Hey, want to hear the most annoying sound in the world? Okay, “most annoying” may be subjective, but according to a 2017 study from NASA’s Langley branch, humans find the buzz of a drone’s propellers to be more annoying than any other machine-based sound on earth, which really limits all the helpful ways we can use drones in big cities.1 But just by changing the shape of the propeller, we may have found a way to make drones–and a lot of other things that use propellers–not just quieter, but way more efficient. For some boating applications, the efficiency was boosted by 20% – 105%! 2 Can this propeller really help everything from quieting tiny drones to helping boats sail further with less fossil fuels? And if just changing the propeller shape makes such a big impact, why haven’t we tried something like this sooner?
Drones: love ‘em or hate ‘em, they are an increasingly commonplace feature of the modern world. Though they’re probably best known for their use in photography (or more infamously as weapons platforms), there’s truly an astounding variety of applications for drone tech, like exploration, forestry, treating weeds on farms, and safety.3 You may have seen Mark Rober’s excellent video on Zipline’s drones delivering lifesaving blood and medicine to distant parts of Rwanda and eventually other cities around the world.4 Speaking of deliveries, one often promised but not-yet-realized opportunity for drones can be found in cargo and shipping services. Amazon has been working for almost a decade to make drone delivery a real possibility 5, but concerns about safety, theft and noise pollution have stymied its development. As I mentioned at the top, humans are very sensitive to the sound of a drone’s propellers. This is because the buzz they make falls into the 100 Hz to 5 kHz audible range, which is the same as a crying baby.6 It could also be compared to the sound made by flying insects, such as wasps or houseflies, which we’re already accustomed to avoiding. This is a shame because drone-based ‘last leg deliveries’ emit 84% less greenhouse gasses, and consume up to 94% less energy per parcel when compared to standard vehicle deliveries.7
Cargo ships are in the same boat, pardon the pun. When shipping large amounts of goods, ocean freight is usually the preferred method because it’s so much cheaper to ship items by water than by air or land. Maritime shipping plays a critical part in global supply chains, accounting for 80% of global trade by volume.8 These large ships rely on the dirtiest kind of fuel – heavy oil fuel – and they can be difficult or expensive to retrofit for clean energy.9 As a result the maritime shipping industry accounts for 3% of global greenhouse gas emissions.9 Now that might not sound too bad, but if maritime shipping were its own country, it’d be the sixth largest CO2 producer in the world.10
Normally drones and cargo ships would be two very different videos, but one, singular, simple innovation might kill two birds with one stone. But, other than shipping us stuff, what could a 3 pound drone and 220,000 ton container ship possibly have in common?
Well, they both use propellers. Interestingly, both MIT’s Lincoln Lab and Sharrow Marine hit upon the same, radically different propeller shape: the toroidal propeller. Instead of the traditional blade design used by most propellers to generate thrust or lift, these use softly slanting donut shapes, called toroids. We’ll get into all the benefits this weird shape provides in a minute, as well as how two different institutions working on very different projects stumbled upon the same solution.
In both cases, the path to the toroidal prop was a winding one. Sharrow Marine’s founder, Gregory Sharrow, was a Berklee College of Music graduate working in video production, when he started tinkering with propellers to make drones less annoying on film sets. He knew the propellers’ tips made most of the noise, but how could he make a propeller blade with no tip?11 This initially lead him to the toroidal shape way back in 2012, and after years of testing and modeling,11 he thought it would be a better fit for maritime applications. And here we are … with their prop now on the market.12
Conversely, according to Lincoln Labs’ Dr. Thomas Sebastian, they were working on ionic propulsion for fixed-wing aircraft when they came across an outdated, turn-of-the-century, ring-shaped wing design. The shape didn’t work well with fixed-wing craft, but Sebastian wondered if that type of shape could be applied to a propeller, and thanks to an intern and a 3D-printer, in a few days the team had developed some working toroidal prototypes. After testing, they found some surprisingly large improvements in noise pollution and efficiency.13
What were their findings? How did simply changing the shape fix both the noise and efficiency issues? As you can clearly see in this test from Sharrow Marine, a standard propeller creates these small swirls, or vortices at the tips of its blades, which can create bubbles or cavitations. Surprisingly, the loudest part of a propelled device isn’t the engine or the motor, but the sound made by these vortices and cavitations.12 Look at this example from Sharrow Marine, and you can see that not only does the Standard Propeller create noticeable tip vortices, it also sprays (or displaces) water in a less concentrated manner as the sharrow prop does, reducing the potential to propel the boat.14 Here the toroidal and standard propellers are moving at the same speed. Sharrow’s toroidal propeller is not just generating far smaller tip vortices, but the dye being pushed out behind it forms a tighter stream. That equates to more power and efficiency for the same amount of energy.15.
This quote from Dr. Sebastian really helps illustrate what we’re seeing in these tests: “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…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.”6
In most cases we have to sacrifice some level of power or efficiency for a quieter ride, but that’s just not the case here. MIT’s best-performing B160 design was not only quieter at a given thrust level than the best standard propeller they tested, but it also produced more thrust .6 The toroidal drone is about twice as quiet as a traditional drone, and they found that the most annoying sounds – that’s 1-5 kHz range – were the most reduced.16
One of the reasons for the sound is the high speed of the propeller tip. As it passes through the fluid (water or gas), it lowers the pressure of the fluid. For ships, this means a poorly designed propellor can lower the pressure enough to actually turn it into vapor- it’s possible to boil 33 F seawater by lowering the pressure. When these gas bubbles burst, called cavitation, they cause a lot of acoustic noise and can actually damage the propellers. This problem is also found in pumps which use a rotor to move liquids through pipes in the chemical processing industry.
As you saw a second ago in Sharrow’s tests, these effects are even more pronounced when it comes to hydrodynamics. The toroidal variant sucks in more water than a traditional propeller, which reduces the amount of water that “slips” out the sides. The result allows the boat to be not just faster, but smoother and more efficient too.12 In testing, Sharrow has doubled the speed a boat can achieve at lower and mid-range RPMs. This broadens the effective rev range of the motor and reduces fuel consumption by somewhere around 20% (that’s significant when we’re talking about fuel prices!).6 Boats also tend to spend the majority of their time at midrange cruising speeds around 4,000 RPMs. As you can see in this graph, the boat equipped with toroidal props managed to be 105% more efficient than your average propeller at this 4,000 RPM sweet spot.2
If just changing the propeller shape made such a big difference, why are we only hearing about it now? Surely, someone else must have been experimenting with non-standard propeller shapes? The answer is, of course, yes. The potential energy savings means that almost every industry with a propeller is toying with its shape. Just like Zipline’s seed-pod-esque propeller design from Mark Rober’s video.3 Airbus, a giant in aviation, is testing their “open rotor” propeller designs, which they claim reduce a passenger plane’s CO2 emissions by 20%.17 And those are just some of the recent examples. I mean … just look back at the Cold War stealth submarine race, where both the US and the Soviet Union continually sought to make their nuclear submarine propellers quieter and quieter. This culminated in the Toshiba-Kongsberg Scandal. Combining Toshiba’s precision machining tools and software with Kongsberg’s Numerical control device, the Soviets were able to craft an ultra-stealthy propeller.18 The US could detect the older, louder subs from 200 miles out, but submarines with the new propeller were only detectable from 10 miles away or less!2020
So, if practically everyone has been working on improving propellers for the last 200 years, then why are we only hearing about these developments now? Sadly, it’s just one of those things that doesn’t usually make it to mainstream headlines. And yet, coincidentally, both Lincoln Lab19 and Sharrow Marine20 recently earned coveted awards in their respective fields. And when a similar design starts making waves in two different industries, it’s hard not to take notice!
As exciting as it is, you probably don’t own a boat, and hopefully you’re not waiting on a drone to deliver you some lifesaving meds. So, you might be wondering, “Why should I care?”
We’ve already covered the benefits drones bring if we can make them quieter, but consider the potential of toroids and larger aircraft. Airtravel accounts for 2.5 – 3.5% of global CO2 emissions, depending on how you tally it. If this tech can be scaled up it presents another avenue for fuel and energy savings.21 … and we’ve already touched on just how much of the global economy relies on massive, diesel-chugging cargo ships. Now, ideally, we’d be able to replace these with something more environmentally friendly, but in the meantime Sharrow Marine’s toroidal propellers are already on the market. A radically more efficient ship means a lot less diesel fumes, heavy metals and carbon being pumped into our seas and skies. If and when these kinds of vessels do make the leap to greener alternatives, the increased efficiency of toroidal propellers is going to be a welcome addition. As the pandemic and so-called “port crunch” inversely showed us, anything that helps these big boats move around the world faster, cheaper and more efficiently could very well reduce prices on all kinds of goods.22
As good as that sounds, we just won’t know if toroidal propeller tech can successfully be scaled-up for big ships and planes until more tests are performed.6 And the same unique shape that gives toroidal props so many benefits comes with manufacturing drawbacks. The novel shape means they’re more complicated to produce compared to traditional propellers, and this issue only increases with scale. It’s not a huge problem for a drone hobbyist to screw up a toroidal propeller 3D print job, but when we’re talking about a 43 ton propeller for a cargo ship, any mistake in the manufacturing process is bound to be very expensive
And before we get too far from expenses, as is often the case with any new technology, these propellers are initially very pricey. Sharrow Marine’s toroidal propeller is going to run you a minimum of $5,000 for a consumer-sized boat,23 which is a full ten times more expensive than a similarly sized, standard-issue propeller.6 Of course, thanks to the fuel-savings, the toroidal propeller could pay for itself in time,24 and a $5K investestment for a minimum 20% fuel savings seems like a no-brainer if we’re just trying to maximize stats. Still, a 1,000% price hike is going to be a hard sell for a buyer. Then again, as the technology matures and the manufacturing process becomes more commonplace, that price will likely come down over time. Same goes for 3D printing, which was critical to the creation of Lincoln Lab’s propeller.25 3D printers have vastly sped up the development process here and elsewhere, and as the tech continues to proliferate we might see tinkerers create even more radically efficient propellers.
At the moment there’s still a lot of testing to do before we know just which applications are right for this tech, but it’s very cool seeing such seemingly simple engineering have an outsized impact like this.
- Initial Investigation into the Psychoacoustic Properties of Small Unmanned Aerial System Noise, NASA Langley Research Center ↩︎
- GALEON 325 GTO / TWIN MERCURY 400 HP OUTBOARDS – Key Collected Data ↩︎
- What is a drone? – Tech Target ↩︎
- Mark Rober, Amazing Invention- This Drone Will Change Everything ↩︎
- Amazon Prime Air prepares for drone deliveries ↩︎
- Toroidal propellers: A noise-killing game changer in air and water ↩︎
- Drones bearing parcels deliver big carbon savings ↩︎
- UNCTAD – Review of Maritime Transport 2022 ↩︎
- How to save the planet from the largest vehicles on Earth ↩︎
- Emissions-free sailing is full steam ahead for ocean-going shipping ↩︎
- Boat Test – The Sharrow Propeller ↩︎
- Sharrow Marine – Our Story ↩︎
- MIT’s latest drone propellers are very quiet and efficient ↩︎
- Sharrow Propellers (2022) – Test Video by BoatTEST.com ↩︎
- Sharrow Propeller EXPLAINED ↩︎
- MIT’s latest drone propellers are very quiet and efficient ↩︎
- Airbus To Flight-Test CFM RISE Open Rotor ↩︎
- Chronology of Events In the Toshiba/Kongsberg Propeller Milling Technology Diversion Case, CIA Reading Room ↩︎
- Six Lincoln Laboratory inventions win R&D 100 Awards ↩︎
- Marine Power Innovation Awards 2022 ↩︎
- Climate change and flying: what share of global CO2 emissions come from aviation? ↩︎
- IMF Blog – How Soaring Shipping Costs Raise Prices Around the World ↩︎
- Sharrow Marine Store ↩︎
- Sharrow Props 46% More Fuel Efficient At 18 Knots On 300-Hp Diesel Engines ↩︎
- MIT Lincoln Laboratory Innovation Highlight – Toroidal Propeller ↩︎