Go to this thread on the forums for great video tutorials.
http://www.subsim.com/radioroom/showthread.php?t=181538
Watch this tutorial to learn how to intercept a convoy.
http://www.subsim.com/radioroom/show...0&postcount=28
When you get a contact report, that's where the convoy is that minute. By the time you get to that position, the convoy has moved away. So you need to figure out an intercept point.