Sort of reminds me of the design of Navy submarine propellers (there known as 'screws'), optimizing for similar noise dampening properties. For a very long time the design of modern submarine screws was one of the closest guarded secrets of submarine design, with submarines docked having cloth shrouds over the screw even in classified facilities.
AFAIK, those weren't torodial, but they had similar long arcs of the blades.
AFAIK, those weren't torodial, but they had similar long arcs of the blades.
For more information: https://americanhistory.si.edu/subs/anglesdangles/taming.htm...