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What is tailwalking and what is wheelies?
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isobars



Joined: 12 Dec 1999
Posts: 20132

PostPosted: Sun Jul 31, 2011 2:35 pm    Post subject: Reply with quote

maxsonic wrote:
.is spin out caused by "cavitation" or by increasing the fin's angle of attack to/past the symmetric airfoil stall point shown in Figure 11-20? I vote for the latter!


I vote for operator error, including loose or damaged or undersized or cheap fin and/or poor technique, all exacerbated by heavy chop and point of sail. Wink

The one time I sailed in extreme winds on absolutely glass-flat water, I TRIED to spin out from sheer curiosity; couldn't do it. I guess that places true cavitation way down the list.

Mike \m/
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chophop



Joined: 16 Apr 1996
Posts: 228

PostPosted: Sun Jul 31, 2011 2:46 pm    Post subject: Reply with quote

I agree with James Douglas' analysis.

The fin is not always true vertical to the water surface. So it is a bit sideways lots of the time. I would also guess that the water passing rapidly under the tip of the fin causes some lift.
For me it is a given that fins cause lift and big "powerful" ones cause more lift than small thin flexible ones.

Wink
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B737



Joined: 27 Mar 2009
Posts: 216
Location: Jersey Shore

PostPosted: Sun Jul 31, 2011 3:47 pm    Post subject: Reply with quote

bred2shred wrote:


Sorry dude, but you're completely wrong. Please review fluid dynamics 101 because you clearly have no clue what you're talking about. By definition, lift is the component of force generated by a fluid flow over a surface which is perpendicular to the fluid's direction of flow. Whereas drag is the component of force generated by a fluid flow over a surface which is parallel to the direction of fluid flow. Therefore, the horizontal force generated by a fin which is perpendicular to the board's center line is, by definition, LIFT. The same goes for a sail- the force that pulls you sideways/forward is lift. The force that limits your top speed is drag.

There is no question that symmetrical foils generate lift (and drag). A sheet of plywood can generate lift. The key is that the foil has an incident angle to the flow (i.e., the foil must be angled to the flow in order to generate lift regardless of whether it is symmetrical or asymmetrical). Asymmetric foils are typically more efficient at generating lift, but a foil does NOT need to be asymmetrical to generate lift.
sm

This... +1

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inarchetype



Joined: 01 Jul 2010
Posts: 31

PostPosted: Sun Jul 31, 2011 7:45 pm    Post subject: Reply with quote

This has been illuminating.... as a non-engineer, I have always wondered about the same thing (mystery of a symmetrical foil oriented parallel to direction of travel creating what people call "lift").

It dawns on me that the key is that it isn't perpendicular to the direction of travel, as lee way ensures that direction of travel is actually at an angle to the longitudinal axis of the board, creating the necessary angle of attack for lift to counteract the lee way.

Light bulb goes on... so thanks.
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boardsurfr



Joined: 23 Aug 2001
Posts: 1246

PostPosted: Sun Jul 31, 2011 8:22 pm    Post subject: Reply with quote

Nice to see a pissing contest about text book formulas. Here's a much simpler way to look at this:
The sail creates "lift" that is more or less perpendicular to the sail (imagine a force arrow pointing straight away from the center of effort). Without a fin, a board would pretty much follow that arrow, going mostly downwind.

The fin keeps the board from slipping sideways. Therefore, it must generate a force that is (in first approximation) equal to the sideward force generated by the sail. Imagine a force arrow on the fin that is pointing towards the wind.

Most of the time while sailing, the fin is attached to the board (if this is not the case, tail walking will not happen, so we can safely ignore the rare exceptions). So take a fin, hold it at the top, and push the center of the fin sideways. Surprise: the fin wants wants to tilt! Attached to the board, it wants to tilt the board so that the windward edge comes out of the water. Longer fin = more leverage => more tilting power. Might have something to do why 70 cm long fins are found on formula gear, but not on wave boards.

So, based on this simplistic analysis, I get the suspicion that the "lift" of the fin (using the aerodynamic term) that is perpendicular to the fin surface creates noticeable "lift" on the windward side of the board simply because the fin is attached to the board on one end.

As to why a symmetric foil generates lift: look at the chicken strap on formula boards. If I understand correctly, it's in the center, and used when going straight downwind (a crazy thing that windsurfers on most other boards never do, except for a very short time when playing in waves). Going straight downwind is the only time when a windsurfer is actually traveling exactly where the nose of the board is pointing. At all other times, the board will also have a certain amount of sideway drift which creates a non-zero angle of attack on the fin, and therefore lift. Usually, we don't notice the sideways drift, but you can sometimes see it in other surfers.

Again, there's a simplistic explanation that may be easier to understand. Think about being on a board with a reasonable large fin and outside footstraps, like a freeride or slalom board, in marginally powered conditions. Often, the easiest way to get planing is to push onto the fin, for example with a foot wrapped around the windward rail. If you want to, think about the water pushing back, which generates the windwards-directed power on the fin. Or if you prefer, think of pushing the fin leewards creates a lower pressure area on the windward side that wants to "suck" the fin back. As before, since the top of the fin is attached to the board, this will torque the board so the windward edge wants to come up.
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jingebritsen



Joined: 21 Aug 2002
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PostPosted: Mon Aug 01, 2011 4:34 am    Post subject: Reply with quote

BTW, flex fins, what ever their size make the sailor have to work harder for control in gusty choppy conditions. stiffer fins do not. for max planing power in glassier venues one goes for lotsa flex. for windier choppier conditions the opposite.
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bred2shred



Joined: 02 May 2000
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PostPosted: Mon Aug 01, 2011 11:14 am    Post subject: Reply with quote

rigitrite wrote:
Since you clearly can't add, the "lift" on either side of a symetric foil is the same, therefore, X - X = 0. So lift = 0, get it...no lift? That's what I'm talking about. Symetric fin does not create lift,... Since there is no pressure differential, there is no "lift".


The only time this statement is true (zero lift) is when there is zero incident angle (leeway angle) resulting in zero pressure differential. This would only be the case when you're either sailing dead downwind or coasting.

Quote:
BTW, when does your fin ever have an incident angle??? Never...since your fins incident angle is always = 0, and the sum of the forces normal to the fin is still ZERO. Thus, no lift.....


So unbelieveably wrong it's silly. Your fin virtually ALWAYS has an incident angle (leeway angle in sailing terms). Every sailing craft in the world has a leeway angle. They have to unless they are only going to sail dead downwind.

If you think your fin produces ZERO lift, then do a very simple experiment - try sailing without it.

sm
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isobars



Joined: 12 Dec 1999
Posts: 20132

PostPosted: Mon Aug 01, 2011 12:07 pm    Post subject: Reply with quote

It seems pretty apparent that some posters still don't understand that aero/hydro-dynamic "lift" on a sail, a fin, an arm stuck out the window, or a paddle dragged in the water is a technical term unrelated to lifting an object vertically against the pull of gravity like we do in the gym's weight room. Others may not understand that lifting a fin vertically, through whatever real-world means -- is not going to make the board nose paw at the sky. To compound the confusion, ever-present hydrodynamic drag on a tracking fin always exerts a downward force on the board nose.

Mike \m/
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noshuzbluz



Joined: 18 May 2000
Posts: 791

PostPosted: Mon Aug 01, 2011 1:40 pm    Post subject: Re: What is tailwalking and what is wheelies? Reply with quote

sl55 wrote:
What exactly is "tailwalking" and what's when they say the board is "doing wheelies"? Not sure if I spelled it right.
Any pics, vids?


This is a brief moment of too much lift. Wheelie/Tail Walking. Which ever you prefer to call it.



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techno900



Joined: 28 Mar 2001
Posts: 3914

PostPosted: Mon Aug 01, 2011 2:26 pm    Post subject: Reply with quote

jingebritsen said
Quote:
BTW, flex fins, what ever their size make the sailor have to work harder for control in gusty choppy conditions. stiffer fins do not. for max planing power in glassier venues one goes for lotsa flex. for windier choppier conditions the opposite.


This used to be the thinking, but it has changed, particularly for formula fins. Soft is the way to go with lots of flex for better control. I don't know specifically why, but that is what is happening with the big fins.

The Drake fin that came with my formula board (starboard 160) is stiff and unforgiving over 15 knots, but OK in light winds. I have a custom Deboichet R13M (M = medium flex). Both fins are 70 cm, but the Deboichet is sooooooooo much better with control when the wind picks up. This is an old formula fin and the ones they are making now are even softer.
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