Toroidal propellers: A noise-killing game changer in air and water• https://newatlas.com by Loz Blain
Radically quieter than traditional propellers in both air and water, they're also showing some huge efficiency gains.
Propellers are designed to take a fluid, generally air or water, and use a rotating motion to push that fluid through. They're evolutions, in a sense, from the Archimedes' screw, which was likely used in ancient Egypt thousands of years before it was described by Archimedes in 234 BCE.
For devices designed to revolve, however, there's been little in terms of revolutionary design changes for an awfully long time; prop-driven aircraft still use twisted-aerofoil bladed props similar in design to the bamboo-copters Chinese kids were enjoying 2,400 years ago, with surprisingly slim gains in efficiency over the wooden props the Wright brothers developed in wind tunnels in 1903. Boats still use screw-style propellers, variants of which can be found as far back as the 1700s.