The problem with sub warfare isn’t the economic part, which is kinda cool in G40. You could HR that every SZ touching water is a convoy, easy.
Europe 1999 AxA was a much different Atlantic map, with convoy zones as little pocket Sea Zones. These let you kill UK and US income, but as in G40, the dynamic of that game was that Germany started with subs but they get cleared and from then on, the Atlantic is a UK/US highway.
Subs are most useful to just beef up your fleets cheaply and create some havoc.
Still you can hit quite a bit of income, though Germany is pretty safe, and there are only 2-3 zones that can do real damage.
During a KJF I’ve put 5 Germany subs on the UK zone, that’s 8. We often roll this over Italian, german, UK, japan income, all of those. Say 6-7 times per game?
It’d be nice if it were more of a dynamic back and forth where there are subs and fleets all roaming the seas but the combat in OOB AxA is too simplistic to model that well, the main dynamic of the game is that the ships (and all units, really) stick together, safety in numbers. But subs cant spread out or defend themselves very well….