When the first radar sets were introduced many users found them to be faulty and found major problems until around about July 1941 if my memory serves me correctly whether or not this is scripted in the game i have no idea but i would have to agree with Kapt Z on that one Around May 1941 surface attacks are hard to execute though it is still possible you are still dealing with the possibility throughout even the early stages of the war of one keen eyed ship or destroyer spotting you. It's best to be in and out as quickly as possible on the surface sometimes even try salvo attacks rather than precision aiming. I would recomend deck gun attacks earlier on in the war especially before the introduction of Flower corvettes in early 1940 because before that if you catch the convoy in the Mid-Atlantic you will find little to no escorts and no armed merchant's which presents free pickings of any ship you like. But always execute the attack at night like the real commanders because of the low visibility of course.
This is my opinion but if you are interested in surface attacks i would recommend looking at Kretschmeer's attack on SC-7 it is a perfect example.
Phewww that was a bit of a mouthfull
