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?

[The Munster]

I'm with you on the Radar Detection Equipment; it was the best thing I've used my renown on so far (Aug, 43). I get a smug feeling when I hear "Radar Signals Detected Sir"..feel I have an edge though I tend to go straight to periscope depth upon hearing this !

[Finnbat]

I am not sure, but correct me if wrong.

First time they start to picking you up with radars the range hangs around 10 kilometers, then improving every year to 25 km (1944)
I was getting pickked with snorkel up from 10 km later days of war.

Periscope detected, seems to happen very easy. Some help did come when the periscope get covering layer uppgrade. I could drive with my XXI boat inside the convoy and carefully keeping periscope very low and was not detected.
I just love that XXI with all uppgraded, it is like stealth machine. (And fast like devil.)

[Skubber]

25 km is a wide flanking maneuver, but I'll give it a try.

Thanks!