Progress is being made on “toroidal propellers.” This will have a significant impact on vessel-generated ocean noise pollution.
Air is compressible, so collapsing tip vortices, and the following blades smashing through the pressure discontinuities generated by the preceding blades are the predominant noise sources.
Water is really not compressible, so the artifact of these conflicting pressure gradients occur when the static pressure of a liquid behind the blade reduces to below the liquid’s vapor pressure, leading to the formation of small vapor-filled cavities in the liquid. This generates “vapor bubbles” that collapse, making most of the propeller noise.
Story by Michael Stocker, Ocean Conservation Research
