Try this:
http://www.subsim.com/radioroom/show...rol+objectives
In your case, get to your objective, and then you can basically wander off anywhere on the map and sink merchants - I know I did. Keep sending in status reports, and when you add up the mission tonnage you'll get a new objective or be allowed to return to base at your discretion if you're out of torps.
Happy hunting!
edit: aw man - beaten to it....