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opening a can of worms
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bluefish1



Joined: 24 Dec 2013
Posts: 1102

PostPosted: Fri Jun 22, 2018 9:07 am    Post subject: opening a can of worms Reply with quote

I’m trying to conceptualizing a fin cutting through water.

Correct me if I’m wrong on these basics.

Lift in windsurfing is defined as “off the water”.
A board creates lift (off the water) by going fast due to the pressure difference as the board rides over it.
The board also creates lift (off the water) as air gets caught under the nose at speed.
The riders weight counteracts the force of the sail.
The fin counteracts the sideways force of the rider’s weight and sail force (when powered up).

In other words, because water is hundreds times more dense than air, just a small fin counteracts the giant sail to balance things correct?

So here’s my question. How does the fin create lift (off the water)? Since a single fin is symmetrical, if pulled through the water without a rider it would just “cut” straight through the water creating no lift. Correct?


So multiple choice. Does the fin create lift (off the water) because…

1. When loaded with the pressure from the sailor, it twists or bends creating lift.
2. Because the board is never perfectly flat, the fin is always at an angle (not perpendicular to the water) creating lift.
3. Even if the board is perfectly flat, it will create lift (off the water) because of the pressure difference on one side of the fin due to the rider applying pressure to one side (I don’t get this, wouldn’t the lift be sideways not up?).
4. It doesn’t really create lift (off the water) just upwind lift.
5. Because the fin is thick at the top and narrow at the bottom, it creates lift.
6. Some of the above
7. None of the above
8. It’s way to complicated for my small mind to comprehend without a fluid dynamics degree.
9. Aliens.
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coachg



Joined: 10 Sep 2000
Posts: 2619

PostPosted: Fri Jun 22, 2018 10:09 am    Post subject: Reply with quote

https://www.youtube.com/watch?v=NLJoKjn8Ngw

https://www.youtube.com/watch?v=OEhYC9IYOSU

Coachg
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gregnw44



Joined: 23 Jul 2008
Posts: 749
Location: Seattle, Wa

PostPosted: Fri Jun 22, 2018 10:15 am    Post subject: Reply with quote

Great question... well written!!

I'm kind of with you. And there's definitely some of #6,7,8,and 9.

But also this - I think the main or real answer to your question is mostly (but possibly not all) #4.

I also remember hearing #5 back in the day, back when fins got better and actually had more technical foil shapes. But I never really "bought much of that"... of course I'm not sailing on a broad reach at 50 mph like they do in Luderitz. so maybe those guys are getting a bit of that.

Back in the "way back" many fins were just cut out pieces of flat plastic, with no profile. And windsurfing still worked similar to how it works now. So that means for general, recreational, basic windsurfing... your fin doesn't even need any fancy profile shape.

But yes, this is a good "can of worms to open"... the way more tech types will have lots of better reasons for you Smile

_________________
Greg
Longboarding since '81
Shortboarding since '84
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bluefish1



Joined: 24 Dec 2013
Posts: 1102

PostPosted: Fri Jun 22, 2018 10:20 am    Post subject: Reply with quote

So coach, does that mean all fins bend? Would a perfectly stiff fin not create lift?
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cgoudie1



Joined: 10 Apr 2006
Posts: 2159
Location: Killer Sturgeon Cove

PostPosted: Fri Jun 22, 2018 11:07 am    Post subject: Reply with quote

I'm not Coach, but with Bernoulli, even a symmetrical stiff wing (fin) will create
lift (which as you surmised is sideways in a windsurfing system) due to Angle of Attack. If you don't believe that, stick a flat piece of non deformable something out the window while someone is driving you down the road and vary the pitch angle. You'll notice the something "pulled" up if you pitch the something with the front of the something up and the back of the something down relative to the incoming wind.

With proper structural implementation airplanes can and do fly upside down (though not as efficiently) this is done with AOA.

-Craig

bluefish1 wrote:
So coach, does that mean all fins bend? Would a perfectly stiff fin not create lift?


Last edited by cgoudie1 on Fri Jun 22, 2018 11:10 am; edited 1 time in total
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coachg



Joined: 10 Sep 2000
Posts: 2619

PostPosted: Fri Jun 22, 2018 11:09 am    Post subject: Reply with quote

I'm sure all windsurfing fins bend to some degree but even if you made a perfectly stiff fin out of metal that doesn't bend the stiff fin will lift because you left one thing out of your list. Yaw. Unless you are directly downwind there will be a bit of yaw applied to the board/fin because of the force of the wind so you will not have an even flow across the fin. I'm surprised Greg didn't point this out as he is an avid longboar sailor and the centerboard of a longboard will give you all the education you need on fins. Sail directly downwind & the centerboard becomes useless drag. Why? Because of even flow over its surface. Start heading across the wind and you will immediately feel the centerboard start to work because of the slight yaw of the board. The sail doesn't pull the board perfectly forward, there is some lateral force applied to the pull.

Coachg
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dvCali



Joined: 23 Aug 2007
Posts: 888

PostPosted: Fri Jun 22, 2018 11:13 am    Post subject: Re: opening a can of worms Reply with quote

The sail generates lift, the fin counteracts the side force generated by the sail ...

Last edited by dvCali on Fri Jun 22, 2018 11:27 am; edited 1 time in total
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bluefish1



Joined: 24 Dec 2013
Posts: 1102

PostPosted: Fri Jun 22, 2018 11:19 am    Post subject: Reply with quote

DvCali, so tail walking is the board going to fast?
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bluefish1



Joined: 24 Dec 2013
Posts: 1102

PostPosted: Fri Jun 22, 2018 11:38 am    Post subject: Reply with quote

coachg, Is yaw twist? The twist of the fin? Or are you saying the board, as in my number 3? The flatness of the board on the water is never always perfectly flat because the board is pitching from side to side.

Sorry I just brought pitch into the discussion. If someone brings up vector, I'm outta here.
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dvCali



Joined: 23 Aug 2007
Posts: 888

PostPosted: Fri Jun 22, 2018 12:01 pm    Post subject: Reply with quote

bluefish1 wrote:
DvCali, so tail walking is the board going to fast?


Tail walking I suspect is more of a consequence of water generated lift (i.e. a "wave" lifts the board, the side force, hold, disappears and the lift of the sail takes the upper hand decreasing the pressure on the mast base ... and you "tail walk" ) ... but I am not clear on it ... and now I have to go do something instead of forum-ing Very Happy


Last edited by dvCali on Fri Jun 22, 2018 4:06 pm; edited 3 times in total
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