On a calm day, a stationary observer will notice there is no wind. However, the same day, an observer onboard a yacht moving under power will experience a stream of air coming from the same direction the boat is heading to and whose velocity equals that of the yacht.
On a windy day, the stationary observer will notice there is air in movement. The wind ‘felt’ by this fixed observer is what is called true wind. However, the wind that the observer onboard the moving yacht will feel will be not the true wind nor the wind due to the boat’s speed, but a wind which results from the combination (addition) of both. This wind is called the apparent wind.
The true wind, the wind generated by the boat speed, and the apparent wind are generally represented by vectors whose direction represents the direction of the wind, and their length the speed. The true wind speed is usually denoted as Vt, the wind speed due to the boat velocity as Vb, and the apparent wind speed as Va. All of them are represented in what is known as the wind triangle.
The angle between the apparent wind and the course sailed is named β (beta), and the angle between the true wind and the course sailed is called γ (gamma).
As soon as a yacht under sails starts to move with an angle to the wind’s direction, the wind will push the boat away from its intended course. The angle between the heading and the course will start to increase until the lift forces generated by the underbody (mainly the keel, centerboard, rudder, or foils) will counter the drifting force. At that moment, the boat will sail at a constant angle of leeway λ.
Some important points to bear in mind are:
- Sailing craft obtains the energy necessary to move from the wind flowing around its sails, i.e., from the apparent wind (direction, angle, gradient). It is, therefore, the apparent wind that determines how to trim the sails.
- For conventional displacement yachts, i.e., yachts whose hulls move through the water by pushing the water aside, Vb is usually smaller than Vt. However, some catamarans, foilers, and other wind-propelled craft types (for example, iceboats) can achieve higher speeds than the true wind speed.
- When sailing in a giving course, when Vt increases, so does Vb (if the yacht has not achieved her critical speed), but Vb increases at a much lower rate than Vt. That means that the ratio Vb/Vt decreases with increasing true wind speed.
- For a fixed value of Vt and the same sailing course, when Vb increases, the apparent wind angle, β, decreases. Thus, when increasing the yacht speed while maintaining the course, the apparent wind direction rotates forward.
- For fixed values of Vt and Vb, turning the yacht into the wind increases Va. In other words, the apparent wind speed, Va, increases when the apparent wind angle, β, decreases.
- A yacht can sail closer to the direction of the true wind if the true wind speed, Vt, increases.
- In a dead run, the apparent wind angle, β, is zero. A yacht that tries to increase its velocity will experience a reduction in the apparent wind speed and, as a consequence, a decrease in the thrust force which will lead to a yacht’s speed reduction. Thus, in a dead run, the speed of a sailing craft is always limited by the true wind speed. In other words, it is impossible to sail faster than the wind at this point of sail.
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