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Jim.od3
Joined: 25 Aug 2012 Posts: 90
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Posted: Mon May 22, 2017 10:28 am Post subject: |
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Thanks, all, for the replies. Good info.
Jim
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windfind
Joined: 18 Mar 1997 Posts: 1901
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Posted: Tue May 23, 2017 10:28 am Post subject: |
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isobars wrote: | I have never yet seen even an ATTEMPT, let aloe a persuasive one, to explain how the sail knows there's any current. Unless you're standing on the bottom, "It's the wind, stupid" that drives a sail into the water, still or moving.
cgoudie1: I've personally experienced that "clew dragged under" effect that Westender mentioned sailing the Wall under high current |
Hi Gang,
It is a mistake to assume a water column moving in the rivers current is a homogenous mass moving the same direction/speed.
The first 2 images below show that when there is swell you have some complex and opposing movement patterns in the water column. So one part of a submerged rig can be carried the opposite direction than the deeper parts of the rig and board.
Plus the parts of the rig exposed to wind force will try to move in yet another direction.
And all of these vectors change with swell size, current strength, water depth and wind velocity.
Bottomline:
The 3rd image below of a rig in the water gives part of the answer to the "clew dragged under effect". Remember all of this is from the perspective of a person who faintly remembers the one oceanography course I took ages ago.
There are lots of forces involved that work differently on the parts of a rig/body above and below the water. But as you can see in the 3rd. image when the clew or, worse yet, the mast end gets into the deep rotation part of a swell things get weird fast.
Incidentally, if you are a surfer you can now visualize why a duck dive as a wave breaks works so amazingly well.
Mike Godsey
iwindsurf.com/ikitesurf.com
Weatheflow.com
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Last edited by windfind on Tue May 23, 2017 3:28 pm; edited 4 times in total |
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merriam2
Joined: 25 Oct 2013 Posts: 33
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Posted: Tue May 23, 2017 12:43 pm Post subject: |
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Very interesting and great explanation as usual! Thanks Mike
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doug_needs_wind
Joined: 02 May 2005 Posts: 35
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Posted: Sat May 27, 2017 12:00 am Post subject: |
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And then, yesterday, Temira reported a "very competant windsurfer" who experienced "great swell" at the wall in steady 35mph (range 25-45 lull-gusts), with a flow of 439 KCFS (I saw ~390-400 on my examination) - Making me wish I had gone, in spite of the serious talk here about the hazards... But then again that wind was not so well predicted, making it hard to plan ahead... time to give up and buy a jet ski.... NOT!
Thanks, all, for the reports, facts and opinions.
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isobars
Joined: 12 Dec 1999 Posts: 20935
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Posted: Sat May 27, 2017 4:34 pm Post subject: |
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Thanks for the explanation, Mike. If I'm reading it correctly, it seems that it's the vertical turbulence generated by current (with help from bottom terrain, I presume), that grabs sails and submerges them ... rather than the horizontal component of the overall river current. I experienced that in Grand Canyon rapids, too, where even a PFD wasn't always a guarantee against being sucked straight down. One could even see upwellings and feel downward pulls (what goes up must come down) even in placid-looking flat stretches of river.
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windfind
Joined: 18 Mar 1997 Posts: 1901
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Posted: Sun May 28, 2017 10:21 am Post subject: |
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isobars wrote: | Thanks for the explanation, Mike. If I'm reading it correctly, it seems that it's the vertical turbulence generated by current (with help from bottom terrain, I presume), that grabs sails and submerges them ... rather than the horizontal component . |
Hi Isobars,
You see the vertical motion of particles even in the deepest ocean. The circular motion of the water particles is not impacted by the bottom unless the waves are in very shallow water less than the 1/20 the wave length of the wave. That is what is shown in the 2nd image above. As water gets even shallower surf will form.
Here is a way to visualize what is happening to your sail hits the water:
1. Once submerged the river current grabs the sail and tries to carry it down river.
2. At the same time the wind tries to blow the parts of your body and board that are above water up river.
3. In most conditions the opposing 2 forces means that your gear is pretty easy to sweep to the surface. And usually in weak to moderate current the circular movement of water particles in the water column is of such diameter that it does not have much of an effect on your sweeping the sail to the surface.
4. However if your position and the current speed and swells are at a critical point where the circular motion of the particles has mostly a downward motion on your sail then you will have great difficulty getting the sail to the water until you drift into a swell area where the motion is more favorable.
I will as Ken M. who is a Gorge sailor and well known UC oceanographer if he has time to explain this better.
The animation below might help visualize all of this. Watch the highlighted particle in the largest circle motion. Then imagine your sail in the area where the particles are moving downward. See the issue?
Now if are lucky on a given day you may never fall in a location where momentarily circular motion exerts this effect.
Mike Godsey
iwindsurf.com/ikitesurf.com
Weatheflow.com
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biffmalibu
Joined: 30 May 2008 Posts: 556
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Posted: Sun May 28, 2017 1:24 pm Post subject: Sucking. |
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OMG, PEOPLE! This does not have to be so complicated! Complications help destroy windsurfing in USA!
Strong current means disorganized and irregular swell or chop.
It also means more friction for surfing. So, it means you will need more power in your sail to overcome that friction in order to continue planing/surfing. And you will have to be a better surfer to maintain your board hull speed. Try getting your feet out of the straps and walking the board, for example.
And, yeah, if you suck at controlled crashing and/or waterstarts, you might have more difficulty getting organized and clearing your sail for a waterstart. If you're weak or you suck, you're going downstream fast. So get strong and don't suck. And then waterstart quickly. Don't F! around crying to yourself about the cold water. Control your crash (unhook, take the rear foot out of the strap for a change, bleed speed upwind before jibing, hold on to something during the crash, etc.).
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windfind
Joined: 18 Mar 1997 Posts: 1901
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Posted: Sun May 28, 2017 1:51 pm Post subject: |
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Hi Biff,
You are right! You don't have to understand the complexity of something to enjoy it. In fact for me as a big part of enjoying windsurfing is that you don't think...you just act.
But there are people who like to understand the complexities of why things happen. And understanding the complexities of something and also mindless enjoying it really are not incompatible.
I guarantee you that the people who designed our modern windsurfing gear did not just enjoy windsurfing. Instead they worked hard to understand the complexity of wind vectors, sail shape, fin design and mast strength etc.
Quote: | Complications help destroy windsurfing in USA! |
And 25 years ago if we had had "complications" like modern feather light carbon booms, carbon masts and sails with huge wind ranges windsurfing would have been far easier to learn and advance. It was the lack of those "complications" that made it so hard for the sport to grow.
Mike Godsey
iwindsurf.com/ikitesurf.com
Weatheflow.com
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jgda
Joined: 19 Jul 1999 Posts: 131
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Posted: Sun May 28, 2017 5:52 pm Post subject: kcfs at boneville |
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I just looked at the DART web site, and saw that the flow was at ~430 kcfs at bonneville dam as well. As Iso said, twice the flow of last year at this time. I have a friend who just retired as a fisheries engineer (including tracking water flow over the years) and absent a warm rain, he thinks it will be another 2 or 3 weeks before flow beigns to drop off.
I am not sure if I have tried to sail with that much current, but I am thinking that Stevenson might be a bit more manageable given the river is pretty wide there vs. up-river at the Hatch, Doug's (and MaryHill)? I would not want to go out though unless I was fully powered (no slogging)
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westender
Joined: 02 Aug 2007 Posts: 1288 Location: Portland / Gorge
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