Apparent Wind 

Reprinted from
Fast Track to Sailing
By Steve and Doris Colgate
Published by McGraw-Hill

Another type of shift, which also causes the need for sail adjustment, is a change in the apparent wind direction. While true wind is what your masthead fly and telltales show when your boat is not moving through the water (at anchor or docked), apparent wind is what you feel and what you see in your telltales and masthead fly when the boat is underway. 

Apparent wind is derived from the combination of wind produced by the boat moving through the air and wind produced by nature (true wind). Cigarette smoke, telltales, and electronic wind-direction indicators all indicate apparent wind direction when you are moving. 

Imagine that you are standing up in a convertible on a windless day. As the convertible starts forward, you begin to feel a breeze on your face that increases as the speed of the car increases. This is like boat speed wind. At 10 mph, you feel a 10-mph breeze on your face. This is apparent wind. 

Now imagine yourself in the same parked car, pointing north with an easterly wind (true wind) of 10 mph. You feel that wind hitting the right side of your face. As the car starts forward you don't feel two different winds-one on the side and one on the front of your face. You feel a resultant wind coming from an angle forward of the true wind. This is apparent wind.

Figure 4-18 shows apparent wind when towing a boat at 6 knots on a dead-calm day. Since there is no true wind, a resultant angle is not produced and the apparent wind is coming from dead ahead at the same speed as the boat: 6 knots. On your first day out on a close-hauled course, you may wonder why the telltales on the shrouds indicate you are almost sailing directly into the wind, while you are technically sailing around 45° off the wind. The telltales are indicating your apparent wind—the resultant angle of your boat's forward motion and the true wind. 

You can demonstrate the force and direction of apparent wind by drawing a parallelogram on graph paper, keeping your boat's speed and the true wind in the same scale. Figure 4-19 shows a boat close-hauled, sailing at 6 knots in a 12-knot true wind. Suppose you know your boat tacks in an 80° arc (the distance it will travel when moving from one tack to another). Therefore, the true wind direction is at half of your tacking range, or 40° off your bow. To find the strength and direction of the wind you feel—the apparent wind—draw a parallelogram on the graph paper using your boat speed (6 knots) and the true wind (12 knots). Then draw a diagonal line through the parallelogram. The diagonal line measures 17 knots of apparent wind by your scale. Now, using a protractor, the apparent wind reads 27° from your heading (versus 40° for the true wind).