Super Secret Settled Science of Sailing?

Oct 19, 2017
7,732
O'Day 19 Littleton, NH
The application of science in sailing is undeniable. The sciences of fluid dynamics, Newtonian laws of motion, conservation of energy, Aerodynamics and lift, friction, leverage, driven harmonic systems, weather, Astronomy, Mathematics, electromagnetism, curved space and non-Euclidean Geometry, it's all here in sailing.
There have recently been some lively debates about some of these aspects of sailing; what surface is fastest on the bottom of a hull, how to correct and correlate varied navigation systems, what does Hull Speed actually mean, balancing lateral resistance with center of effort, even how much, if any, flotation is needed in your boat, have all recently been debated. Then there is the whole problem of epistemology, how do we know what we know and if it is reasonable to question what is often considered "settled science".
I recently stated that science should never be settled. There were a couple of very good rebuttal points brought up around that statement. Example: it is considered settled that the Earth is not flat. Great point. Then there is the response that "settled" or not, we treat certain well developed ideas as settled because they hold true with use and they have acceptance from the science community. Absolutely.
I know there are a lot of great minds and an amazing level of education floating around on sailboats. I am astonished at the number of engineers and scientists on this forum. I am hoping to tap into some of that resource for some discussions into the "reality" or "myth" of science around sailing.
To kick it off, I thought it would be interesting to explore a little about the science of lift.
It has always seemed counter intuitive to discover that the fluid racing through a restriction in a pathway, the curve of a full sail, the displacement of a moving hull, can actually cause a lessening of pressure such that lift is created.
I wonder if the atmospheric pressure on the convex side of a sail is really lower than the pressure on the concave side or is it only experienced by the surface over which the air is racing? What I mean is, obviously the pressure on the convex surface of the sail is lower than the surface on the concave side of the sail because the sail is drawn into the low pressure area to equalize that pressure variance, but if a pressure gauge were flowing freely along with the air moving over the surface (no appreciable air movement relative to the gauge), would it feel the same drop in pressure as is felt on the surface of the sail?
You see, it had been explained to me that the directed flow of air meant that the sum total of pressure vectors could not push as hard or as long on the surface of the sail because of its movement across it, resulting in lift. However, some of the images I've seen in a couple of threads here on SBO, show regions of low pressure independent of air movement across the surface. That would mean there was a very real loss of atmospheric pressure for any object, regardless of its movement with or against the air flow. So, why? Why does a dynamic that seems to compress the gas by forcing it through a restriction, actually result in lowers psi and ultimately, lift?

- Will (Dragonfly)
 
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May 25, 2012
4,333
john alden caravelle 42 sturgeon bay, wis
me, i found the answers to all, and i do mean all, those sailing science questions, with nice pictures, and drawings well written and more in one book. 'high performance sailing' by frank bethwaite
 
Oct 19, 2017
7,732
O'Day 19 Littleton, NH
me, i found the answers to all, and i do mean all, those sailing science questions, with nice pictures, and drawings well written and more in one book. 'high performance sailing' by frank bethwaite
Great to know Jon, I will definitely look into getting a copy of that book. However, while I am interested in the answers, I'm also interested in the whole question and answer process. I'm interested in the science and experimentation around the answers. How do we know the answers are "THE" answers? You know, until recently, my lifetime, Ptolemaic astronomy mathematics was still being used in planetariums. The mechanics of the projector made the mathematics based on geocentric and circular star systems more efficient.
Funny, of you built a model of Ptolomy's system and placed the pedestal under the sun instead of the earth, you would get a solar system with planets that moved in elliptical orbits around the Sun. This is, in part, about how we know the answers we know are right and how we got there.
It is also about the discussion and, of course, about the answers.

- Will (Dragonfly)
 
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Feb 2, 2006
464
Hunter Legend 35 Kingston
I know your hoping to drum up some good discussion here on the forum, but Frank B.'s book is fascinating in many ways, but particularly in his back stories and explanations on how he "noticed" behaviours and phenomenons. He's a pilot as well as a sailor, and some of his stories of how he came to various realizations are great ( bioluminescence and foil shapes!). He explains his process of observations and experiments to show how he supports his determinations.

Books by C. A. Marchaj, while a little dated and a little heavy on the technical, are fantastic resources for sailors.

Chris
 
Nov 26, 2012
1,653
Hunter 34 Berkeley
The pressure really is lower. This is conservation of energy. As a reminder “energy can neither be created nor destroyed. Only changed from one form to another” in the case of a fluid the total energy is the atmospheric pressure + temp + velocity. When you increase one of those types of energy you must borrow it from another so that the total stays the same. Therefore, velocity increase causes a pressure decrease. That’s it.
 
Mar 20, 2015
3,094
C&C 30 Mk1 Winnipeg
I recently stated that science should never be settled.
Sailing science is settled but people just don't want to admit it... science this science that.. earth is flat and i'm proving it in my steam powered DIY "rocket"...
It's all blah blah blah.

The truth is...like most things.. sailing is simply FM

;)jk please please realize i'm joking. :wahwah:


How do we know the answers are "THE" answers?
<Serious hat on>
You mention what you were taught....
How about the Bohr model being taught in school when it was obsolete in 1925 ?
Why ?
Sometimes we "know" something because the the actual current info is too complicated for most humans.

People like simple answers and solutions... even if they are flawed.

So easier just to say.. it is all FnMagic
 
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Jan 11, 2014
11,321
Sabre 362 113 Fair Haven, NY
I'm interested in the science and experimentation around the answers. How do we know the answers are "THE" answers?
The nature and strength of the scientific method is the understanding that all answers are only an approximation of the truth. It is impossible to prove a theory, so the method is to form a null hypothesis that can be disproven. What science does is to rule out alternative explanations, that is what confuses folks and why many lose faith in the process because what is known today is challenged tomorrow.

Using Will's example why sails work, the early theories were about the differential pressures on the sails caused lift. We can't prove that this differential is what causes sails to work, but we can measure the pressure on both sides of the sail. The hypothesis is that there are pressure differentials on the sides of the sails and that this causes lift. The first step in the process of supporting this theory is to establish that there are pressure differentials. It is impractical to test every conceivable condition that might cause pressure differentials, so science looks for instances in which there is no differential, i.e., the null hypothesis.

Thus (and very crudely stated) the hypothesis is that air flowing over a curved surface causes a pressure differential. We can't prove that, what we can do is demonstrate that there is a pressure differential and that when there is no differential there is no lift. The null hypothesis is that there is no pressure differential with air flowing over a properly curved surface. The experiment then is to form a properly curved surface and blow air over it. If we find a pressure differential then we have failed to support the null hypothesis and we can then assume that air blowing over a curved surface causes a pressure differential. However, if we find there is no pressure differential, then we have supported the null hypothesis that air blowing over a properly curved surface causes a pressure differential. Assuming we are really interested in this phenomena, we would construct experiments to confirm the initial findings and would vary some of the conditions, for example increasing air velocity, perhaps in low velocity flows there is no differential or in high velocity flows there would be no differential. We would then retest our null hypothesis (no differential in airflow) under these different conditions. Eventually we would have enough data over enough trials to conclude that with a high probability of being correct, that air flow over a curved surface causes a pressure differential. But, we have never proved that it does, we only demonstrated that under a collection of conditions sufficiently broad that there is not an absence of a pressure differential.

Understanding the concept of the null hypothesis takes some head scratching and you ponder it, the null hypothesis can be understood. Trips up grad students all the time. There are probably some better, more easily understood explanations about the null hypothesis out there on the internet.

That last statement is really a hypothesis, "the foregoing description of the null hypothesis is not the best explanation." The null hypothesis is "the foregoing description of the null hypothesis is the best explanation of the null hypothesis." Skipping over the methodological concerns, in order to prove the the hypothesis (this is the best) it is necessary to evaluate all descriptions of the null hypothesis. A daunting task. However, to disprove the null hypothesis, all that is necessary to find one explanation that is better than this explanation and therefore we have demonstrated that this is not the best. But we haven't found the best explanation yet.

Unfortunately, this kind of rigorous thinking drives many to the brink of distraction and often yields apparently contradictory results. Thus folks rely on intuition, feelings, and anecdotal data rather than the scientific findings that evolve over time.

To answer Will's initial question, it is not pressure differentials, it is laminar airflow over the sails and laminar water flow over the hull and appendages that cause lift.

Note to Phil: We need a head scratching emoji to go with the beer emoji. ;)

Edit: Another note to Phil, I think we might also need an "eyes glazed over" emoji.
 
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Feb 14, 2014
7,399
Hunter 430 Waveland, MS
Good Stuff all.

Science it the study of natural phenomena. That study becomes Science when observers discovered their observations would repeat.

The early observers, defined their observations in a simplistic form or Model. Perhaps a crude beginning, but fairly good Models.

After many others took those crude models, tested them, refined them a bit, and all agreed it was a GOOD model.
Those Models were given the Elevated Status of becoming a Scientific...

LAW

I would suggest, to this thread, 3 views of Scientific Models.
1) MACRO Models
2) MICRO Models.
3) LAWS
_______
The sciences of fluid dynamics, Newtonian laws of motion, conservation of energy, Aerodynamics and lift, friction, leverage, driven harmonic systems, weather, Astronomy, Mathematics, electromagnetism, curved space and non-Euclidean Geometry, it's all here in sailing.
Will noted the Macro Model, Laws and even a PURE Science called Mathematics.
_____
The current Sailing Subject of Lift, is a Micro model of Fluid Dynamics. Sailing motion in two fluids, Air and Water.
Conflating Fluid Statics forces like Buoyancy, with Dynamics or Motion through a fluid can be confusing.
______
I am not setting any rules, but just offering a nomenclature to help understand the conversation.
Jim...
 
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Mar 26, 2011
3,399
Corsair F-24 MK I Deale, MD
We're expecting a rigorous treatment of fluid dynamics in a forum post? Neat.

Forget suction-based lift for a moment. Think about conservation of momentum. The more air the sails turn, the more momentum must be transferred to the boat and the sea. Chew on that one for a while.
 
Jan 11, 2014
11,321
Sabre 362 113 Fair Haven, NY
The more air the sails turn, the more momentum must be transferred to the boat and the sea.
Thus in warm moist less dense air a boat can carry a larger sail for the same given windspeed. Or a boat sailing in high mountain lake reefs later.
 

jviss

.
Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
The most interesting thing about the theory of lift is that there is no general agreement on the mechanism, even today. For example:
https://www.thenational.ae/uae/science/the-secret-to-airplane-flight-no-one-really-knows-1.358230

NASA publishes an educational series that points out what it considers several
"incorrect theories of lift:"
https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html

I have pondered this topic for years, for both aircraft and sailboats, and I have come to the conclusion that there is not one, single mechanism, but several mechanisms that contribute to lift, including effects explained both by Newton and Bernoulli. They are happening simultaneously.
 
Feb 14, 2014
7,399
Hunter 430 Waveland, MS
momentum must be transferred to the boat and the sea. Chew
Chomp chomp...
You need to include AIR also. Loss of Momentum or Energy transferred to the Air is real. Thus we trim the sails.

I love the pictures of a crew racing with their boats. They are normally seen peering forward and not concerned with their bodies LOSING boat Momentum back to the AIR.

A good example of AIR momentum LOSS is bicycle Racing. The cyclist IS CONCERNED, thus the shape of his helmet, handle bar position and even the low momentum loss of his clothes fabric.

Drag is the common term for the LOSS of momentum.

LIFT is used to describe a force caused by Fluids flowing over an non linear shape. That force is caused by differences in Pressure on opposite sides of that shape.

Our MACRO Model of LIFT is simple and works well with Sailing. Complicated MIRCO LIFT models are normally insignificant and those momentum terms can be ignored.
Jim...
 
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jviss

.
Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
Less Dense?
Warm air is less dense, but add moisture? Density is Weight/Volume.
But the Viscous Drag of Air INCREASES as temperature increases.
Jim...
"At 18 atomic units, water vapor is lighter than diatomic Oxygen (32 units) and diatomic Nitrogen (28 units). Thus at a constant temperature, the more water vapor that displaces the other gases, the less dense that air will become."
http://www.theweatherprediction.com/habyhints/260/
 
Feb 14, 2014
7,399
Hunter 430 Waveland, MS
Density of a gaseous MIXTURE is...
Sum of the Weight percentages of the Gases /Total Volume they jointly Occupy.
Period.

Moist AIR has Greater Specific Volume than Dry Air.
Now back to LIFT.
Jim...

PS: Weather prediction?:confused:
 
Jan 11, 2014
11,321
Sabre 362 113 Fair Haven, NY
Density of a gaseous MIXTURE is...
Sum of the Weight percentages of the Gases /Total Volume they jointly Occupy.
Period.

Moist AIR has Greater Specific Volume than Dry Air.
Now back to LIFT.
Jim...

PS: Weather prediction?:confused:
Jim, you're a weather guy, you know that cumulus clouds form from warm moist air rising. If moist air wasn't lighter, less dense, than dry air, clouds wouldn't form. the water vapor would settle over the ground and we'd be perpetually in a state of fog. Or water would never evaporate and the air would be very dry.
 
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