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.