[ExI] Toroidal propellers: A game changer in air and water

[BillK]

These strangely-shaped twisted-toroid propellers look like a
revolutionary (sorry) advance for the aviation and marine sectors.
Radically quieter than traditional propellers in both air and water,
they're also showing some huge efficiency gains.
<https://newatlas.com/aircraft/toroidal-quiet-propellers/>

Looks good! Still very early in development though, so improvements
can be expected.

BillK