The sun has burned through the early morning marine layer, and the breeze is gentle and warm enough for me to abandon my hoodie. It looks like a perfect day to head out onto the Pacific Ocean. But as soon as we exit the harbor walls at Marina Del Rey, near Los Angeles, the 29-foot sport-fishing boat starts to heave.

“We have some great waves out here today,” says Kelsey Albina, one of my guides for the day. I’m glad I haven’t had lunch yet, I think, as we thrust through waves, rolling from side to side. Then the captain taps a touchscreen on the helm. Like a movie set at lunch time, everything seems to freeze. The swaying stops. I can stand again without clutching the railing. The boat still rocks gently front to back as it crests and descends each wave, but the violence of a moment ago is gone.

I walk steadily to the back of the boat and look under a deck hatch to see the device responsible. In what would have been empty, or storage space, a white metal version of a beach ball sits suspended in a cradle. Inside that, a 500-pound, doughnut-shaped flywheel is spinning in a vacuum, clocking 8,450 revolutions every minute. This gyroscope, about the size of a large microwave, is keeping the large boat stable—a boon for sailors (like me) who don’t have their sea legs yet. This is the Seakeeper, made by the company of the same name.

Inside what looks like a beach ball, a 500-pound, donut-shaped flywheel spins in a vacuum, clocking 8,450 revolutions every minute to counteract the force of the waves. Seakeeper Inc.

Any spinning object works as a gyroscope, moving to counteract any force that tries to change its orientation. That’s how a spinning top stays upright, even when flicked. Such freaky physics makes gyros ideal for stabilizing satellites or, in miniature, for guidance on ships and missiles, where they provide navigation systems with a steady frame of reference. The designer of the 1967 Gryo-X used one to make the narrow vehicle stand on two wheels. The Gyrowheel offered the kids of 2011 a way to ride a bike without training wheels. (You won’t find either for sale today.)

Here, Seakeeper’s gyro serves to keep things steady on the high seas. As the boat rolls left or right, the gyro swings forward and backward in that cradle. That delivers torque, or rotational force, to the port or starboard, counteracting the motion of the entire boat. (You can try an experiment for yourself—sit on a swivel chair and hold a spinning bike wheel in front of you, like these folks at the University of Texas.)