I could see doing that on a larger map, but I’m concerned that the current map is too small for that. Then again, the game includes air bases, so maybe naval bases/ports aren’t exactly out of the question. Either way, I like the specifics of your suggestion.
i like the convoy zone optional rule, however i believe there should be a qualification that the axis still only gain the income on an actual roll of a 1. this would represent the fact that subs can’t steal cargo, their sole goal was to destroy merchant ships not seize them.