Which year? (Asume early war since you are in a VIIB?)
Which mod or stock SH3?
In real life (Works also well in SH3/GWX/NYGM as far as I can tell):
1939-1942: Kaleuns would stay ON the surface at the visibility limit and follow the convoy gathering data through plotting and radioing BDU. Then at night, once BDU gives the signal and other U-Boats are closeby, they would run on the surface on a dogleg curve approach to go in for the kill.
1943-1945: Stay near the surface, varying between periscope depth and 30-40 metres (For effective sonar listening) to gather target data (Speed/course) from the convoy, while making a right-angles approach (Bring the enemy to your 270º or 90º relative bearing and turn towards/against the convoy until the bearing is constant) till maximum effective shooting range (~3000 metres), then fire a salvo with FATs whenever available.
Hope that helps