Physics of Windsurfing 

Thanks Nico!

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.

​@@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.

@@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.

@@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.

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?

Thanks Cookie!. I love your videos too.

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

You're very welcome!

Very welcome!

You're welcome!

You're most welcome!

Thanks. Glad you liked it ☺️

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?

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?

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.

Thanks. I've been thinking about it.

Thanks man! There are more coming 😊

@@3denlightenment I'll watch them.

🤙👍👍

Glad it was helpful!

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

You are welcome!

You're welcome!

Glad you liked it!

You're welcome. More on the way

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.

@@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).

​ @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/

@@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.

@@cyrilcivet7683 it's hard work though!

Thanks!

Glad you liked it!

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

@@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?

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.

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.

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.

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.

@@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.

@@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!

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

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

@@ripwind Awesome! Glad to hear 🙂

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

Thanks!

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

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.

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.

You're welcome. Glad to be of help ☺️

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.

Glad you enjoyed it

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

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

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

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

Thank you! Cheers!

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

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

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.

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

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.

Glad to be of help 😊 Good luck with your sailing!

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

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.

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!

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).

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! :-)

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.

@@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?

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

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?

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.

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

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?

​@@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.

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.

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.

@@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?

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

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

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

@@OKuusava Quite true.

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

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

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

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

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