Physics of Windsurfing

[@guymoseley5848]

Good as far as it goes. I don't normally comment on these things but I feel some of the inaccuracies are so glaring I have to say something. Let’s start with the positives. The graphics are great and so is your systematic explanation (although flawed in some places). For a beginner this introduces some difficult concepts in an understandable way but starts to unravel when you talk about lift and steering. If you put the correct numbers in the wing simulator accounting for apparent wind angle and the correct sheeting angle for the speed of the apparent wind then you get a very different result. This is how you explain the fact that at full speed on a broad reach, a speed sailor is almost fully sheeted in. In your model this could never happen because you could never generate an apparent wind sufficiently strong at such an angel. Then we get on to steering which deserves its own video. Pressure on your feet is irrelevant to steering with the sail from a physics point of view. Your feet are only transferring the force from the sail. This is then complicated by foot steering which is where you use the board moving over the water to steer like you would a surf board. Both of these are effected by the speed of the board and the relationship between the Center of Effort (CE) of the sail and the Center of Lateral Resistance (CLR) of the board. As a beginner foot steering is all but irrelevant. As an intermediate/ advanced sailor, sail and foot steering is used constantly in combination. At high speeds, the setup of the board and sail also become more important as you can tune the relationship between the CE and CLR to give a setup for different things. Hope this helps some one.

[@ScottMillerCoaching]

Glad to see someone bringing up these points, Guy Moseley. I agree that the graphics in the video are nicely done. But I was also confused by a few parts of the video, like the wing simulator and the comments about lift overall. - In the NASA simulation, why use negative numbers for Angle-deg and Camber-%c? - The fact that this results in "negative lift" seems like it's being presented in a misleading way, as if "negative lift" is supposed to mean "not moving the board upwind." But that's not what the simulation is showing. - Why not use positive numbers in the simulation? We're looking down at the sail and the water from above in the simulation, right? If I'm not mistaken, positive numbers would simply represent a board and sail on a port tack, instead of the starboard tack shown in the video. And the lift value would be positive in that case. Not that it matters, really, since in this simulator, positive and negative lift values simply represent different directions of travel perpendicular to the apparent wind, not necessarily upwind or downwind. - The "CORRECTION" that the author added to the video description only adds to my confusion. If the orange arrow is meant to represent the force of lift, why would it ever point "downwind" and not perpendicular to the airflow? - And I also wondered why the explanation of steering ignored the way that tilting the sail moves its Center of Effort in relation to the boards Center of Lateral Resistance. Again, the "Additional NOTE" added to the description only adds to my confusion. If the video and this "Additional NOTE" are correct, why is it standard practice to move the back foot up closer to the mast when performing a basic, non-planing tack?

[@Nico_GER7]

This is fabulous!

[@3denlightenment]

Thanks Nico!

[@koerper51]

Nico. Does you have informed you about physics of windsurfing till now. Or is it only a feeling thing for you? Is it important to know abaout ? Maybe you can do Video about that. I feel Windsurfing but I'm also interested in that physics.

[@RIWmag]

​@@3denlightenment and Nico : I graduated in physics and did my thesis on aerodynamics applied to windsurfing sails. I appreciate the initiative but personally I consider this video technically uneducating: it is wrong to explain the force of the wind on the sail based on static physics. The sail is a wing and must be explained with the laws of aerodynamics, and the same goes for the fin. Where does the info about the 45 degrees come from? Why does not it take into consideration the sail profile? Now you a have a large public with no notions of physics who believe they have understood information that is in fact so inaccurate that it is wrong.

[@3denlightenment]

@@RIWmag Thanks RIWmag for the review. I prepared the video to simplify the basic understanding of the physics occuring during the sailing process. I focused on the Newtonian physics as that is the simplest to understand and predominates the forces involved. The Bernoulli forces acting on the sail are partially captured by the formulas presented but as you know, it is not a comprehensive explanation but was not meant to be. It is not wrong. I use the resulting angle of 45 degrees in my explaination since that is the approximate angle resulting from Newtonian forces, and I always say "about 45 degrees". As you may know, the forces acting on the wing are somewhat controversial (www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/) but I am interested in making a more comprehensive explanation of the physics as a part II and would be very interested in reading your thesis if it was publically available. Please let me know if there was any way to review it. Thanks.

[@RIWmag]

@@3denlightenment thank you for your reply. My thesis was written in 1998 in Italian and on floppy disks. Many years later I discovered that they were not readable any more and therefore now only the version printed on paper is available. Anyway I did not invent anything about the physics. My thesis took the existing physics knowledge applied on sailing and added a comparison of experiments done on the sea with a real windsurf sail and done in the wind tunnel with scientific measures. Sorry but I do not agree at all with your approach starting from Newtonian forces and avoiding the Bernoulli forces. You first should use Aerodynamics to determine the existing forces and then you could split those forces into vectors, not the other way round. You do not need to review my thesis, you just need to take one book explaining the physics of the sailing (there are many ones available) and start to create your videos from there. I really appreciate your video skills and I really appreciate your intention, but your simplifications on this topic are too crude and lead to some explanation that conveys scientifically wrong content.

[@gregdyer1380]

Great presentation and detail. Just one thing... if the board is displacing water, what you say about steering is true. When lift from the fin and speed of the board across the surface of the water causes it to plane (not displace) it steers more like a skateboard, responding to rail to rail pressure. In addition the rig/sail is tilted towards the back of the board as the wetted area of the board decreases due to upward lift from the fin, so the centre of effort in the rig moves back as the board accelerates. This is why the windsurfer starts by standing just behind the mast, but later moves to the back of the board into the foot straps.

[@3denlightenment]

Yes, you are very right. I should have emphasized this is the approach in low wind conditions.

[@wildbill805]

Nice attempt at a basic explanation, but it misses Bernoulli‘s principle entirely. The odd fact is: all sailing craft are sucked long more than they are pushed along. The suction on the Lee side of the sail is slightly greater than the pressure on the Windward side. en.wikipedia.org/wiki/Forces_on_sails Some other useful concepts & terms are: center of effort, center of lateral resistance. These are the imaginary points around which the vessel will turn. If the center of effort is moved behind the center of lateral resistance, the vessel will turn upwind; vice versa if the CE is moved ahead of the CLR. Another point, when the board is planing, dynamics change massively and turning relates mostly to angle of the board versus the water. The sail can be held in the same place but foot pressure will cause most of the turning.

[@3denlightenment]

Thanks wildbill805. Yes, it was meant to be a basic explanation. As you know it can be quite complicated and confusing. With the NASA simulator, I was trying to show that the force on the sail is still in a leeward direction and doesn't magically pull the board upwind. Maybe it was a bit confusing. Perhaps another video of clarification or perhaps even a non-layman's version is required. Thanks for the link.

[@richardturnnidge]

@@3denlightenment This is a critical point about the profile of a sail, and even taught at beginner level. Otherwise, all sails would be flat, not curved (3 dimensionally).

[@3denlightenment]

​ @Richard Turnnidge Keep in mind there are cam-free sails which do not have a curvature yet are still able to generate significant force due to the "angle of attack" of the sail against the apparent wind. Under these conditions the Newtonian forces against the sail are significant. Also, keep in mind there remains some disagreement as to the primary forces acting to generate lift www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/

[@cyrilcivet7683]

@@richardturnnidge Having sailed a cambered sail whose cams refused to rotate, sailed just fine back to the beach with the sail inverted. The curvature is helpful but not necessary. Likewise, airplanes can fly inverted with the curvature in the "wrong" direction.

[@richardturnnidge]

@@cyrilcivet7683 it's hard work though!

[@bertbosman2710]

Hi Marc/Mark, a planing board is hardly steered with the sail, but by canting the board.

[@3denlightenment]

Thanks. I agree. I should have made it more clear that I wasn't talking about a planning board.

[@guycribb5874]

Fantastic presentation, thank you. The steering section is the best explanation I've seen for beginners, indeed its what I've been teaching for years rather than the 'lateral resistance' explanation I've heard some people give. And the physics stuff is fantastic. What I'd like to see next is 'how foils work' and I'd happily collaborate with you on this! Thanks, Guy

[@3denlightenment]

Hi Guy! I've loved your videos as well. Thanks, I'm glad you liked it. I don't see why we couldn't do one on foiling. Do you have aspects of foiling you'd like to cover, or simply how it generates lift?

[@nathanvee6135]

Guy! You been MIA on the tubes. Love your stuff. SO HELPFUL. Did you ever try Paul Mindnich's designs (BenchMark Board Design and Tri-Hull Dynamics)? I recently met him in HoodRiver and he was spitting some interesting factoids about his design. I've also been pondering why the board shapes are not modeling some of the ski boats or modern sailing hulls. Thoughts?

[@antc5010]

Guy, the so called lateral resistance stuff is physics. What is omitted in this explanation is where the centre of lateral resistance (determined from the board) lies in relation to the centre of effort (determined from the sail). If COE is forward of the COR, given nothing else changes, the board bears away. Moving COE behind the COR the board rounds up. Maybe a bit much for beginners, but it still lies at the heart of sailing physics.

[@speedquestwindsurfing]

Loving the graphics. Really well done. Holy math!

[@3denlightenment]

Thanks man! There are more coming 😊

[@speedquestwindsurfing]

@@3denlightenment I'll watch them.

[@ripwind]

This is a great video. The physicist in me is loving it. Would really love to see a similar video on the physics of a foil. Know the magic is in the reduction of drag via surface area but would greatly enjoy a detailed explanation.. Thanks!!

[@3denlightenment]

Thanks! I'll think about a foil video. It shouldn't be too difficult

[@ripwind]

@@3denlightenment really enjoyed the part about the density of air/water. Today as the clouds blew by I thought about this.. ☁️

[@3denlightenment]

@@ripwind Awesome! Glad to hear 🙂

[@effrosynimoraiti7246]

Nice job! While I was at high school my uncle (physicists) was explaining to me sailings physics on a notebook. It’s nice to see a simulator!

[@jantheman334]

Looks like a lot of effort to design it in 3D. Thanks for explaining with engineers eyes😉🙌 great stuff

[@3denlightenment]

You're welcome. Glad you like it. :-)

[@mikethezipper]

This is an incredible video. So much work went into this. Great job 👏🏻 incredible illustrations. Love it

[@3denlightenment]

Glad you liked it!

[@richie694]

Excellent ! Finally a video that explains this simply and clearly from first principles !

[@3denlightenment]

Thanks. Glad you liked it ☺️

[@Tonijo]

It's good to visualize these mechanics for an even better and faster understanding of windsurfing.

[@hi-wind9335]

I’m a windsurfer . When I go down wind by high wind with waive or slalom boards, I do just load backward foot to change direction. I do not push sail to forward and not load my front foot like this video. The sequence is for light wind by long board having daggerboard.

[@3denlightenment]

I would agree with you. I should have emphasized that was for light winds and especially for a board with a daggerboard.

[@Cookiesports]

Great graphics, really cool to see the analogies we use as instructors shown this way; que ball, and apparent wind for example. Can’t imagine it’s as quick to make somethings like this as it is for me when I jump on a board with a few cameras and a mic. 😉

[@3denlightenment]

Thanks Cookie!. I love your videos too.

[@MontanaPreston]

Well done! Thanks so much for taking the time to make this!

[@whenyoudiporeosinmilktheyb8329]

I had no idea that steering works by feet preasure. This is really helpful thank you so much!

[@amihaynevo6486]

I doubt. It's the movement of the sail compared to the board that creates a rotational torque. will also happen if your feet dont move.

[@pmoleri]

Only partially. Most of the steering (while not planning) comes from the relative position of the sail compared to the board. That's why moving the sail forward pushes the nose downwind, and backward pushes the back downwind and the board upwind.

[@3denlightenment]

It's not about foot movement, it's about foot pressure. Since there is no rudder, it is the change in foot pressure that turns the board. If you changed the angle of the sail, the pressure through the mast is always at the same point in the board. There is also no rotational torque through the universal joint since it is free to rotate in all directions and rotational energy is not transferred to the board. Only through the resistance transferred through your feet results in a change in pressure resulting in change in direction.

[@pmoleri]

@@3denlightenment there's torque going on in relation to the center of lateral resistance. The angle of the sail changes how much the sail pushes through you and your feet and how much it pushes through the mast foot. When tilt it forward is mostly pushing through the mast foot. I think the video explanation is oversimplified, giving the impression that sail angle only affects feet pressure distribution and only feet pressure affects steering. Both are partially true, but each of them can be used independently. I can steer changing foot pressure while keeping the sail in the same angle (changing my body angle). Or I can sail with only one foot at a fixed spot and still steer the board with the sail, provided that I'm very good at keeping the balance. Ultimately, I think that sail angle is the one most effective at steering while proper feet pressure also contributes to the process.

[@kaushikmerchant5971]

@@3denlightenment Thank you for clarifying. I am a complete beginner so totally loved your video, the tone of your voice and all the comments and clarifications in this section. A follow up video about the physics of turning, foot pressure and sail angle would be great!

[@immer_here]

to 1.: The forward force is actually greatest at a much smaller angle. At 45° the sail would definitly already stall. Otherwise also airplanes would use a 45° angle for their wings, isn't it? to 5.: It's not about the feet where you steer to, it's more about whether the pressure point of the sail is behind or in front of the fin. If it's behind the fin, you luff.

[@3denlightenment]

Thanks for your comments/questions. 1. From a conservation of energy perspective, the sail at 45 degrees would provide the greatest forward force, but as you know, people don't sail with their sail at 45 degrees. This is primarily due to the apparent wind and the forward drag on the sail when it is at 45 degrees. Airplanes don't have their wings at 45 degrees because they do not just want to go straight up. The wing on a plane is almost flat, and will angle slightly upward on take-off when it is necessary to get off the ground quickly. The width of the wing also provides resistance to falling down and at the same time providing lift from the Bernoulli effect. 5. The wind provides the pressure which is translated to the board through to the three points of contact with the board i.e. the sailor's two feet and the mast base. The pressure on the mast base is constant for any given angle of the sail, so the only variable is the pressure on each of the feet. If your back foot is behind the fin, that is perfectly fine, but it is also possible to steer the board if both your feet are forward of the fin. Additional pressure on the back foot will force the fin through the water in a downwind direction.

[@Tomseeable]

Acknowledge the attempt to simplify however, in my opinion, it is far easier (queue Bernoulli’s principle) to just explain how an aircraft wing works, then explain that any form of sail, be it windsurf, dinghy, yacht etc is simply an aircraft wing in a vertical rather than a horizontal position. The sail produces power ‘lift’ just like an aircraft wing, and is trimmed (sheeted in) as the speed of the air flowing over the sail increases, just like an aircraft wing. Like an aircraft wing a sail produces power (lift) due to its profile encouraging a build-up of high air pressure on the inside (windward side) of the sail - the underside of an aircraft wing - and a lower/reduced air pressure on the outside (leeward side) of the sail - or upper surface of wing. The higher pressure attempts to move to the low but the sail is in the way! air pushes against sail and power (lift) is produced. When instructing beginners I find 9/10 times they just ‘get it’ if you take a little time to explain the theory correctly from the get-go. It’s cool for most having windsurfing likened to ‘flying’ on the water!...

[@3denlightenment]

Thanks Tom. I agree that a sail is very much like a wing and that the wing or sail provides lift in the classic physics sense. In the simulator section, I was trying to clarify the fact that people hear the word "lift" and they think that the board is pulled upwind. In fact, there are no forces on the sail that pull lit upwind. All forces as shown by the simulator will push or pull it either downwind or forward. There is definitely differences in pressure on either side of the sail, but this may be due to Newtonian forces and/or Bernoullian forces. In this video, I chose to emphasize the Newtonian approach. See these two brief papers on the different schools of thought. Higgins 2020: www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/ Babinski 2013: www3.eng.cam.ac.uk/outreach/Project-resources/Wind-turbine/howwingswork.pdf

[@henrykwdk]

@@3denlightenment But you can't just stick to the Newtonian forces and ignore the Bernoulli forces! It simply makes for a wrong model, as they are both present. So, it's not and/or, it's AND unless you go straight downwind. Our models reflect reality by omitting certain factors that are deemed inessential. If you omit an essential factor, like, here, the Bernoulli pull on the sail, you do it at your own peril, as your model will simply be at fault. Having just read the Scientific American article, I see the same confusion permeating most of it. I just fail to understand why it has to be a conflict of two beliefs rather than an acknowledgment of the two forces working in concert. Sailors have known it for generations - why not scientists?

[@henrykwdk]

BTW, the Bernoulli lift accounts not for an insignificant difference, as you state, but for 50-60% of the total driving force at certain courses w. respect to the wind. Dismissing that makes for a major flaw.

[@Lozzie74]

The interplay between speed and pointing ability is critical, explaining why windsurfers make poor ground upwind. Might be worth explaining the optimum speed angle, which is quite far off the wind.

[@3denlightenment]

Yes, I would agree. Might be worth another quick video. Thanks, Lozzie74!

[@3denlightenment]

@@OKuusava Quite true.

[@johnfraser4591]

The physics of windsurfing, what a daunting task! I noticed that in the NASA simulator, you implied that the lift force is downwind, but you drew the arrow across the wind. The sailor in that diagram was positioned to be sailing high upwind. As you showed earlier in the video, if the lift force was perpendicular to the sail then there would be a component taking the board cross/down wind and one going close to the wind. So the windsurfer is pulled upwind while the fin resists the cross/down wind force. I also found the free body diagram of the fin a bit confusing because it was broken down into components and not just perpendicular. The sail stalling also seems to be what limits your angle upwind not really the forward resistance of the fin. I did learn, though, that the the angle we can sail upwind changes with apparent wind. I had not thought of that!

[@3denlightenment]

Thanks John, I get what you are saying about the arrow pointing across the wind and not exactly downwind. Thanks for that. And, yes, even though the lift was negative, it is still the driving force taking the sailor upwind. I was trying to clarify the "lift" in terms of a pulling force...but I may have unintentionally added another level of confusion to it.

[@ondrejvojta3882]

It is well explained but very simplified as the sail isn't really being pushed by the wind but rather pulled by the low pressure created on the downwind side of the sail.

[@3denlightenment]

That is a common misconception which I will try to clarify in an upcoming video.

[@maxcastiglia]

That has been proven to be only a partial explanation, and incomplete in itself. Some people call it a fallacy, this physics professor at the end of this excellent video corrects what he says at the beginning and calls them "partial truths", meaning that there is not a single simple and easy explanation for lift. There is one real fallacy in what is probably the most common old explanation for lift to be found in flying manuals: the air particles separating at the leading edge (more precisely at the stagnation point) DO NOT meet again at the trailing edge, that concept of particles meeting at the TE is totally false indeed and demonstrated by wind tunnel measurements (there is mention of it in the video below). I suggest you take a look at the video, very interesting; there are lots more videos to be found on the web that debate that very common and surpassed theory of lift.. kzbin.info/www/bejne/hneVY3-DbaaVf6s

[@robbiecouch9897]

This was really helpful! I didn't really understand what apparent wind was before this, or that it could change your no-sail zone. Thank you!

[@chrisfarmer5632]

Amazing, I struggled to figure this out on a beach, trust a physicist to explain it simply 😂 thank you for the video.

[@3denlightenment]

You're welcome. Glad to be of help ☺️

[@StenellaFr]

amazing animation. it makes everything clear!

[@ПавликГерман]

We need more videos like this!

[@windaddict]

Great video! I have always had my doubts about "lift" in windsurfing. This cleared it up! Thanks a lot for putting in the work to make this!

[@3denlightenment]

Very welcome!

[@karesala8885]

Awsome explanation, congratulations!

[@petersoulcarve9146]

That is a fantastic start of a hopefully long series as the more you go into the important details the more you will have to explain. Could you do the next one about the physics of windfoil?

[@3denlightenment]

Thanks. I've been thinking about it.

[@olissvans758]

Please do one on wakeboarding! Just got into the sport and was wondering how it worked

[@3denlightenment]

Thanks 👍. I'll add that idea to the list

[@robtangent4664]

"The curve profile of the sail does not lift the sail upwind". For a given sail, when wind becomes lighter we would "bag" the sail to help gain power. I assume bagging the sail would create a curvier profile. Does it mean a baggy sail reduces the upwind efficiency?

[@3denlightenment]

When the sail is "bagged" this would provide more of a curve and therefore would increase the drag. Depending on the angle of direction relative to the apparent wind, the increased curvature may "catch" more direct wind force, but will vary with direction of travel. When the wind blows over the curved sail it will create a slight low pressure area from the coanda effect on the leaward side of the sail. This will result in an additional but slight pull forward and downwind (not spoken about in the video). The angle of the fin (daggerboard) in the water will translate all of the forward forces upwind. So, all of that to say that the increased curvature can provide an additional forward force but, yes, will also create more drag. The increased bagginess can also provide a bit more of a delay in responsiveness which may be desired if the winds are gusty.

[@boatsandoutboards9070]

Great video and great& simple explanation without loosing the audience.

[@henq]

Explaining turning upwind and downwind with the pressure of the back/front foot leaves out the ESSENTIAL reason for the change in direction: the center of the sail's forces in relation to the lateral force the fin and the rails of the board provide. I can turn upwind or downwind with zero change in feet pressure, even standing on 1 leg (litteraly) , and than tilting the sails slightly front or aft does turn the board upwind or downwind. Windsurfers DO use foot pressure in stearing, when the board is plaining: back foot pressure is used on the inside of the board (lee side rails) to turn the board towards broad reach, to initiate a gybe in full speed. Front foot pressure is used on the wind side of the board to turn the board windward. You see, the rails is shaped in such a way that it helps the board to turn when not flat.

[@3denlightenment]

Thanks for the input. I agree that the center of "effort" on the sail in relation to the center of lateral resistance provided by the fin are essential in describing the pressures driving the steering of the board and I should have discussed this. The steering section of the video was meant to focus on that experienced by beginners in low wind and provides some practical advice to accelerate a turn. I also agree that under higher speeds, the rail provides a significant contribution to a carving turn through the water. When it comes to pressure on the feet, this remains important and I'm not surprised that you can turn the board even on one foot. That is because the pressure shifts between your one foot and the mast base due to the change in the center of effort when the sail is shifted forward and aft. So, although not discussed in the video, the mast base also provides pressure on the board. Perhaps a video to clarify steering would be beneficial. Thanks for your input.

[@BriceSoniano]

Mind blowing! thank you so much for your very clear explanation!!!

[@3denlightenment]

You're most welcome!

[@christianamstutz9308]

Just great. Thank you very much for the hight quality contribution

[@3denlightenment]

You're very welcome!

[@marchynes8450]

Thanks, as I can sail fairly well, and read the wind, I could never explain it

[@michaelp8856]

thanks so much for the time you put into this - very helpful

[@islamabouelata6575]

How are we then sailing upwind if the profile of the sail is not providing any lift?

[@3denlightenment]

The force of the wind against the sail provides forward motion, and the angle of the fin in the water, allows the forward motion to be translated upwind. It is a bit confusing because the term 'lift' is also used in aerodynamic wing theory whereby the wind actually lifts the wing upwards due to a combination of Bernoulli's principles and Newtons third law (didn't discuss this directly in the video). However, when sailing, there is actually a 'lift' force in the physics sense, but it is in the negative direction i.e. downwind. So, I was saying that the wind doesn't lift the sail upwind in the common sense of the word 'lift' i.e. in a upward direction, but from a physics perspective, there is still 'lift' (negative) from a wing theory perspective. I hope I haven't confused things more. Let me know, and I'll try to answer your question more clearly.

[@henrykwdk]

@@3denlightenment Sorry, but you have. More on this later.

[@uehavshie_podcast]

Very cool, scientific and old-school-style video. 8-) Thanks!

[@3denlightenment]

Glad you liked it!

[@Aijse]

Very nice! Would be great to extend it with the forces that get your board planing. How does the luff side of your board get lifted? How is this affected by the fin and position of the mast base. Thanks!

[@3denlightenment]

Thanks and thanks for the suggestions. Those would be good for a Part II.

[@bazbell7500]

Very interesting and great edited! What happens when you tighten the battens and as a result change the angle of the wing of the sail? Does it affect the angle of the particle "hitting" the sail and change the angle you can surf upwind? Are you sure that the nasa simulator is a good reference? Does it consider the fake wind and the movment of the object (moving sail vs still wing of airplane)? And can you make another video about foil windsurfing?

[@3denlightenment]

Thanks very much and interesting questions. I would think that if you tighten the battons you are enhancing the stiffness of the sail and therefore will enhance the responsiveness of your sailing experience. The change in angle of the sail as a result of tightening the battons will have a minimal effect relative to the general angle of the sail. The maximum upwind angle should also be minimally affected as adjustments can be made to the general angle of the sail to compensate. A more dramatic difference will be noticed with an increase in fin/dagger board size. I'm sure the NASA simulator is fine, but it doesn't distinguish the ambient wind from the apparent wind, but would primarily consider the apparent wind since when it comes to aircraft, the speed of the plane, and therefore apparent wind, will be relatively high compared to the ambient wind. The discussion of lift around the NASA simulator has presented a point of confusion since the term "lift" also has specific meaning in aerodynamic theory, a point which I will clarify in another upcoming video. Thanks for the suggestion regarding the foil windsurfing. I will seriously consider that! Thanks again.

[@henrykwdk]

Baz, it's not the battens that determine the angle (of attack? I'm not sure what you mean); it's you, the sailor! The battens may influence the profile (depth, shape) of the sail, but have no influence on the angle of attack, as it's measured between the direction of the apparent wind (not fake! For us, it's so real like you wouldn't believe, and the particles hitting the sail are exactly that on a macro scale) and the plane from mast to leach. Tightening the battens may influence how close you can surf upwind, though, especially in stronger wind. If a profile is too deep, the wind will make a dent in it on the leeward side, get the airflow out of shape and so greatly diminish the Bernoulli pull. And I understand your doubt about the NASA simulator as a reference. It's actually as good as it gets, provided we don't misinterpret it like the video does.

[@henrykwdk]

@@3denlightenment The NASA simulator does not distinguish between the ambient wind and the apparent wind because it's already been calculated into the apparent wind; your words, actually, although you're not pushing the point through sufficiently enough in the video; more on that later. And it's not the simulator that presents a point of confusion; it's your interpretation of the diagram. More on this later, too. And if 'lift' sounds confusing (as it may), why not call it 'drive' or 'thrust' when it comes to windsurfing?

[@directsound]

Nice work!

[@whatzup1993m]

Very nice explanation and simulation! Don‘t you think there can be some lifting forces threw pulling the sail down so that it not stands totally horizontal, like when you are going really fast? Can you mabey explain the reason of spin outs? Thank you!

[@3denlightenment]

Yes, definitely. Maybe a video on spinouts would be worth it. Thanks.

[@bradkubota6968]

Causes of spinouts are many and complex. It's at the limits of hydrodynamic lift. Crap is going on with a fin at high windsurfing speeds that doesn't happen in air until .95 mach or AT ALL.

[@paulosilva-dm1qb]

The part wich says that the sahpae of the sail won´provide lift is not clear enough it din´t consider the apparent wind..Other wise we wouldn´t need a sail with a shape. Also it din´t took into account that the leech of the sail contributes to it´s performance. So why would one need a loose or close one?...I don´t doubt the research but i think it should go deeper..

[@3denlightenment]

That is the key question. Do we need a sail with a shape? The physics analysis would say that it is not needed. Many sails have no camber (curve shape) and they sail perfectly well. If the leech is loose, then the sail will absorb the force of the wind and there will be less overall power generated through the sail. A future video could go into this. Thanks.

[@johnfraser4591]

Hello Paulo, this article from North Sails discusses sail twist. www.northsails.com/sailing/en/2019/12/how-sails-work-north-sails-blue-paper#:~:text=Since%20the%20purpose%20of%20the,possible%20pressure%20difference%20across%20it.

[@naturepro5643]

Great tutorial, good basic for understanding, how forces on sail and fin really work. I imagine putting the similar wing like airplane on top of the mast just for lift up or support fin-foil, we could eventually fly...what do you think?

[@3denlightenment]

Thanks nature pro... Interesting idea! Well, if the wings were large enough it would likely work in theory!

[@filipkowalski2183]

very educating! thanks a lot!

[@yus.c.2231]

If the upwind force is not made by the profile shape, then where does the force come from? I understood there is resistance from the water to fin. But that to not a force to push the board upward. If the profile shape cannot provide upward force, there will be only one downward force from the wind. It will push the board downward against the resistance of the fin. The board will go backward. Anyone can help? I cannot understand this part. Thanks

[@marklewis7306]

Yes, this is confusing. The force is always downwind and forward. The angle of the fin provides resistance downwind but the board is moving forward, and if it is pointing upwind, the fin is angled so it will slice through the water in an upwind direction. The result is that the sailor moves upwind. If there was no fin, everything would move downwind. Hopefully that helps.

[@yus.c.2231]

@@marklewis7306 the fin provides resistance to water. But why the board is not going backward downwind 45 degree instead

[@3denlightenment]

@@yus.c.2231 This is because of the angle of the sail to the wind. Have a look at the analogy to the billiard balls and you can see the force drives it forward or across the wind.

[@gillesaribiers8612]

Very clear, very precise, very informative, great job, thank you so much for your contribution, itmakes me feel suddunly a bit less dummy !!!

[@3denlightenment]

You're welcome!

[@frodriguezalmada]

Very useful information. Thanks 👍

[@matsg8905]

Can you do videos for winging/wing foiling and/or kite boarding. This is amazing animation.

[@sergijmorgun4236]

Sumptuously. Many thanks!!! Revolution is Coming - Wing and Foil. It would be nice to do the same for the wing.

[@3denlightenment]

I was thinking of getting one of those. Might be worth another video!

[@AlexanderMorales-wz8ce]

Great job.. thank you for your contribution

[@fish-bama]

very well made, thanks. I like the dog sticking out the window ☺

[@markopanger412]

Does that means that theoretically the faster you go the less upwind you can go. If the sail is designed in a way the wind is optimal at 45° this means that by increasing the speed you would eventually go at most perpendicular to the true wind (apparent wind + true wind hits the sail at 45°). This further means that for windfoiling (a lot of apparent wind due to true wind speed vs windsurfer speed) sails with a lot of shape (cambered) won't work as you need to sheet in a lot. Due to that "shallow" sails are preferable. Thoughts?

[@3denlightenment]

In response to your first paragraph, the answer is in theory yes. At the peak point of sail you would at most be able to sail perpendicular to the true wind. So, for windfoiling, the speed is greater and therefore the apparent wind has a greater influence. As far as cambered sails go, I would think that you are correct; however, there seems to be some debate as to the influence of the curved sail and the Bernoulli effect vs the direct force on the sail surface (Newtonian effect). I would think at that angle to the apparent wind there would be greater drag from the curved sail and therefore a non-cambered sail may work better at higher speeds.

[@julianasiqueira9001]

OMG! this is amazing! Iwould like to know if I can use this video and translate it to Portuguese for my students! I'm a windsurf teacher and sometimes is too difficult to explain the physics of windsurfing.

[@3denlightenment]

Sure. Go ahead and translate the video to Portuguese. :-)

[@MyKroolik]

Can i Translate it to Polish and use it the same way? Maybe me and @Juliana Siqueira could uplod those videos to this cahnnel, so people colould learn it in their own language?

[@3denlightenment]

​@@MyKroolik @Juliana Siqueira Sure, you can translate it to Polish. Once you have translated it and would like to upload it to this channel, just let me know.

[@LwDWilco]

Very good explanation 👏

[@johnfromportland]

Great video! I appreciate the billiard ball metaphor and dynamic force equations. Toward the end when you are explaining turning upwind and downwind, I always thought this had to do with the center of mass(?) of the board relative to the combined forces from the foot pressure of the sailor and the sail reaction on the mast base. If those forces are balanced about the board center you don't turn, but if they move fore or aft of the center of mass of the board the board will rotate. Is that right?

[@3denlightenment]

Yes, that is correct it involves combined forces from the sailor's weight and pressure from the sail on the mast foot and the additional pressure from the sail translated through the sailor. I didn't speak about the pressure on the mast base as that is constant for a given angle of sail. When you are sailing along, all is in balance, so if you throw that balance off a bit by tilting the sail forwards or back, the imbalance is translated through to the sailors stance and additional pressure one side or the other changes the direction of the board. If there is a dagger board, all of this movement will be much slower and the board will tend to pivot more around the center (where the dagger board is) rather than from the back (where the fin is).

[@henrykwdk]

John, I appreciate your question mark. It's not the center of mass; it's rather the geometrical center of the lateral profile (and our rotation axis) vs. the geometrical center of the sail. The former is not very well explained in the video, and the foot and mast base are just transmitters of the force on the sail. Think of a weather vane (except it has a fixed rotation axis while ours is moveable) with a tail (we call it the rig :-) ) that can rotate in its plane. You get the picture? Happy sailing! :-)

[@jamicaruma7257]

👌very nicely explained and animated ✨👍

[@gregjohnson514]

Good stuff, thank you

[@patriciagoser2890]

Many Thanks for this educational video!!

[@3denlightenment]

Glad it was helpful!

[@volkerweitbrecht7458]

Great video, and very interesting discussion below! Why can Kite-Surfers go higher upwind than windsurfers?

[@3denlightenment]

That is an interesting question that requires a bit of research, but I imagine it is because you are free to maneuver the edge and angle of the board to very steep angles while still being strongly pulled.

[@nathanvee6135]

nice man! would be awesome to do a hydrodynamic model, as it relates to board construction and performance in turns. I've been learning the sport recently and wondering why the board shapes are not modeling some of the ski boats or modern sailing hulls. Maybe it's just easier to manufacture a flat, fat, rounded rail? I met Paul Mindnich recently of BenchMark Board Design and Tri-Hull Dynamics and the stats on his board designs are impressive.

[@3denlightenment]

Thanks. A hydrodynamic model related board hull shape would be interesting. I'll add it to the list. Thanks.

[@jsibleytube]

Great vid, thanks. Please consider doing one on how to optimize upwind progress.

[@3denlightenment]

That's a great idea. From my experience, the greater the surface area of the fin and the presence of a daggerboard make a very significant difference. If I were manufacturers I would increase the surface area of the fin. Of course, depending on the design, it can interfere with maneuverability but something for them to think about. I will also think about another video. Thanks.

[@mark2talk2u]

Really nicely done video. Where are you standing in the opening and closing scenes? Looks like some places I know on the Great Lakes

[@3denlightenment]

Thanks, mark2talk2u! Close, I am in Ontario, on a beach on Golden Lake.

[@miedwiesport]

great video! Im waiting for 'physics of windfoils' that will be interesting too! cheers

[@kevinalmeida7159]

wow the fin is not the only water factor there is weight transfer and drag and rail provides more control than the fin and the rail is used for speed the fin for tacking or other slower maneuvers. what makes me mad is i either cant properly set up my sail or the rig is to heavy for my winds..... try to find sail neutral in 13 MPH or less winds and my rig is to heavy! in gust of 17 it almost gets weightless.... this is not good enough i really need a tiny rig set but my board is to advanced for me.... i really should have started with a beginner kit. i was hoping i would have missed something in physics but it did not cover anything i didnt know already and noticed it was missing some factors. like even the water smooth or chop are factors and leaning mast forward or tilt back can also change its power not just pulling in sail. fin length and daggerboards and speed also add a ton of stability

[@3denlightenment]

A balance of board size and sail size are important especially if you are beginning. If you are not able to water start yet, then having a board that it boyant enough to allow you to uphaul is essential. The selection of sail size will depend on the wind speed, your skill in the harness, and the conditions of the water. If your sail is too heavy, is it possible there is water in your mast? The larger the sail the heavier it will be. Also, some lower-end sails will be heavier than the more expensive high-end ones. It is also natural for the rig to feel heavy in light winds and become almost weightless when the wind speed increases and the board starts to plane. All the best.

[@rndlab2]

@Klaas Voget, maybe you would like to comment on the new shape features introduced to the Grip 82 model used in this video :)

[@3denlightenment]

Haha...Ok. I'll check it out.

[@gykul]

great video, but i think that in the sail lift profile you have a mistake , if you change the camber diraction in the simulator you do have lift

[@3denlightenment]

Yes, but then the camber is not the direction that it is while sailing. A note of clarification on lift has been added below the video. Thanks

[@melihapaydn2137]

There is a lot going on but basic physics and presentation is nice thanks

[@3denlightenment]

Thanks. Glad you liked it. There's more coming

[@kikobeats]

This is GOLD, thanks a lot

[@3denlightenment]

🤙👍👍

[@senhadjimohammedkhatir1813]

I wish know about harness in 3D, please do it

[@3denlightenment]

You mean the physics forces on the harness or how to use a harness?

[@senhadjimohammedkhatir1813]

@@3denlightenment yes, please

[@emiliandarie7277]

Ciao ciao, nice video where at minute 0:45-0:50 you talk about how the red force is not perpendicular and you decompose it , but what about the yellow force which has the same quality of not being perpendicular and if you decompose it you would get other forces that would kinda amplify the downwind total force and one that would be totally opposed the force pointing forward...at least this is how I see this particular part of video

[@3denlightenment]

The red arrow represents the force on the sail and the yellow arrow is the resulting momentum of the wind after it strikes the sail. The momentum of the wind will be in a different direction and will be slightly diminished after it strikes the sail. I probably should have drawn the two yellow arrows different sizes to show this. The red arrow is the only resultant force on the sail. The yellow arrow shows the direction of the wind.

[@pfadi691]

Hey! I am writing my master thesis about the physics of windsurfing in regard of teaching :). Over all I am totally agree with wildbill805 but I think you should clarify your group of adressees or rather your objective. From that depends your balance of scientic correctness and didactical approach. Additionally, the model of air particles that transfer there momentum (NOT force!) to the sail is a bit old fashion. I may see your didactical attempt; its to pave the way for your billard analogy of the resolution of forces which I actually really like but you pay, in my point of view, a price for it. People might think that there has to be a force which is pointing towards the direction of the moving billard ball but its not! In that moment where the billard cue leaves the ball the acceleration of the ball (due to your muscles force) stops. After Newtons law the ball moves from now on with constant velocity (means v=const. -> a=0). If it clashes with another ball it transfers its momentum (m*v). The clashed ball is accalerated from v=0 to v=constant in a sec as the billard cue acclerated the first ball in the beginning, so you could say "Well if the velocity of a mass was changed by time, there had to act an force!" Yes, thats right! But just for that moment and afterwords no anymore. So the transfer of momentum causes that a force (pair) apllies to the balls whereby the balls leave there place with constant velocity (a=0). Just to clarify, during windsurfing for example: if the wind blows with a constant velocity in your sail and your are not changing its angle of attack than you are driving with constant velocity because there is a force pointing towards your moving direction but its in balance with the friction forces of water and air. So in that case, there is a force pointing in the moving direction but its in balance with friction. And after Newton first law the board drives with constant velocity as if there is no force apllied. Omg, that was a lot of explanation and sorry for my bad punctuation.... hope I could make my point :) Cheers

[@3denlightenment]

A bit of a late reply, but thanks for this. I would agree with you regarding force vs momentum, I should have made that more clear. I was trying to speak in more of a common language, but I should have clarified. My next video should clarify this.

[@GlasgowSmile91]

Really helpful video, I hope that it receives enough views, so you can afford a proper microphone :D

[@3denlightenment]

Haha...yes, I hope so too :-)

[@caetzar]

Excellent video!

[@Critikal-thinking]

the lif of the wind is not downwind nor upwind, its perpendicular to the direction of the wind, at optimal lift angle. (min:6:00) or am i missing something here?,, the negative lift force is not going downwind.... in any case the lift force adds force to the sail and the windsurfer can use it, or direct it as he chooses.

[@3denlightenment]

Thanks Gab Co. The question being answered in that portion is whether the lift force "lifts" the board upwind. You are right, the overall lift force is perpendicular to the direction of the apparent wind but it is always in a downwind direction, never upwind, and therefore the air flowing over the sail does not actually "lift" it up wind, as has been said in other videos. I was just trying to clarify that point. Yes, the lift force adds force to the sail which the windsurfer can use or direct as he chooses.

[@cbassrips]

Hey Mark, great video! Let’s do one on kiting!

[@marklewis7306]

Sounds good! I've bought two kites...but I don't know how to surf yet! Wish you were here!

[@cbassrips]

@@marklewis7306 Sounds like a trip to Whitehorse is in order.

[@alexgosan5707]

its not the area of the sail but the projected area of the sail thats relevant you explain it a on the fin but i think for people with no knowledge it might still be missleading

[@3denlightenment]

Thanks Alex, I totally agree. I might try to include that in an upcoming video.

[@adolfallesch7648]

What is the shear force on the universal joint?

[@3denlightenment]

It is complex. If you assume an 8 m/s wind (~30 km/hr), and a 7 m2 sail, the force on the sail would be 269 N, which is translated through to the universal joint which is receiving most of that pressure. Of course, this will vary depending on the angle of the sail but it is also counterbalanced by the bodyweight of the sailor hooked into the harness, and the angle of pull on the harness, and the directional speed of the board. ...so it is complex.

[@IliaBroudno]

I think the pool ball analogy is a bad one. If the wind hits the sail at 45 degree angle or less, the reason it pushes the sail cross wind instead of downwind is due to the shape of the sail (same way an air-plane wing generates lift due to it's shape) if the sail was flat, it would be pushed downwind still when it was held at 45 degree to the wind.

[@3denlightenment]

There is some discussion over whether lift is the Bernoulli effect resulting from the curve of the sail or the Newtonian effect from the air molecules striking the sail. There are a number of papers and videos discussing the difference. I will also discuss the difference and present this further in an upcoming video.

[@IliaBroudno]

@@3denlightenment If we take the board and the fin out of the equation and replace the sail with a rigid flat object, according to your version it should still be moved cross wind when it's held 45 degrees to the wind. Correct? I'll be honest i didn't test that but I think it will be moved downwind. Would you say that experiment would settle the argument for us?

[@3denlightenment]

@@IliaBroudno Yes, that is correct. I will try to demonstrate that in my next video.

[@IliaBroudno]

That should be interesting. How would you set up the experiment? Remember we are taking the board and fins out of the equation. Meaning you can't stop the object from moving downwind. Meaning if you turn a board at 45 degree to the wind and fix it on a railway car for example that's not going to prove much. The railway car will move along railways. You can set up railways cross wind and it will of course cross wind. You don't need pool ball analogy or molecule model to explain that. You just add the vectors. So how would you set up the experiment? Meaning how would you fix the board at 45 degrees without also preventing it from moving downwind?

[@jamicusx3]

Great vid!

[@ThanosSofroniou]

This is extremely interesting. Where did the idea of the video come from...from a windsurfer or a physicist? Or a physicist who windsurfs!!!

[@rodfer5406]

Excellent. Do you have one on the physics of surfing?

[@3denlightenment]

Sorry. Just started this channel. Perhaps in the future.

[@denkennstenich4988]

Thanks, great explanation! I would be interested in more details about feet and hands position, body weight distribution and set-up like boom height or mast position. I was actually thinking about building-up a multi-body simulation model of a windsurfer + gear to investigate, but it soon became clear that I would have to invest a lot of time. No problem in general, but I would have to do it during working hours haha!

[@3denlightenment]

Thanks Den. Those finer details in the positioning of body and hands, boom height, etc make for subtle differences in the mechanics and forces involved. They also depend on wind and water conditions and body size etc. It would be interesting to do...but yes, a bit of time.

[@henrykwdk]

Den, you have yourself an interesting project, but - unless you live in California - why don't you just wait till winter and spend the same amount of time windsurfing instead? It's even more interesting, and doing it during working hours would add a nice touch :-). And if you still want to do your project, I suggest you drop the windsurfer; he/she is a major source of drag :-). Concerning your question: understanding the basic physics of sailing will give you some very valuable hints with respect to what you are asking. Also, there are plenty of videos out there that deal with exactly those important details. Happy surfing and happy hunting!

[@ivorcheung]

Great video, I know how to push downwind now. 👍

[@monkey-life]

This explains everything😀!

[@zackariad9567]

The first part of the video is inaccurate because as you gain speed, the apparent wind changes direction. Therefore, you can close the sail to produce the same force.

[@3denlightenment]

It is theoretically correct, but you are right, as the apparent wind changes direction the sail can be closed to produce the same force, and that is what was said at the 5:00 to 5:12 minute point in the video. I probably should have emphasised that a bit more. Thanks,.

[@conqueststrategicmarketing7933]

Nice video! Thank you

[@romanniki5610]

well explained, thank you

[@512k]

The hand positionning effect on the sail physic and interacting with the board behavior is a point missing I think.

[@3denlightenment]

I would agree. Could be worth another video

[@512k]

@@3denlightenment I wish I had know sooner how powerfull it is to turn downwind with the sails just with moving your front hand back. I now think it should be teach right away on the beach to the beginners, not only pushing the sail back/forward to turn

[@paulfelix158]

Thank you! Now I understand!!!

[@nexik939]

Damn, that's awesome! Can U do a kitesurfing one?

[@alexgosan5707]

for kitesurfing the principle is exactly the same. as long as the kite is on 12 you pretty much have a windsurfsail thats lying above you instead of standing in front. and when you move the kite its gets "moving wind" (dont know the proper english term) like the windsurfer does when he moves. of course you also get the moving wind from moving as a kitesurfer yourself. for kitesurfer the effect it has is a bit more power for the kite and it rotates your windwindow (because it simply makes the winddirection rotate a bit) long story short. the faster you go the less you can go upwind

[@KDalgliesh]

I don't follow the logic on the conclusion regarding lift. The example shown on the NASA simulator used negative camber on the wing. Of course, the simulator will show 'negative' lift for this case, but only because the convention it is using, is that 'positive' lift is in the 'up' direction on the screen. Even a flat plate turned at an angle to the wind will produce lift, just like you explained with the perpendicular vector in the billiards balls example. The NASA simulator even has an option for flat plates you can test it pretty easily.

[@KDalgliesh]

I watched it again. You mentioned 'lift up-wind' a few times which doesn't make sense to me - I think we're getting a bit mixed up on terminology. Lift is the vector perpendicular to the wind direction. As another commenter mentioned; the apparent wind direction changes this

[@3denlightenment]

To simulate the shape of the sail in the NASA simulator you have to have a negative camber. This positioning simulates the real-world condition of the sail against the wind. If you look at the shape of an airplane wing it generates lift from the air moving over it. The curved profile of the sail is like an upside-down wing relative to the wind blowing over it. As a result of this, as well as the angle that the wind hits the sail...the sail provides a negative lift i.e. pulls the sail downwind.

[@3denlightenment]

@@KDalgliesh I agree. The Bernoulli principle applies lift perpendicular to the curved surface. The only reason why I speak about the "lift up-wind" is only to counter other videos out there which speak about how the sailor can sail upwind due to the curved shape of the sail, which is not true. I agree with you that lift is perpendicular, and in this case, it would be in a direction downwind. So the end result is that there is a push from the front or windward side of the sail and a pull from the leeward side of the sail all in the downwind direction.

[@pietropapili6598]

Very useful, thank you!

[@lukeheatley4148]

It never occurred me to that the reason you don't get washed downwind is that water is 800 times more dense that the air. Seems so stupid not to have thought of that. Although at a standstill your fin's V is zero (assuming no current) while your sail's V is the wind speed. And once you get moving, everything changes anyway.

[@3denlightenment]

Yes, and once you start moving, the resistance force against the fin is activated.

[@marianagonzalez8580]

Loved it!

[@3denlightenment]

Thanks!

[@teeropita3208]

Αmazing thank you so much for the information

[@sashalukyanov]

love this!

[@3denlightenment]

Glad to hear it!

[@crony1100]

Wow thank you, this is perfect

[@idanfarkash8080]

thank you. excellent video