Understand Apparent Wind when Sailing
Wind awareness is a standout amongst the basic skills that you need to develop to sail a boat. If you do not know where the wind is coming from, then you are unlikely to trim your sails correctly. This can be very inefficient, meaning that your boat will be sailing much slower than it could, or even not sailing at all. To put it simply, you cannot sail if you do not know where the wind is coming from. The good news is that a basic understanding of wind is easy to develop.
Steps
- Develop your wind sensing skills while on land. Rather than start sailing without having developed accurate awareness of wind, start developing this skill earlier. You can do this by simply paying attention to the wind daily. If you are not moving (or moving slowly), then you do not have to bother about the difference between apparent and true wind. Check your local weather forecast services often to gain the ability to guess the wind’s speed. Not only the average speed is important, but also the speed of gusts.
- Get used to detecting the wind’s direction. You were born with a few wind detection sensors. In fact, your whole body can feel the wind. Yet, many people struggle to determine where exactly the wind is coming from. If you are on or close to land, you can use your eyes to observe the movement of trees, flags and other objects. If you do not have any of these objects around, you can use this ear technique [1]. Pay attention to the wind blowing on your ears. Unless you are facing the wind, you will feel it stronger on either side. Then simply turn your head until you start to feel it on both ears. This is where the wind is coming from.
- Understand the effect of movement on how you feel the wind. Imagine one of those days when there is no wind at all. That is, the air is completely static around you. If you start moving –for example, if you are riding a bike— you are likely to feel ‘the wind’, even though in reality there is no wind. The wind that you feel while in motion is called apparent wind. In this particular situation, the apparent wind would be coming towards you, while as there is no true wind. Now, think of another situation. You are riding your bike on a road and the wind is blowing perpendicular to the road. You will now feel two mixed effects. First, the true wind coming on your side and second, the effect of your own motion, which feels as if the wind were coming from ahead. But in reality, you only feel one direction, that of the apparent wind. In this case, this direction will be the combination of the two separate effects. Therefore, the direction that you feel is not the direction of the true wind, nor is it simply against you. This exact effect occurs when you are on a sailboat.
- Roughly estimate the direction of the true wind while on a sailboat. Now that you understand the difference between true and apparent wind, how would you determine where the true wind is coming from? If you are on a sailboat that is moving, then you can only feel the apparent wind. However, you know that the boat is moving forward, and therefore, that this will make you feel as if the wind were coming more from the bow. In other words, the apparent wind’s direction is shifted towards the bow [2]. For example, if you feel the apparent wind at a 45-degrees angle, then you could think that the true wind could possibly be almost perpendicular to the boat. How much of a change in the wind’s direction is produced by the boat’s motion will depend on how fast the boat is sailing and on the speed of the true wind. For example, if you are moving at 1 knot and the wind speed is 15 knots, then your movement will have a negligible effect on the direction of the apparent wind (apparent and true wind will be almost identical).
- Understand your instruments on board. When you are on a sailboat, you can only feel the apparent wind. If you look at a flag or teller, they will also be moving at the boat’s speed and will be pointing in the direction of the apparent wind. A wind meter will be no different. These meters simply read the speed of impeller (or pitot tube) and the direction of a vane, both usually installed on top of the mast. If the boat is moving, the vane will be oriented in the direction of the apparent wind, so this is what it will be measuring.
- Determine the direction and speed of the true wind more accurately. Most of the time, a rough estimate of the direction of the true wind is enough. However, you might still want to have a more precise measure of its direction. One way of doing it is to use vectors. In order to perform this analysis, you will need a GPS, and also a wind speedometer. First, read the course and speed over the ground (COG and SOG) from the GPS. Then, read the speed and direction of the apparent wind from the wind meter. On a piece of paper, draw a vector representing the opposite direction of the boat, which will represent the boat’s wind. For example, if you are heading South, draw it pointing North. The length of this vector should represent boat’s speed (you can use centimeters or inches, for example, depending on the size of your paper). Now, starting from the same origin, draw another vector representing the apparent wind’s direction [3]. As previously, the length of this vector should represent the wind speed. Now, you only need to remember a bit of elementary vector analysis. The apparent wind (measured by the wind meter) is the resultant of two vectors, true wind, and the boat’s wind. Now that you have apparent and boat wind vectors, you can determine the missing vector (true wind) by drawing a third arrow from the tip of the boat’s vector to the tip of the apparent wind one. This will give you the direction of the true wind. You can then determine its speed by measuring the length of this vector.
