good discussion on swaylocks:
http://www.swaylocks.com/forum/gforum.c ... t=homepage
thick fins
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willli
- Ripper (more than 100 posts)
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HW, I used one waaaay back in the day on a Bing bonzer I rode. I have no idea what happened to it or who made it. figured you'd pick up on the reference to flight and Newton.
my own theory is surfboards (foam n glass) transitioned (evolved) from "hull" shapes to "wing" shapes. skegs are necessary to control the "wing" and can become the dominant surface depending on how much of the board is actually in contact with the water, which changes all the time. I've thought a lot about "boost" and the curvature of the wave and AOA of the board driving on rail, and in those conditions the rail itself 'becomes' what we would traditionally think is a thick skeg, all base and drive, and the skeg becomes the "board".
How's that for a chemo induced revelation(delusion).
my own theory is surfboards (foam n glass) transitioned (evolved) from "hull" shapes to "wing" shapes. skegs are necessary to control the "wing" and can become the dominant surface depending on how much of the board is actually in contact with the water, which changes all the time. I've thought a lot about "boost" and the curvature of the wave and AOA of the board driving on rail, and in those conditions the rail itself 'becomes' what we would traditionally think is a thick skeg, all base and drive, and the skeg becomes the "board".
How's that for a chemo induced revelation(delusion).
Re: in thick or out thin?
There is a page on the internet that summarizes the results (trends) of extensive studies of foils by NACA. Unfortunately, I don't have the URL, nor (since I'm reorganizing everything, which means I can't find anything) do I have my printed copy of the summary in hand at the moment. However, as I recall, maximum lift typically occurs with sections with thickness to chord ratios of 13-15 percent.headwax wrote:Thanks willlli... geee I thought kneelos rambled on!
Worth a read, but I kept looking for the synopsis and the conclusion....
but near the end...
another poor misguided soulThere is a common misperception that the lift generated by a foil (wing or fin) moving through a fluid medium (air or water) is due to pressure differences (a la the Bernoulli Effect). Nothing could be further from the truth!
As a foil moves through the respective medium, it deflects that medium and, in accordance with the Laws of Newtonian physics (in particular the third Law, 'for every action there is an equal and opposite reaction'), lift is generated.
A thicker fin deflects more water than a thin fin and that is why it provides more lift.
Check out http://www.aa.washington.edu/faculty/eberhardt/lift.htm
...and I thought Bryan had come up with the reference himself
The value of a thicker section seems to be the smoother (i.e. more "rounded" nature of the forward portion of the foil, which reduces the curvature of the flow--especially around the leading edge) and hence reducing the accelerations the flow must make while following the contour of the foil. However, too thick a foil and the flow faces a greater difficulty in avoiding separation in the flow farther aft due to the curved nature of the foil section.
On the other hand, thicker sections generally result in more drag (assuming the flow is not separated on either a thin or thick foil), and--as best I remember--10 percent is mentioned as a reasonable compromise when reducing drag is important.
Another consideration when using foils in water is the distribution of low pressure over the low-pressure side of the foil. Foil sections with a very low presssure somewhere along the foil tend to promote ventilation (the entrainment of air from the sea surface onto the foil) and cavitation (a pressure so low that the combination of temperature and pressure is sufficient to cause some of the water flowing over the foil to vaporize [i.e. "boil"]--resulting a cavity filled with water vapor, rather than liquid water). In general, thicker foils tend to produce greater reductions in "peaks" of low pressure than do thinner sections (except, perhaps, near the onset of separated flow--or stalling). However, in sea water, speeds well in excess of 25 mph are typically required (and a foil well below the sea surface) in order to produce cavitation, while ventilation can occur at much slower speeds and primarily occurs with foils close to the sea surface.
[And finally--for what it's worth--Newton's Second Law: "Force = Temporal rate of change of momentum" is also crucial to understanding the physics of the flow around a wing]
mtb
Experience gained is in proportion to equipment ruined.
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Bryan Jackson
- Ripper (more than 100 posts)
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The level of thinking, sophistication, and value of information regarding surf craft design on Swaylocks is so far advanced compared to the ksusa site that it is like comparing apples and oranges. (Although at one time kneelos may have been independent and progressive thinkers in the surfing world, nowadays they tend to be a bunch of sheep if not outright ignoramuses!) 
By no means am I trying to imply that I am an expert on this subject (I actually know very little in technical terms). But the fact remains that the subject of how foils work (i.e., provide lift) is far more complicated than most of us have been led to believe (i.e., lift is provided merely by water/air molecules moving faster over a curved surface) and that if we are to advance our understanding of fin and board dynamics and thus design we must get past that rather simplistic notion.
At the very least, one would have to admit that there is a very active and lively debate in the world of aerodynamics involving opposing/differing viewpoints as to what provides lift, and that the principles/laws discovered by both Bernoulli and Newton are invoked by different authorities on the subject. (It is interesting to also note that neither Bernoulli nor Newton were trying to explain the principles/physics of flight nor the dynamics of foils!).
Anyway, for those of you who retain an open, skeptical and/or inquiring mind, further discussion of this topic can be found at
http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html
http://www.av8n.com/how/htm/airfoils.html
http://230nsc1.phy-astr.gsu.edu/hbase/f ... rfoil.html
Or do your own damn web search!
By no means am I trying to imply that I am an expert on this subject (I actually know very little in technical terms). But the fact remains that the subject of how foils work (i.e., provide lift) is far more complicated than most of us have been led to believe (i.e., lift is provided merely by water/air molecules moving faster over a curved surface) and that if we are to advance our understanding of fin and board dynamics and thus design we must get past that rather simplistic notion.
At the very least, one would have to admit that there is a very active and lively debate in the world of aerodynamics involving opposing/differing viewpoints as to what provides lift, and that the principles/laws discovered by both Bernoulli and Newton are invoked by different authorities on the subject. (It is interesting to also note that neither Bernoulli nor Newton were trying to explain the principles/physics of flight nor the dynamics of foils!).
Anyway, for those of you who retain an open, skeptical and/or inquiring mind, further discussion of this topic can be found at
http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html
http://www.av8n.com/how/htm/airfoils.html
http://230nsc1.phy-astr.gsu.edu/hbase/f ... rfoil.html
Or do your own damn web search!
A vote in favor of a momentum-based approach...
I disagree that the "why" of how things work is relatively unimportant and it is sufficient to just know that some particular feature/thing "works". One of the primary differences between man and the lower animals is his curiosity as to why things work--and that the results of gaining this knowledge lead to much quicker and more profound solutions to many problems than just experimentation alone.
In the case of understanding "why" a wing/fin/foil (henceforth referred to as a "wff") works, I favor a conceptual model based on the conservation of momentum (rather than the Bernoulli Equation) for the following reasons (assuming that we are talking about a macroscopic, non-relativistic, non-quantum mechanical environment):
1. Momentum is always conserved; energy is not always conserved.
2. Bernoulli's Equation is derived from Newton's Laws--but only after making certain simplifying assumptions. Hence situations adequately described by the Bernoulli Equation represent only a subset of the conditions that are governed by Newton's Laws, and Newton's Laws (in their most general form) cannot be deduced from Bernoulli's Equation.
For example, Newton's Laws apply to the boundary layer flow around a wff as well as the larger-scale flow; Bernoulli's equation does not. Why is the boundary layer important? Because the development and evolution of the boundary layer is critical to describing the separation of flow and the onset and evolution of stalling. Moreover, in the absence of viscosity (and the development of a boundary layer), there would not be any circulation generated around the wff (it would be a pure potential flow), and hence no lift would be generated (however, once viscosity "does it's thing" in establishing the boundary layer, the circulation that leads to "lift" can continue even in the absence of viscosity).
Having said that, however, I am more than willing to concede that Bernoulli's Equation is a convenient simplification of Newton's Laws for the computation of the "many-body" problem (in this case the bulk motion of fluids such as air or water for which one can use statistical approximations such as density, etc.), versus the more general and more precise Navier-Stokes equations (which also follow from Newton's Laws but without the approximations of Bernoulli's equation) when the assumptions involved in it's derivation are adequately fulfilled. But it is also necessary to keep in mind that one must still use some additional means/method (and/or assumptions that may turn out not to be valid) to compute the flow field around the wff before Bernoulli's equation can be applied.
3. A momentum-based conceptual model is easy to visualize (and compute/estimate the effects of) in the case of a planing hull; it is hard (at least for me) to visualize how to apply Bernoulli's Equation to this case. Nevertheless the two situations (wff vs planing hull) have much in common since both generate lift by adding momentum to the water orthogonal to the bulk flow.
Since the streamline passing just above the top of a wff ("top" referring to the side creating a force directed away from the wff), and the streamline passing just below the bottom of a wff parallel each other just aft of the trailing edge, the upper and lower surfaces of a wff should contribute equally to the lift if: (a) the speeds of the flow on each side are equal, (b) the stagnation point on the leading edge lies at the dividing point between the upper and lower faces of the wff, and (c) the regions affected by the presence of the flow on each side of the wff are of equal thickness.
In fact, as it turns out, the slope of the lift coefficient (as a function of the angle-of-attack) of a typical wff (i.e. flow over two sides) is roughly twice (better approx: 2.1 x) that of a flat planing hull (i.e. flow over one side of the hull). Hence at first glance, it would thus seem that both sides of the wff might be contributing aproximately equally to the lift.
Actual measurements (and/or proper theoretical calculations), however, show that much more of the lift comes from the upper surface of the wff than from the lower surface. The reason for this is that at a positive angle-of-attack, the stagnation point/line on the leading edge that separates the flow over the wff from that going under the wff moves below and aft of the leading edge of the wff. Hence at positive angles-of-attack the flow over the wff "steals" from the flow otherwise going under the wff, thus increasing the flux of water (and speed of the flow) over the wff, and reducing the flux of water (and speed of the flow) under the foil (thus robbing from Peter--the lower face--to pay Paul--the upper face--so to speak, in calculating the momentum change contributed by each side).
In short, while the precise details between the two situations differ to some degree, a momentum based conceptual model allows one to use the same basic approach to visualizing and computing the lift generated by either a wff ) operating embedded in a fluid (e.g. an air or water medium) and the lift generated by planing hull (operating at the interface between two fluids--air and water--of greatly different density).
mtb
In the case of understanding "why" a wing/fin/foil (henceforth referred to as a "wff") works, I favor a conceptual model based on the conservation of momentum (rather than the Bernoulli Equation) for the following reasons (assuming that we are talking about a macroscopic, non-relativistic, non-quantum mechanical environment):
1. Momentum is always conserved; energy is not always conserved.
2. Bernoulli's Equation is derived from Newton's Laws--but only after making certain simplifying assumptions. Hence situations adequately described by the Bernoulli Equation represent only a subset of the conditions that are governed by Newton's Laws, and Newton's Laws (in their most general form) cannot be deduced from Bernoulli's Equation.
For example, Newton's Laws apply to the boundary layer flow around a wff as well as the larger-scale flow; Bernoulli's equation does not. Why is the boundary layer important? Because the development and evolution of the boundary layer is critical to describing the separation of flow and the onset and evolution of stalling. Moreover, in the absence of viscosity (and the development of a boundary layer), there would not be any circulation generated around the wff (it would be a pure potential flow), and hence no lift would be generated (however, once viscosity "does it's thing" in establishing the boundary layer, the circulation that leads to "lift" can continue even in the absence of viscosity).
Having said that, however, I am more than willing to concede that Bernoulli's Equation is a convenient simplification of Newton's Laws for the computation of the "many-body" problem (in this case the bulk motion of fluids such as air or water for which one can use statistical approximations such as density, etc.), versus the more general and more precise Navier-Stokes equations (which also follow from Newton's Laws but without the approximations of Bernoulli's equation) when the assumptions involved in it's derivation are adequately fulfilled. But it is also necessary to keep in mind that one must still use some additional means/method (and/or assumptions that may turn out not to be valid) to compute the flow field around the wff before Bernoulli's equation can be applied.
3. A momentum-based conceptual model is easy to visualize (and compute/estimate the effects of) in the case of a planing hull; it is hard (at least for me) to visualize how to apply Bernoulli's Equation to this case. Nevertheless the two situations (wff vs planing hull) have much in common since both generate lift by adding momentum to the water orthogonal to the bulk flow.
Since the streamline passing just above the top of a wff ("top" referring to the side creating a force directed away from the wff), and the streamline passing just below the bottom of a wff parallel each other just aft of the trailing edge, the upper and lower surfaces of a wff should contribute equally to the lift if: (a) the speeds of the flow on each side are equal, (b) the stagnation point on the leading edge lies at the dividing point between the upper and lower faces of the wff, and (c) the regions affected by the presence of the flow on each side of the wff are of equal thickness.
In fact, as it turns out, the slope of the lift coefficient (as a function of the angle-of-attack) of a typical wff (i.e. flow over two sides) is roughly twice (better approx: 2.1 x) that of a flat planing hull (i.e. flow over one side of the hull). Hence at first glance, it would thus seem that both sides of the wff might be contributing aproximately equally to the lift.
Actual measurements (and/or proper theoretical calculations), however, show that much more of the lift comes from the upper surface of the wff than from the lower surface. The reason for this is that at a positive angle-of-attack, the stagnation point/line on the leading edge that separates the flow over the wff from that going under the wff moves below and aft of the leading edge of the wff. Hence at positive angles-of-attack the flow over the wff "steals" from the flow otherwise going under the wff, thus increasing the flux of water (and speed of the flow) over the wff, and reducing the flux of water (and speed of the flow) under the foil (thus robbing from Peter--the lower face--to pay Paul--the upper face--so to speak, in calculating the momentum change contributed by each side).
In short, while the precise details between the two situations differ to some degree, a momentum based conceptual model allows one to use the same basic approach to visualizing and computing the lift generated by either a wff ) operating embedded in a fluid (e.g. an air or water medium) and the lift generated by planing hull (operating at the interface between two fluids--air and water--of greatly different density).
mtb
Experience gained is in proportion to equipment ruined.
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Shelfbreak
- Ripper (more than 100 posts)
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- Joined: Mon Aug 09, 2004 6:29 pm
- Location: Sunshine Coast
Although I have no technical comment to add to the discussion I support the view that it is helpful to understand why a certain modification might have worked or not worked. As an outsider I find it very hard to identify where the trial and error approach to surfboard design and hydrodynamic theory are reconciled.
I assume many innovations have been inspired by designer's views on water flow etc. The boards we ride now are testament to these improvements but I wonder if some of them actually work for different reasons than their designers envisaged.
I assume many innovations have been inspired by designer's views on water flow etc. The boards we ride now are testament to these improvements but I wonder if some of them actually work for different reasons than their designers envisaged.
Shelfbreak
Here's a post that addresses some points regarding foil and lift:
viewtopic.php?p=17041#16957
viewtopic.php?p=17041#16957
Maybe, but in a Positivist (or even Post-Positivist, Critical Realist world*) we progress by having an idea about something, then testing the idea in a logical fashion, hopefully reducing the amount of effort and dead ends in the process.it doesn't really matter "why" it's the "does it go" that matters. Unless youre trying to convince someone else that something works
Humans are really good at intuitively finding patterns, so we intuit complex relationships. Unfortunately we are really poor at comprehending relationships that are twice removed (i.e. further than A->B) or non-linear.
Sure it's great to be gung-ho and to trust your senses and instincts to bring you to a conclusion, but there's always a grave risk that the conclusion may be erroneous (like espousing twin hulls above all other forms). It may be possible to logically move it from "doesn't go" to "Wow" through application of logical, (hacksaw wrought) changes, but it's a poor idea to fire the jigsaw up without considering where/why to cut
* it doesn't matter what you call it, as long as it has waves
There is an interesting speech on systemics and modelling at http://www3.interscience.wiley.com/cgi- ... 1697/START
MTB,
Thank you for sharing your depth of knowledge.
I can email copy of the above paper if you wish. Just PM me
Here is brief exerpt:
"Understanding complex systems requires mastery of concepts such as feedback, stocks and flows, time delays, and nonlinearity. Research shows that these concepts are highly counterintuitive and poorly understood. It also shows how they can be taught and learned. Doing so requires the use of formal models and simulations to test our mental models and develop our intuition about complex systems. Yet, though essential, these concepts and tools are not sufficient. Becoming an effective systems thinker also requires the rigorous and disciplined use of scientific inquiry skills so that we can uncover our hidden assumptions and biases. It requires respect and empathy for others and other viewpoints. Most important, and most difficult to learn, systems thinking requires understanding that all models are wrong and humility about the limitations of our knowledge. Such humility is essential in creating an environment in which we can learn about the complex systems in which we are embedded and work effectively to create the world we truly desire. "
By the way "systems" may be anything from the global environment to the flow of water out of a pipe - the concept of a system with a defined boundary is highly artificial since many outside factors may influence our object of desire, but artificially closed systems provide a practical limit to modelling.
Oh yeah, Bernoulli, Newton, Positivist etc etc.
It's very important to realise that these are not explanations of reality. They are merely models that propose to fit observations. The academic argument is more about reputation than what's correct. We learnt a model of the atom that was quite well accepted back then but has now proven inadequate, whence new (String Theory) models.
Don't assume the map is the territory - it's just a model that facilitates our (sheepish) thinking.
Here is brief exerpt:
"Understanding complex systems requires mastery of concepts such as feedback, stocks and flows, time delays, and nonlinearity. Research shows that these concepts are highly counterintuitive and poorly understood. It also shows how they can be taught and learned. Doing so requires the use of formal models and simulations to test our mental models and develop our intuition about complex systems. Yet, though essential, these concepts and tools are not sufficient. Becoming an effective systems thinker also requires the rigorous and disciplined use of scientific inquiry skills so that we can uncover our hidden assumptions and biases. It requires respect and empathy for others and other viewpoints. Most important, and most difficult to learn, systems thinking requires understanding that all models are wrong and humility about the limitations of our knowledge. Such humility is essential in creating an environment in which we can learn about the complex systems in which we are embedded and work effectively to create the world we truly desire. "
By the way "systems" may be anything from the global environment to the flow of water out of a pipe - the concept of a system with a defined boundary is highly artificial since many outside factors may influence our object of desire, but artificially closed systems provide a practical limit to modelling.
Oh yeah, Bernoulli, Newton, Positivist etc etc.
It's very important to realise that these are not explanations of reality. They are merely models that propose to fit observations. The academic argument is more about reputation than what's correct. We learnt a model of the atom that was quite well accepted back then but has now proven inadequate, whence new (String Theory) models.
Don't assume the map is the territory - it's just a model that facilitates our (sheepish) thinking.