Sailing 16 years ago 1,519,698 views
Northern California has a storied, 500-year history of sailing. But despite this rich heritage, scientists and boat designers continue to learn more each day about what makes a sail boat move. Contrary to what you might expect, the physics of sailing still present some mysteries to modern sailors.
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Visit me on FB: https://www.facebook.com/LeonardoPereznieto Visit: http://www.fineartebooks.com/ for free drawing...
The "The Physics of Sailing - KQED QUEST" video is part of the Sealing, tipps, boating category, which contains similar videos like this one.
10. comment for The Physics of Sailing - KQED QUEST
aka F.M.L.
20. comment for The Physics of Sailing - KQED QUEST
The wheels do not generate lift?
30. comment for The Physics of Sailing - KQED QUEST
Here, Explaining the physics behind the world of Sailing!
I should say - A Very nicely illustrated video piece - As compared to any of the boring lecturers speeches/ the books on college library shelf racks!
Reflecting on the new times.
Well I call it "An Era Of Real Thought Sharing"!!
Thanks for sharing.
Increase in velocity results in decrease in pressure and temperature.
50. comment for The Physics of Sailing - KQED QUEST
The only part i really got much form was the bit at @4:16 where he blew on the paper, and demonstrated how 'lift' works, same as aeroplane wings etc.
Other than that, i want to know how one sails INTO the wind(IIRC, they said it was not directly, so yeah. As close to the direction of the wind as possible). I want to see what kind of mechanical system they have to harness that energy which allows it to move into the wind..
http://1.bp.blogspot.com/-bkRxU2KQ2fM/VVESGlmwyUI/AAAAAAAAFCo/F7yqcCywU1I/s1600/Slide2%2Bcopy.jpg
Sometimes they were even broken up for their valuable lumber. This preferable to losing the ships to Teredo worms. This was inevitable as copper plating wasn't mounted on the bottom of the hulls to save money, speed construction, keep the cargo weight high and keep the the hull speed high. If someone reads a script that is pertaining to a subject that they don't know well, is it fair to ridicule the orator? It depends on how well they were paid!
LOL
Correctly analyzed, lift is caused by streamline curvature, which is not taken into account for by Bernoulli's equation, which only analyzes velocity and pressure gradients ALONG the streamline (from mouth to far end of paper), not PERPENDICULAR to it (from paper surface outward).
Well there goes my "theory" up in smoke.
Best thing if you're nervous is to try sailing a little dinghy with the simplest kind of sail, then you're directly holding the main sheet and can feel the power of the wind, you can let out the sail to reduce heel but soon get a rate for getting as much speed as possible and at the same time learn all the important basics of how the wind interacts with the sail.
I've sailed Norfolk Broads yachts and they're pretty stable, quite wide so don't lean much and the rig is optimised for that style of sailing.
Modern racing yachts are designed for high speed over comfort so lean, unless you sail a catamaran for stability.
All sailing boats use same principles to sail, Older ones would have used lots of ballast instead of deep keels.
Also those square sails are needed in such huge ships, you cant quite easily correct weight by shifting passengers in a boat of that size.
Square sails do in fact work fine for going to windward, they're just a little less efficient at it than a small sailing yacht with Bermuda rig.
'I am that I am'.
Existing out of necessity.
We always try to image a chicken/egg scenario because that's our experience of life. But what if rather than there originally being nothing, there was originally something - originally life rather than non-existence?
It had to be one of the two, and we're here! Which makes more sense?
Put nothing in a vacuum and leave nothing there for 100 quadragintillion**999 years - What will you find when you open it?
A universe??
If you do, something was put in there.
Now imagine existence in the abstract sense. Why would that need to come from anything? That's an oxymoron.
If there's existence; there was always existence.
As far as the modern world goes:
Airplanes: Quickest without equal. Can only land in aiports. Logistics heavy mode of transportation and therefore, most expensive.
Trains: Inexpensive and fast, but limited to where the rails go.
Cars: Relatively fast and can go anywhere the roads are(or in extreme cases, are not), but heavily affected by traffic and weather conditions and is more expensive than trains.
Ships: Most inexpensive for intercontinental transportation of lots of goods, can go anywhere the water is deep enough. Limited to the waters, slowest transportation on the list.
No clue what he is on.
100. comment for The Physics of Sailing - KQED QUEST
http://www.real-world-physics-problems.com/physics-of-sailing.html
The wing and the sail simply divert air in the opposite direction and then Newton's third law takes over.
Jon, you need to read more carefully, I didn't say I believe Bernoulli accounts for all the lift, I said I believe it's a combination of that and Newtons, meaning the air is also flowing down and being pushed down depending on the angle of the wing or sail and that also contributes. That is why an airplane can fly upside down and also have symmetrical wings (Newtons Law). Symmetrical wings would obviously just take advantage of the downward force of the air and have no advantage of the pressure differential of Bernoulli. But if you look it up, wings that are cambered have advantages such as lower stall speeds etc.
Bernoulli is not strong enough to produce the lift, besides, how do you account for the fact that some airplanes have symmetrical wings? Shouldn't the curved surfaces cancel each other out and produce no lift at all?
It's funny to see the "water tunnel" demonstration of High School Bernoulli, because it demonstrates that the simplified theory is nonsense! The simplified theory says that the two flows -- over the straight side and the curved side of the old-fashioned asymmetrical airfoil -- arrive at the trailing edge simultaneously, predicting the higher speed over the curved side. In real life -- and in the underwater demo in the video -- that doesn't happen. And in real life, symmetrical airfoils work just fine, thanks, and airplanes fly upside-down pretty well, too! Disappointing presentation of "science"!
Fortunately, Newton's laws of motion facilitate a simple AND accurate explanation of lift, including symmetrical foils and inverted flight. Why not find some physicists who can talk about THAT on camera?
And finally, the famous curved piece of paper has NO flow over its bottom side, so it only proves that there is a pressure drop with increased flow, not that we need asymmetrically curved wings to get airborne.
Shame on KQED for this! Fortunately, the Internet has a number of accurate explanations of BOTH how airplines REALLY fly AND how sailboats sail. But not here.
As such, the video is perfectly fine for an intro to the topic. It would help when criticizing these pop sci videos to have some basic understanding of the subject yourself.
This is simply not corrrect: "best scientists in the world never nail it down perfectly."
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Many amateurs debate it. The pros I have consulted with and read only differ on small details and sometimes on emphasis, but the major phenomena are similarly very well understood by the folks in the business.
... See my other lengthy post to you and Jon
The 61 lines I see is what I posted. There was a glitch and I re posted it, then deleted a short portion that was the error.
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I understand and I see you have fallen prey to the many amateur discussions.
No, the pressure difference on the wing or sail IS the lift and causes the heel. Think carefully about it. In air (or any fluid) the only thing you have at your disposal to create a force is the fluid itself and the only way to get a force is by pressure differences. This must be understood first. There can be no other cause for forces (except for more detail when explaining just how that pressure difference comes about).
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People (pros) who have actually MEASURED the pressures have proved this is lift. People who tried to CALCULATE the pressure difference using long-standing bad assumptions got the wrong answer. This proved their assumptions were in error, not the correct concept of lift.
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What Bernoulli revealed to us is that in a classical flow, say along a pipe (where no energy is either added or extracted), there is a relationship between velocity and pressure. This is easily understood if you realize that a higher pressure region will push (accelerate) fluid toward a lower pressure region. Fast air doesn't create the lower pressure; it is being pushed toward it by a higher pressure elsewhere.
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Bernoulli's EQUATION only applies in a single stream, called a streamline. He revealed that as a bit of air moves along A PATH, pressure differences ALONG THAT PATH affect its speed (velocity) IN THAT PATH as you would expect.
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The error comes when you extend that to the two DIFFERENT paths AND also assume that the air on both sides of the foil arrive at the trailing edge at the same time (the equal transit-time theory). Using those two incorrect assumptions gives the wrong result -- EVEN THOUGH there are videos on-line showing this method today! Sigh! Two wrong assumptions don't make a right. In addition, the wing is indeed doing work on the fluid and, therefore adding energy...bad assumption abound.
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IF you use the Bernoulli concepts correctly in CALCULATIONS you do get the correct result, but the math is complex and completely unnecessary for understanding the phenomena happening.
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For sail boats, this is a darn good reference, though it doesn't get into the nitty-gritty of how pressures are changed:
http://www.real-world-physics-problems.com/physics-of-sailing.html
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. If you want authoritative sources, try any one, or all of these for a good understanding. While each author will emphasize different things or use slightly different ways to describe some things, the basic story is the same:
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An example of someone who started by doing his homework:
Peter Eastwell - teacher
http://www.scienceeducationreview.com/open_access/eastwell-bernoulli.pdf
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Weltner in PDF - "Misinterpretations of Bernoulli's Law":
http://user.uni-frankfurt.de/~weltner/Misinterpretations%20of%20Bernoullis%20Law%202011%20internet.pdf
Weltner as a web page:
http://www-stud.rbi.informatik.uni-frankfurt.de/~plass/MIS/mis6.html
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Anderson & Eberhardt AAPT paper: The Newtonian Description of Lift of a Wing-Revised 2009:
http://home.comcast.net/~clipper-108/Lift_AAPT.pdf
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NASA Glenn Research Center. This is a series of pages you move through:
http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html
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Or, AllStar is another good source:
http://www.allstar.fiu.edu/aero/airflylvl3.htm
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If you prefer videos...
Video lecture explaining the Bernoulli Principle. If you understand Newton, you'll clearly understand Bernoiulli after this video.
Dr Holger Babinsky, Cambridge University Engineering Department
https://www.youtube.com/watch?v=XWdNEGr53Gw
His missing slides HERE (Click the Download Icon for the complete set of slides):
https://docs.google.com/file/d/0B0JABuFvb_G_MkpBZHJmRGo3UkU/edit?usp=sharing
This is the 2003 article he mentions in the video:
http://www3.eng.cam.ac.uk/outreach/Project-resources/Senior-glider/howwingswork.pdf
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Doug McLean Boeing Technical Fellow, retired; gets rather esoteric later in the video, but starts simply for the most part. (I've talked with him): Common Misconceptions in Aerodynamics.
https://www.youtube.com/watch?v=QKCK4lJLQHU
For the "Newton" part of lift, watch the result of the pressure difference, in the GREEN VIDEO:
... http://amasci.com/wing/lasrWing.gif Quite impressive!
For the setup used to get that green video, watch part this 2 of 5 from 10:20 to 12:59
https://www.youtube.com/watch?v=H-qyrqxuH4Y
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David Bentley Australian Air Force Cadets:
http://219sqn.aafc.org.au/Flight/Principles%20of%20Flight%20-%20web.pdf
Dave Bentley shows slowing of upper air & other misconseptions:
http://219sqn.aafc.org.au/Flight/Simple%20Aerodynamics-How%20planes%20fly.pdf
Dave Bentley Wings Don't Suck
http://219sqn.aafc.org.au/Flight/Wings%20don't%20suck-How%20planes%20really%20fly.pdf
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Cheers, ScienceAdvisorSteve
http://www.challengerillinois.org/
In that response, you attributed all the lift effect (and presumably the heeling effect, too) to the difference in pressure between the two sides of the sail. I've read that people who've measured those pressures found them significantly inadequate to explain the lift that's experienced -- e.g., the pressure difference between top and bottom of a plane's wing wouldn't suffice to keep the plane in the air. I haven't worked hard to reconcile every conflicting statement, and I've also taken "the Coanda effect" on faith.
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Now, I appologize. You went much further after your Newton summary than I intended. I wasn't looking for sailing instructions.
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I was holding back the way I asked about your Newton explanation. I understand lift well, but wanted to see how you framed it.
What you say is correct, but doesn't explain the pressure difference that causes the "lift" force in the first place, which, then causes the air to be "thrown" rearward. That explanation is sort of backwards --- or "If "A" always causes "B" and we can see "B" happening, then "A" must be happening even though we can't see or explain why it is happening". See what I mean?
It says that since air is thrust rearward, there must be an equal and opposite force forward that is accompanying it, but doesn't explain what causes what (or even talks about the forward force in reality). This, "what" is, of cource, the pressure difference that actually is the "lift" force that is transferred to the mast and movers the boat --- not the reward moving air as you know.
That version of the "Newton Explanation" is only half the complete story of "HOW".
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Your reference to Coanda is technically incorrect. Many mistakenly call the effect Coanda for normal airflow over the convex side of a sail/wing, but Coanda Effect is reserved for the narrowly defined effect seen by a high velocity jet, or sheet of air directed (forced) along one side of a curved surface (it is not defined [by Coanda himself] as any air moving past due to the ordinary motion of the surface through the air). The normal airflow around a moving sail or wing is not the Coanda Effect proper, although it obviously occurs for similar reasons. This is a common misconception, though I admit perhaps a bit of a nit to pick.
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BTW it's Newton's THIRD Law (equal and opposite) that you refer to.
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The web site posted in another comment
http://www.real-world-physics-problems.com/physics-of-sailing.html
is excelent at describing all the sailboat forces, though it glosses over the fine detail of the cause of the pressure differences resulting in lift. That's ok with me. The vast majority of discussions/disagreements about what causes lift is only among amateurs...
I also have my own set of authoritative on-line sources and wanted to see if you have any others.
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Cheers, ScienceAdvisorSteve
http://www.challengerillinois.org/
The simple Newtonian explanation of lift involves his 2nd Law of Motion: For every action there's an equal and opposite reaction. So, in order to push a sailboat forwards, the sails have to "throw" an equivalent amount of air backwards. Specifically (and way more complex than necessarily), the forward momentum (mv) imparted to the boat equals the net (resultant) momentum (mv) imparted to the air that's deflected "aft".
And an airplane wing has to deflect enough air DOWN to hold the airplane UP. And a centerboard or keel has to deflect enough water to leeward (downwind) to hold the boat on track, pushing to windward (upwind).
There is one semi-complex part of what a sail does, which invokes the Coanda effect. Basically, through that effect, a sail can not only deflect or "curve" the airflow that HITS the sail (on the INside of the sail's curvature), but it can also deflect or "curve" the airflow that MISSES the sail and curves around the OUTside of the curve.
When the sail is too in-line with the wind, we first lose the force from the inside of the curve as the sail transforms into a flag. That's luffing. When the sail is too perpendicular to the wind, we first lose the force from deflecting the wind along the OUTside of the curve, because the Coanda effect has limits, and the wind will not curve around a sail that's perpendicular to the wind. That's stalling, and it happens similarly with airplane wings, keels, centerboards, and rudders.
Both of those airflows leave the leach (TE) of the sail at a much more "aft" or "rearwards" angle than they come at the luff (LE) of the sail. The resultant of that change in direction is partly sideways (downwind) and partly forward, and the keel or centerboard resolves that force into an effective forward force, and a sideways force that's mostly converted to heel (tipping). The forward force is conceptually identical (to Newton) to the forward force we'd get from throwing sandbags overboard, over the back of the boat. Except instead of throwing sand aft, we're throwing air aft.
The ratio of sideways force to forward force is not constant, but changes with the angle of the boat to the wind -- the Point of Sail. When cutting across the wind (Beam Reach), the sails are quite far out, so the sideways force is modest compared to the forward force.
When we sail upwind, we do several things that create bad effects (more heel and slower speed), in return for being able to point higher, i.e., closer to the wind. It's a compromise, trying to maximize our Velocity Made Good to Windward. E.g., we have to use a flatter sail shape than the high-power shape we use on a reach, and we trim in tight, with the boom more-or-less on the centerline. None of that is fast, but it lets most boats sail at a decent speed around 45 degrees off the true wind, which we couldn't do without those adjustments. And when we sail that way, the sails are probably pushing us sideways around 10 times harder than they're pushing us forwards -- but it's worth it.
Also, which internet sources you recommend.
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Cheers
Current accelerates because of low pressure on one side of the airfoil compared to other side, not vice versa. Pressure is set up by viscous force in a boundary layer. There is no lifting force in superfluids.
+jolllyroger1 is correct. Now go find some real evidence to refute that, or even better spend a few hundred hours on both square rigger and a modern yacht and actually find out for yourself.
generally a modern sail boats main sail can be let out to approximately 45 degrees give or take a few....
a square rigger can generally turn it's sails 45 degrees give or take .... now back when the Nina pinta and Santa Maria came to America those had round tub like hulls and we're literally some of the worst ships available. ..... viking ships were long canoe like ships with square sails
in posting here a link to viking replica square sailers and if you have ender addled you will know that these are sailing up wind ... yes they are beating into the wind and are capable of what a fore aft rig is. ....
just imagine a Lug Sail..... it's square and it can point into the wind. .... any good sailor knows how to point with any rig.....
If you want verifiable examples of tall ships outrunning modern yachts, Pride of Baltimore I outran several racing yachts the first time she went out according to the crew that was onboard. It's very easy to believe given her hull, rig, and size if you're familiar with sailing vessels. The crew in the documentary Pride: Legacy of the Baltimore Clipper talk about that briefly. Another example is HMS Rose sailing into Boston shortly after her construction. That's mentioned in detail in an article written for a publication of the San Diego maritime museum written by her designer who was on board at the time. You''re welcome to go find it if you like, that's the verifiable stuff that comes to mind.
But it's quite easy to see how an authentic replica of a fast sailing tall ship like a Baltimore clipper can outrun a modern yacht in many conditions. Vessels like this were very sharp with massively powerful hulls and huge rigs. They're also over twice the size of your average modern sailing yacht. Add a decent breeze and you can outstrip an average yacht, even an average racing yacht, in a variety of conditions, especially when you have stronger breezes and rougher seas which favor bigger heavier boats. This stops applying, of course, with hardcore racing yachts like volvo 70s or anything that's on foils. This is coming from a student of naval architecture who races yachts inshore and offshore and an experienced tall ship sailor.
in order to cancel a force, and equal and opposite force must be applied. it does add another vector, which like you say is equal and opposite to any that arnt directly inline with its thin ends
Now let's take the left side turn it so the wing points straight down .... Now you will see the wing is the keel and the elevator is the rudder ..... Turn the rudder the keel changes angle making lift
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http://www.real-world-physics-problems.com/physics-of-sailing.html
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However, some people want to understand just how "a keel keeps the boat moving the way the boat is going. and keeps the boat up right", that is, the detailed physics.
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Hans has the correct idea.
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For those interested, much more detail, An EXCELLENT sail boat explanation site:
http://www.real-world-physics-problems.com/physics-of-sailing.html
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Cheers
That Web Page has an excellent explanation of both the sail and keel, the drag components, the sideways motion of the keel *and* pointing out that the keel and sail forces are offset and produce the unavoidable torque-moment causing the tilt.
I give it a A+ (;-) --
Cheers
Hopefully decidiousrex read it also.
http://www.real-world-physics-problems.com/physics-of-sailing.html
But it's not you misunderstanding, it's that their description of the keel is very misleading. A keel's purpose is less to generate force and more to ensure unidirectional movement. Think of it this way, go grab a credit card, driver's license, library card, etc, and fill up a sink with water. Slide the card through it so that the skinny part is "cutting" through the water. Moves pretty easy right? Now try to move it side-to-side. Doesn't work so well. It takes a much greater force to do it. That's the principle of a keel. It doesn't add another vector, it simply cancels out anything but the desired one so that the only motion is in the forward/backward direction.
Without a keel a sailboat would still go forward, only you will notice the boat will also shift and hop sideways, which is bad for navigation and the boat and also is incredibly dangerous.
... lets put it this way, modern sails would never give enough wind to a Man O War...
yea, thats probably it
was likely a race committee motor boat, a start line, and boats starting. when there's racing, boats easily come within a foot of one another.
That is an excellent point. It's all the more reason why good tech writers are important.
Engineers and most programmers DO understand how things work. So much so that it is nearly incomprehensible to them that there are people who do not.
Nothing at all wrong with being a scientist, engineer, or programmer. Some of my best friends are. Of course, I wouldn't want my sister to marry one ;)
Yes, I was a tech writer for 30 years when I wasn't teaching sailing.
Thanks
this kind of 'lift' theory of aerodynamics has (in the past decade) been quite disputed in the field of aerodynamics anyway, as many experiments with airfoil (wing) designs which should not create any lift according to this theory - do!
so this is not the whole story. it may play a part (maybe even a large part) but its not a complete explanation of whats going on.
Comment One: if the keel of the boat in the video fell off, and the sailors wanted to move forward in any direction other than downwind, the more they tried to head in away direction other than downwind, the more trouble they would have in making in real forward progress. and that is because of something the video makers did not even talk about, which is that one of the main purposes of the keel is to counter act the force of the wind pushing the boat over. when you are headed up wind, as the wind meets the sails, the wind tends to push to boat over on its side (heeling), and that prevents two real problems. Problem 1: the keel keeps the wind from successfully knocking the boat over. sailboats do not move well if at all when knocked over. If you lost the keel, the wind would knock the boat over if you tried to go upwind. The wind would not knock the boat over if the boat only wanted to go downwind; the wind would just drag the boat downwind. Problem 2: the sail is most effective when the sail is perpendicular to face of the water, and less effective as the sail moves to a parallel position relative to the water. The keel helps to keep the mast vertical and the sails perpendicular to the face of the water. If the sails where parallel to the face of the water, the wind would tend to pull the mast down into the water, effectively pushing the boat over. So, both of these problems are avoided with the boat in this video having a keel, and if the keel was lost, the boat would instantly face these two problems, if they tried to go anywhere but downwind.
Comment Two: if the keel on the boat in this video was lost, the sailors still have a tool they can use to address the two problems pointed out in comment one: they can (try to) use the rudder. So, let's say the wind is blowing from the north, to the south, and the sailors are headed west. they are on what's called a "port tack", headed northwest, the wind is coming over the port (left) side of the boat. Suddenly, the bolts that hold the keel to the bottom of the boat fail, and the keel finds its way to the bottom of the bay. In a manner of seconds, the boat would first lean over to the right a great deal. This would make the rudder useless, and the design of this particular boat (and most modern sloopes) would make the bow of the boat going directly into the wind almost immediately, which would stop all forward motion of the boat at the same time. The rudder continues to be useless. The sails would instantly start flapping like they were possessed. Then the wind would start to push the boat backwards, and not too long after that the boat would start to "fall off" probably to the right (because it was previously on a port tack). As the boat continued to turn right, at some point, the sails would fill with air, stop flapping and the boat would move some combination of forward and to the right. As soon as water starts to flow over the rudder again, the rudder now can be used to steer the boat to the left and counteract the force of the wind pushing the boat to the right. Not very effectively, mind you, but it will work somewhat. Instead of heading northwest like the boat was doing with the keel, the boat could probably head somewhere between west and southwest without the keel. Tricky business here, because if you try to head back to the north, the excessive force on the rudder would tend to break its linkage. The sailors would be wise to forgo "tacking" (turning the front of the boat through the eye of the wind) and stick to "gybes" (turning the back of the boat through the eye of the wind).
If you want to have some real fun seeing how this works, take out a windsurfer (which does not have a rudder) and leave the dagger board up, and see how much you really can control the direction of a board just by altering the sail position relative to the wind. Before long you'll be saying, "Keels? Rudders? I don't need no stinking Keels and Rudders!"
btw, my favorite boat to sail is my Prindle catamaran, that has no keel whatsoever, of any kind. on my boat, the rudders force the hulls to carve a track in the water in the direction I point them, which is most often at odds with the direction the sails want to push the boat. The only exception to this is when the boat is headed downwind; then the sails are dragging (not lifting) the boat and this whole vector thing becomes moot (I think). A concession here - I am pretty sure the hulls take on the role of producing these vector things that the keels produce, but that is physics beyond me.
Fair winds, and following seas!
In those days, the central timber laid down to which all of the ribs and other parts were attached was called the keel. It is from the term that the more modern wing-shaped keep got it's name.
The Dutch, having to sail very shallow waters, built boats with very broad and shallow hulls with a system of lee boards on each side that could be lowered and raised to fit conditions. As these boats often had to travel through canals, often by towing, this worked very well for them.
As a sailing instructor using lasers, I can verify what you say is true, especially with shallow hull boats like the laser.
Larger boats with more hull in the water, they can still be sailed although not as efficiently. Keep in mind that , in the age of sail, very few boats had deep keels or dagger boards. When they had 14 or 15 feet of hull in the water, they didn't need them. No, they did not point as well as modern boats, but they still were sailed around the world.
not saying its the rule rather than the exception, just interesting how that would relate to the theory in this video.
We see this all the time when we land at the beach in our Laser 4000. The moment the daggerboard is taken out to land the boat only has the momentum to carry it where the bow is pointing. all further force from the sails is in the direction of the force of the wind. We almost always end up crabbing in for the last few meters. Without a dagger board or keel you can't balance the lift provided by the sails with the drag required to cancel out the sideforce and the boat goes sideways.
Luv and Peace.
While Bernoullis can describe some of what is happening, the fundamental law behind lift is newtons 3rd. watch the flow as it comes off the wing, its at a different angle, meaning a force was imparted on the air by the wing, and thus vice versa
(im an aerospace engineering student and even people in the aero business get this wrong (including the most of the internet), and this isnt coming from me, this is coming from Dr Iain Dupere at Manchester University)
1) Bernoulli's equation is a description of the momentum equation under three very specific conditions: inviscid (all viscous forces are ignored), steady (i.e. time invariant) and incompressible (actually not too bad here although interestingly +Kronstadt Sailor also comments that fluids are all slightly compressible). In fact it is easily understood, if the only two forces acting on a fluid are pressure-area and gravity, then for the flow to accelerate it must either see a drop in pressure (i.e. it accelerates from high to low pressure) or a drop in height or both. Far from causing low pressure with high speed, in fact the low pressure creates the high speed.
2) Bernoulli's equation comes from consideration of a stream tube and so only applies along a streamline and not to more than one streamline.
Bernoulli then, definitely falls down in this application, on two counts (inviscid and more than one streamline (the flow either side of the sail is on two different streamlines).
Someone else refers to Potential flow and, in particular, the Kutta condition. This is a common approximation, in which the flow is assumed inviscid apart from the generation of vorticity. This is a fudge since Kelvin's law shows that vorticity cannot be created in inviscid, incompressible flow. To have the same constant on either side of the sail requires uniform horizontal flow which has no vorticity.
All this leads to the well documented result that, in the event that Bernoull's equation is correctly applied to a wing (or sail) the actual lift will be zero!!
The Kutta condition does give a crude estimate of the lift (and was used before the days of CFD) and does give some indication of why the viscous effects are important in this case, however, Bernoulli does not strictly apply (the flow violates one of the conditions) and certainly is a poor explanation without some reference to the Kutta condition. As pointed out, it is better to say that the flow leaving the sail follows the path of the sail due to a subtle but important viscous effect (this could also be described crudely Mathematically as Kutta's condition) and the turning of the flow gives rise to the lift force through Newton's second law (force causing a rate of change of momentum) and third law (equal and opposite reaction - flow is pushed in one direction by the sail causing a force on the sail in the opposite direction).
If +Kronstadt Sailor wishes to come along to the University of Manchester I will happily not only teach him this, but show him how this works.
PS Application of Multivariable calculus leads to zero lift! precisely because Bernoulli does not apply in the strict sense of the term to this case.
In fact, square riggers, while not as efficient to wind as a modern sailing boat, could sail at a reasonably close angle to the wind.
I know that they were building a galleon from authentic materials and techniques about 15 years ago in Rochefort on the French West coast. It was quite well under way when I saw it, and all the frames were in place, with the bottom planking of the hull completed, and planking was progressing up the sides. I would expect it to be finished by now, although I know they were having financial difficulties which may have slowed it down.
Oh - it is finished. I just found ...
http://discover-poitou-charentes.com/what-to-see/visit-the-hermione-at-rochefort/
I think most "ancient" ships built these days are for film props, such as the bounty.
http://en.wikipedia.org/wiki/Bounty_%281960_ship%29
As a side question, do you know of any good places to learn wooden shipbuilding (shipwright skills)? I am very interested in replicas of the classic sailing ships and wooden ships/yachts/boats based off of the old sailing vessels.
Moving on to a discussion I had recently about sailing yachts to sail trading vessels. Sailing yachts are efficient at using the wind available to a point but then you are comparing a sport car to a house mover the power to weight ratio is showing us that the sports car is as fast as possible and the house mover is moving as much as possible. When the house mover has more power so is less efficient. But again compare the date, the difference of technology, the difference of purpose of sailing vessels. As I have already stated, there are vessels, clippers being the favorite that are the high speed, high valve product carrier and sailing record holders for speed.
Modern naval architecture is a compromise of speed, stability, cost and how well it completes its purpose as well as exploiting every chance to make a vessel as efficient as possible through new technology. Today ships are slowing down and are carrying unthinkable masses of cargo.
To be honest the modern cargo ship and yacht should be able to look at a flying P-Liner and see that it is the pinnacle of human exploitation of modern physics and design. To move multi thousand tonne vessels at comparable speed to modern monohulls. Which is even more impressive when you think of the much greater winds they handled out at sea, a later period square rigged vessel will trump the top speed in the monohull class.
And if there is to be a mass scale of sailing cargo vessels again they well bear more resemblance to a Fly P than a yacht or cargo ship of today.
Not that any of this reply is necessary to this particular comment on this video as well as any offensive I might of given in expressing myself so righteously but I am proud of my career and its origins which I have as my hobby.
My understanding is that sailboats and ships of the Age of Sail were able to sail into the wind, but they were not efficient at it in the same way as modern sailboats designed with computers and modern physics can be.
Well, I've contemplated my reply, and my assertion is still the same. The exact wording in the video clip that I objected to was this ; "Square riggers [... did various things ...]. But these ships of old had their limitations. They were slow, and they only went in one basic direction - with the wind"
These ships went to other countries, and came back again, not always by taking a different route home to make the best of the wind patterns, but frequently in both directions along the same route, such as from Europe around the Cape of Good Hope to Australia, then back the same way, so the statement that "they only went one way" is counter-intuitive. More than that, however, it's wrong. See the explanation by Alex Newman, who has first hand experience and knowledge of these boats, below.
BTW, you might care to reconsider your own assertion that I make you wish you had cancer. Be careful what you wish for. As one who does, I can tell you that it's no matter for glib comment.
Exactly right and well explained, too. :)
The second way to set squares is to achieve the airfoil like a modern sloop does but instead of haveing the mast as the leading edge of the sail they are projected out by the yards and then the luff (leading edge) is twerked (actual term for over tensioning a line) then the trailing edge is slacked a touch, this makes your wing shape. The major problem to going high upwind is the fact of the shrouds prevent the yards from achieving the same angle you can apply to a boom when closed hauled.
On the 60' Brigintine I sail in we have got 8 knots at 35º off the wind in under 20 knots of wind
And 9 knots with 12 knot of wind while running, and for a cargo style sailing ship of it's size and at about 30 ton with less than 1000 sq. ft of sail.
Modern sailing is just using a simpler rig and boats designed for sailing not trading
When teaching sailing, the #1 rule I teach is "Stay on the boat." That applies to everyone. You cannot sail the boat, help others, or do much if you are not on the boat.
A piece if the book I wrote for students (So You Wanna Be A Sailor?) is at:
http://www.boatsafe.com/nauticalknowhow/overboar.htm
where i learnt to sail (Mediterranean) that was how they did it. there must be a life preserver for each person, but they dont need to wear that the whole time unless your in (or it becomes) bad or dangerous conditions.