Toroidal props.

The Sharrow Propeller

Interesting concept... tough to beat on a rock and straighten after you ding an ear.

I'm not sure if you should run it on an engine or kneel before it and pray.

Interesting design and agree with Clay T on fixing it when damaged
In Capt Steve Video he defines propeller slip as propeller efficiency they are not the same. Efficiency compares engine applied power to prop delivered power. He correctly describes slip related to pitch.