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opening a can of worms
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nw30



Joined: 21 Dec 2008
Posts: 6485
Location: The eye of the universe, Cen. Cal. coast
PostPosted: Sat Jun 23, 2018 10:47 am    Post subject: Reply with quote
Lots of confusion with the term "lift", to the layman, lift means "up" or "vertical", but that is not always the case, there is also "horizontal lift", which is a more accurate application to w'surfing. Reread my other post on the second page, that might help.
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Ugly_Bird



Joined: 04 Nov 2008
Posts: 335
PostPosted: Sat Jun 23, 2018 10:51 am    Post subject: Re: opening a can of worms Reply with quote
U2U2U2 wrote:
Ugly_Bird wrote:
bluefish1 wrote:

(I don’t get this, wouldn’t the lift be sideways not up?).


Would be if a fin had asymmetric cross-section (like a plane wing).


Only about 10years old now ASYMETTRICAL fins on multi fined boards, and even longer on surf boards , Cross section .. what is that ?


Cross section is the shape when you cut the fin across. Smile
For multifin boards the fins can be asymmetrical as they equalize each other, not the single fin. Smile
Single asymmetrical fin would help carve jibe on one tack and mess it up on the other Very Happy

Andrei
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dvCali



Joined: 23 Aug 2007
Posts: 1314
PostPosted: Sat Jun 23, 2018 11:16 am    Post subject: Reply with quote
coachg wrote:
bluefish1 wrote:
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.


The flatness of the board is roll. Yaw is the board sliding slightly sideways-think of spin out to a lesser degree. There are two factors acting on the fin. The force of the wind & the direction of the board. When these line up-straight down wind-you have no lift. We can not plane going straight down wind because there is an even/equal flow over the fin. Start going across the wind & you have an uneven flow over the fin. The wind is pulling in one direction while the board is moving in another so you have an uneven flow over the fin.

Coachg

Only the sail (and board volume) generates lift (i.e. a vertical force), the fin generates only a horizontal force, it cannot do anything else. It is a (symmetrical) vertical foil and the force it generates is perpendicular to it. You need an asymmetrical horizontal foil to generate lift, as it obviously happens with hydrofoils.

What keeps you planing going straight downwind, is the drag of the Cat style sails we use. Use a spinnaker, or a square sail, a kite, or a Cat style sail with lots of lift (it will suck anywhere else) and downwind planing will happen ...
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J64TWB



Joined: 24 Dec 2013
Posts: 1685
PostPosted: Sat Jun 23, 2018 1:42 pm    Post subject: Reply with quote
So tail walking has nothing to do with fin? It's mast base pressure and sail overpowered?
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dvCali



Joined: 23 Aug 2007
Posts: 1314
PostPosted: Sat Jun 23, 2018 2:06 pm    Post subject: Reply with quote
bluefish1 wrote:
So tail walking has nothing to do with fin? It's mast base pressure and sail overpowered?

Your board "climbs" out of the water because of sail lift. The fin offers lateral resistance. If the fin is long the sail can keep lifting the board out of the water because your lateral resistance does not disappear. You can "foil", i.e. get the hull out of the water, for a very few seconds with a long 50-70 cm fin. But again it is not the fin that is lifting you, it is the sail and the fin keeps you in balance for a few magic moments. If the fin had lift you would hydrofoil ... but it does not and you sink back to the surface of the water Sad

My guess is something similar happens when tail walking, the board "climbs" out of the water because it can (it got a "long" fin) until some of the forces involved abruptly changes and you lift the front "catastrophically". How exactly does that happen, as other have said, would require some very serious modeling. Certainly outside my expertise!!!!!
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rigitrite



Joined: 19 Sep 2007
Posts: 520
Location: Kansas City
PostPosted: Sat Jun 23, 2018 2:43 pm    Post subject: Reply with quote
Quote:
the fin generates only a horizontal force, it cannot do anything else. It is a (symmetrical) vertical foil and the force it generates is perpendicular to it. You need an asymmetrical horizontal foil to generate lift, as it obviously happens with hydrofoils.


dcalvi, I agree with your general premise, but symmetrical foils can create lift. Fore example, some helicopter rotor blades are symmetrical foil cross sections (just like a windsurfing fin), and create lift by the angle of attack relative to the rotation plane being changed by the swash plate. The dive planes on a submarine are also symmetrical and create true lift by doing the same thing. I've had it explained that a windsurfing does the same thing, and that that the fin only appears to be aligned with the direction of travel, but in fact, is at an angle of attack of a couple of degrees, due to the difference between true and apparent wind.
I personally am skeptical of this explanation, but it is possible. I agree with you that the sail is doing all the lifting and is what causes you to go upwind. A better fin simply prevents you from losing ground to side slip. This is the viscous forces dominating explanation to which I was referring.
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cgoudie1



Joined: 10 Apr 2006
Posts: 2599
Location: Killer Sturgeon Cove
PostPosted: Sat Jun 23, 2018 4:23 pm    Post subject: Reply with quote
+2

Your sail is also producing "lift" that is (mostly) not vertical. That's what
sucks you along.

-Craig

nw30 wrote:
Lots of confusion with the term "lift", to the layman, lift means "up" or "vertical", but that is not always the case, there is also "horizontal lift", which is a more accurate application to w'surfing. Reread my other post on the second page, that might help.
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U2U2U2



Joined: 06 Jul 2001
Posts: 5467
Location: Shipsterns Bluff, Tasmania. Colorado
PostPosted: Sat Jun 23, 2018 7:46 pm    Post subject: Re: opening a can of worms Reply with quote
Ugly_Bird wrote:
U2U2U2 wrote:
Ugly_Bird wrote:
bluefish1 wrote:

(I don’t get this, wouldn’t the lift be sideways not up?).


Would be if a fin had asymmetric cross-section (like a plane wing).


Only about 10years old now ASYMETTRICAL fins on multi fined boards, and even longer on surf boards , Cross section .. what is that ?


Cross section is the shape when you cut the fin across. Smile
For multifin boards the fins can be asymmetrical as they equalize each other, not the single fin. Smile
Single asymmetrical fin would help carve jibe on one tack and mess it up on the other Very Happy

Andrei


Thanks I would never have guessed . So why then dont all tri and quad boards have asymettrical fins ?
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U2U2U2



Joined: 06 Jul 2001
Posts: 5467
Location: Shipsterns Bluff, Tasmania. Colorado
PostPosted: Sat Jun 23, 2018 7:49 pm    Post subject: Reply with quote
bluefish1 wrote:
So tail walking has nothing to do with fin? It's mast base pressure and sail overpowered?


Tail walking has mostly to do with the fin, mostly its too big.

Many other factors can induce it, POS fin, fin with gouges and nicks. Plus incorrect sailor position , heavy back foot, and it not being leap year
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J64TWB



Joined: 24 Dec 2013
Posts: 1685
PostPosted: Sat Jun 23, 2018 9:50 pm    Post subject: Reply with quote
I have tailwalked sub planing, trying to get going way overpowered. Must be some other factors.
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