In 2011, Gavin Allwright was living in a village outside Fukushima, Japan, with his wife and three children, when a powerful tsunami destroyed the coastline, splintering homes into debris, crashing a 150-foot fishing boat into the roof of his wife’s parents’ house and setting off a power-plant accident that became the worst nuclear disaster since Chernobyl.

Allwright had a background in sustainable development, especially as it relates to shipping. In his travels in East Africa and Bangladesh, he had watched traditional sails and masts replaced by outboard motors. The move locked people into a cycle of working to buy fuel, damaging their lives and the environment. In Japan, Allwright had been living a quiet life, running a sustainable farm and dabbling in consulting. Now, it seemed, environmental disaster had followed him there.

To escape the aftermath, the family moved to Allwright’s hometown on the outskirts of London. But Allwright couldn’t stop thinking about the Fukushima disaster. To him, it was a dramatic display of technology going wrong, further proof that the world we built is unsustainable.

He thought about shipping. It produces 2.9 percent of global carbon-dioxide emissions, almost as much as the entire continent of South America. With every country benefiting from global trade, it could be argued that shipping is everybody’s responsibility, but it is treated as if it is nobody’s. In the vast but liminal space of the ocean, cargo vessels — some of the largest machines on the planet — have generally operated in obscurity. The industry’s greenhouse-gas emissions have only grown as world trade has expanded, about 10 percent in the last six years. Shipowners, charterers and regulators have done little about the situation.

Allwright had previously spent 10 years working with a group that tried to build small cargo ships that would run on wind power to eliminate their carbon footprint. It underscored for him that sails aren’t a relic of the past. At the most fundamental level, the way modern sails work is similar to the way sails did a thousand years ago: As wind moves against their curves, it creates a high-pressure system on one side and a low-pressure system on the other, resulting in a forward thrust that pushes the ship along. But the design, materials and size of modern sails, along with the ships’ movements, allow them to harness significantly more power from the wind than the cloth sails of the past — enough so that they can move a huge cargo vessel. In conjunction with fuel, modern sails can power ships with something close to the speed and predictability to which the global economy is accustomed.

The group Allwright worked with never managed to get the ships built. Looking back on it, he believed it was a commercial failure, not a technical one. In 2014, he started the International Windship Association, a trade association, bringing together disparate groups of inventors, researchers and others who wanted to get modern wind propulsion on cargo vessels — not to replace fuel entirely but to require considerably less of it.

Climate change, Allwright told anyone who would listen, would create intolerable pressures. He would point them to books and reports by scientific organizations like the Intergovernmental Panel on Climate Change that outlined what would happen if the world stayed on its current trajectory, sending average temperatures up 3 degrees or more: vicious wars over resources, mass refugee migrations, major cities engulfed by rising seas. And because this was a crowd of businesspeople, he would mention, too, that all of that would be catastrophic for the economy.

This preaching of sustainability was heard, at first, as an act of aggression. Shipping executives would walk out of meetings and slam the doors as they left. When he brought up numerical targets for carbon-dioxide emissions from shipping, someone shouted that it would never happen. “It’s a fantasy!” another yelled. Then, in the last couple of years, something shifted. The industry has been facing more pressure to emit less carbon, but one of the most talked-about methods of reducing shipping’s carbon footprint — using alternative fuels such as hydrogen — is costly and difficult to pull off. Wind propulsion, on the other hand, is already available.

The fact that shipping is contributing significantly to climate change has become so well understood among shipowners that Allwright has been able to delete the two or three slides in his presentation that outlined the industry’s carbon emissions and its impact on the environment. An exception is presentations in the United States, where there are inevitably audience members who don’t believe in man-made climate change. For them, he keeps the environmental slides in, while emphasizing the economic argument: Fuel can be expensive, especially if, in the future, the price of oil spikes, taxes on carbon emissions drive up its cost or the industry is forced to shift to green fuels. Wind is free. If wind-propulsion technologies could be offered cheaply enough, the reduction on fuel use from wind-assisted ships may well make them more cost-effective than conventional combustion-engine ones.

Allwright’s trade association has grown, with about 40 companies now developing wind-propulsion technologies. They include a Finnish company that is installing sails on existing vessels, as well as businesses in Britain, France, China and Japan. Fifteen large wind-assisted ships are already on the water. Another five are expected to go into sea trials and enter the market soon, and more than 20 are in late-stage prototypes.

Yet unanswered is whether the sails can be made cheaply enough — and can save enough fuel — to make it worthwhile to install them. “Can we harness the wind?” Allwright asked. “Yes. Can we do it at a cost that is tolerable to the market? That’s the question we’re answering now.”

One afternoon in late April, I boarded the MV Afros, one of the wind-propulsion ships already in use, which is working as a cargo vessel and providing an early proof of concept. A conventional combustion-engine ship has a life span of 20 to 30 years, and this initial stage of modern wind propulsion largely involves retrofitting sails onto existing ships, using wind to cut down on some fuel use until the ships are scrapped. The MV Afros was fitted with sails from the start, using Flettner rotors, a technology that has existed since the 1920s. The rotors — spinning cylinders powered by the ship’s diesel-fueled generator — are mounted on the deck of the ship. Though they bear no resemblance to the sails of old, they function like them: Wind splits into high- and low-pressure systems when it hits the rotors, creating thrust that pulls the ships forward.

The MV Afros, named after a Greek word for the white foam on the tip of a wave, had just pulled into one of the berths of St. Bernard Port in New Orleans — its first discharge stop after a 34-day voyage from Gwangyang, South Korea — to unload a cargo of cold-rolled steel coils. I stood on the deck and peered down into one of the holds, where a pair of dockworkers in high-visibility vests stood deep in the bottom, scarcely taller than the coils of steel and dwarfed by the cavernous scale of a ship built to carry more than 63,000 metric tons of goods.

The rotors were on the starboard side: four columns painted in white, each 52 feet tall and seven feet in diameter — wide enough for a seafarer to crawl in and make repairs. I clambered around the deck, past heaving cranes and over thick, greasy coils of mooring rope, and ran into three deckhands in hard hats and orange jumpsuits. They boarded the Afros just four days earlier, during a brief stop at the Panama Canal.

I asked them what they thought of the sails. They were, at first, confused. Then I gestured at the rotors. They told me it was the first time they’d seen such a sight or heard the deep whir that comes from the rotors at full spin. Their first thought was excitement. “High-tech,” one of them said. The next thought was of obligation. “Of course, it adds to our work,” he noted — a new set of maintenance hassles to deal with. “But when it comes to the environment, it helps.”

The Afros was the brainchild of Costas Apodiakos, a member of the third generation of a Greek shipping family. He first realized something had to be done about the industry’s pollution in the late 1970s while on an apprenticeship on a vessel docked in Alexandria, Egypt, where a beautiful sunset struck a discordant note with his surroundings: acrid water slicked with oil and chemicals; trash, tossed over the side of the ship, drifting with the current.

He became interested in using sails on his cargo ships, but it took another 20 years before wind-propulsion materials became light and cheap enough and the technology advanced enough — and nearly another 20 years before Apodiakos could develop, test and install rotors on the Afros, which is managed by a company started by his family. That was in 2018. In the three and half years since, the Afros has been bouncing to ports around the world — 59 of them so far — carting sand, fertilizers, iron ore, manganese, soybean meal, steel slabs and sulfur.

Apodiakos’s wind-propulsion company, Anemoi Marine Technologies, is now beginning to sell its rotor sails to others, for $350,000 to $1.1 million apiece, depending on the size, with another $500,000 in installation costs. An Anemoi representative says the company is planning to install three rotors on a customer’s ship in 2022.

On the bridge overlooking the deck and sails, Antonios Mandas, the second officer, showed me the controls for the rotors. I was surprised at how simple they were. There are four yellow remotes, one for each rotor. Each remote has eight big black rubber buttons used to control the trolleys that slide the rotors along the deck, so they can be moved out of the way of cranes at port. On the ship’s main control panel, a small, mint green console the size and shape of a pulpit controls the speed and movement of the rotors in the open ocean. There is a monitor displaying data such as wind direction and a key for a manual override. Most of the time, the crew leaves it on automatic.

The rotor sails, unrecognizable to most people in the shipping trade, garner quite a bit of attention. Mandas said that people on other ships radio the Afros after seeing them from afar, asking what, exactly, those things are. He tells them they help save fuel, add speed and reduce environmental damage. Longshoremen at the ports inquire about how they work; some wonder if they’re wind turbines or some kind of cargo that the ship will be discharging. Off the coast of Vancouver, members of the Canadian Coast Guard, fascinated by the sails, asked the officers to turn them on so they could see them spin.

The attention isn’t always positive, however. Mandas told me that, at a port in China, he was confronted by one of the stevedores. Stevedores board ships to operate the cranes mounted on deck, and the rotor sails seemed to be partially blocking this stevedore’s line of sight. He couldn’t understand what the man was saying, but screaming and gesticulation is something of a universal language: Mandas gathered that the man was annoyed.

Flettner rotors like the ones on the MV Afros are among the first in the water and can be installed in a matter of hours, but their reduction in fuel consumption is limited; they save an average of 3 percent to 15 percent on fuel. Other sails in the works are projected to save as much as 30 percent in fuel use, on average. They will be bigger and more powerful, with more sophisticated automation, maybe even augmented-reality cameras to compensate for sails blocking the view of the horizon.

Some of the ships under development use soft, square sails stacked onto masts, like the famously fast clipper ships of the 19th century, but with sleeker, larger designs. Others look nothing like the ships of old. One design calls for a narrow-bodied ship rising high above the water, so that part of the hull itself functions as a sail. Another has a line of smooth, hard, upward-reaching sails along its center, arranged like the plates of a stegosaurus. A fourth features rigid rectangular sails that would retract to allow ship-to-shore cranes to pull containers off at port. There are even plans to fit cargo ships with huge kites that unfurl ahead of them, pulling the ship along on a good wind.

Over the past several years, some important players in the shipping industry have begun to invest in decarbonization, including wind propulsion. A.P. Moller-Maersk, one of the largest vessel operators in the world, has studied technologies that could help, as has Cargill, the food-and-agriculture company, which is a major charterer of ships. “We really see this as a massive change coming at us, and we better be part of that,” Jan Dieleman, the head of ocean transportation for Cargill, said. Cargill looked at the wind-propulsion technologies available, including kites and rotor sails, and made a notably bold move: It chose to work with BAR Technologies, a start-up in Portsmouth, England, committing to pay for the company’s first sails — and the cost of installing them on one of the ships that Cargill charters — with the help of a grant from the European Union. Beyond that, Cargill aims to charter at least 20 new wind-assisted ships over the next couple of years.

Portsmouth, which sits at the mouth of a natural harbor on England’s southern coast, was built on sail: Many centuries ago, at a time when international trade felt like the rarest of miracles, a French merchant with a fleet of sailing ships developed the port. In 1194, Portsmouth was formally chartered as a market town. But its sailing ships lost their dominance a long time ago, as newer vessels became increasingly large and bulky — powered first by coal, then by oil. The modern ships outgrew the harbor and moved to wider and deeper ports on the outskirts of town. What was once the thriving commercial and naval center was renamed Old Portsmouth. Rather than being filled with warships and schooners, the piers of Old Portsmouth are now crowded with ferries and leisure yachts, their white masts tilting with the sway of the water.

In 2014, Ben Ainslie, a knighted Olympic sailor, mounted a campaign costing more than $100 million to win the America’s Cup sailing regatta from a headquarters in Old Portsmouth, building a modern glass-and-concrete structure, six stories high, to house 150 employees, including engineers, sailors and boat builders. While sails had disappeared from commercial shipping, the America’s Cup had become the most technologically advanced regatta in the world — an engineering contest as much as a racing one — and a showcase for innovation in wind propulsion. Ainslie’s building came to dominate the skyline of Old Portsmouth.

Early on, Simon Schofield, the naval architect who led the engineering team, and Martin Whitmarsh, the chief executive for Ben Ainslie Racing, had a wild notion: What if they could funnel the technological innovations from the America’s Cup campaign into bringing wind propulsion back to cargo vessels? They knew other companies were trying it and thought, with their engineering background, they could do it better.

Schofield set up shop in a small room next to the Ben Ainslie Racing cafeteria, gathered a few of the engineers from the racing team and began designing, under the name BAR Technologies. The rigid sails they came up with, named WindWings, look like vertically mounted airplane wings, rising 120 feet or more, and consist of a steel frame wrapped in lightweight composite fibers hardened in resin. They’re meant to be installed on a ship in groups of three to five, rotating and changing shape to catch the wind so that the vessel can harness more free power from any given gust.

Schofield grew up on Mersea Island on England’s eastern coast, the son of a boatbuilder. His father made him his first sailboat — a single-masted green wooden dinghy named Polo — when he was 6. Schofield can’t remember a time before he was sailing Mersea’s creeks and estuaries, throwing barbecues with his friends on marshy banks; leaping into one another’s boats; slapping his homework shut on a Wednesday night to race, with as many as a hundred other kids in dinghies, during Mersea’s weekly regattas. It was on his boat that he learned to read the wind in the ripples on the water and catch a breeze as it slid along the curve of a cloud. He found sailing to be like a game of chess: You have to see far enough into the horizon to plot your route across the water from one gust of wind to the next.

Today’s seafarers no longer know their way around wind; conventional combustion-engine ships mostly take the shortest straight-line route to their destination. Part of an effective wind-propulsion system is software that can guide the ship onto routes that might not be the shortest — but, because of how the wind is moving, could be more fuel-efficient. During a windy stretch, a cargo vessel could turn off its engines and be propelled entirely by the wind. Modern wind propulsion, then, depends on teaching software, not seafarers, to gauge the best route.

By the time I visited Old Portsmouth, the Ben Ainslie Racing team had moved out of its building, largely leaving it to Schofield, now the company’s chief technology officer, and his workers. BAR Technologies commandeered the top floor, an oval open-plan room with near-360-degree views overlooking the harbor and the historic rowhouses. Schofield’s team had grown to 25, with a new hire coming in nearly every week. Mechanical and systems engineers, computational-fluid-dynamics analysts and software developers worked in a quiet hush in front of wide monitors.

Bright yellow masking tape was arranged in loops on the floor — the central section 33 feet wide, with two sections 16 feet wide on either side — running nearly half the length of the office. These were outlines, at full scale, of BAR Technologies’ WindWings. The sails are designed to be fitted onto tankers and dry-bulk carriers, which transport unpackaged goods such as grain or coal. Schofield looked up the building’s measurements and worked out that the office floor was 100 feet high, meaning the WindWings would be 24 feet higher. I looked up, trying to imagine a sail rising to that height.

When Schofield first took on the task of making a sail fit for cargo vessels, he thought it would be easy: sort of like a yacht sail, but bigger. But sails for a cargo ship need to be optimized for factors beyond just speed. There were ports to consider, seafarers, shipowners, manufacturers, regulations, the placement of hatches, bird collisions, the not-uncommon prospect of a 36-foot wave breaking across the deck in the middle of a storm.

The design they ended up with, the WindWing, can be installed as a retrofit on existing cargo vessels or fitted onto newly built ships. The sail is designed to pivot automatically, using sensors to gauge the speed and direction of the wind, to catch it and to make sure the ship keeps moving forward. During a storm, or when the wind is blowing too strongly, the sails would automatically turn off, whipping in the wind without harnessing it. To allow the ship to go under a bridge, or while it is at port, the wings would fold into themselves, then lower, flat, onto the surface of the deck — a 15-minute process — so they would be clear of the cargo hatches, cranes and railings of the ship.

While few ultramodern wind ships exist yet in physical form, eye-catching renderings are legion, depicting futuristic hulls that could never fit into any existing port or spindly sails mounted onto the top of a cruise ship that look more like antennae than something that can propel a vessel. Schofield resisted the temptation to put out a rendering of the WindWing before completing the mathematical models needed to back up the sail’s performance. He says many cargo-sail renderings risk not being able to stand up to technical scrutiny: “Very quickly it becomes no more than a cartoon.”

Schofield’s hope is to overdeliver later — thus maintaining the credibility of the design, the company and the whole idea of wind propulsion. “The shipping industry is conservative,” he said. “Lose their trust, and it will be harder to convince them the next time.” By the time the company’s designs hit the water, Schofield’s engineers expect average fuel savings of 30 percent.

Cargill and the E.U. will fund the first WindWings system to be retrofitted onto an enormous 750-foot dry-bulk carrier currently traveling the world. The sails will be assembled at a shipyard in China and mounted onto the ship by the summer of 2022.

The price for the first prototype set is more expensive than John Cooper, BAR Technologies’ chief executive, would like. He wouldn’t disclose the price, saying only that he believes the cost will fall by the time the sails are in mass production and that shipowners will see a full return on investment in just over five years. (By the company’s calculations, a 210-deadweight-ton dry-bulk vessel retrofitted with three 164-foot sails could save around $1.5 million in fuel costs a year. A larger ore carrier, with five sails, could save about $2.5 million.) Cooper’s ability to pull this off could determine whether BAR Technologies will, in coming years, be making just a handful of sails or thousands.