New propellers with looped blades could help make the oceans quiet again, sparing both marine life and the climate

• The propellers of the world's ships and boats create a constant drone that disrupts ocean life.
• New propeller designs based on loops could one day quiet that drone and be more fuel efficient.
• Sharrow Marine sells them for small boats but would need to scale up to reach the shipping industry.

Imagine you're a whale. You spend your entire life in the open water. You can see well enough, but you mostly listen.

Songs of other whales help you communicate and travel with your pod. Rhythmic waves crashing on distant shores orient you on migrations that span thousands of miles each year.

But a distant drone, like white noise, is starting to obscure the sounds you rely on.

It's the constant hum of all the cargo, cruise, and charter ships crisscrossing the ocean and trailing the coasts. Their noise is growing more thunderous each year as their propellers blare into the ocean void.

"A ship traveling from Tokyo to Shanghai is going to have direct sound impacts in that area but could potentially send noise all the way across the ocean to Los Angeles," Ben Halpern, a marine ecologist at the University of California, Santa Barbara, told Insider.

At any given moment, thousands of commercial ships are traveling over the oceans.

"You can get quite a substantial hum of all those propellers that can potentially cover up that natural noise," Halpern said.

It's not just whales that suffer. Many marine creatures rely on sound to communicate, navigate, find food, or avoid predators.

Research indicates that the overpowering hum of ships can kill off sea-slug larvae, make humpback whales stop singing, and cause schools of bluefin tuna to change direction and become "uncoordinated." Other studies have found that the temporary disappearance of shipping traffic during COVID-19 lockdowns brought a resurgence in endangered pink dolphins near Hong Kong and, after 9/11, reduced stress among North Atlantic right whales.

Much of this underwater noise comes from ships' propellers. But it's not from the mechanics of the propeller motors — it's the physics at the tips of the propeller blades.

That's why engineers like Tommy Sebastian, a senior staffer at MIT Lincoln Laboratory, are tinkering with unconventional propeller shapes. New loop-based models are designed to not only reduce noise pollution and help protect marine life but also increase energy efficiency — a win-win-win for the oceans, the climate, and the shipping industry.

Propellers that close the loop cut out some of the noise

When a blade propeller spins in the air (like the one on the left in the above image), it generates regions of high and low pressure that meet at the blade's tip to create a vortex.

It's these so-called tip vortices that make a lot of noise, both in the air and underwater.

When you replace the blade with a loop, there's no tip. The high- and low-pressure regions meet along the edge of the loop, distributing the vortices more widely so that they dissipate faster and therefore more quietly.

That's what Sebastian realized when he put loop propellers on a drone.

Hear how much of a difference it makes:

"I think this gives people another dimension to explore in propeller design," Sebastian said.

Bringing the closed loop to water

Greg Sharrow was trying to make quieter drones to film live classical music when he realized that his loopy design, which aimed to limit tip vortices, had an advantage in the water too.

As a boat propeller spins, it creates similar high- and low-pressure regions in water as a drone propeller does in air. These pressure changes create cavities of air that form bubbles.

This process, called cavitation, is very loud — Kathy Metcalf, the president and CEO of the trade association Chamber of Shipping of America, said it accounts for most of a boat propeller's noise.

Sharrow said his loop-based boat propellers are about 20 decibels softer than traditional propellers because both tip vortices and cavitation are "eliminated or significantly reduced."

But to determine the noise reduction for ocean life, Halpern said that it's crucial to measure the frequencies, in hertz, of the propeller noise. Marine animals can hear a much wider range of frequencies than humans can.

Sharrow began selling propellers to recreational boaters in 2020 through a company he founded called Sharrow Marine.

Sharrow plans to run third-party tests to better assess the extent of his propellers' underwater noise reduction. He said he aims to publish the results in the next year.

Loops could be more energy efficient, too

Metcalf described cavitation as lost energy.

Testing led by the company BoatTEST found that the Sharrow Propeller was up to 30% more efficient than a standard blade propeller.

Efficiency doesn't just save money — it can also help ships use less fuel and emit less heat-trapping gasses into the atmosphere.

The Organisation for Economic Co-operation and Development has said that about 90% of traded goods are transported by ocean shipping, which represents 2.9% of global greenhouse-gas emissions.

Sharrow hopes to expand into the shipping industry as his company scales up manufacturing.

"We have been actively designing propellers of 10 meters in diameter for many years and working collaboratively with shipping-container companies around the world to apply the technology for container ships," Sharrow said.

He said that Sharrow Marine produces up to 1,300 propellers a month at its facilities in Detroit and that the company had months' worth of back orders to catch up on through a supply collaboration with Yamaha. His plan is to scale up to make hundreds of thousands of propellers each year.

The future of shipping could be much cleaner and quieter

Though loop propellers aren't widely used for commercial shipping vessels — the main culprits of underwater noise from propellers — they could one day replace those ships' noisy screw propellers or traditional blade propellers.

Quieter propellers are just one part of an impending reckoning for the shipping industry.

To reduce emissions, cargo ships have to switch to low-emission fuel sources like green methanol. That would involve redesigning new ships, retrofitting old ones, and building the coastal infrastructure to refuel them.

Nobody's requiring shipping companies to do that, so they aren't rushing to do it. But Metcalf said there's "a very quiet push" to turn sustainability guidelines from the UN's International Maritime Organization into a mandatory code or market instrument to kick-start the fuel transition.

Reducing underwater noise could go hand in hand with that transformation.

"There's a synergy there," Metcalf said. "You can improve your energy efficiency and reduce your underwater noise with the same kind of things."

Strategies with such synergy include reshaping bows and hulls, maintaining a clean hull and undamaged propellers, and adding extra structures to decrease cavitation around the propeller. Some of those options are operational, and some involve retrofitting the ship. All can help save energy, reduce noise, and make greener shipping.

"Propellers are a big piece of the solution, but they're not the only piece," Metcalf said.

April 19, 2023: This story has been updated to clarify the findings of the study on stress among North Atlantic right whales.

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