Could U-boat milk cows sustain Atlantic patrols?

Could Germany use milk-cow submarines and surface supply ships to sustain distant U-boat patrols?

Not reliably; the supply vessels face the same barrier, so attack boats must preserve enough fuel and stores to return without them.

The same exit problem that traps attack boats also applies to the large supply submarines known as milk cows. They would have to cross the watched North Sea largely underwater and could be lost before reaching their rendezvous areas. U-boats that escaped would then need to retain enough fuel and stores to return without assuming resupply would arrive. That shortens patrols and prevents the distant, sustained campaign on which the historical Atlantic offensive depended.