Seems I might have found a way to deal with these pesky radars.
Feb 1943
I got a contact report for a small convoy as I was heading for Freetown. Plotted a normal intercept but then went around a 25 km circle centered on the intercept point, and steamed a little faster to compensate and be there on time. This seems to be a tiny bit beyond their radar range... Notice I plotted a first intercept from the contact report but then made contact (''receiving radar signals'') and they were way off the predicted course, hence the two plots on the map.
Yea, call me a cheater, I play with map contact updates... Otherwise full realism and manual targeting though.
Finished off a damaged passenger/cargo with the guns after killing everything else (including the escorting A&B).
Small convoy obliterated.
Extra rum rations for all... Cheers !