Milk Cows - any way to collapse patrols back into one?

It's a specially built resupply sub which carries lots of fuel and torpedoes out into a predetermined area. When the regular U-boats run low they (theoretically) could rendesvous with the 'Milch Kuh' and thus stay on patrol a lot longer.

They also experimented with surface resupply ships. Both were in great danger from Allied planes, which by mid-war patrolled pretty much the whole Atlantic.