The sub isn’t a bad unit, its a bad STACK.
You can have 1-2 of the sub in any fleet or tooling around the board, because of the destroyer and “canal” rules, the subs can utilize the board in a different way than the other units, and are effectively invulnerable to an enemy without a destroyer.
The problem is that you can’t just buy a bunch of subs and call that a strategy. They don’t block movement, they cant hit planes, and in the face of a single destroyer (with help) the entire stack would come crashing down. Combined with their weak defense, it means that there are much more beneficial ways to use your money than buying subs, and if you only have 1 shot at beating the allies, a big stack of transports or tanks does the job much more clearly.
“Well you could say that for any unit and call it a fun game if you buy nothing but, however, I’m looking for a solid philosophy or a purposeful awareness to utilizing the submarine to the best of it’s capabilities. Now what about Germany taking the Southern France IC and building 3 subs there per turn? They would be great fodder when helping a German bomber stack hit any American ships that try and park off Gibraltar (mostly for their ability to go through that strait without controlling Gibraltar).”
It seems pretty cool, but it takes risk and time to get these subs in the water turn 2 and fighting turn 3. You could also do Yugo or Romania and get the subs out nearly as early. Unf, the subs at either factory med are very ill placed to strike out into the Atlantic. We see a lot of games where UK dominates the Mediterranean, but using german $$ to stop this domination doesn’t really alter the strategic game. Every $$ you spend on those subs is a tank that will not be rolling against Moscow. Subs aren’t enough to stop a dedicated invasion force, or to clear the med without the Italians helping.
Now, if that stack of subs (whether its off the skaggerak or in the med) can attack the US/UK invasion stack sitting outside Gibraltar, that’s worth something. Usually the US under commits that first invasion stack, and with only 1 AC and 1CA + 1 DD, a sub stack could ward off the invasion. If the US cant move forward, it cant really do anything.
However, this only buys you 1-2 turns against a decisive euroinvasion, which is time you can also buy using Italian Infantry that cost you nothing…
That is the problem with this subrush strategy; making it offensive. At best, its a standoff strategy. You cant control the Atlantic with subs, you cant do with a few like the allies can.
The abs best use of the sub isn’t in combat at all; its buying and preserving them early game so that by J4/I4 you are blocking substantial income from the axis and forcing them to waste time hunting you all over the board. This however, isn’t decisive either as Germany only has 5 income to take. Japan has lots to take, but lots of sub hunters. Italy has plenty to take, but its income is irrelevant to victory by that juncture. its a long game plan.