Generally speaking, I never attack on the surface.
I think I've made a couple surface attacks in 1939 or 1940 when merchants are not armed, or in very bad weather.
But once the merchants have guns (1941), it becomes suicide to attack on the surface, because once you are detected they will light you up with spotlights and then everyone in the convoy with a gun will start shooting it at you.
My preferred method of attack is to intercept the convoy ahead, slide past the lead escort at silent speed, then wait and maneuver until the convoy passes over the top of me. I mark myself in between two columns and fire fore and aft tubs as targets pass by. Typically I only shoot at the columns directly in front of and behind me, so I can't miss. Unless something is particularly juicy a column over then I might.
I secure from silent running as soon as the shooting starts, so my guys can reload 2 or 3 shots more as more targets parade past me. Usually I can rip the convoy from stem to stern before the escorts figure out the fox is in the middle of the henhouse and it's time to dive.
However, once hedgehogs make the scene, it's best to abandon convoy attacks from within the convoy and fire from 5000+ meters away with pattern running torpedoes.
Unless you can lure the escorts away and kill them with homing torpedoes.
Steve