Note: Most of the content of this post is from the “School of Sailing” website. You can use the link to pursue building your sailing knowledge at that site.
True and Apparent Wind
There are two kinds of wind; true and apparent. We call the wind that blows across the land or water, the true wind. This is the wind talked about in the weather forecast of 10-15 knots for instance. It is the wind we feel when we are outside at rest and not moving.
As you might guess the other kind of wind, apparent, is the wind that is generated by our movement in combination with the true wind. The only time there is no apparent wind is when we are at rest and only feeling the effects of the true wind. When we move and the wind also moves the total wind we feel is the apparent wind. Stationary objects only feel true wind while all objects in motion feel apparent wind.
Let’s talk about some examples and start with the situation where there is no true wind. This is a day when it is completely calm with no detectable wind speed when we are standing still.
In this illustration we have calm conditions so we motor ahead at 5 knots producing an apparent wind of 5 knots from straight ahead.
On the next day, tied up at the dock, we have a true wind of 10 knots constant blowing across our port beam.
No wind from our boat’s forward motion. The true wind is 10 knots and that’s what we feel on the boat. What happens when we go sailing?
Lucky us! The wind will just blow us off the dock. We raise sails and move ahead on a close reach at 5 knots. We know the true wind is 10 knots and, since we will be moving forward we will be producing 5 knots of wind ourselves. How do these combine? We are combining our boat speed and direction with the true wind speed and direction.
We see the combined effects of the true wind and our boat’s motion forward. This produces the apparent wind, what we actually feel while sailing on the boat. Boats always sail in the apparent wind. Here, note the apparent wind is stronger than the true wind (the arrow is longer) and is coming from further towards the bow.
What if we fall off the wind and head down on a run? Let’s see what the combination of boat speed and direction along with the true wind produce now.
As we turn away from the wind the boat slows down. It slows down because the apparent wind drops and the sails become less efficient. In this simple example we are going in the same direction as the wind so we just subtract our boat speed from the true wind speed and that gives us the correct apparent wind speed. If we have a wind speed indicator aboard it would show the same reading.