So I've been thinking about particle velocities in waves and how they affect kite boarding. Water flows back towards the oncoming wave and then follows the wave as it passes, so particles make a (nearly ) circular vertical motion at the surface.
So if I'm riding in front of the wave and the wind is blowing from behind the wave, I'd expect a little extra power as there's a counter-current to the wind. And a bit less power if I'm behind the wave. Strangely I find the opposite is true and I tend to be more powered at the back of the crest. On lighter days of course, when you're trying to milk every bit of wind - powered up I don't really think about it.
Has anyone else noticed this? I can't quite get my head around it.
on light days you can gleen some extra knots by timing the push/suck back of the white wash. Get it wrong and it works against you. get it right and yourl be cranking upwind. It also depends on the cross beach current flow in the shallows.
It's the same as a current that flows against the wind. The current is pushing you and the board up wind so you get a boost in power in the kite due to the "apparent wind " increase.
Ie if you have a 15 knot easterly and a 5 knot current running out of a river etc from the west, your apparent wind will be around 20 knots.
This was exactly the case at Caloundra last Sunday. If the tide were flowing in rather then out, the current would have be in the same direction as the 15 knot breeze and the apparent wind would have been 10 knots! Unkiteable for me.
Yeah I get the water flowing up the wave and therefore against the wind but, the boost from gravity, that is falling down the face of the wave, in the same direction as the wind over comes this and give you and apparent wind decrease at the kite.
Select to expand quoteReevesy said...
It's the same as a current that flows against the wind. The current is pushing you and the board up wind so you get a boost in power in the kite due to the "apparent wind " increase.
Ie if you have a 15 knot easterly and a 5 knot current running out of a river etc from the west, your apparent wind will be around 20 knots.
This was exactly the case at Caloundra last Sunday. If the tide were flowing in rather then out, the current would have be in the same direction as the 15 knot breeze and the apparent wind would have been 10 knots! Unkiteable for me.
Perfect conditions for a light wind kite promo video
Select to expand quoteReevesy said...
Yeah I get the water flowing up the wave and therefore against the wind but, the boost from gravity, that is falling down the face of the wave, in the same direction as the wind over comes this and give you and apparent wind decrease at the kite.
I wondered that too, maybe it's the angle of the surface. ie the flow is not that strong but angle makes a difference to the way the board works against the water.
If i remember correctly from studies, the theory is that particles in water are "stationary" and do not move but the pulse of the wave is moving and passing the energy from particle to particle. or something like that.
Irrespective of me being accurate on that point (as I've not bothered looking before posting) IF let's say the surface particles are flowing "upwind" (which should cause an increase of apparent wind), this is only a small percentage of individual molecular movement in a BULK MASS. The energy of the BULK MASS is travelling in the "downwind" direction, which would cause the lack of power. The reason I believe you feel more power behind the crest is due to the fact that you're behind the BULK MASS and it's driving force and you are probably getting some assistance from the turbulence behind the moving body. That being said, the flow regime is also highly turbulent so it's hard to accurately predict the behaviour. I haven't actually looked at the fluid mechanics behind this so it is possible that I am wrong.
