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
