Use the radio guy for your sonar. I don't think sending radio messages effects game at all. With the standard 8km atmosphere, its hard to make surface attacks because you see ships at the same time you do. Then, in late war, they always see you before you see them.
In GWX, you actually enjoy the historic see them first in the early war.
Try brushing up on this topic:
http://www.subsim.com/radioroom/show...attack+convoys
It may give you some insight on how to make an approch. Approch is everything when attacking a group. Getting in close enough, submerging, then waiting at 20 - 14 meters silent while they come to you.
Try it out, can't hurt. Might want to get rid of your IID before making other convoy attacks, I don't think that boat was really disgned to do that.