Hello,
which year ? From 1943 on .. lots of radar installed even on older ships, and lots of "sightings"
But they will not exactly know what that blip is. In reality, running surfaced at the same speed and course as a convoy, they probably would not even find you by radar.
And radar does not work under 1000 meters, so you are pretty secure in fog. And - Flower class corvettes and sloops with their 15-16 knots are slower than you, so you can run away from those.
When i ended the Gibraltar single mission i ran surfaced through the fog with 18 knots, zig-zagging past destroyers which might have seen me for 5 seconds, until i was gone again in the fog. If this hiding works in SH5 ... good !
Greetings,
Catfish