My strategy is a little different, but this is what I do, and it works
When you first get word of a convoy by radio, go way in front of it's course(while surfaced). When you believe you are right in their path, go to silent running and completely stop(periscope depth). Then you let the convoy come to you:p When the ships are rolling past you, raise ur periscope and fire away
This strategy worked for me last night, as I sank 5 ships out of a convoy.
It's always better to be too far ahead of the convoy, because then you can just stop and let them come to you.