SAN FRANCISCO — What happens in the San Francisco Bay every Thursday night does not look real or even possible, and it certainly does not look safe. Dozens of people skim across the waters at speeds of up to 40 miles per hour, kites far ahead and their boards floating two feet above the chop of the waves. They move so smoothly and quietly that some riders are on their cellphones.

They call it hydrofoiling — or, if you’re hip to it, foiling.

And every week, the foilers of San Francisco race.

“It’s like flying,” said Ariel Poler, a 50-year-old start-up investor, standing by the winged doors of his Tesla and pulling on body armor and a helmet. “The board doesn’t touch the water. It’s like an airplane wing.

“It’s like a powder day,” he added, referring to snow skiing.

He walked into the water, strapped his feet onto the small, oddly shaped board and latched his kite onto his harness. Then he leaned back in the water like an overturned turtle. As the kite caught wind, Poler popped up and blasted toward Alcatraz Island. 

The Silicon Valley elite have long loved extreme water sports (there is an entire tech and kitesurfing conference circuit). Now they have upgraded them. And suddenly, the tech world is participating in a sport most of the country has never seen.

In workshops around San Francisco and Hawaii, craftsmen and technologists are hooking hydrofoils — a lifting surface attached to the bottom of a watercraft — onto seemingly everything. Versions include a jetfoiler, which looks like a surfboard with a rudder and a tiny hidden motor; the SUP foil, a stand-up paddle board with a foil; and the kitefoil, which is like a kiteboard but has a foil.

The Oracle co-founder Larry Ellison made foils famous when he allowed them on the America’s Cup boats in 2013, leading to a panic over cost and safety. Since then, private investors around Silicon Valley have been bankrolling the technology.

“Now we all foil,” Poler said.

Just as it did for the America’s Cup, foiling adds both speed and danger. 

“Falling from four feet in the air at 40 miles an hour, the wipeouts are pretty spectacular,” said Joey Pasquale, 34, a tugboat captain and foiler. “I broke two ribs earlier this year.”

It was a typical cold summer night in San Francisco as the racers competed.

Stretching against his van was Johnny Heineken, 29, one of the world’s top foilers, and a mechanical engineer for Alphabet Inc.’s experimental technologies lab, X.

“It’s a combination of tactical sailing and a high-performance action sport,” said Heineken, who wore a straw hat and Tevas. “And then it just feels great. You fly around the bay.”

Nico Parlier, 22, a professional foiler visiting from France, said that the French racers had been using the technology for years but that Americans took it more seriously.

“They took it to a different level,” Parlier said. “It’s not a very cheap sport, and people here are very wealthy, so it’s a good combination.”

The man who got Silicon Valley into the sport in the first place was Don Montague, a foil craftsman and the tech world’s foiling fixer.

“They look you up, or they have a friend, and all of a sudden you’re hanging out with Larry and Sergey,” Montague said, referring to Google’s founders Larry Page and Sergey Brin.

At a massive former naval base in Alameda, Calif., Montague has a staff of 10 working on inflatable foils, jetfoils and giant foil boats. His investors and clients include the Google founders and its former chief executive Eric Schmidt. 

Montague, with messy brown hair and a surfer’s drawl, had made a name for himself building kitesurfing gear, selling more than 20,000 of those boards (he claims to have named the sport). In 2013, he founded a wind-power company and sold it to Google. But a motorized inflatable jetfoil is more complicated than a simple kiteboard, so he moved from Hawaii to the Bay Area and assembled a staff of mechanical engineers and designers.

“I had to come here to build the team,” Montague said. “This is the foiling spot for America.”

Montague is a regular on the private-island and yacht circuit with people like the Virgin Group founder Richard Branson and Google’s Page, whose islands are close enough to foil between. There is some competition in the small community. When Google’s Brin surfed with two girls on his board, Montague said, Branson took a photo with three.

“It’s just way better than golfing,” Montague said.

Now, once a week, Montague said, he takes Page on a four-hour kitefoiling trip, with a chase boat and a water scooter in tow. Long trips are easier by foil, since the board is not bouncing on every wave. 

“There’s less wear on the body because you’re not absorbing the chop,” Montague said.

Montague’s jetfoil goes to market this spring, selling for around $5,000. His competition is the Lift eFoil, which costs $12,000 and will ship in September (with a five-month waiting list).

Just a few miles north of Montague in El Sobrante, Calif., is the craftsman Mike Zajicek, who foilers say makes the best in the world.

“To get a foil from Zajicek takes a year,” Montague said. “If he likes you.”

For the past few years, Zajicek has made his hydrofoils by hand, selling them for around $6,000 each. In total, he estimates he has sold 110 foils, and has 50 in construction. This year, the demand skyrocketed. He said hundreds are on a waiting list.

“Now the whole world is banging on my door, and I have to attempt to answer emails I have no answers for,” Zajicek said. 

What kind of questions?

“Like, ‘When am I getting my board?’” he said.

Zajicek’s house and workshop in El Sobrante are covered in wisteria. Inside, he has laid out layers of carbon fiber. An intern is in charge of duties such as altering the weld nuts. Nothing was originally built to be a foilboard, so Zajicek came up with his design through trial and error.

“Its limitless in the choices you can make,” he said. “More than 100 different pieces go in this thing. I kept fiddling, combining different wings and masts and fuselages.”

He cuts the carbon fiber with scissors, brushes each layer with epoxy resin, then carefully applies the next layer. He puts on “some pretty powerful reading glasses” to aid in his precision. After 20 layers, which usually takes him about three hours, he puts the foil in an oven.

He tests each foil, flexing it between his hands for give and tension. He takes most of them on the water himself. Then he sends the board out to the buyer.

Back in San Francisco Bay, the foilers were wrapping up. The water was cold and choppy. The foilers made a nearby windsurfer look as if he were standing still. Some foilers fell, their kites sweeping down as the chase boat went to pick them up.

Afterward, they shared a beer in the parking lot before heading to the nearby St. Francis Yacht Club for burgers and fish tacos. The club decided to classify the foils as yachts to allow the peculiar new athletes in.

They brag to one another about gaming their corporate calendar systems for more foiling time. Stefaans Viljoen, 40, tailgating before joining the rest of the foilers at the club, had not won that night’s race but said he would be back in the water by morning.

“I have my C.T.O. book Fridays as busy 11 a.m. onward,” Viljoen said of his chief technology officer.