The only chance the U baoats had would have been if the enraged Fuhrerbefehl to scrap the surface fleet after Barents Sea in December 1942 and diverting of all production in to U boats, would have taken place 1 1/2 years earlier.
This could only have happened if Bismarck and PE had both been sunk for no loss to the British in June 1941, then a livid Hitler accelerating Doenitz's promotion and putting all resources in to U boats, there might have been just enough new ones available and manned in action in the Atlantic by March 1943 to totally sever the supply line to the UK, thereby delaying D Day for months.
This could only have happened if Bismarck and PE had both been sunk for no loss to the British in June 1941, then a livid Hitler accelerating Doenitz's promotion and putting all resources in to U boats, there might have been just enough new ones available and manned in action in the Atlantic by March 1943 to totally sever the supply line to the UK, thereby delaying D Day for months.