Trouble surviving.. How smart is SH5?

[Brandemihl]

I sunk two brit merchs off the coast of brit. 2nd (or 3rd?) mission campaign TWoS.
I am getting spotted from way out. A little torpedo like boat and 2 destroyers. They always know where to find me.

The questions I suppose I have:

1) What distance between myself and a warship should I go to silent running?
2) Can fishing boats (etc) radio warships and give my coordinates?
3) Are radio transmissions triangulated?
4) Any tips for traveling/evading? I've played other uboat games and didn't have this much trouble being found so much.

Any help appreciated!