Sending contacts hourly seems to be pointless for me. If the Convoy is actually maintaining course and speed unchanged for 24 hours. It should be enough to report every 24 hours of a position of the convoy.
Exclusions to report BDU about changes of convoy course or speed should be done only if that is actually necessary. (otherwise 1 report every 24 hour should be enough) The u-boat commander shouldn't be risking of losing stealth every hour by surfacing, while he shadows a convoy.
About messages getting lost thing ? Ahm.. now how exactly are they getting lost if the message IS actually sent ? If the u-boats would have used post pigeons to carry the messages, this loss would be self explanatory in more ways, rather that radio wave is lost ?
If I was a u-boat commander in RL I wouldn't want to stay in eternal visual contact with the convoy and sending contact every hour, thus increasing the chance that something can go unplanned. It's not always the aircraft that brings great threat to a u-boat, but also destroyers/escorts. I'm not sure wether "Huff Duff antennas" (radio direction finders ) work in GWX but as you know, destroyers also had fair chances to detect a u-boat that is sending reports with info about convoy..
I would be shadowing convoy in periscope view following it by hydrophones, surfacing only to catch up with the convoy to visual range say every 6 hours, to check if the course and speed is the same. I would also do this at night more often than in clear daylight..