Back off!
6km is too close for shadowing, except at night and even then you might get spotted by an escort and then all hell will break loose and even if you get away you've lost ground, I stay 10 to 15km away and match their speed from a position slightly ahead and make a quick dive to listen every 2 hours, if they change course during that time, and they will at some point, I can adjust my course and speed and still be in a good position to make an attack when the weather clears/night falls/or i've had enough of shadowing
