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In page Propeller:
"Twisted-toroid (ring-shaped) propellers, first invented over 120 years ago,[citation needed] replace the blades with a-circular rings. They are significantly quieter (particularly at audible frequencies) and more efficient than traditional propellers for both air and water applications. The design distributes vortices generated by the propeller across the entire shape, causing them to dissipate faster in the atmosphere.[1][2]
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