I try to get ahead of the convoy on their supposed course, it's quite difficult sometimes as they often zig-zag (realism 100%, no map contact update).
Periscop depth, silent running and hydrophon check when i think it's good.
When i see the leading escort i try to guess the distance between my sub and the escort, usually around 8500m. So convoy is around 10.000m away but i can't usually see them at this moment. 10.000 m means that a 7kts convoy need around 45 min to do this distance. I start immediately to go to -155m depth at 3kts ( i think escorts can hear you if you make 4kts). It took 12 min to reach this depth with a type VIIB. As i need 22min to get back to periscop depth at 2 kts, i will spend 5 min at -155m at 2kts. During this manoeuver i try to keep leading escort in my 0 with the help of hydrophons and when i reach -45m i try to get perpendicular to the convoy route.
So when i get back to PD the convoy is near, and most of the time i'm in a very good position to attack. The last seconds before seeing the convoy are very exciting! Then i fire two salvos of two torpedoes for ships up to 5000tons or single shot for light targets.
This works in jun 41 with absolut calms seas (0 m/s), hope it helps!
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