Enemy radar - How far does it "see"?

[Skubber]

It is Feb 1944. I am surfaced and trying to flank a convoy and suddenly the destroyers all start charging after me like hounds after a rabbit.

"Radar signals detected sir!"

Oh, so that's how they found me in this pea-soup fog.

I recently spent my hard-won renown on the latest radar detection equipment - thank goodness!

So, after four separate deep dives to escape the inevitable depthcharge attacks, I finally manage to flank the convoy. Only now it's raining, too.

I get inside the convoy, only this late in the war the convoys are packed together like cigarettes. Squeezing between them, I am lucky to spy some targets at just over 300m. A large tanker goes down, and also a liberty ship.

Then a destroyer pops into my field of view as I'm panning around looking for more targets. (In the fog, I had no idea he was there. Soundman did not pick him up in time.)

"Destroyer on attack run, sir!"

Okay, somehow he misses my periscope, and we get away.
Boy, I don't want to try THAT again. (And neither does my crew.)

So, this is a long preamble to a very simple question.
In the past, I could flank the convoy by simply keeping their smokestacks in sight.

How far out do I have to be now to escape enemy radar detection while I'm trying to get forward of the convoy and into a shooting position?