Currently, a single aircraft can destroy any number of subs in Big World (if the subs are controlled by StrongAI). What should happen is that the subs submerge as soon as possible since the subs have no way of hitting back on the airplanes.
The patch makes the StrongAI submerge its subs if the opponent only has airplanes.
I have tested the patch works in Big World, when playing USA. In the first turn, Germany has 3-4 subs stacked in the Caribbean. USA could previously send a single bomber to defeat the wolf pack. Now, the subs submerge at the end of the first round of combat.
The patch may break some tests. As mentioned in some other tests, many of the tests fail even in head for me. Let me know how you want me to handle this problem.