An unusual late WWII design for the German Kriegsmarine was the midget submarine called UGS Manta. A hybrid design of different research bureaus, the idea was to provide a naval vessel with the capabilities of a midget submarine and the speed of a hydrofoil boat.

Hellmuth Walter, a German engineer who pioneered research on underwater airindependent propulsion (AIP) had built a submarine with remarkable underwater speeds. Similar technology was proposed for this design but due to competing interests and costs it was doubtful this could be viable even during wartime. Although an interesting design, it was never built and only a model remained of its existence.

Credit to these good sources for further reading:

http://www.hisutton.com/MantaUBoat...

https://www.navalencyclopedia.com/ww...

(In German)
https://de.wikipedia.org/wiki/Manta_(...)

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