[Col7777]

First thanks to Sergbuto for providing the means to do this.

Using the mission editor:

Start by placing Serg's (Submerged British) sub on the map.
Then create a group from the sub, naming it (your choice).
Add whatever ships to the map then attach them to the group.
Add the way-points for the convoy.
After you've added the convoy group then add your Sub.

I thought by doing this, it might alleviate the problem of the convoy creating havoc if you happen to sink the lead ship, in this case the lead ship is the submarine which is out of sight to a certain extent.

Then who knows you may even be attacked from the sub just to add to the drama.

I have tested this and it does work.