Kiteboard Theory - Thread 1: Apparent Wind

Postby James on Thu Jul 26, 2007 3:34 am

Generally, apparent wind is the wind that you feel when you are moving. It is a combination of 1) the true wind and 2) the wind caused by your forward motion. Here are some examples:

a) This example is so simple it's almost cheating: If the true wind is 0 and you are driving a car east at 100 km/h, then the apparent wind is 100 km/h from the east (from straight ahead). Dogs that stick their heads out car windows understand.
b) Another simple example: If you continue driving east at the same speed, and you encounter true wind blowing from the north at 100 km/h (a cross wind), then the apparent wind will be from the NE at about 141 km/h. BTW, that would make for a very hairy drive.
c) Here's a more complex and relevant example: A 15 mph NW wind is blowing at Jericho. A kiteboarder is riding out toward West Vancouver at 20 mph. His direction is such that he is heading 10 degrees upwind of a beam reach (a beam reach is exactly 90 degrees to the true wind). Vector diagrams and trigonometry can be used to determine that the apparent wind is 27 mph, and it is blowing toward the kiteboarder at an angle of about 33 degrees from his course (the direction that he is riding through the water).

Here is a link to simple apparent wind vector diagrams: http://gpsactionreplay.free.fr/index.php?menu=1&choice=7
Here is a more detailed link: http://en.wikipedia.org/wiki/Apparent_wind

Basically, the faster you go, the more the apparent wind is caused by your speed, and the less it is caused by the true wind. If you're walking at only 2 mph across a 10 mph true wind, then the apparent wind will be coming from almost the same direction as the true wind. But if you ride a scooter at 40 mph across the same 10 mph true wind, then the apparent wind will be coming from almost straight ahead.

My example c) above actually represents a possible and likely real-world scenario (provided that the kiteboarder is riding a very flat, efficient board like a Door or Spleene Session; otherwise he wouldn't be able to go 10 degrees upwind at 20 mph in a 15 mph wind). Now, here are some important things to consider:

1) The apparent wind is the only wind that matters to a kite or anything else that is moving. True wind only applies to something that is stuck in one place, like an anchored boat or somebody standing still on the beach flying a trainer kite.
2) In the example, the apparent wind is 27 mph, which is 1.8 times the speed of the true wind. And that creates about 1.8 x 1.8 = 3.24 times as much force from the kite, because the aerodynamic force is roughly a squared function of the wind speed. That is one of the reasons that it is so much easier to keep moving than it is to start moving. When you get up and start riding, the apparent wind speed increases, which has a dramatic effect on the force (often erroneously referred to as power) that the kite produces. Sailing craft that have very little drag, such as ice boats, can sail at many times the true wind speed, resulting in very strong apparent wind. They have small sails that are able to generate huge forces at high speed, but only small forces when the iceboat is going slowly, so accelerating up to high speed takes a while.
3) Similarly, it is the apparent wind that sometimes causes novice riders to crash and burn at high speeds if they are not able to modulate their speed properly. The faster they go, the faster the apparent wind becomes, which causes larger force from their kite, so they go even faster, and so on.
4) Now, back to the example of the kiteboarder at Jericho, specifically about the angle of the apparent wind: In the example, the apparent wind is blowing over the rider and the kite at about 33 degrees from straight ahead. That's a pretty forward angle, and the faster the rider goes, the farther forward that apparent wind angle will become. Unfortunately, there are practical limitations to how close to the wind direction a kiteboarder can go, so if the he goes too fast, he'll have to bear off (change direction in a downwind direction), which will prevent him from staying upwind.

So, going faster generates more force, or pull from the kite, but going too fast makes it impossible to go upwind. Effective upwind riding involves careful, and hopefully intuitive, speed modulation to maintain optimal apparent wind speed and angle. In relatively strong wind, it's usually best to edge hard enough to keep your speed to the minimum required to keep your board efficiently planing (which is a matter of experience and feel). In lighter wind though, you might find that it's sometimes necessary to ride a bit faster to generate faster apparent wind and therefore maintain adequate force from the kite to keep riding.

Here are a couple of other considerations:
5) Unlike a car or sailboat, a kiteboarder is able to generate extra apparent wind by diving or sining the kite. During a hard dive, such as when water-starting, it is the dramatically increased apparent wind flowing over the kite that increases its force so much.
6) Apparent wind direction is the factor that makes upwind riding dificult in light wind. Here's why: Any given kiteboard has a minimum speed that it will efficiently keep the rider up and planing. For most kiteboards I'd guess that's around 10 mph (which is 8-1/2 knots or 16 km/h), regardless of how hard the wind is blowing. If it's blowing 30 mph (true wind), the apparent wind direction won't be radically different from the true wind direction when riding at 10 mph. But if the true wind speed is only 10 mph or less, riding at 10 mph (to keep the board planing) has a such a major effect on the apparent wind direction that going upwind, or even staying upwind, might be impossible. Body dragging is a good way to get upwind if the wind is really light because body dragging is so slow that it doesn't have a big adverse effect on the apparent wind direction.

Regards,
James

©2007