GILFORD, N.H. — An empty runaway boat speeding in circles on New Hampshire’s largest lake was brought safely to a stop by a teenager who jumped aboard from a personal watercraft.

Rich Bono, who captured the events on video, said he was on the dock Wednesday in Lake Winnipesaukee’s Smith Cove when he heard some commotion in what is usually a quiet, no-wake zone.

“I can hear an engine revving, and I looked down the end of the dock toward the noise and saw a boat circling, circling, circling, and no one was in it,” he said. “Obviously, that’s not good.”

Bono later learned that the boat’s operator, a sailing instructor, had reached into the water to pick up a tennis ball used for teaching when one of the students’ sailboats tipped over. The sailboat’s mast hit the motor boat’s throttle, sending the instructor overboard and the boat into a spiral.

Brady Procon, 17, hopped on the back of his neighbor’s personal watercraft. They pulled alongside the runaway boat, and Procon jumped onto it and cut the engine.

“Brady was a hero,” Bono said in an interview Monday.

Though there were multiple children in sailboats and other vessels docked nearby, no one was injured, nor was any property damaged, Bono said.

“That boat was under power, throttled up,” he said. “Motors on boats are like meat grinders, they’re not very forgiving if someone gets hit.”

Procon, who is joining the U.S. Navy in the fall, told WMUR-TV the experience was both scary and fun.

“I’d do it again,” he said.