Home . Articles . News . Technique . Understanding Apparent Wind

Understanding Apparent Wind

2010 July 1
What is the difference between true and apparent wind, and does it matter?

One of the more esoteric concepts beginning sailors find themselves trying to wrap their brains around is the phenomenon of apparent wind. What is apparent wind? And no, it's not when you look outside, see the trees swaying in the breeze and say, "Apparently it's windy."

Loosely defined, apparent wind is the wind we feel and experience when we are in motion. It's a combination of the actual wind (true wind) that blows over land and sea and the wind created by our moving forward. The popular analogy is that of riding a bike. Get on a bike and start pedaling. That wind you feel on your face is apparent wind. Even if there is a slight wind at your back (the true wind) you can still feel the wind coming from in front of you (the apparent wind).

Now, increase that following wind and, at the same bike speed, you'll start feeling less of a headwind coming at you. That's because apparent wind is a combination of the true wind and the wind created by speeding forward. The important thing to realize here, however, is that as you travel faster the apparent wind not only increases in speed but it also changes the angle of the wind.

On a boat this can be easily demonstrated and felt by first sailing upwind, and then bearing off to a downwind course. On a close reach and higher you'll feel the wind in your face, the boat speeding along, and you may even want to reach for your jacket. Turn down to run with the wind and it's as if you are sailing in different weather. Suddenly the fresh breeze is gone, the boat feels slower and you'll soon be shedding that jacket.

Because of this relationship between true wind and apparent wind, and its speed and angle, we get such things as relative velocities and vector sums. And for many of us this is the point where the eyes roll into the back of the head and our brains begin to shut down. But we need not concern ourselves with advanced math-that's why we have digital instruments.

The main points we want to understand is that when sailing any course other than directly downwind (or, obviously, in the no-sail zone) the apparent wind will always come from farther ahead than the true wind, and that when sailing any course from beam reach to close hauled the apparent wind will be stronger (higher velocity) than the true wind. The vector diagrams in photograph above and the illustration to the left show the relationship between true wind, the wind from sailing forward and apparent wind.

So, what does this mean to our sailing? Well, the wind we feel on our face is the same wind the sails feel. So we sail to the apparent wind, and not to the true wind. Moreover, we can take advantage of apparent wind to sail faster.

On slow, heavy boats the difference between the true and apparent wind may be small. But on light, fast boats-think multihulls and planing dinghies-it's a different story. You may hear sailors talk about boats making their own wind. What they're referring to is boats that can sail faster than the true wind. Remember, the faster you go the stronger the apparent wind, so the faster the boat goes the more wind it gets and the faster it goes. This is also why sailing dead downwind is typically slower than heading up and harnessing the power of apparent wind.

Iceboats, which have very little resistance between hull and ice, are very good at making their own wind and will often sail two to four times faster than the true wind. This self-perpetuating speed may lead one to believe that boats are capable of breaking the laws of physics to become perpetual motion machines, and the we have to ask ourselves, "Where does it end?"

Well, let's go back to our iceboat example. As the iceboat picks up speed the apparent wind increases and also moves forward. So the sailor sheets in for the new wind angle and picks up more speed, further increasing the apparent wind and moving it even farther forward. Soon the sailor is sheeted in as tight as possible and is sailing as close to the apparent wind as possible. If the apparent wind moves any more forward the boat will essentially find itself in the no-sail zone and the sail will begin to luff and the boat will lose speed. So there is a limit on just how fast a boat can go relative to the wind.