I suppose everyone has there favourite method of attack but personally I never attack a convoy on the surface regardless of the weather. I track it position and course on the surface, then get well ahead and submerge right in its track and at 90 degrees to it. I then keep a periscope watch until I see the lead escort and then go to 25m(just in case he zigzags over my conning tower by chance) and rig for silent running. As soon as he passes me by I pop back up to periscope depth and look for the merchies. I'll pick out 2 I like and fire 2 eels at each. Then hit full flank, deoth 100m and turn in the direction the convoy is sailing. Soon as I reach about 60m rig for silent running at 1 knot and slowly angle my self away from the convoy. Best button I ever discovered was the one at the sonar station where you can track the nearest warship
I just keep turning away from him at 10 degrees rudder