"Claims:
1. Toroidal propeller comprising: a hub supporting a plurality of elongate propeller elements in which a tip of a leading propeller element curves into contact with a trailing propeller element to form a closed structure with increased stiffness and reduced acoustic signature.
2. The toroidal propeller of claim 1 having two or more propeller elements."
That's it. Honestly, that's pretty easy to get around, even without arguing against its validity from prior art in the submarine world.