You need to do an end-around. Assuming you picked up the convoy on sonar or radar, but it is still outside visual range, you must begin to plot the course. I will often slow down and start putting marks at the end of the detection line (or lines). After a few sweeps of the sonar/radar, you will have the convoy's heading. Note its a good idea to not change the zoom level on the nav map while doing this step.
Once you have a few plots, you can draw a line that gives you the best fit, this is the convoy's course. It is quite possible to get a pretty good idea of the convoy's speed at this point too if you like. Then you can extend this line to extrapolate where the convoy will be at a future time. I often extend the line 50nm or more.
Now which way the convoy is moving will determine you you next proceed. Ideally the convoy is moving towards you. Go to periscope depth, silent running and set up your position. Quite often the convoy is moving some other direction, forcing you to plot an intercept.
While there are a lot of circumstances and situations here, the general idea is to maneuver outside the detection range of the convoy, surface, plot a course parallel to the convoys course, but outside their detection range, say 10nm, then travel at high speed. every now and then submerge, obtain a sonar bearing to ensure the convoy has not changed course, then surface, so on so forth. While submerged you can make more marks and adjust your extrapolated line for more accuracy.
The end goal here being that you will eventually get far enough ahead to maneuver back into the path of the convoy, submerge and go silent, all before the convoy can see you. Then it simply becomes a waiting game for the convoy to sail right into your torpedos.
The caveat here being if the convoy changes course you have to figure out the new heading etc, sometimes you will even lose the convoy altogether if it changed course, say right after you did a hydrophone check.
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