Understanding Apparent Wind

Good sailing is a continuous process of building sailing skills.
John Ellsworth explains what apparent wind is
and how to account for changes.
First appeared in SAIL magazine • All rights reserved • © 2006 John Ellsworth

Determining the apparent-wind direction, as opposed to true wind direction, is a skill all sailors should have. The reason? Knowing how and why apparent wind changes is important for good steering and proper sail trim.

The components

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.

Four variables

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.

Points of sail

As a boat's heading changes, look at what happens to the apparent wind. You are going to see these same effects while sailing. When you are beating, apparent wind is going to be stronger and more forward than true wind (A). When you are running downwind or reaching (C, D), apparent wind is weaker than true wind. When the boat is going directly downwind, you merely feel less wind velocity.

The difference in apparent wind can affect your comfort. In Figure 1, the boat sailing close-hauled (A) experiences an apparent wind of 14 knots. When the boat is running (D), the apparent wind is only 5 knots. Nine knots may mean the difference between putting on and taking off a sweater or windbreaker.

Suppose you are sailing and using ripples on the water as your true-wind indicators. The ripples indicate that the true wind is directly off the beam. Even though the true wind is abeam, you are sailing on a close reach because of the apparent wind.

The effect of boatspeed

Boatspeed changes also affect apparent wind (Fig. 2). An increase in boatspeed, if true-wind speed and direction remain constant, shifts the apparent wind forward; a decrease shifts it aft.

If boatspeed increases and true-wind speed and direction stay constant, apparent wind moves forward and increases. If boatspeed decreases, apparent-wind velocity diminishes and its direction shifts aft.

Here's where you can use this information to sail more efficiently. If you are accelerating down a wave, you know the apparent wind moves forward. So you should either fall off (steer away from the wind) or sheet in the sails to maintain proper trim.

As you are moving up the next wave, boatspeed slows and the apparent wind moves aft. This means that as you begin to slow, you should either head up (steer toward the wind) or ease your sails to maintain proper sail trim.

Changes in wind speed

A decrease in the true-wind velocity shifts apparent wind forward, with boatspeed and course constant; an increase usually shifts it aft (Fig. 3). If the true wind decreases, the apparent wind moves forward and decreases.

Keep these relationships in mind when wind-speed changes take place, as in a series of lulls and gusts. If you are sailing to windward and the true wind increases temporarily in a gust, the apparent wind shifts aft. If you are close-hauled, stay properly trimmed by heading up when the gust hits and the apparent wind moves aft. When the gust subsides, the apparent wind moves forward again, and you should return to your original course. If you perform this maneuver properly, you work to windward at maximum efficiency.

Understanding apparent wind is essential to efficient sailing. Remember, a slight change in either your boatspeed or heading, or in the speed or direction of the true wind, affects the apparent wind. When these changes occur, make sure you either retrim your sails or change your course to accommodate the new direction and speed of the apparent wind.

–diagrams by Tadami Takahashi