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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Tue Jun 19, 2012 1:17 pm 
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johnney and lobo,
i believe that you are both correct.
however that does not alter the point i was trying to make.
as long as there is an attraction between your butt and the earth (gravity) there will be some divergance in the flow field exiting the board.

you are certainly correct that the additional flow conditions (i.e. true velocity of the board or kite traction force, etc) should be added.
however you cannot simply toss out the ones that dont fit your model.
....unless you believe the magnitudes are negligable in this case.

yes/no?
-bill


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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Tue Jun 19, 2012 1:52 pm 
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zfennell wrote:
johnney and lobo,
i believe that you are both correct.
however that does not alter the point i was trying to make.
as long as there is an attraction between your butt and the earth (gravity) there will be some divergance in the flow field exiting the board.

you are certainly correct that the additional flow conditions (i.e. true velocity of the board or kite traction force, etc) should be added.
however you cannot simply toss out the ones that dont fit your model.
....unless you believe the magnitudes are negligable in this case.

yes/no?
-bill



Since a kite/surfboard is not a displacement hull and works on plane, we create less depth of wake than other craft. So at fin depth, (particularily for race fins) it must be considered that the overwhelming majority of the cross sectional area of the fin is moving through motionless(ish) water (relative to board speed)

As for the boundary layer between the board and water I have a VERY basic knowledge of the mechanics of planing which is enough to tell me that I truly know nothing and would not be surprised at all by any direction of flow in this boundary layer. But I think this this is where board shape not fin geometry becomes the important factor.


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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Tue Jun 19, 2012 3:48 pm 
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Johnny Rotten wrote:

Since a kite/surfboard is not a displacement hull and works on plane, we create less depth of wake than other craft. So at fin depth, (particularily for race fins) it must be considered that the overwhelming majority of the cross sectional area of the fin is moving through motionless(ish) water (relative to board speed)

As for the boundary layer between the board and water I have a VERY basic knowledge of the mechanics of planing which is enough to tell me that I truly know nothing and would not be surprised at all by any direction of flow in this boundary layer. But I think this this is where board shape not fin geometry becomes the important factor.



“Since a kite/surfboard is not a displacement hull and works on plane, we create less depth of wake than other craft. “.....

This may be true, but I don’t think its relevant.
Because your kite board is not a displacement device, the forces to keep your board on the surface must come from other means. Specifically, both the kite and the flow past the bottom of the board keep you on top of the water. The specific flow path taken past the bottom of the board should be the point of this discussion.


“So at fin depth, (particularily for race fins) it must be considered that the overwhelming majority of the cross sectional area of the fin is moving through motionless(ish) water (relative to board speed) “....

A very good point, longer fins would certainly minimize the interaction between the fins and bottom surface of the board. However, what your describing is the ‘potential flow’ or the extent of the pressure field created by the board. Historically, the pressure field of a submerged body can be ‘felt’ 1-5 diameters upstream and 10-20 diameters downstream. Not sure where our case fits , but I’m with you on this one.

“As for the boundary layer between the board and water I have a VERY basic knowledge of the mechanics of planing which is enough to tell me that I truly know nothing and would not be surprised at all by any direction of flow in this boundary layer.”....

Planning forces are typically calculated independent of the boundary layer.
I’m sure the boundary layer is of marginal thickness and will not provide much impact on the flow-field.
The boundary layer certainly contributes to the skin friction (laminar/turbulent) , impacts the flow separation point and resultant wake of the board. All factors which contribute to the overall drag. But not likely to influence the streamlines (flow path)

.”But I think this is where board shape not fin geometry becomes the important factor.”
....
Yes, I was never proposing the use of fins to alter the flow.
My point was/is “where” to locate the fins to maximize alignment with the flow.

The flow past the board is not well contrained or defined as you would imagine for flow inside a pipe. Even if all of the steady-state scenarios we have discussed are appropriate, I’m sure we’re only scratching the surface here

-bill


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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Tue Jun 19, 2012 6:35 pm 
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zfennell,

with all due respect i think you are complicating things a little :P

Two main things to consider:

- drift angle (if it didn't exist, any sailing craft would always go straight downwind); it's by far the determining factor in the angles of attack "felt" by the fins of a board sailing in a straight line. (whatever happens inside the boundary layer of the bottom of the board is negligible when compared to this, specially for longer fins as the ones used in raceboards.
- the drag curve in relation to angle of attack is parabolic.


so imagine the two following cases (board sailing in a straight line of course):

1. Parallel front fins, angle of attack in the fins (alpha) = drift angle (beta)

2. Toed in front fins (toe in angle "teta"),
alpha(leeward fin) = beta +teta
alpha (windward fin) = beta -teta

So, even though the sum of both alphas remains the same, in the toed in case the drag will be higher than in the parallel case because the drag gained by the leeward fin will be more than the drag lost by the windward fin. (because the drag curve is parabolic, not linear)

Now, as Johnny said, a surfboard is different animal, and a surfboard riding (not sailing) waves is from another planet, and some of your insights are probably right in that case!


Attachments:
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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Tue Jun 19, 2012 8:56 pm 
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thanks,
i know this may be painful,
we probably have different backgrounds and tend to use different termimology.
so things tend to get sidetracked.

I still seem to be failing at making my single point.
its possible/likely that the flow over the bottom of the board is not made of parallel stream lines everywhere.

the blackdiamond link is a good place to define terminology, but does not address the flow field for locations away from the centerline of the boat.

I do accept johnneys ascertion that the depth of the fins may effectively remove them from the flow field of the board.
i also believe that the pull of the kite reduces the lift needed for the board to remain on plane.

however, i have a hard time accepting that boards and boats and planes are subject to different rules of physics.

if they are different animals, as you suggest.
I'll let you provide the free-body diagram that explains why the pressure (lift) on the bottom of your board compells the flow to travel directly from front to back.
....in particular along the side rails, with no attempt to travel laterally

in the end you may be right, but i'm not convinced you know why.


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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Tue Jun 19, 2012 9:49 pm 
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Lobodomar is saying the direction of flow is never paralell to the centerline of the board when you're sailing upwind, it's on an angle (beta in the picture above, usually called the leeway angle in sailing). The board doesn't travel in the direction it's pointing, it travels at a slightly downwind angle from where the centerline is pointing.

He's also saying: Maybe the flow also diverges away from centerline, but who cares? The fins are too deep to notice.

Interesting topic!


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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Tue Jun 19, 2012 10:31 pm 
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zfennell wrote:
thanks,

I'll let you provide the free-body diagram that explains why the pressure (lift) on the bottom of your board compells the flow to travel directly from front to back.
....in particular along the side rails, with no attempt to travel laterally


I never said the flow travels directly from front to back - the whole point about drift angle is exactly that because of it the flow is never aligned with the board


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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Tue Jun 19, 2012 10:37 pm 
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davesails7 wrote:
Lobodomar is saying the direction of flow is never paralell to the centerline of the board when you're sailing upwind, it's on an angle (beta in the picture above, usually called the leeway angle in sailing). The board doesn't travel in the direction it's pointing, it travels at a slightly downwind angle from where the centerline is pointing.

He's also saying: Maybe the flow also diverges away from centerline, but who cares? The fins are too deep to notice.

Interesting topic!


Thanks for the "translation"!
Actually the drift or leeway angle is present not only when sailing upwind, but downwind as well (but is then smaller).
It's zero only when the board is travelling directly downwind (in other words, never)! :thumb:


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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Wed Jun 20, 2012 12:04 am 
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thank you both.
i should have been clearer.

i am quite willing to accept the concept of drift angle.
your diagram indicates the flow is parallel to the direction of travel..
(regradless of the direction the boat is pointed)

so ....moving on.
is the flow parallell to the direction of travel everywhere ?


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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Wed Jun 20, 2012 8:35 am 
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Imagine a sailboat with a completely flat bottom at the sea, and let's set the current to zero so we don't mix things up.
If the boat didn't have a keel, it would go straight downwind because of the action of the wind on the sail.
The presence of the keel is what allows it to drive it's way out of the straight downwind path
However even with the keel the boat will still travel downwind of it's (and the keel's) longitudinal axis, because the keel's efficiency is far from 100%

The angle between the longitudinal axis of the boat-keel system and the actual direction of travel is the drift angle. Free flow will come aligned with the direction of travel.

In the real world, of course there will be interferences affecting the actual flow, specially close to the bottom of the board and the back fin.

But I would say that, basically, the flow that matters for most of the lift and drag generated by the fins will come aligned with the freeflow, yes, specially in the front fins (and considering how thin and long the front fins are, and how distant they are from the tail, I'd say it's pretty aligned with the freeflow also in the back fin)


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