Now we’re getting somewhere!

Global Shipping Traffic

When I first tossed my fins into the pool on this ocean noise pollution thing in 1992, it was a really arcane inquiry. The “elevator pitch” was pretty long. But over the years the issue has been floating up to the surface, and unlike chemical and plastic pollution (the other two big ocean pollution issues), what we like to say is “when you stop the noise, it goes away!”

Now it’s really easy to say “the ocean is ten times louder now than it was just 50 years ago due to trans-oceanic shipping alone!” and people get the gist.

So are we making any progress toward taking the noise away? Actually, yes! While it has been up on my sonar for a little while, progress is being made on “toroidal propellers.” I was working on quiet fans and propellers with Pax Scientific as a physics consultant between 1998 and 2007 designing test environments, instruments, and performance-metrics. Their products are really quiet, and really efficient, but they don’t have the instantaneous thrust or pressure gradient profiles required for many common commercial/industrial gas and fluid transfer or drive applications.

But only a couple of years ago the concept of “toroidal propellers” came across my desk. Engineers at MIT had designed a toroidal air-propeller that significantly quiets down the typical four-fan drone. And Sharrow Marine is manufacturing toroidal marine propellers for watercraft.

The preponderance of noise from propellors is due to cavitation. This occurs when the pressure differential between the leading and trailing surfaces of the propellor blade exceeds the tendency of the fluid or gas to smoothly (and quietly) establish equilibrium. Given that propellers are by design exploiting this differential to generate thrust, noise is a natural byproduct of their operation.

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.

This mechanism is called “cavitation,” and there is enough energy in it that it can eat away at the steel of the propellers – requiring their replacement after a number of years.

On the other hand, toroidal propellers redirect the pressure discontinuities back into the system input so they don’t “get the opportunity” to collapse, entangle, or interfere with the desired function of the propeller.

All of this noise is a dominant symptom of inefficiency, so the decrease in noise from the toroidal propellers is also and indicator of an increase in efficacy – in terms of both fuel consumption and drive  efficiencies, with a dominant artifact of greatly extending the service life of the propellers.

While the MIT air propellers are making their way into recreational, commercial, and military drones, the use of toroidal marine propellers – currently entering into the recreational and small commercial vessel market, may have a wide avenue to become the standard in large cargo, military, and industrial markets.

This will have a significant impact on vessel-generated ocean noise pollution.

Yea!