Coriolis Deflection on 10 km Downwinders – Why Your GPS Track Curves
You plan a 10km downwind run. You check the map. You draw a straight line from Point A to Point B. You launch. You feel like you are riding perfectly downwind with the swell. Yet, when you check your GPS track later, it is a curve. You drifted miles off course.
Why? You fought the Coriolis Effect (indirectly) and the Ekman Transport.
The Wind vs. The Water
The wingfoil wind blows (roughly) along the isobars. But the water does not moves in the same direction as the wind.
Due to the Coriolis Effect (the rotation of the Earth), the surface water is deflected.
- Northern Hemisphere: Water moves 45° to the Right of the wind.
- Southern Hemisphere: Water moves 45° to the Left of the wind.
This is called Ekman Transport.
The Drift Factor
Imagine the wind direction is North to South. You want to ride South.
- The Wind: Pushes you South.
- The Current: Pushes you Southeast (in the Northern Hemisphere).
Your board is in the water. It is locked into the current. Even if you point your nose downwind, the entire body of water you are riding on is sliding to the right. Over a 10km run, this drift adds up.
The "Crab" Effect
To stay on your straight-line course, you unconsciously compensate. You have to "crab" or steer slightly left to fight the current drift. If you just surf the bumps naturally (following the flow), you will end up landing miles down-drift of your target.
The Wind Veer
To make matters worse, the wind direction itself often shifts over a long distance due to surface friction changes (land vs. deep sea). This is called "Veering" (clockwise shift) or "Backing" (counter-clockwise).
- Near Shore: Friction slows the wind, backing it (turning it left).
- Offshore: Friction drops, veering it (turning it right).
As you ride 10km out to sea, the wind angle slowly changes relative to your board. You feel like you are going straight, but the wind grid is warping around you.
Summary for Navigation
On a long downwinder, do not trust your eyes or the waves.
- Check the Hemisphere: Know which way the current will push (Right in North, Left in South).
- Aim Up-Drift: Aim for a point slightly up-current of your destination.
- Trust the GPS: Use a watch. If it says you are deviating, correct it early. The Coriolis Effect is silent, but it moves the ocean underneath your feet.