If you find yourself being pulled away from shore by a powerful rip current, what should you do? The traditional advice has been to swim parallel to the shore in order to get outside the current. But after an extensive study of these currents — by using instruments and floating in them himself — Jamie MacMahan has concluded that your odds are better if you stay still and just tread water.

Dr. MacMahan, an oceanography professor at the Naval Postgraduate School in Monterey, Calif., found that the conventional image of a rip current is inaccurate — that it’s actually not a long plume of fast-moving water running out to sea. Instead, rip currents more closely resemble whirlpools, with strong, persistent eddies that circulate throughout the surf zone, Dr. MacMahan reports in an article to be published in Marine Geology. If you swim parallel to the shore, he concludes, there’s a 50-percent chance you’ll end up be swimming into a stronger current. But if you just tread water, he says, there’s a 90 percent chance of being returned to shore within about three minutes.

Dr. MacMahan went swimming and deployed sensors in rip currents at Sand City in Monterey Bay, California; at Truc Vert, France, and at Perranporth in the United Kingdom. I asked him to tell Lab readers what he’d found. His report:

I have jumped in a lot of rip currents all over the world and always end up back onshore. Folks always ask me what it feels like. The answer is . . .nothing. Rip currents are similar to rivers. If you have ever floated downriver by tubing, you do not notice the flow until you look at the bank to estimate your speed, but you feel nothing. Only when you miss your end spot or want to take a break and start to swim back up river do you now notice the current. Even then you do not feel it, you just notice that you are not making any progress. This deception catches swimmers off guard and then they start to swim against unknown flow pattern and make no progress.

Waves come in sets or groups — you hear surfers say the largest wave is the seventh wave. The wave heights will periodically get larger and smaller about every two minutes. Rip channels are deeper places along the beach. If you are in the rip channel and the largest wave occurs, it will cause you to lose footing with the sea bed, allowing the rip current to start to move you. In addition, there is another offshore motion occurring with the wave groups that creates an offshore flow when the waves are large in the group. When the waves are small in the group, this motion is onshore, and at times reduce the rip current flow. This fluctuation on top of the rip current can catch swimmers off guard, because for a few minutes there is small onshore flow — and then a few minutes later it is very fast offshore.

Rip currents are often mistakenly called rip tides, but they’re not tidal currents. In fact, Dr. MacMahan says, rip currents are generally fastest at low tide on American beaches. These currents are one of the chief hazards of swimming. As my colleague Cornelia Dean reported, each year in America about 100 people drown because of rip currents, and more than 50,000 swimmers are rescued from the currents by lifeguards.

Have you ever gotten in trouble by exhausting yourself fighting a rip current? Have you ever tried just treading water and gotten carried back to shore?