the three Xs I have marked. Giving me a rough bearing and an angle. I take note of the angle.
#3: I surface and run at flank speed for 30 mins. The dive to periscope depth to take another reading. I adjust course again to keep them at 0(zero)
#4: Repeat step 2. Take note the new angle. Here I make a decision. Of the angle has gotten smaller they have moved further away and can out run me. If it is slightly larger then they are heading away but I am over taking them. If it is alot larger then they are heading towards me and I ahold prepare for contact.
Once I have visual contact I make a run at periscope depth towards the convoy to get a firing solution. Course and speed. Once I have those I mark a relative course on the map and head away from the convoy to do and end around.
Once I am far enough away I surface turn to a relative parallel course and make flank speed. All the while making sure I can still see them. Once I get to where I feel I can make an attack run, normally once it's dark. I turn 90 into them at flank. I run till a am close enough that I can dive and still get in range (this might require speed changes as the faster you go te easier you are to see as you are crossing the escorts bow), then I dive to PD and wait.
I may or may not recheck my solution, often times I do.
And that's normally what I do in a nut shell, but it's really all about what makes it easy for you. I hate math therefore I use the NAV tools given by the game. But whatever suits you.
Good luck Captain!
