You’re at anchor in an idyllic cove, the sun having just dipped below the horizon. The water is perfectly still, a glassy mirror reflecting the last warm colors of the day. You’re finishing a lovely beverage, soaking in the quiet, and thinking about turning in for the night.
Then it starts. A gentle, almost imperceptible roll at first. Then a bit more. Your stomach begins to take notice. Your boat is moving on a slow-period beam wave… but there’s no wind and no current. Nothing obvious to explain it. Still, the motion continues, slowly building into a relentless, nauseating side-to-side rocking. Your serene evening has suddenly transformed into a low-grade amusement park ride—one you definitely didn’t sign up for.
Welcome to the worst night’s sleep you’ll ever have. Welcome to the seiche.
What Is a Seiche?
A seiche (pronounced sā(t)SH) is a standing wave that forms in a semi-enclosed or enclosed body of water. Unlike rolling waves generated by wind or passing boats, a seiche is caused by periodic forces that excite the natural resonance of the water body. This forcing can come from wind, atmospheric pressure changes, or waves entering through a narrow inlet.
Here on Lake Macatawa near Holland, Michigan, the seiche is driven by wave energy from Lake Michigan transferring through the inlet. Wind-driven waves pass the mouth of the inlet, varying the water pressure there. This causes the inlet to act almost like a piston, creating an oscillating pressure differential that pushes and pulls the smaller lake back and forth. The system behaves much like a Helmholtz resonator: think of blowing across the mouth of a bottle to make a tone. The water in the inlet acts as the air in bottle’s neck, and the lake is the resonant cavity—the bottle.
The result is a nausea-inducing wave that rocks the boat gently but relentlessly, often for hours or even days, without any visible cause on the surface. And it’s not just at anchor—seiches can affect boats in marinas and at docks, too. Even when your boat is tied up and the water looks calm, the same oscillating forces can make the boat roll, much fto the surprise of anyone expecting a perfectly still evening.
Why It Feels So Weird
Every boater knows beam-to waves never feel good. Even small or slow-period oscillations are amplified in this orientation, turning what looks like a calm anchorage into a constant, stomach-churning roll.
Seiches are unusual because they create this motion in the absence of wind or current. With wind or current, an anchored boat tends to align with the waves. Without such forces, you simply bob on the seiche in whatever orientation the boat started. What’s worse, your brain sees calm water outside while your body feels rolling motion inside. If the seiche happens to hit your boat’s resonant frequency, all hell can break loose below decks while the surface looks calm and serene.
Seiches can vary in period depending on the size and shape of the water body. Smaller lakes or coves tend to have shorter, more rapid oscillations; larger lakes produce slower, longer waves. The amplitude of the motion depends on the forcing and how well it matches the water body’s natural frequency. Some calm anchorages may feel like a gentle cradle, while others feel more like going ten rounds with Mike Tyson.
Living With a Seiche
Since launching our new-to-us boat on August 5th, we’ve experienced this phenomenon on and off in our boat slip. It’s a gentle reminder that even in the most serene anchorages or well-protected marinas, nature has its own surprises. Glassy water may look like a mirror, but below the surface, physics is always at play.
If you encounter a seiche, consider these practical tips:
- Secure loose items. Anything not tied down can slide or tip with repeated side-to-side motion.
- Adjust your orientation. Pointing the boat into the waves can reduce rolling. Sometimes relocating to another part of the anchorage or dock changes the amplitude and improves the “ride.”
- Brace your body. Even small oscillations can make sensitive stomachs queasy. Lying low in the hull and along the centerline of the vessel helps minimize discomfort.
- Use handholds. Moving around the boat can feel disorienting during a seiche. Keep a hand on something solid at all times—safety first!
- Check for timing. Seiches often follow periodic cycles, sometimes lasting for hours. Understanding these rhythms can make life below decks easier.
- Be patient. Unlike wind or current waves, seiches can persist with minimal surface indication. There’s little to do other than ride it out and adjust your habits.
- Adjust lines and fenders. In a marina slip, you may be able to find a “sweet spot” where roll is minimized. Longer lines are generally better than shorter ones, allowing the boat to move with the oscillation rather than fight it. You’re unlikely to cancel it entirely, but this is about survival at the dock.
- Consider an antiemetic. For those prone to motion sickness, over-the-counter options like meclizine (Bonine) or dimenhydrinate (Dramamine), or natural remedies like ginger, can help manage nausea during extended seiche events.
If possible, take the opportunity to go ashore. A seiche is a perfect excuse for a shore adventure off the boat: visit the marina’s facilities, spend a day at the pool, explore the town.
