For those interested in aviation history/ I had never heard about this one.
This is super interesting and totally bizarre, so I did a little research. The idea is that a single blade would be more efficient than multiple blades because the single blade is always travelling through undisturbed air. The propeller is counterweighted and mounted eccentrically on the hub to keep it balanced. It also has fore-aft pivot so the prop pitch self-adjusts to the most efficient angle - you can move the tip of the blade forward and back several inches with your hand.
Apparently the design worked; in 1939 the Everel prop was tested on a Taylorcraft in a race and won by quite a bit. However, shortly after the introduction of the prop, powerful 50HP engines were developed which rendered the efficiency gains of the single blade moot. Considering that the balance of the prop was very fickle in changing weather, the already mechanical complex prop just wasn't worth the effort, so the design never caught on.
It's a nifty bit of engineering and a cool piece of history.