Toroidal propellers: A noise-killing game changer in air and water

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Interesting @judd I would see those on offshore power boats.

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Cool!
I wonder if its because they have no tips to shed vortices from?

Seems to be: its from MIT

This closed-form structure
minimizes the drag effects of swirling air tunnels (i.e.,
vortices) created at the tips of blades and strengthens
the overall stiffness of the propeller. These features
reduce the propeller’s acoustic signature.

EDIT: and the original article says it - DOH!
“The key thing that we thought was making the propellers quieter, was the fact that you’re now distributing the vortices that are being generated by the propeller across the whole shape of it, instead of just at the tip,” says Sebastian. “Which then makes it effectively dissipate faster in the atmosphere. That vortex doesn’t propagate as far, so you’re less likely to hear it.”

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