The Type XXI's postwar influence varied by navy

Which postwar programme made the best use of the Type XXI: Soviet Zulu, American Barracuda, or British Porpoise submarines?

The Soviet programme is judged to have made the greatest use of it after wartime disruption to its research base, while Britain and the United States selected useful features within their own design traditions.

The question compares Soviet Zulu, American Barracuda, and British Porpoise submarines, but the premise that all three owe their development chiefly to the Type XXI is challenged. British hull design is traced through domestic U- and V-class experience, with selected German ideas considered alongside that lineage. Britain's wartime submarine-against-submarine success is also offered as a reason for confidence in its own designs. American submariners and engineers are described as recognising useful technology while retaining confidence in their own boats. Soviet research had suffered repeated moves, personnel losses, and disruption during the war, making German work more useful to restarting development. The Soviet Union is judged to have made the best and most extensive use of the Type XXI because wartime disruption left it more dependent on that technology.