Wednesday, 14 May 2008

True and Apparent Wind

Here is an interesting article from author/sailor John Ellsworth on this question. Every good sailor becomes familiar with this phenomena and with experience is able to understand the difference between the two.

Imagine you're sailing close-hauled on port tack, on a course of 270 degrees. Your masthead wind indicator shows the wind is coming at you at 30 degrees off the port bow. You tack over to starboard and sail close-hauled due south, or 180 degrees. Now your masthead wind indicator is pointing 30 degrees off the starboard bow-a 60-degree spread from port close-hauled. Why does this new course of 180 degrees differ 90 degrees from the last course and not 60 degrees, what the masthead indicator shows as the difference between the two tacks?

The answer is that you are dealing with apparent wind. Apparent wind is the wind shown by a masthead indicator and telltales, the wind you feel on your face while you are sailing.

True wind is the wind you feel when you are stationary. Weather systems, topographic features, and thermal effects can influence the strength and direction of true wind. Look at flags flying ashore or ripples on the water to determine true-wind direction. Your boat's speed over the bottom and your course combine with true-wind speed and direction to determine the velocity and direction of apparent wind.

On a still day, try coasting down a hill on a bicycle. Your forward motion causes the breeze you feel on your face. Similarly, when a boat moves forward at 5 knots in no wind, the forward motion creates an apparent wind of 5 knots moving in the direction opposite to the boat's course.
Two work as one. The wind that blows on your face when you are in motion and to which you trim your sails is apparent wind. It is a combination of true-wind velocity and direction and the boat's speed and course. When you are sailing close-hauled, the true wind is most likely about 45 degrees off your windward bow. But your forward motion results in apparent wind coming from a direction that is slightly forward of the true wind and with a speed that is greater than the true wind.

A change in either your heading or your speed affects the apparent wind. Similarly, a change in either true-wind speed or direction can also change the direction and strength of apparent wind.
Assuming a constant true wind, see how apparent wind changes at various points of sail (Fig. 1).

Apparent wind angles on the opposite tack are a mirror image. One way to find apparent wind is to draw a parallelogram using the true-wind speed and direction and boat course and boat speed as two sides.

The apparent-wind angle is the diagonal vector that bisects the parallelogram. The length of the bisecting diagonal is the apparent-wind velocity expressed in the same units as boat speed and true-wind speed.

Reproduced courtesy of John Ellsworth - you can visit him on his website

You can read more about wind direction and sailing with it in my ebook 'The Voyage of the Little Ship 'Tere Moana' downloadable from my website for sailors


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