sailing - page 10
2.10 Wind
Wind is the very basis of sailing. Without wind, you're not going anywhere. It is therefore essential to feel the wind and to know how to determine its direction, its speed, but also its changes in order to adapt your sail trim. But in order to do so, we must first understand that there are two types of wind: the real wind and the apparent wind. The wind you feel on your sails or the wind you feel on your face is not the same as the wind that waves the water or slams a flag on land.
True wind
True wind is the direction the wind is indeed blowing on the planet, as witnessed by flags, smoke, trees bending, ripples on the water etc. If your vessel were standing still, at anchor or securely in the slip, you would measure true wind speed and direction. The weather report will tell you the true wind speed and direction. However as we know, true wind is always fluctuating depending upon; weather conditions and turbulence of landmasses, tall buildings, cliffs and mountains, etc. Use true wind speed forecasts as an approximation, but be aware that it can change (shift) dramatically.
Apparent wind
When you are on your bike and pedaling, the wind you feel on your face is the apparent wind . In the same way, when you put your hand through the window in the car, it is again the apparent wind. In a boat, it is the wind you feel when you are sailing. As you move forward, the boat creates its own wind (just like in a car or bicycle), which is called the speed wind. The real wind is the meteorological wind, the one that blows over the elements. At sea, you will observe it when your boat is stationary. In the end, the apparent wind is the sum of the speed wind - the wind you feel on your face as you gain speed - and the real wind, which blows over the sea (photo 2.9.6). It is materialized by the orientation of the wind vane or pennons on your sail.
The apparent wind has the below characteristics:
- Is always shifted further forward of the boat than the true wind, except when sailing downwind.
- Speed increases the effect of wind speed on the direction and velocity of the apparent wind.
- Is always lighter when sailing away from the true wind than when sailing towards it, except in the case of a very fast boat.
photo 2.9.6
Bernoulli's principle
Make yourself a rectangular tube of paper, put it on a table, and blow through it. As you do so, the paper will collapse down, then spring back up again when you run out of breath. What's happening? When a fluid flows from one place to another, it has to conserve its energy. In other words, there has to be as much energy at the end as there was at the start. We know this from the fundamental law of physics called the conservation of energy, which explains that you can't create or destroy energy, only change it from one form into another. Think about the air flowing through your homemade tube. The air just outside the tube, just where you're blowing, has three types of energy: potential energy, kinetic energy, and energy because of its pressure. The air in the middle of the tube has the same three types of energy. However, because the air is moving faster there, its kinetic energy must be greater. Since we can't have created energy out of nothing, there must have been a reduction in one of the other two types of energy. You're blowing straight across a table so the air doesn't rise or fall—and doesn't change its potential energy. The only place we can compensate for the extra kinetic energy is in the fluid's pressure. As the air speeds up, its pressure goes down. Since the air inside the tube is at a lower pressure than the air above it, the tube collapses until you stop blowing. Stated simply, Bernoulli's principle (pronounced Bur-noo-ee's) simply reminds us that the total energy in a moving fluid is constant. But you're likely to see it described a different way: if a fluid speeds up, its pressure goes down (and vice-versa).
The need for smooth airflow
Just like an airplane getting lifted vertically into the sky by its wings, a sailboat gets lifted horizontally through the wind across the surface of the water.
A sailboat is driven forwards with the wind 30-45° on the beam. When air flows along a sail (or an airplane wing) the shape of the sail forces the air flow on leeward side to take a longer path than on the windward side. Therefore the air has to increase its velocity on the leeward side of the sail resulting in a lower pressure than on the windward side (Bernoulli). In effect a sailboat may be sucked through the water due to the low pressure on the leeward side. Conversely a slight increase in the pressure will act on the windward side. The total sail force F may be split into two components, namely lift F1 (drive force) and drag F2 (heeling force) as shown on the photo 2.9.7. The lift acts at right angle to the wind and the drag acts in the wind direction. Both lift and drag increase with wind speed but drag increasesfaster. As a consequence different sailshapes have optimum drift/drag ratios at different wind speeds. When sailing to windward (beating/close reaching) lift should be maximized and drag minimized. With the wind abaft the beam (broad reaching and running), however, drag works in the right direction and contributes to boat speed."
photo 2.9.7
Similar to steady flight, we've got to keep the air flowing over the sails smoothly. Anytime the angle of the wind across the lifting side of the sail becomes too great, the sail stalls. Wind molecules detach from the lifting side of the sail and turbulent flow is produced and thus reducing the lift. The result is not as dramatic as in an airplane but you will get a noticeable loss in speed if your sail is producing turbulent flow of air when set at the wrong angle. On below photos 2.9.8 and 2.9.9. watch the airflow as you increase the sail angle to the wind. At the beginning (photo 2.9.8) it is set to its optimum, but as you pull in the sail, watch the turbulent flow begin to spawn (photo 2.9.9).
photo 2.9.8
photo 2.9.9
