You should mostly make use of your hydrophones to locate targets in a 40 km range (with rough sea your sonar guy may only detect at lower range). Be sure to dive below periscope depth and listen for a while, if you rely on eyeball alone you will probably never make encounters in the Atlantic, even if on a convoy route.
Staying near ports is a sure way to encounter targets, but mostly patrol ships too...so you are asking for trouble
Consult your Captain's Log frequently and watch for convoy reports there, then anticipate a possible interception.
Inside your patrol zone (as for your current mission) any encounter chance is increased.