Recognizing the difference that speed makes to wind direction
The wind that blows against you as a result of your passage through still air is known as the apparent wind. Boats create their own wind when they’re underway, and if there is a natural wind blowing, it will be modified in force and direction by the boat’s forward movement to become the apparent wind, as observed from the boat. The wind hitting a sailboat’s sails or her steering vane is the apparent wind, and that—not the “true wind”—is the wind by which she is steered.In a dead calm, the apparent wind created by a boat’s movement comes from dead ahead. The faster she goes, the stronger the apparent wind. Conversely, if a boat lies dead in the water when a wind is blowing, the wind she experiences will come from the true-wind direction. However, as soon as she starts moving forward, the apparent wind direction will draw ahead; the faster she moves, the more it will move ahead.As the wind moves ahead, a sailboat must pull off more to keep her sails full; therefore, a boat going to windward at 6 knots cannot point quite as close to the true wind as a boat doing only 4 knots. Luckily, the difference is small, so there is no chance of the slower boat reaching a windward destination ahead of the faster boat because she can point slightly higher.It’s interesting to observe the effect of the apparent wind when you’re sailing dead downwind in a light breeze. If the wind is blowing at 5 knots from astern and your forward speed is 5 knots, the apparent wind will cancel out the true wind. You will fall into dead air until the boat slows down and the true wind can once more overwhelm the apparent wind. This holds true no matter how much sail you raise. The answer is to change course and tack downwind on a series of broad reaches, thereby generating an apparent wind at an angle that provides useful drive instead of merely canceling out the true wind.See also Wind and Altitude.
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