Hi Scott,
I'm new to SH4 myself, having played a lot of SH3.
How about some more info? Which year? What depth are you at? Do you leave your periscope up the whole time? Are you attacking during daytime or nighttime?
I've attacked more than a few convoys in the past few days (as a newbie to SH4) and have had much success. I love surfacing and gunning the ships down after I've wounded 'em. I hate wasting torps to finish off a ship when a few HE shells can do the trick.