Submarine technology is one of the few areas in which World War 2 Germany had any sort of advantage over the Allies. The Type XXI U-Boat is one of the most commonly known examples; it was one of the first (but not the first) submarines designed for sustained underwater performance; it was capable of 16-18 knots underwater, about double what other U-Boats could do. That they sank a grand total of zero Allied vessels during their career is more a symptom of German military incompetence and industrial issues than any severe problems with the concept. (Of course, it would take until 1955 for a ~*t