Cant do submarine school convoy attack mission

[spriterfight12]

So i have some struggles to understand the basics of convoy attacks, i am unable to complete the mission because whenever i position myself the convoy is faster than me submerged and 1/2 speed, the problem i am afraid to use flank speed to get in position because the destroyers will hear me. I am around 7 km away (4.3 miles) and when i get close the convoy changes course and i have to repositon myself but i am too slow to get in position and cant use faster speeds. Anyway could someone tell me some tips and the ranges that a destroyer can see,hear me, also when we must to submerge and use the radar to track the convoy