Question

What was a "cow sub" in WWII?

Answer 1

If I remember correctly they were special subs modified to carry more fuel, torpedoes and food to supply, at sea, attack submarines and so freeing them from the need to go back to their base.

Explanation:

The special submarine used to resupply other subs had also the advantege to be difficult to spot being able to submerge in case of danger.