[merc4ulfate]

The easiest way is to go in at night on the surface. Unless their visual and or radar sensors are set to see you at farther ranges like later in the war they certainly will not hear you with sonar.

Getting in close to shore is ok as long as there are no shore batteries.

I have slipped past many escorts on the surface but as soon as you submerge they can hear you if they are within their sensor range for sonar.

I have ran into convoys at night slipping past escorts and the outer merchants without being seen. I try to hit the tankers first and then flank to exit the convoy on the other side while we reload for a circle round to attack again.