Type XXI ambition could not overcome logistics and ASW mass

The Type XXI pursued the idea of a submarine able to remain submerged for days and travel faster underwater than on the surface. Designers had to work out much of the detail under bombing and looming defeat. Unreliable snorkels and underpowered diesels left the advertised range, speed, and endurance largely theoretical. Claims that these boats could have cut Allied logistics enough to force a retreat to Normandy therefore depended on performance the programme had not delivered. Even if some boats had worked, the assessment is that their small numbers would have faced an overwhelming concentration of Allied frigates and other anti-submarine systems. This is a counterfactual judgment about the forces they would have encountered. A former enthusiast for the design also conceded that it would not have reversed Germany's fortunes.