Now, Operation Overlord was dependant upon construction of enough landing craft to take part in the operations and secure lines of communications to UK.
For all-statistic lovers, how much more shipping should have been sank by KM (and to lesser extent, IJN) to delay the invasion by one year? More sinkings would result in less extensive buildup in UK as well as delaying the construction of landing crafts due to more steel and resources diverted to escort and merchant ship build-up.