Apparent wind
Feb. 2nd, 2014 07:19 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
khiemtranThe third big concept to grasp when learning to sail is that of apparent wind. While knowing how to read the sails will always let you set them at the right angle, understanding apparent wind will help you predict how the wind will "change" (or at least appear to). Something that's increasingly important as you start putting your whole body over the side in stronger breezes.
The key to apparent wind is that at any given time the wind experienced by the boat will be made up of two components. One is the "true" wind, as would be felt by a stationary observer. The second is the "headwind", produced by the forward motion of the boat. (It's easy to visualize headwind if you imagine yourself sitting on a bike on a still day. When stationary, there's no wind at all. When you start pedalling, you can feel a wind that always comes from directly in front of you, no matter which way you head. That's the headwind.)
It turns out, the only wind your sails care about it is the apparent one, which is the sum of the true wind and the headwind. In practice, this means that the only time you'll really feel just the true wind is when you're stationary; and as you increase speed the apparent wind will seem to shift more and more towards the front of the boat.
This has a number of interesting effects. For instance, if you're sailing close-hauled into the wind (i.e. as close as you can without your sails stalling) and your speed increases, you will find the wind will shift closer to your heading and you'll need to bear away (turn further away from the wind) to keep the critical angle. It will seem as if the wind is changing, but actually it's only your headwind picking up.
Likewise, if something slows down your boat (say you hit a wave), the headwind will diminish and the apparent wind will shift back towards the true wind. To maintain the optimal angle, you need to luff up (turn into the wind). All of this quickly becomes second nature, as does learning how to guess what the true wind will be, just by observing the apparent wind and the speed of the boat.
The next trick is that the true wind itself will always be changing. As well as changing direction, there will also be gusts and lulls. Whenever a gust hits, the true wind will increase and the direction of the apparent wind will shift back towards it. This means when you feel a gust, the normal reaction is to turn a little towards it. And if the wind drops, you need to bear away. (In practice, since the boat will take a little while to turn, you may also be letting the sail in and out, since all the sail really cares about is its angle to the wind, not its angle to the boat.)
Of course, there are also times when the headwind and true wind will cancel each other out. If you're drifting with a current on a low wind day for example, and the true wind happens to match your velocity, then your apparent wind will be zero, which means your sails will also be zero use for getting out of trouble.
On the other hand, the magical thing about apparent wind is that if you get the true wind and headwind aligned at an angle where they don't cancel as the headwind increases (which will typically be with the true wind coming from the side of the boat), the apparent wind can be much greater than the true wind and will keep building and building as you speed up. This is the reason that sailboats can go faster than the (true) wind (sometimes very much faster), as long as they don't sail directly with it. Something that is astonishing to non-sailors, but manifestly obvious to those who have sailed before.
And finally, of course, the fun really starts when the true wind picks up and the differences between the gusts and lulls becomes amplified. The force of the wind on the boat increases with the square of the wind velocity (because sails are two dimensional) and so the difference between ten and fifteen knots is a doubling of force and from fifteen to twenty is a doubling again. The more the true wind fluctuates, the harder you'll have to work to compensate; and in a small dinghy in a (relatively) big wind, you'll really have to work. Oh, and if happen to lose some speed or gain some unexpectedly in one of the shifts, then your apparent wind is going to shift around too. But at least now, you should be able to predict which way it's going move. And, once you've capsized, you'll only have to deal with the true wind for a while...
The key to apparent wind is that at any given time the wind experienced by the boat will be made up of two components. One is the "true" wind, as would be felt by a stationary observer. The second is the "headwind", produced by the forward motion of the boat. (It's easy to visualize headwind if you imagine yourself sitting on a bike on a still day. When stationary, there's no wind at all. When you start pedalling, you can feel a wind that always comes from directly in front of you, no matter which way you head. That's the headwind.)
It turns out, the only wind your sails care about it is the apparent one, which is the sum of the true wind and the headwind. In practice, this means that the only time you'll really feel just the true wind is when you're stationary; and as you increase speed the apparent wind will seem to shift more and more towards the front of the boat.
This has a number of interesting effects. For instance, if you're sailing close-hauled into the wind (i.e. as close as you can without your sails stalling) and your speed increases, you will find the wind will shift closer to your heading and you'll need to bear away (turn further away from the wind) to keep the critical angle. It will seem as if the wind is changing, but actually it's only your headwind picking up.
Likewise, if something slows down your boat (say you hit a wave), the headwind will diminish and the apparent wind will shift back towards the true wind. To maintain the optimal angle, you need to luff up (turn into the wind). All of this quickly becomes second nature, as does learning how to guess what the true wind will be, just by observing the apparent wind and the speed of the boat.
The next trick is that the true wind itself will always be changing. As well as changing direction, there will also be gusts and lulls. Whenever a gust hits, the true wind will increase and the direction of the apparent wind will shift back towards it. This means when you feel a gust, the normal reaction is to turn a little towards it. And if the wind drops, you need to bear away. (In practice, since the boat will take a little while to turn, you may also be letting the sail in and out, since all the sail really cares about is its angle to the wind, not its angle to the boat.)
Of course, there are also times when the headwind and true wind will cancel each other out. If you're drifting with a current on a low wind day for example, and the true wind happens to match your velocity, then your apparent wind will be zero, which means your sails will also be zero use for getting out of trouble.
On the other hand, the magical thing about apparent wind is that if you get the true wind and headwind aligned at an angle where they don't cancel as the headwind increases (which will typically be with the true wind coming from the side of the boat), the apparent wind can be much greater than the true wind and will keep building and building as you speed up. This is the reason that sailboats can go faster than the (true) wind (sometimes very much faster), as long as they don't sail directly with it. Something that is astonishing to non-sailors, but manifestly obvious to those who have sailed before.
And finally, of course, the fun really starts when the true wind picks up and the differences between the gusts and lulls becomes amplified. The force of the wind on the boat increases with the square of the wind velocity (because sails are two dimensional) and so the difference between ten and fifteen knots is a doubling of force and from fifteen to twenty is a doubling again. The more the true wind fluctuates, the harder you'll have to work to compensate; and in a small dinghy in a (relatively) big wind, you'll really have to work. Oh, and if happen to lose some speed or gain some unexpectedly in one of the shifts, then your apparent wind is going to shift around too. But at least now, you should be able to predict which way it's going move. And, once you've capsized, you'll only have to deal with the true wind for a while...
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Date: 2014-02-03 12:33 am (UTC)Seriously, this is *amazing*. This is some serious magic.
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