How do I get inside a convoy?

I am playing in July 1941 in a type VIIC. I have sighted a nice convoy consisting 17 merchants and a Southhampton cruiser in the dead center, flanked by Flower Corvette, Tribal Destroyer, Black Swan Frigate, and something else I can't identify on the far side.

Now I have several questions. First, around the escorts there are rings and sectors on the F5 map (I'm playing with "God mode" turned on - I tried it with it off and I'm not good enough yet). What do they mean? There is a very small circle, and at the outside perimeter a very large circle. Then in between there's a sort of triangular sector in front with a medium-sized circle but with a triangular cut out at the back. I have yet to find what that means.

The reason I ask is once I was submerged waiting for escorts to pass me by, and I passed through the outer ring without detection and if I recall the middle ring (with the triangular sectors) but once that inner ring touched my boat he turned and came right at me at ramming speed. So in my effort to sneak into attack position on this convoy, I need to know what those circles mean and which ones I can enter safely and which I should avoid.

I am outside all the circles right now, and with the escorts darting around, I have no clue the best way to sneak past them to fire off my eels.

Last question I have is with the Southhampton inside the convoy in the dead center, and weighing in at 10,725 tons, it seems like an important target. But would a U-boot commander go after a capital ship or is merchant shipping my only authorized target? Even though I don't play with realism at 100%, I do try to conduct the game like a real WWII kaleu would do it. So should I go for the cruiser, or just take out a couple of the merchants before I'm swarmed by four escorts? (not looking forward to that)