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fluid mechanics?

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by morrison » Wed May 15, 2019 1:19 pm

I have been wondering,
if a river runs against the wind like at the spit,
is it better,worse or the same to have 5knts of current or 5knts of wind?
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by telus022420 » Wed May 15, 2019 6:14 pm

From my Gorge experience its best to have the current opposite of the wind. West winds are the best.

Same will hold true at Locarno; it easier to plane when the wind is from the West and outgoing tide.
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by morrison » Wed May 15, 2019 6:41 pm

telus022420 wrote:From my Gorge experience its best to have the current opposite of the wind. West winds are the best.

Same will hold true at Locarno; it easier to plane when the wind is from the West and outgoing tide.


True, but what I'm pondering is do you get more energy from 5knts (arbitary value)of wind in the sail/kite or from the current acting on the board?
Is 13knts of wind in 5knts of current the same as 18knts of wind in 0knts of current?
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by grantmac » Wed May 15, 2019 6:54 pm

I know on very large ships it's 10 kts worth of "wind" for every 1kt of current when coming alongside.
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by Ryan » Thu May 16, 2019 1:08 pm

You might find this interesting..https://www.youtube.com/watch?v=q2il8Fagbyk
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by Slappy » Thu May 16, 2019 5:16 pm

5 knots of current is way better because it will produce nice big clean swell when running against the wind.

5 knots of wind will give you much smaller and much more blown out waves.
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by morrison » Thu May 16, 2019 5:58 pm

Ryan wrote:You might find this interesting..https://www.youtube.com/watch?v=q2il8Fagbyk


Very thought provoking.
My thoughts are that water trumps wind for a given speed because of relative density and phase(liquid vs gas), but I lack the background to be sure.
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by max.spock » Fri May 17, 2019 10:32 am

Some considerations: in general to get going there must be a balance between underwater forces and above water forces. Water is about 1000x denser than air therefore a sail must be about 1000x larger than a fin to produce similar force.
The current will push you upwind until the sail produces a force strong enough to balance the force produced by the fin.
It seems to me that the relative speed between wind and water surface is what matters but I'll keep thinking about it..
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by Ryan » Fri May 17, 2019 11:39 am

There is no difference, it's the relative motion between the wind and water that counts.
Sailing happens when there is a force on the sail and a force on the fin, in approximately opposite directions. The sail is bigger than the fin due to air being less dense, the fin is supposed to balance the forces of the sail, it's called the symmetry of sailing.

There is lots of discussion online, one being, https://worldbuilding.stackexchange.com ... g-currents
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by gabrielb » Fri May 17, 2019 1:16 pm

a sail in the air or in the water... energy has to come from somewhere, neat.
more interested in ...
:ugeek:
eh.png
eh.png (48.88 KiB) Viewed 1956 times

https://isec.org/sec/


maybe not. the catamaran in river current is 8-)
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