help me when im in trouble or assist me on sinking a convoy
 

Is there a mod i can call for backup

Not really...don't think there is one...if you send a contact report and you're lucky the Luftwaffe might come to help ...there are some mods for AI controlled friendly U-boats but you can only place them and give them waypoints -no calling for backup.Just try to sink the biggest ships or prefferably tankers.And if you don't have many torps loaded (they're in reserves) make sure you blow up the engine room/fuel or oil room so the ships will stay behind the convoy,giving you time to re-load torps or surface with the DG once the escorts are too far.

That would be awesome if such a mod was available. Only the Allied AI warships sorta come to the rescue when merchants are attacked but only if you're within the perimeter of their patrol zones.