Starfox Strategy
I noticed the same phenomenon of subs engaging in long battles and tying down a surface fleet but did not have the need or insight to realize how it could be used to revive the super sub stack strategy. All I can say to Starfox is brilliant, I am glad I was not playing the UK against you when you first used it; but I look forward to a patch that "fixes" it someday. Meanwhile, if you play by a house rule of only six subs to a stack the phenomenon just means they all get sunk if found by a sizable surface fleet (albeit giving you the opportunity to crush the enemy surface fleet with whatever is in range).
In answer to the earlier query about subs in the actual war sinking warships:
That was how the Japanese used them and they proved nearly useless. It was not that they never scored, it was just too infrequent to be of any consequence. Meanwhile, the US pursued an agressive convoy raiding strategy with excellent results, as did the Germans with less long-term success but only due to the overwhelming capacity of the US to respond with air and sea power. Just as WWII proved that airpower was decisive in the seas, it also proved that WWII era subs were not very useful in surface engagements but extremely useful as raiders.
More modern stealthier subs on the other hand should be able to pick off a capital ship more easily, especially when operating alone or in a very small group. Has anyone played DD to the early fifties to see if the best subs might have this capacity? It might be useful to know when planning a long term USSR strategy.