What is "apparent wind" ?

Apparent wind is the wind you can feel when on a boat. This wind is different from the real wind, as we're going to see.

Real wind versus Apparent wind

We will distinguish between:

  • Real wind : the wind observed on steady ground
  • Apparent wind : the wind feeled on a boat while moving

These two winds may differ quite a lot. The higher the boat speed, the higher they'll differ. A simple example is cylcing. If you ride a bicycle with no real wind, you feel an apparent wind which is exactly opposed to your speed direction. If you ride a bicyle at 20 km/h facing a 10 km/h wind, you feel an apparent wind which is 20+10=30 km/h. And with a 10 km/h real wind from rear, the apparent wind is 20-10=10 km/h.

Some Maths...

Apparent wind is the addition of two vectors:
  • Real wind vector
  • Speed wind vector: the opposite of the speed vector

Consequences of apparent wind on sailing

The main consequence of apparent wind while sailing is that in many cases, whatever its course, a boat is "facing" wind. This is particularly true for fast sailing devices such as windsufing gear which are planning faster than wind. In this particular case, we can demonstrate that whatever your course is (even deep downwind), you are always facing apparentwind (I mean wind comes front of you, not from rear).

For slower sailing boats, apparent wind will vary more depending on the course. With upwind or square courses, apparent wind comes from front. But for downwind courses, the apparent wind may come from rear. This is particularly true for absolute downwind course (going in the wind direction), of course.

Besides this direction effect, the other effect is that the speed of apparent wind may vary quite a bit from real wind speed.

Knowing apparent wind may help to best set your sail. Depending on its direction and its speed, you'll set your sail direction and sail power in a specific manner. It's well known for formula racers in windsufing, since they flatten their sail upwind (because of an apparent wind very much facing and high) and they set a loose uphaul tension downwind to have more power (because apparent wind is less from front, and its speed decreases).

Some examples through true GPS tracks

Let's see now some true example computed by GpsActionReplay based on true GPS tracks. For this purpose, wind direction were automatically detected by GpsAR, and wind speed were entenred manually.Of course, these two parameters are provided as an average for the whole course, and should be adjusted in case of change within sailing time...

Sailboard downwindSail boat downwindSailt boat upwind

As the data from the first graph come from a windsurfing track, you'll see that the apparent wind comes from front, and that its speed may be quite a lot higher than the real wind's one. In other words, as it is known, windsurfing is an "apparent wind generator". It's the reason why windsurfers always have their sail almost fully hauled on, even downwind. This phenomenon is obvious in the following "video" from the Finian Maynard's overall speed world record at 46.6 knots: