Regarding the logic of submarine warfare, it needs somebody who practices it successfully to advise here. Contrary to what might be expected, you want high visibility on your sub packs.... so they get attacked. A stack of 9 units is about the right visibility so the dumb UK and USA AIs will find you easily.
You want to be attacked for 2 key reasons:
a) Donitz gives your U-boats a Defense Bonus.
b) Critically, if attacked, you can simply rush via normal movement any nearby wolf pack - and if it reaches battle zone while combat there still on - then there is a 99% chance that your reinforcement Wolfpack will enter the battle. And getting 2nd Wolfpack to join fray is te key to winning.
But if combat results from your U-boats attacking enemy fleet, there is a 99% chance that any 2nd Wolfpack sent there via normal movement will NOT join battle, and a 50% chance that any Wolfpack sent there via Sea Interdiction will also not join battle.
To be successful with this U-boat tactic one needs to work with the game mechanics, and not against them because some stats conflict with the proper thinking.
I've been using 6 unit stacks for most part but it seems like Commanders method is even more beneficial.
Just six U-boats is rather weak when attacked by an 18-unit mixed fleet having 3-4 CVs and ~6 DDs. But mostly the decision about 6-unit Wolfpacks or 9 unit Wolfpacks comes down to organizational efficiency. If having 60 U-boat flotillas, at 6/stack, that is 10 fleets that need managing.
At 9-unit stacks that is only 6 fleets (54 units) plus 6 sitting as spares (or repairing). Managing 6 fleets is far preferable to managing 9 or 10 fleets. And because U-boats have separate areas, it only is actually managing 3 stacks per major area. Well, 3 is about the limit of my management abilities without getting into overload.
But - importantly - combining three 9-unit fleets - gives 27 total which is as close as you can get to the theoretical maximum battle optimum size. If six unit Wolf Packs, you would have either 24 or 30 total, which is not really the best.
Finally, leaders matter a lot. If running 6-unit Wolfpacks, you still need a Grand Admiral to command when 5 wolfpacks in same sea zone. But what you goanna do when only 2 wolfpacks come together? Somebody immediately must be promoted to Vice Admiral. But what if another combination of two wolfpacks comes together? That pair of stacks also needs a Vice-Admiral. Well, soon we have nearly every one of your 6-unit stacks led by a Vice-Admiral - which is a 50% waste of "being too over high in command". However, definitely start with 6-unit Wolfpacks using Rear Admirals for max exp gain in 1939-40.
But by 1941 a 6-unit wolfpack can probably not last without significant losses while it waits for reinforcements to arrive. And the whole strategy is about "not retreating" but instead increasing the battle until the enemy attacker decides to retreat. That is when the crucial time of "AI ineptness" is reached, and most of the enemy fleet destruction occurs because the AI is strategically slow to ever retreat.
SUMMARY: To be clear low sub visibility has no application in sub warfare unless we are discussing single subs. But single subs (and groups of 3) get annihilated almost immediately once attacked.
So we are discussing Wolf Packs - which have considerable total visibility. And as we need 18 U-boat flotillas to handle the org loss of battle with any 18-unit surface fleet, we are discussing huge visibility. And watching the battle display, we see the normal - the targeted U-boats keep getting re-targeted, meaning there is no advantage at all to having low individual visibility identical to the soldier standing next to you.
