Convoy report without visual contact

Ahoi!
I use Realism- and gameplay-related hardcode fixes for SH3.EXE by h.sie and it has a Wolfpack Mod. When I find a convoy and send a contact report, it might happen BdU orders a wolf-pack with AI U-boats to collectively attack the convoy. However you must be in visual range with I think at least one merchant of the convoy and sailing on the surface to be able to send a contact report. When escorts have radar they can detect me long before I could just sight a merchant so it's nearly impossible to send the report without being attacked by destroyers.
I wonder if there's a way to send a report without being visual contact with a merchant or at shallow depth. I read U-boats could send and receive messages when sailing in a depth of 15-20 m so that would be historically accurate, too.
Thanks in advance.