I would love to see ship convoys but it would require an additional alternate resupply system.
A ship convoy system could be designed to use the total number of ships in the pipeline instead of the number of ships to arrive at the port. If a convoy of ten ships is wiped out, then it would respawn at it's start point on the 10 minute clock and head for it's destination again. The server checks the status of the convoy every ten minutes and counts the surviving ships. If they're all alive, then the base receives all it's resupply, if not, then it's resupply is reduced.
To prevent gaming the system by leaving a single ship in the convoy, thus reducing the resupplies over long periods, all destroyed ships are reset at the surviving ship location on the ten minute resupply mark. Thus, a convoy suddenly has 1 to 9 replacement ships fading into existence. (btw, a submarine would have nine new targets.)
A ship convoy of this type, traveling 20 miles would take 2 hrs to reach it's destination.