There has recently been a lot of attention given to "Toroidal Propellers".
MIT has a patent. There are also expensive boat "Toroidal Propellers".
Link with links to triloop and biloop stl experimental models at printables.com and thingiverse. Also shows rails and profiles for sweeps.

https://www.youtube.com/watch?v=hby6U47oV_k

- Brian

So MIT has patented the "dounut" shape:-)
Thrust and Noise may be better or worse than more traditional designs?
The video shows an attempt to reproduce the loop designs, using rudimentary propeller design knowledge.