Using Silent Editor, it would seem the radar detector ranges in the stock game are laughable. You can see farther than they can pick up contacts in most conditions.
After reading Iron Coffins and how they would get contacts as low impulses that grow in intensity beyond visual range, even with the Metox, I've altered them to be a bit more "realistic".
Stock:
Metox 4000m
Borkum 5000m
Naxos 7500m
Tunis 10000m
Moded:
Metox 9000m
Borkum 10000m
Naxos 25000m
Tunis 30000m
I altered the later radar set ranges, too. But, like maillemaker said, you sweep a destroyer, or worse, a task force with radar, chances are your boat is minced meat. The only thing radar is good for is getting a fix on the lone merchant speeding through the fog, as long as the waves aren't breaking over the tower however.