GeorgeAmbush said:
I use stacks of 6 and retreat from the long battles... But sofar I can only remember getting a dutch destroyer from 4 or 5 games (with all doctrines researched).
Fair enough that Sub´s doesn´t clean the oceans for enemys, but IMHO they were more balanced in HOI2
let me give you some advice for doomsday

for surface fleet engagments with subs.
Like you said - research all doctrines. Use stacks of 8 to 12 subs ... Try to battle outside UK coastal waters cause you'll prolly avoid his naval bombers.
Set your missions on night. Use your best wolfpack leaders.
Use naval bombers to soften up the enemy.
Build up some surface ships to soften up the enemy before you move in with subs.
Use subs to finish of the enemy - in that way you have the best chances. If you are lucky you can get a shot at close distance and the enemy will lack many ships to fire back...
subs in doomsday are more supportive units for grand naval operations. So it's cool to have some stacks you can use during combat.
Basicly subs can sink some ships - but they are not that effective. The better judgement will be to build a combined fleet with surface ships and subs.
Subs are also good interceptors for invasion fleets. Especially if you play Germany. It will help your NAV's to secure the seas around France, Ireland and England.
And always remember - if your enemy has his big ships waiting in harbour - low on screens - you should put your subs to guard that harbour cause when they flee - you'll be killing his ships - I once sank 2 US Carriers by doing this

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ps = something I still have to try - attach CVL's to subs ... perhaps your positioning is enhanced so the subs can start the battle at close distance.