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how can we ride 40-50 mph in only 20-25 mph of wind?

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spork
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Postby spork » Fri Jun 09, 2006 1:58 am

Some years back I had an email exchange going with a guy that raced ice boats and had experimented with building wings that he would get inside while on ice skates. It sounds like this is what you're talking about. Am I right?

My recollection is that he was trying to avoid the issue of the pilot disrupting the flow (and causing drag); which became a greater issue as the wind strength increased, and the wing size decreased.

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Postby FarQueLot » Fri Jun 09, 2006 2:10 am

Whilst i have no physical proof I have felt the wind on my face when headding directly down wind on my kite buggy. It is definitly possiible and even managed to do it on 2 wheels one time. It does take a lot of skill and kite control, but it ain't theory. You are really sailing and flying the kite on the edge, and it takes a very efficient kite and low friction craft.
This was only for short distances but was not done on a whip but a bear away run and was definitly maintainable on smooth surfaces.
This maybe wrong but I have heard that the skiffs they use in Sydney Harbour racing where the first and only craft to round a triangular course at an average speed of faster than the windspeed. This may cause the BS meter to quaver a bit.
F

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Postby spork » Fri Jun 09, 2006 2:20 am

In theory you could not maintain a steady state direct downwind course faster than the wind in your kite-buggy with the kite held in one spot.

In theory you COULD maintain a downwind component (VMG) faster than the wind by tacking down-wind. Whether that could be achieved in a kite-buggy in real life I don't know.

I'm nearly certain it's not possible to do it by going directly down wind while "tacking" your kite side to side, but I haven't done the math, so I wouldn't swear to it just yet.

EDIT:

It occurs to me that you could feel the wind in your face while going directly downwind, with kite in one spot due to wind gradient. If the wind aloft was significantly stronger than the wind at your level (which is common) that could happen. But it wouldn't count as going directly downwind faster than the wind as only part of the craft is beating the local wind speed.
Last edited by spork on Fri Jun 09, 2006 2:23 am, edited 1 time in total.

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Postby alpower » Fri Jun 09, 2006 2:22 am

I stick by the fact that you can not go dead down wind faster than the speed of the wind. The minute you aren't going dead down wind (even 1 degree) you are generating apparent wind. As the boat moves faster you will generate more apparent wind but in order to go faster than the true wind you have to "pull" the apparent wind forward of the beam. If the apparent wind (not the true wind) is at or behind the beam you will not go faster than the speed of the true wind. And how fast you go has everything to do with apparent wind. Everything else is about reducing drag which allows the apparent wind to go up.

I look forward to either a scientific explanation or an actual demonstration of how anything can go downwind faster than the wind without some other energy input (including waves).

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Postby spork » Fri Jun 09, 2006 2:26 am

alpower wrote:I stick by the fact that you can not go dead down wind faster than the speed of the wind.
You mean "I stick by my CLAIM that you can not..."

You didn't answer my question. Wanna bet?
alpower wrote:I look forward to either a scientific explanation or an actual demonstration of how anything can go downwind faster than the wind without some other energy input (including waves).
No problem. Before I cough it up how much we putting on it? How's $100K sound?

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Postby ryansurf1 » Fri Jun 09, 2006 2:45 am

sorry to dumb down your debate on what appears a more controversial arguement about the ability to exceed downwind speed...but....

i just wanted to be sure imagineparadise had a grasp of fact....

excerpt from the posted article:

"McKeon says it needs 18 or 19 knots to gain momentum, then can sail at three times the wind speed.

In three weeks of training the team has reached a maximum speed of 40 knots in 15 knots of wind".

Sounds like multiples of actual wind speed. Is video really necessary?

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Postby Attila » Fri Jun 09, 2006 3:13 am

First I'd like to say that imo, Spork gives the most articulate and "scientific" explanations. Also, I agree with him that without using the idea of leverage (like with the spokes), it is not possible to go DW faster than wind.

Second, I’d like to point out something that a lot of people talked about but doesn’t sound write: “usingâ€

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Postby spork » Fri Jun 09, 2006 3:26 am

Attila wrote: In an ideal situation without friction wouldn’t the speed reach infinity?
Yes, but you'd have to continue to sheet the sail in. As your speed approached infinity the sail would approach 0 angle with the hull.

This ideal situation assumes no drag from the hull, and no drag (parasite or induced) from the sail. Essentially we're talking about both a sail and a keel that have infinite L/D.

Now the problem with going infinity mph is that things get a little dicey around light-speed. But I hope we're not going into that :o

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Postby BJZ » Fri Jun 09, 2006 4:05 am

Speed will be limited severly by board drag. Ice boats hit speeds over 150 mph by removing drag. Kites seem pretty efficient already. Work on the board and surface friction and turbulence.

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Postby steveb » Fri Jun 09, 2006 4:20 am

You can go very deep down wind faster than the true wind. If you had ever raced a formula board or an ice boat you'd experience it. On snow with a kite and a well waxed pair of ski's you can get a similar feeling.
Lets say you're stood still and the wind is perpendicular to your shoulders at 2 mph. When you're stood still you'll feel the wind on the side of your face. If you start to walk forward you now add apparent wind. The direction of the apparent wind, which is caused by your movement through the air. is from in front of your nose. As you increase speed the wind direction moves from your shoulder forward towards the direction you're travelling, the faster you go the more apparent wind you create the more the wind moves in front of you. At approx 3.5 mph the wind will be dead on your nose.So in sailing/kiting terms you start with your kite on the true wind, as you pick up speed the wind moves more forward so you have to bear off the wind,you increase speed, create more apparent wind which is more on your nose and you have to bear off the wind again, etc, etc, etc. If you ever watch an ice boat of a formula race board when they are up to speed the sail is always sheeted in to the max as though they are going upwind, that's because basically the wind is on their nose regardless of the direction they are travelling. If you decelerate you have to head up into the wind again, gain speed and bear off on the apparent again. So the downwind course can be very deep. The slowing factor is resistance, of the equipment you use.
A kite with skis and thin hard snow will give you this feeling, you'll end up with the kite locked low, the faster you go the deeper downwind you run.
Joey Levens clocked 67mph this winter on Stormboarding, I think the winds were in the 30's and the kite was a 9m. Apparent wind will scare the shit out of you around the point your vision starts to blure.


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