Looking at the KM's UBoot arm, I couldn't help but notice how stagnant they were, design-wise, until the introduction of the XXIII and XXI. The designs they used were both obsolescent and underpowered. The Type VII was a crappy coastal boat pressed into oceanic service, and the Type IX was barely better than the USN's prewar Porpoise class. Sure, they built a shitton of them (especially the Type VII), but having a lot of crappy boats leads to having a lot of dead crews (which unsurprisingly actually happened).
Main issue was bad throw weight, with only 4 bow tubes (compared to 6 in their US and British contemporaries). Also, speed and endurance was bad throughout the war, with engines and motors being basically the same from 1940 to 1944. Not to mention that some of the changes to the boats actually decreased their underwater speed and maneuvrability (the 3,7 cm Flakzwilling mounted on late-war Type IX, while an excellent weapon, was enormously heavy and drag-prone, and so was the assortment of 3,7 cm and 2 cm guns from the Type VII). Little to no hydrodynamic refinement went into them.
Why didn't the KM adopt something like the Type XII, with its 6 bow tubes? Why didn't they keep streamlining the boats?