Super Secret Settled Science of Sailing?
- Thread starter Will Gilmore
- Start date
The discussion of density of a gas and the viscosity of moist to dry air leads naturally into a discussion of air resistance (drag). Different surfaces/substances have different coefficients of drag. Moist air vs dry air probably have different coefficients of drag. Low pressure air, I would expect, would be less viscous (easier to move out of the way per unit mass) than high pressure air.
I would think the temperature and moisture content of the air would be related to atmospheric pressure, but it isn't necessarily reflective of local atmospheric pressure. What I mean is, you can have a low pressure system with cold air and a high pressure system with warm air, but they won't stay that way.
What does that mean to how well your sails work? I have no idea, but I do know that friction is the biggest limiting factor to sail performance. I know this because there are plenty of examples of hyper-fast sailing vehicles that achieve their speed over a traditional sailboat, simply by reducing that friction. Ice boats routinely exceed 40 knots.
Reducing friction across the foil surface has the effect of improving speed as well as laminar (smooth parallel lines of) flow. The question of surface treatment to retain attachment and reduced friction has recently been discussed. I came to no definitive conclusions from that thread.
It was "unsettling"
- Will (Dragonfly)
I'm not a pilot but I'm pretty sure that home runs fly out the park more frequently in warm, humid weather!
This term Coefficient is used to simplify a MICRO Model to an acceptable and practical..
MACRO Model. Like a Friction Factor or Drag Coefficients.
Many time FACTORS are not constants and vary with some other fluid mechanics dependency. [of course after the null hypothesis is assumed true
]Coefficients are normally a Constant, selected by the BEST use in the Sailing situation.
I suggest we use the Macro Models for Sailing discussions.
Sounds like you have the Right Idea to me.
Greater Fluid Drag over one side of a shape, causes LIFT forces in directions of the Greater Drag.
Jim...
Yes, let's suspend the discussion until everyone involved buys and reads the $55, 468 page paperback.bethwaite, bethwaite, bethwaite....... maybe, just maybe, doing the required reading before discussion group wood help
I guess you're saying this is all figured-out by Frank, "settled science," so to speak?
yes, absolutely, positively, the best read on using these toys we all love so very much. no other book comes close.
my cred......
sailing: 60 years
miles on the water: well over 1,000,000
highly trained merchant mariner
alden caravelle 42
ascow
cal 20
trinka 12
formula 242 ocean racer
starcraft 14'
1948 shell lake 14'
this is the book that we/you all will want to read.
you can listen to my opinion as you want
my cred......
sailing: 60 years
miles on the water: well over 1,000,000
highly trained merchant mariner
alden caravelle 42
ascow
cal 20
trinka 12
formula 242 ocean racer
starcraft 14'
1948 shell lake 14'
this is the book that we/you all will want to read.
you can listen to my opinion as you want
Yes, Jon, I get that, I understand and appreciate a recommendation from a qualified source, and the value of a review. ('Though I haven't been able to find a detailed review of the book yet.)
But, as @Will Gilmore indicated in the introductory post, this is a discussion. I don't think it's fair to even subtly try to squelch the dialogue by asserting it's all figured out, "nothing more to see here, go home."
Besides, perhaps there are those who disagree with Bethwaite? I don't know, not having read the book, 'though I have read many on flying and sailing theory, performance sails, and so on. I may buy it, but maybe not. Who knows.
Also, a discussion like this suffers from appeals to authority, either indirectly, by pointing to an 'ultimate source,' like Bethwaite, or directly.
But, as @Will Gilmore indicated in the introductory post, this is a discussion. I don't think it's fair to even subtly try to squelch the dialogue by asserting it's all figured out, "nothing more to see here, go home."
Besides, perhaps there are those who disagree with Bethwaite? I don't know, not having read the book, 'though I have read many on flying and sailing theory, performance sails, and so on. I may buy it, but maybe not. Who knows.
Also, a discussion like this suffers from appeals to authority, either indirectly, by pointing to an 'ultimate source,' like Bethwaite, or directly.
all fair jviss. in this discussion i am sharing that there is this wealth of knowledge available to you all that you will love to absorb i think.
Here is how I would describe scientific “knowing.”
I found a wind tunnel and an instrumented wing in my fluid mechanics lab in college. The instrumentation consisted of 10 holes in the curved top and bottom surfaces of the wing. Because these holes were at right angles to the air flow, they sensed the pressure in the flow as it moved over the top of the wing. Tubes from the holes run out of the end of the wing where I could measure the difference between the stream pressure on the wing at the 20 points and the atmospheric pressure of still air in the lab. I measured these pressures for different angles of the wing to the air flow in the tunnel (angles of attack) and different wind speeds in the tunnel.
The pressure profiles over the wing were extremely repeatable for the same wind velocity, and angle of attack. That is strong evidence that my pressure profiles were “correct”. I could then calculate the aerodynamic lift and drag of the wing for each angle and velocity. (This analysis was for pressure profiles only - aerodynamic. It left out the effect of friction of the air on the wing which contributes to total drag.) When I researched other experiment results published from different labs around the world, their pressure profiles, and calculated lift and drag agreed with each other and with me. When a super majority of published papers agree, (say 90%) and those disagreeing pose no show stoppers, then I would call the results “settled science.”
Interestingly, there is no simple equation or model to explain precisely these wind tunnel results, and apply them to a sail. Kutta and Zhoukowski did a decent job of modeling the flow around my experimental wing (also ignoring the friction, or viscosity of the air on the wing.) Their model also explains the vortices that flow off the end of a wing, and the added drag these vortices produce.
A sail is more difficult because:
1. The wind speed over water varies considerably from the bottom to the top of the sail. (We call the result of this fact “twist.”)
2. A sail varies in shape from top to bottom, going from small to large, and varying in curvature.
3. Unlike a wing, a sail sheds vortices from both the top and the bottom.
Modern numerical analysis produces some reasonably accurate models of air flowing over sails, and of the resulting forces by computer brute force. Fluid mechanics of a grid of small areas of the sail are analyzed by solving the Navier-Stokes equations of each box of the grid, and harmonizing each box with those next to it over successive iterations This computation intensive task, that can take hours on a modern processor, results in the pretty pictures you can see online. I would call these efforts engineering to produce better sails, rather than science.
I found a wind tunnel and an instrumented wing in my fluid mechanics lab in college. The instrumentation consisted of 10 holes in the curved top and bottom surfaces of the wing. Because these holes were at right angles to the air flow, they sensed the pressure in the flow as it moved over the top of the wing. Tubes from the holes run out of the end of the wing where I could measure the difference between the stream pressure on the wing at the 20 points and the atmospheric pressure of still air in the lab. I measured these pressures for different angles of the wing to the air flow in the tunnel (angles of attack) and different wind speeds in the tunnel.
The pressure profiles over the wing were extremely repeatable for the same wind velocity, and angle of attack. That is strong evidence that my pressure profiles were “correct”. I could then calculate the aerodynamic lift and drag of the wing for each angle and velocity. (This analysis was for pressure profiles only - aerodynamic. It left out the effect of friction of the air on the wing which contributes to total drag.) When I researched other experiment results published from different labs around the world, their pressure profiles, and calculated lift and drag agreed with each other and with me. When a super majority of published papers agree, (say 90%) and those disagreeing pose no show stoppers, then I would call the results “settled science.”
Interestingly, there is no simple equation or model to explain precisely these wind tunnel results, and apply them to a sail. Kutta and Zhoukowski did a decent job of modeling the flow around my experimental wing (also ignoring the friction, or viscosity of the air on the wing.) Their model also explains the vortices that flow off the end of a wing, and the added drag these vortices produce.
A sail is more difficult because:
1. The wind speed over water varies considerably from the bottom to the top of the sail. (We call the result of this fact “twist.”)
2. A sail varies in shape from top to bottom, going from small to large, and varying in curvature.
3. Unlike a wing, a sail sheds vortices from both the top and the bottom.
Modern numerical analysis produces some reasonably accurate models of air flowing over sails, and of the resulting forces by computer brute force. Fluid mechanics of a grid of small areas of the sail are analyzed by solving the Navier-Stokes equations of each box of the grid, and harmonizing each box with those next to it over successive iterations This computation intensive task, that can take hours on a modern processor, results in the pretty pictures you can see online. I would call these efforts engineering to produce better sails, rather than science.
I certainly plan to follow Jon's recommendation and get this book, he has proven his stature in the sailing community to me, but not before this thread has petered out and is forgotten. I leave tomorrow for Scotland to enjoy Scallywagger's hospitality since we swapped places to stay. Reading a treatise on sailing wouldn't go over well with my traveling companions while we are there. Heck, I have to sneak my sailboat time as it is.
It is astonishing how much knowledge is here without the book or some other definitive source to refer to. I also believe that knowledge has its own limitations simply because, once one believes himself knowledgeable, changing perspectives or relearning knew replacement knowledge becomes exponentially hard. It needs to be hard to shift paradigms and embrace new technology or we, as a society, would find it too difficult to move as one while we fought over who was right and quickly embraced ideas that hadn't been well vetted.
At times, it is stifling, like shifting to Newtonian laws from Archimede's teachings or imprisoning or even killing some of our best and brightest because they tried to open our eyes to a universe we had been taught was unacceptable.
- Will (Dragonfly)
It is astonishing how much knowledge is here without the book or some other definitive source to refer to. I also believe that knowledge has its own limitations simply because, once one believes himself knowledgeable, changing perspectives or relearning knew replacement knowledge becomes exponentially hard. It needs to be hard to shift paradigms and embrace new technology or we, as a society, would find it too difficult to move as one while we fought over who was right and quickly embraced ideas that hadn't been well vetted.
At times, it is stifling, like shifting to Newtonian laws from Archimede's teachings or imprisoning or even killing some of our best and brightest because they tried to open our eyes to a universe we had been taught was unacceptable.
- Will (Dragonfly)
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I would describe that as Repeatable Experimentation, using a few dependent variables.
By using Numerical Methods to solve Liner differential equations, you confirmed your MACRO model.
That is good science, Modeled in the Lab.
Here is why the word Settled is "unsettling" to me.
Example:
After reading @David in Sandusky technical report....
Oh well David Solved it all...
Call it David's Law.
No more wing design or investigation to do. Dang.
But here is what is key to David's Lab study...
He knew the Basic Macro Model and showed limited dependency on few variables.
Are there more dependent variables to find?
I now wonder about inducing small vibrations on the air foil, or perhaps small surface induced pulses of air?
The key to David's study was...
it removed or confirmed a few aspects, opening the way forward for others to explore and not duplicate.
____
Thinking about the Sail as an air Foil, the only LIFT, I contend, is shaping the flexible Sail material.
I would suggest Vertical LIFT of my boat would be insignificant.
My contention eliminates Air flow significance, in at least one of the X,Y,Z directions.
Jim...
I wouldn't call it settled science, I would call it a consensus of measurement results. Being able to repeat a measurement doesn't say anything about the underlying mechanism, does it? The pressure difference might be coincidental to, and not causal of the lift. We may never know.
David nice work with repeatable results.
I always cringe a bit at the phrase "settled science." It was once settled science that the vapors caused illness. Sooner or later someone is going to come along and look at the problem from a slightly different perspective that will yield a deeper understanding of the phenomena and another round of settled science.
In the world of aeronautics, it has only been in recent years that wing tip vortices have been noted and addressed with those vertical winglets. At some point someone will look airflow over sails and have a better idea. That's what science does. It comes up with a good explanation for a while until a better one comes along.
After reading this paragraph, it kept gnawing at me. Yeah, it is engineering, but the test to see if the changes work is still science and the scientific method.
If I'm understanding this correctly, the process starts with a bunch of data points and then an equation (model) is developed that attempts to replicate the data and predict data under different circumstances, for example more or less air velocity. This gets a little funky because the goal is to have no differences between observed phenomena and predicted/modeled phenomena, sort of opposite of the social science stuff I'm most familiar with.
The hypothesis would be that there is no difference between the predicted and observed data and the null hypothesis is there is a significant difference between the observed and predicted data. If there is a significant difference between the observed and modeled data, then we accept the null hypothesis and reject the hypothesis, loosely translate the model doesn't work and back to the drawing board we go.
sails
sails behind masts
changes of sail shape
sloop rigs in wind tunnels with smoke
gaff rig sails
all the math used
wings vs. sails
ground effects
twist
university studies of sails
separation and reattachment of flow behind masts
pressure fields on airflow and changes made by changing attack angle
force changes with angle changes
leach separation zone
200 telltails on each side of sails
then put all this on a vessel in a seaway
round masts , oval masts
chapter 17
sails behind masts
changes of sail shape
sloop rigs in wind tunnels with smoke
gaff rig sails
all the math used
wings vs. sails
ground effects
twist
university studies of sails
separation and reattachment of flow behind masts
pressure fields on airflow and changes made by changing attack angle
force changes with angle changes
leach separation zone
200 telltails on each side of sails
then put all this on a vessel in a seaway
round masts , oval masts
chapter 17
Here is how I would describe scientific “knowing.”
I found a wind tunnel and an instrumented wing in my fluid mechanics lab in college. The instrumentation consisted of 10 holes in the curved top surface of the wing. Because these holes were at right angles to the air flow, they sensed the pressure in the flow as it moved over the top of the wing. Tubes from the holes run out of the end of the wing where I could measure the difference between the stream pressure on the wing at the 10 points and the atmospheric pressure of still air in the lab. I measured these pressures for different angles of the wing to the air flow in the tunnel (angles of attack) and different wind speeds in the tunnel.
The pressure profiles over the wing were extremely repeatable for the same wind velocity, and angle of attack. That is strong evidence that my pressure profiles were “correct”. I could then calculate the aerodynamic lift and drag of the wing for each angle and velocity. (This analysis was for pressure profiles only - aerodynamic. It left out the effect of friction of the air on the wing which contributes to total drag.) When I researched other experiment results published from different labs around the world, their pressure profiles, and calculated lift and drag agreed with each other and with me. When a super majority of published papers agree, (say 90%) and those disagreeing pose no show stoppers, then I would call the results “settled science.”
I found a wind tunnel and an instrumented wing in my fluid mechanics lab in college. The instrumentation consisted of 10 holes in the curved top surface of the wing. Because these holes were at right angles to the air flow, they sensed the pressure in the flow as it moved over the top of the wing. Tubes from the holes run out of the end of the wing where I could measure the difference between the stream pressure on the wing at the 10 points and the atmospheric pressure of still air in the lab. I measured these pressures for different angles of the wing to the air flow in the tunnel (angles of attack) and different wind speeds in the tunnel.
The pressure profiles over the wing were extremely repeatable for the same wind velocity, and angle of attack. That is strong evidence that my pressure profiles were “correct”. I could then calculate the aerodynamic lift and drag of the wing for each angle and velocity. (This analysis was for pressure profiles only - aerodynamic. It left out the effect of friction of the air on the wing which contributes to total drag.) When I researched other experiment results published from different labs around the world, their pressure profiles, and calculated lift and drag agreed with each other and with me. When a super majority of published papers agree, (say 90%) and those disagreeing pose no show stoppers, then I would call the results “settled science.”