Watch this first ► • The Physics of Boats Learn more at Waterlust.com Join marine physicist Dr. Patrick Rynne as he explores the science behind hydrofoils.
I've watched countless TH-cam videos both short and long and detailed on how planes fly, and none have been more clear and intuitive than this one. Thanks much!
Excellent, intuitive description. Your analogies are spot on. As someone with a masters degree in ocean engineering (and more exposure to three dimensional hydrodynamics than I care to remember!) this explanation nicely fills in the gap left in many undergraduate courses about the Bernoulli principle. “Yes professor, I know that’s what happens to pressure and velocity, but WHY does it happen?” . Well done, again! Each of your videos is themselves worthy of a subscription !
As a curmudgeon who is highly critical of people who explain fluid mechanical concepts incorrectly, I'm really impressed with how this explanation combines accurate explanations with layperson-friendly concepts. Is there any way you could do a video on how hydrofoils maintain stability in pitch and roll axes? That one still baffles me a bit.
Omg, ur explanation is so good. It's literally life changing for me. I was actually a physics major and all my life, I've been misled to believe that the faster speed causes the lesser pressure, which I could never make sense of. And this has tormented me for years. Now I finally get it thanks to your vid. @07:42 The way Bernoulis principle was taught in school was completely wrong!
Hey man, I just wanted to say that was an excellent video. I really appreciate it and the students in my class will have no idea but they’ll understand it much easier. Thank you.!
They are actually pretty simple. Front wing pushes upwards. Tail pushes down. It creates a strong lift on the mast and the board comes up out of the water. It’s basically just a glider. If you get the foil to the surface and let it drop, it will glide until the board hits the water. How people ride them is basically taking all your weight off it to let it climb, then putting your weight back on to glide. What I’m most excited for is some type of hybrid foil. Generally the thicker the foil, the more low speed lift you have. And the thunder, the less low speed lift you have. The thick foil has a low top speed. The thin foil has a high top speed. Lately, Armstrong has introduced foils with high camber that drastically reduce the stall speed but also reduce the top speed. I’d love to see a wing that can change certain features as you ride. Maybe a bit too much to ask. Great information!
I've been trying to get my head around this for months, but I think I finally get it from this video. Tempted to try a book, though I've no post-school Maths/Physics, so am a bit wary of whether I'll understand it. None the less - thank you so much for making this video!
In our experience, most of the time what people think is foil cavitation is actually foil ventilation. Cavitation happens when the local fluid pressure is so low that the local water shifts from liquid to gas phase. It requires a very large pressure gradient and is most common on boat propellers. Ventilation is when air near the surface gets entrained and sucked down onto the foil surface, causing a sudden loss of lift. In our experience, ventilation is very common on kites, sailboats, wings and surfing foils, as they typically operate very near the surface. We’ve even done some camera experiments where you can see wing tip vortices suck air down from a few meters behind a rider, and the air gets pulled all the way to the wing. It’s fascinating!
@@Waterlust Thank you for the reply. I get what you're saying about the pumping oscillation with forward angle, but have always been mystified when I blow air over the top of a curved piece of paper (wing simulation) and not only is there lift, but also enough forward vector, to the point where the paper will hit me in the face if done just right. Maybe that's because the where I'm holding is really a pivot point and the lift vector pivots on that point? Btw, your tune for the df65 is perfect ;-) Thank you for that as well!
Not sure if this has been asked but what part of a wave creates the most lift for a foil especially small waves where exactly would be the best spot to ride the foil for maximum energy
I love the traffic analogy and also the clarification on dual causality - many people think of it as a one way relationship but it’s much more like electromagnetic fields or chickens that cause eggs and vice versa!
Great question! Hull resistance is typically decomposed into different categories at different speeds. At low speeds, hull resistance is dominated by wave resistance. At high speeds, hull resistance becomes increasingly dominated by skin friction. They really are two distinct regimes. At low speeds, we’d say the existence of waves and the resistance they produce are occurring simultaneously, you can’t have one without the other. The creation of the wave is caused by the boat exerting a force onto the water, and that force is most of the total resistance. We wouldn’t say there is “one-way causality” or that one creates the other, they must occur together. Hope that helps!
Thank you for very detailed explanation. But could you also explain why hydrofoil blades can be placed at the aft of board - comparing to planes where wings are located more/less in the center of gravity? Or how is it done that momentum coming from drag of sail or kite does not swing board into the water.
great question! In general, the center of gravity of the rider has to be vertically aligned with the center of pressure on the foils. The fuselage usually is positioned towards the back, but the center of pressure can be forward of that, especially if the front wing is large. Hope that info helps!
Thank you so much for this video! My brain disagrees about math being awesome and I did horribly in physics, but your car metaphors and explanations helped immensely!
if you introduced air at the thickest part of the cord while the foil is in motion would that increase lift and reduce drag? one or the other or neither?/
Great informative video! I'm curious why all foiling surfboards seem to have wings with negative dihedral. It seems to me that positive dihedral, like in an airplane, would be more stable.
Great question! Positive dihedral helps stability (like in planes), but it also brings the foil tip closer to the water surface, which increases the risk of ventilation (sucking air down). Tip vortices are incredibly effective at sucking air down to the foil surface and causing catastrophic stall which often results in a loss of lift and a crash!
It depends on what kind of application. If it’s a wind powered foiling craft like a sailboat, kite or wing surfer, the forward motion is caused by the forward thrust produced by the wind. If it’s a foil surfing on a wave, the forward motion is produced by the wave energy in the same way conventional surfing works. If it’s a foiler pumping around flat water, the forward motion is produced by the articulating motion of the foil itself, much like how a bird moves forward by flapping its wings.
Pere-Andreu Ubach de Fuentes I'd also like to know if this (very well articulated) presentation is correct. I had read that this concept had been disproven by someone demonstrating that a 'symmetrical' foil with no rake (angle of attack) also produced 'lift' though i'm not qualified in this area myself. In my own head i think of a hydrofoil/aircraft wing as simply a surface angled to re-direct the climate to create the opposite 'lift' force and that the shape/hump is to provide flow re-attachment to reduced drag.
A foil does not need to be curved with a teardrop shape and rounded leading edge to work. Simple flat plates can produce lift and their characteristics are well documented in the scientific/engineering literature. However, they do need to operate at an angle, as do symmetric foils, in order to produce lift. But while inclined flat plates can create lift, they also produce a lot of drag which makes them less efficient than a more optimized shape. A well designed foil will tend to have a high lift coefficient and a low drag coefficient, or a high “lift to drag ratio”.
A rounded edge reduces the likelihood of what is called “flow separation” or “boundary layer separation”. If the foil is shaped with too sharp a leading edge or is angled too much, the water isn’t able to follow the foil surface, which can lead to a messy and chaotic flow field (called turbulent flow), which produces far less lift and more drag. We think of the hydrofoil as gently redirecting water downward, but there are limits. If the foil tries to bend the water too much, turbulence occurs. This is like when an airplane wing “stalls”
Hello, my friend, your explanations are really interesting, but there are other things I want to learn, for example, you explained the lifting force, but where does the pushing force come from?In the videos you have shown, another vehicle, for example, a boat or a kite or a sail, creates a forward thrust, but I watched some videos, any vehicle didnt pull .
The pushing force can come from a variety of sources: an engine, a sail, a paddle, or the push from a wave. As long as the forward thrust from one of those sources is sufficiently strong, a foil can fly! Some talented foilers can produce forward thrust without any of these by “pumping” the foil in the same way a bird flaps its wings.
Great question with a somewhat complex answer. Different foil systems have different mechanisms to control the “ride height”, or to make sure the foil doesn’t fly out of the water. The simplest method is used on foil boards. Here, the rider uses leg pressure to keep things balanced. If they need to foil higher, they put more pressure on the back leg, if they need to come down, they put pressure on the front. It sounds difficult, but with a well-designed foil, it’s actually quite manageable! For boats that have these same kinds of “T” foils (shaped like an upside down “T”), they don’t have the ability to quickly shift crew weight like boards can, so they often use some kind of mechanism to control an articulating flap on the rear edge of the foil, similar to flaps on airplanes. The Moth sailboat is a classic example of this. They use what is called a “wand” on the front of the boat that connects to the wing flap through an internal mechanism. When the boat is low in the water, the wand is fully submersed, pushing it at a large angle, which creates a large angle on the flap, which helps drive more lift. As the boat flies higher, less of the wand is in the water, which reduces the angle of the flap. When properly tuned, this creates a self-maintaining control system so the sailor can comfortably sit and not have to shift weight to maintain a proper ride height. The final kinds of foils are called “surface piercing” foils, which are the ones on the catamarans in the video and are used now in Americas Cup boats, IMOCA’s, etc.. Instead of a “T”, the foil is a curved "S" wing shape and there are one on each side of the boat. These kinds of foils are more inherently stable because as they fly higher out of the water, less lifting surface area remains submerged. When properly tuned, these are more or less self-regulating designs, and the boat naturally locks into a ride height proportional to its speed and the size of the foil. Hope that info helps!
@@Waterlust Thanks! I used to windsurf a lot, before this came about. Foil boards seemed like they add yet another degree of freedom you have to control, and I was right. Seems possible. I'll bet it helps to already know how to windsurf? I was aware of the "wand" method the Moth uses, and surface piercing.
@@bentonjackson8698 if you already know how to windsurf, you gotta try a foil. We’d recommend either a windsurf foil or wingsurfing. The gear has gotten much more user friendly and it’s an absolute blast! Go for it!!!!
@@Waterlust I haven't windsurfed in at least 10 years, but I haven't ruled it out. I broke my foot snowboarding the year kiteboarding became popular. I never really got back into it. I have some old windsurfing buddies who talk about foil boarding. I suspect foils would not handle weeds very well, and we have a eurasian milfoil problem. They're bad enough on a windsurfing fin, I can imagine what a clump of weeds would do to a foil.
Why don’t the bubbles on the upper surface of the trailing edge of the wing (6:47) move along with the fluid? Shouldn’t they behave like little cars or little boxes? Thanks.
In that visualization, they’re using a pipe ahead of the foil that lets out small bubbles that travel with the flow to help observers see what the flow is doing. They call this a “tracer”, and it’s never a perfect representation of the fluid itself. The holes in the pipe are spaced apart, and we’re guessing the reason we’re seeing a gap in bubble coverage on the upper surface as the flow passes is partly because there weren’t enough bubbles upstream to give adequate coverage. If the bubbles were universally distributed everywhere in the flow, we expect you would see more of them back there. There is also another interesting phenomenon called an “adverse pressure gradient”, which can cause a reversal in flow from the trailing edge towards the leading edge of the foil, resulting in what is called “flow separation.” If this is the cause, then the fluid we’re seeing without bubbles originated off the trailing edge and moved from right to left into that region. Hope that info helps! Great question!
@@Waterlust Thanks, but it seems like there’s a generous amount of bubbles to fill that void. And there seems to be plenty of bubbles just downstream of the trailing edge that could fill the void if they were moving upstream.
Since the principles of hydrodynamics and aerodynamics have so much commonality, why don't I see the use of vortex generators (VGs) on hydrofoils in the same way they are used on aircraft to increase efficiency of lift and lower stall speed? I would think these could be used to increase efficiency when pumping the board where the laminar flow across the foil is most likely to separate and "stall" the foil.
Great question. We don’t have first-hand experience with vortex generators, but If you google “vortex generator hydrofoil”, you can find some academic papers, so it is done in some contexts. We’re not sure if the Reynolds number associated with a surf hydrofoil aligns with the kind of flows where these work…they may only be effective in very specific Reynolds number regimes. But certainly worth looking into!
Seems like in simple terms they are needed. Water is so dense you can design a high performance foil with a pretty slow stall speed. VG’s being a slow speed enhancer, what mode of foiling do you need it that doesn’t trade off some cruise speed? I’ve thought about it as well and I can only imagine effective VG’s wou Mlk d be tiny.
WATCHED 2 VIDEOS N IM HOOKED AND SUBSCRIBED LOL...... VERY GOOD CONTENT, AND DELIVERY.....EVEN I GOT IT ALL LOL.....KOOL MERCEDES TOO, THAT BLACK N WHITE LONG SLEEVE IS NICE TOO.
Stabilizers and the pros/cons of different foil configurations is a fascinating subject, but for this video we wanted to stay focused on the fundamental physics of how foils product lift and reduce drag. Maybe we should make a new one about stabilizers 🤔
None, really, except for fluid density (and, subsequently, Reynolds numbers/turbulence effects). Low speed airfoils operating in air or another gas can discount compressibility, and liquids are also not compressible. If you have a good understanding of low speed airfoils, you also have a good understanding of hydrofoils.
To lift an object out of the water, the foils need to generate lift equal to the weight of the object. This is embodied in a simple mathematical equation, Lift = wing area * coefficient of lift (an arbitrary dimensionless number that evaluates the "efficiency" of the airfoil) * fluid density * (.5 * (velocity of the flow squared)). If that number adds up to more than the weight of the object in question, you're flying, either in water or air. So, if you want to fly at a slower speed you can design a more efficient shape that has a greater coefficient of lift (although some very complex physics provides a limit to that), or build bigger wing to compensate for the slower flow. That's why airplanes that want to fly at slow speeds have big complex flaps (bigger coefficient of lift, although at the cost of drag), and have big wings compared to the airplane's weight.
In regards to your explanation as to why bubbles are not seen on the upper trailing edge of the wing, it seems like there’s a generous amount of bubbles that were available to fill that void. And there seems to be plenty of bubbles just downstream of the trailing edge that could fill the void if they were indeed moving upstream. Are you convinced that these are the accurate explanations. Thanks.
OK. That's how we are staying above water. But how we aren't diving down? How is this jumping and pumping leg moves keeping us above water? How is vertical jumping on the payload platform converting to horizontal movement forward?
There is a misconception that foils are faster than planing surfaces such as surfboards and power boats. Foils are not FASTER. Surfboards and sailboards are slower with a foil. Power boats are significantly slower with the addition of foils. Foils are, instead, more EFFICIENT in low power environments. SPEED and EFFICIENCY are two different things. Cheers
We would say that foils produce less drag than planning craft for most speed regimes. Less drag results in higher speeds when the same thrust is applied. In that sense, foils are “faster”, but really we’re saying they’re more efficient at being faster. Planning hulls, under the right conditions, can reach the highest speeds ever achieved by watercraft, but the thrust requirements are large. The current speed sailing record is held by a planning craft, however there are a number of next-generation foil designs under development that could beat it. Will be exciting to watch…
@@Waterlust Yes, you just expounded the definition of efficiency. Foils are not even close to the speed and acceleration of planing craft. They have their place, for sure, in their efficiency. Cheers
@@Waterlust When you analyze the stats for speed sailing, it's evident that foils are both the solution and the problem. Compared to land and ice sailing, foils are almost half as slow - 125mph for land sailing and 75mph for foils. This shows just how much drag the foils are producing - 50mph worth of drag. Can foils be designed to match these speeds? I doubt it. Because, in a sense, foils are the equivalent of gears, which convert top speed into efficiency at medium to lower speeds. Unless foils can be made to ever reduce in accordance with higher speed, they cannot reach those higher speeds achieved by a planing hull. Cheers again.
@@robertcain3426 agreed, foils will always have a hard limit due to the friction they produce. Unlike a planning hull that can reduce its wetted surface area at higher speeds, foils must always be submerged in order to function. In order to go faster, the foil must be smaller, higher aspect ratio, etc…the problem with this is that smaller foils are less capable at slower speeds, and every foiling craft starts from rest. We’ve seen on the AC racing some boats beings “towed up” onto the foil in light winds. Depending on how the rules are interpreted, this may be an option that opens the door for small, higher speed foils. There are also some interesting kite based record attempts in the works like the SP80 project that seem well poised to break the 500 meter speed record of 65 knots held by Sailrocket for the last 10+ years. Land and ice sailing are different beasts as they have significantly less drag, though much can be learned from their rigs with respect to high apparent wind sails.
@@Waterlust Yes. That is a conundrum; the size of the foil. I have to dissagree with your comment about the high aspect foils in relation to high speed because HAR foils are about power and efficiency. Whereas LAR foils are about speed. For some reason this is a forgotten fact about LAR foils. This is another conumdrum. So eeither foil is reduced or becomes more of a LAR to go faster.
@@potsmkr87 Totally! A foil doesn't work by deflecting water doenward and then pushing upward the payload as a reaction! If that were the case we would simply use flat plates! And we don't!
@@potsmkr87 , by stating that, he is invoking the principle of conservation of quantity of movement. But that principle is not at play here, because the foil doesn't change its vertical velocity, and neither water's overall vertical velocity isn't changed if you measure the velocity of water of a section sufficiently upstream and sufficiently downstream from the foil. No, that's not how a foil works!
@@Waterlust I insist: conservation of momentum has nothing to do with lift! As there is no upward momentum gained by the foil, nor the fuselage nor the payload. Upward force is not the same as upward momentum!
Great question! Pumping a foil is similar to a bird flapping a wing. The underlying physics of how the foil itself creates lift and minimizes drag is the same, but there is a lot more complexity around the pumping motion and how it affects the overall flow field. Too much to cover in a comments section, perhaps we should make a video that dives into all the fun details!
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I've watched countless TH-cam videos both short and long and detailed on how planes fly, and none have been more clear and intuitive than this one. Thanks much!
So glad it was helpful for you! More ocean science videos like this coming, stay tuned!
Awesome foil explanation! It’s easy to feel how fun foils can be and thanks to you we can understand a bit better what’s going on under the surface!
So it works exactly the same way an airplane wing works. Just replace air with water.
Excellent, intuitive description. Your analogies are spot on. As someone with a masters degree in ocean engineering (and more exposure to three dimensional hydrodynamics than I care to remember!) this explanation nicely fills in the gap left in many undergraduate courses about the Bernoulli principle. “Yes professor, I know that’s what happens to pressure and velocity, but WHY does it happen?” . Well done, again! Each of your videos is themselves worthy of a subscription !
As a curmudgeon who is highly critical of people who explain fluid mechanical concepts incorrectly, I'm really impressed with how this explanation combines accurate explanations with layperson-friendly concepts. Is there any way you could do a video on how hydrofoils maintain stability in pitch and roll axes? That one still baffles me a bit.
Omg, ur explanation is so good. It's literally life changing for me.
I was actually a physics major and all my life, I've been misled to believe that the faster speed causes the lesser pressure, which I could never make sense of. And this has tormented me for years. Now I finally get it thanks to your vid.
@07:42 The way Bernoulis principle was taught in school was completely wrong!
Hey man, I just wanted to say that was an excellent video. I really appreciate it and the students in my class will have no idea but they’ll understand it much easier. Thank you.!
You’re very welcome, more educational content like this coming, stay tuned!
Thank you so much first for making it easy to understand and 2nd for choosing the topic.
Right on, glad you enjoyed it!
That was the best way i think anyone could have every explained a hydrofoil. I hope you teach somewhere. Thank you for the insight.
They are actually pretty simple. Front wing pushes upwards. Tail pushes down. It creates a strong lift on the mast and the board comes up out of the water.
It’s basically just a glider. If you get the foil to the surface and let it drop, it will glide until the board hits the water. How people ride them is basically taking all your weight off it to let it climb, then putting your weight back on to glide.
What I’m most excited for is some type of hybrid foil. Generally the thicker the foil, the more low speed lift you have. And the thunder, the less low speed lift you have. The thick foil has a low top speed. The thin foil has a high top speed.
Lately, Armstrong has introduced foils with high camber that drastically reduce the stall speed but also reduce the top speed. I’d love to see a wing that can change certain features as you ride. Maybe a bit too much to ask.
Great information!
I've been trying to get my head around this for months, but I think I finally get it from this video. Tempted to try a book, though I've no post-school Maths/Physics, so am a bit wary of whether I'll understand it. None the less - thank you so much for making this video!
We’d say the text we cite at the end by Doug McLean is the best resource for general readers. Happy to answer any questions here too!
You explained this very very very well. Using that traffic analogy was a golden move!!! Thank you!!
Glad you enjoyed it! The interplay between pressure and velocity in a fluid is super tricky to visualize, glad the metaphor helped!!
Excellent. I would be curious to see another video about methods of stabilizing and modifying the flight of foils in the water. Thanks.
Thanks for the easy and understandable explanation and the references to the books.
Super informative video, thank you for the content! This is the principle we utilise with our monofins.
Such a great explanation!! Easy to follow and digest
Glad you enjoyed it!!
Thank you for unveiling the mystery of these things for me.
Awesome video!!
Excellent easy to understand video. Thank you!
your intro was just what i needed to hear. Look forward to yakking. keep sharing your innovation.
Mind blown! Thank you for this fantastic explanation !
Absolutely amazing vid. Thanks.
The fuselage connects the foil wing and stabilizer, what you call the fuselage is the mast. Good video!
Awesome explanation, could you also explain foil cavitation that would be great!
In our experience, most of the time what people think is foil cavitation is actually foil ventilation. Cavitation happens when the local fluid pressure is so low that the local water shifts from liquid to gas phase. It requires a very large pressure gradient and is most common on boat propellers. Ventilation is when air near the surface gets entrained and sucked down onto the foil surface, causing a sudden loss of lift. In our experience, ventilation is very common on kites, sailboats, wings and surfing foils, as they typically operate very near the surface. We’ve even done some camera experiments where you can see wing tip vortices suck air down from a few meters behind a rider, and the air gets pulled all the way to the wing. It’s fascinating!
Awesome! Thanks Patrick!
Great explanation, thank you.
Thank you Patrick, can you help explain why there is forward speed / lift generated, not just vertical lift?
@@Waterlust Thank you for the reply. I get what you're saying about the pumping oscillation with forward angle, but have always been mystified when I blow air over the top of a curved piece of paper (wing simulation) and not only is there lift, but also enough forward vector, to the point where the paper will hit me in the face if done just right. Maybe that's because the where I'm holding is really a pivot point and the lift vector pivots on that point?
Btw, your tune for the df65 is perfect ;-) Thank you for that as well!
Excellent video. As an educator AND prospective wing foiler, I am doubly impressed!
Not sure if this has been asked but what part of a wave creates the most lift for a foil especially small waves where exactly would be the best spot to ride the foil for maximum energy
I love the traffic analogy and also the clarification on dual causality - many people think of it as a one way relationship but it’s much more like electromagnetic fields or chickens that cause eggs and vice versa!
Are waves a consequence of hull resistance or a cause of it?
As an experienced recreational foiler this was a really great watch. Thank you.
Great question! Hull resistance is typically decomposed into different categories at different speeds. At low speeds, hull resistance is dominated by wave resistance. At high speeds, hull resistance becomes increasingly dominated by skin friction. They really are two distinct regimes. At low speeds, we’d say the existence of waves and the resistance they produce are occurring simultaneously, you can’t have one without the other. The creation of the wave is caused by the boat exerting a force onto the water, and that force is most of the total resistance. We wouldn’t say there is “one-way causality” or that one creates the other, they must occur together. Hope that helps!
@@Waterlust that is super thank you.
Great stuff!
Thank you for very detailed explanation. But could you also explain why hydrofoil blades can be placed at the aft of board - comparing to planes where wings are located more/less in the center of gravity? Or how is it done that momentum coming from drag of sail or kite does not swing board into the water.
great question! In general, the center of gravity of the rider has to be vertically aligned with the center of pressure on the foils. The fuselage usually is positioned towards the back, but the center of pressure can be forward of that, especially if the front wing is large. Hope that info helps!
One of the best explanations ever
I hope I don't remember the title of the first book (someone remind me in the future pls)
Great video!! Good company also, I love my Waterlust swim wear. 💙 🌍
Btw, can you elaborate on how the thrust is provided when surfing on a hydrofoil board?
Why would the foil pumping technique make you go forward?
Thank you very much
Thank you so much for this video! My brain disagrees about math being awesome and I did horribly in physics, but your car metaphors and explanations helped immensely!
if you introduced air at the thickest part of the cord while the foil is in motion would that increase lift and reduce drag? one or the other or neither?/
You're really impressed with yourself!
Will someone give the name of the text books mentioned in the rnd
Great informative video! I'm curious why all foiling surfboards seem to have wings with negative dihedral. It seems to me that positive dihedral, like in an airplane, would be more stable.
Great question! Positive dihedral helps stability (like in planes), but it also brings the foil tip closer to the water surface, which increases the risk of ventilation (sucking air down). Tip vortices are incredibly effective at sucking air down to the foil surface and causing catastrophic stall which often results in a loss of lift and a crash!
Totally missed the question that i had. Im curious about how it keeps continuous forward motion with no motor, is their something im missing?
It depends on what kind of application. If it’s a wind powered foiling craft like a sailboat, kite or wing surfer, the forward motion is caused by the forward thrust produced by the wind. If it’s a foil surfing on a wave, the forward motion is produced by the wave energy in the same way conventional surfing works. If it’s a foiler pumping around flat water, the forward motion is produced by the articulating motion of the foil itself, much like how a bird moves forward by flapping its wings.
Pere-Andreu Ubach de Fuentes I'd also like to know if this (very well articulated) presentation is correct. I had read that this concept had been disproven by someone demonstrating that a 'symmetrical' foil with no rake (angle of attack) also produced 'lift' though i'm not qualified in this area myself. In my own head i think of a hydrofoil/aircraft wing as simply a surface angled to re-direct the climate to create the opposite 'lift' force and that the shape/hump is to provide flow re-attachment to reduced drag.
A foil does not need to be curved with a teardrop shape and rounded leading edge to work. Simple flat plates can produce lift and their characteristics are well documented in the scientific/engineering literature. However, they do need to operate at an angle, as do symmetric foils, in order to produce lift. But while inclined flat plates can create lift, they also produce a lot of drag which makes them less efficient than a more optimized shape. A well designed foil will tend to have a high lift coefficient and a low drag coefficient, or a high “lift to drag ratio”.
Perfect!
Thanks.
Grey to me.
Why is the leading edge of an aerofoil or hydrofoil rounded? And not sharper?
A rounded edge reduces the likelihood of what is called “flow separation” or “boundary layer separation”. If the foil is shaped with too sharp a leading edge or is angled too much, the water isn’t able to follow the foil surface, which can lead to a messy and chaotic flow field (called turbulent flow), which produces far less lift and more drag. We think of the hydrofoil as gently redirecting water downward, but there are limits. If the foil tries to bend the water too much, turbulence occurs. This is like when an airplane wing “stalls”
@@Waterlust
Got it. Thanks!!
Hello, my friend, your explanations are really interesting, but there are other things I want to learn, for example, you explained the lifting force, but where does the pushing force come from?In the videos you have shown, another vehicle, for example, a boat or a kite or a sail, creates a forward thrust, but I watched some videos, any vehicle didnt pull .
The pushing force can come from a variety of sources: an engine, a sail, a paddle, or the push from a wave. As long as the forward thrust from one of those sources is sufficiently strong, a foil can fly! Some talented foilers can produce forward thrust without any of these by “pumping” the foil in the same way a bird flaps its wings.
Amazing
Can you recommend any books explaining this topic? Academic level or imma say "regular" :D
We’d say the text we cite at the end of the video by Doug McLean is the best resource for general readers.
I don’t understand the stability. How does it maintain a specific depth?
Great question with a somewhat complex answer. Different foil systems have different mechanisms to control the “ride height”, or to make sure the foil doesn’t fly out of the water. The simplest method is used on foil boards. Here, the rider uses leg pressure to keep things balanced. If they need to foil higher, they put more pressure on the back leg, if they need to come down, they put pressure on the front. It sounds difficult, but with a well-designed foil, it’s actually quite manageable!
For boats that have these same kinds of “T” foils (shaped like an upside down “T”), they don’t have the ability to quickly shift crew weight like boards can, so they often use some kind of mechanism to control an articulating flap on the rear edge of the foil, similar to flaps on airplanes. The Moth sailboat is a classic example of this. They use what is called a “wand” on the front of the boat that connects to the wing flap through an internal mechanism. When the boat is low in the water, the wand is fully submersed, pushing it at a large angle, which creates a large angle on the flap, which helps drive more lift. As the boat flies higher, less of the wand is in the water, which reduces the angle of the flap. When properly tuned, this creates a self-maintaining control system so the sailor can comfortably sit and not have to shift weight to maintain a proper ride height.
The final kinds of foils are called “surface piercing” foils, which are the ones on the catamarans in the video and are used now in Americas Cup boats, IMOCA’s, etc.. Instead of a “T”, the foil is a curved "S" wing shape and there are one on each side of the boat. These kinds of foils are more inherently stable because as they fly higher out of the water, less lifting surface area remains submerged. When properly tuned, these are more or less self-regulating designs, and the boat naturally locks into a ride height proportional to its speed and the size of the foil.
Hope that info helps!
@@Waterlust Thanks! I used to windsurf a lot, before this came about. Foil boards seemed like they add yet another degree of freedom you have to control, and I was right. Seems possible. I'll bet it helps to already know how to windsurf?
I was aware of the "wand" method the Moth uses, and surface piercing.
@@bentonjackson8698 if you already know how to windsurf, you gotta try a foil. We’d recommend either a windsurf foil or wingsurfing. The gear has gotten much more user friendly and it’s an absolute blast! Go for it!!!!
@@Waterlust I haven't windsurfed in at least 10 years, but I haven't ruled it out. I broke my foot snowboarding the year kiteboarding became popular. I never really got back into it. I have some old windsurfing buddies who talk about foil boarding. I suspect foils would not handle weeds very well, and we have a eurasian milfoil problem. They're bad enough on a windsurfing fin, I can imagine what a clump of weeds would do to a foil.
@@bentonjackson8698 weeds are definitely a problem! Here in Miami in the summer we get lots of sargassum and it makes foiling almost impossible.
Why don’t the bubbles on the upper surface of the trailing edge of the wing (6:47) move along with the fluid? Shouldn’t they behave like little cars or little boxes? Thanks.
In that visualization, they’re using a pipe ahead of the foil that lets out small bubbles that travel with the flow to help observers see what the flow is doing. They call this a “tracer”, and it’s never a perfect representation of the fluid itself. The holes in the pipe are spaced apart, and we’re guessing the reason we’re seeing a gap in bubble coverage on the upper surface as the flow passes is partly because there weren’t enough bubbles upstream to give adequate coverage. If the bubbles were universally distributed everywhere in the flow, we expect you would see more of them back there.
There is also another interesting phenomenon called an “adverse pressure gradient”, which can cause a reversal in flow from the trailing edge towards the leading edge of the foil, resulting in what is called “flow separation.” If this is the cause, then the fluid we’re seeing without bubbles originated off the trailing edge and moved from right to left into that region.
Hope that info helps! Great question!
@@Waterlust Thanks, but it seems like there’s a generous amount of bubbles to fill that void. And there seems to be plenty of bubbles just downstream of the trailing edge that could fill the void if they were moving upstream.
Since the principles of hydrodynamics and aerodynamics have so much commonality, why don't I see the use of vortex generators (VGs) on hydrofoils in the same way they are used on aircraft to increase efficiency of lift and lower stall speed? I would think these could be used to increase efficiency when pumping the board where the laminar flow across the foil is most likely to separate and "stall" the foil.
Great question. We don’t have first-hand experience with vortex generators, but If you google “vortex generator hydrofoil”, you can find some academic papers, so it is done in some contexts. We’re not sure if the Reynolds number associated with a surf hydrofoil aligns with the kind of flows where these work…they may only be effective in very specific Reynolds number regimes. But certainly worth looking into!
Seems like in simple terms they are needed. Water is so dense you can design a high performance foil with a pretty slow stall speed. VG’s being a slow speed enhancer, what mode of foiling do you need it that doesn’t trade off some cruise speed? I’ve thought about it as well and I can only imagine effective VG’s wou Mlk d be tiny.
WATCHED 2 VIDEOS N IM HOOKED AND SUBSCRIBED LOL...... VERY GOOD CONTENT, AND DELIVERY.....EVEN I GOT IT ALL LOL.....KOOL MERCEDES TOO, THAT BLACK N WHITE LONG SLEEVE IS NICE TOO.
Nothing about trimming or stabilizers?
Stabilizers and the pros/cons of different foil configurations is a fascinating subject, but for this video we wanted to stay focused on the fundamental physics of how foils product lift and reduce drag. Maybe we should make a new one about stabilizers 🤔
I kinda understand airodynamics of wings. What is the difference to hydrodynamics? Pressure differences sounds familiar to me.
None, really, except for fluid density (and, subsequently, Reynolds numbers/turbulence effects). Low speed airfoils operating in air or another gas can discount compressibility, and liquids are also not compressible. If you have a good understanding of low speed airfoils, you also have a good understanding of hydrofoils.
At 6:47 it looks like there is a notable voided space on the upper side of the wing.
That looks like a void because none of the tracer bubbles go there, but there is still fluid present.
@@Waterlust Why don’t the bubbles move along with the fluid? Shouldn’t they behave like little cars or little boxes? Thanks.
I knew all that already. What I want to know is how "pumping" works to provide forward thrust on a foil board.
2:15 I can't find the link?
Oh just seen it, thanks!
Is there a minimum speed required for using a hydrofoil?
Probably not, but the slower you go, the bigger the foil needs to be, if it gets to big the foil will make you slower.
@@Waterlust are these hydro foils dangerous? Will I tip over if I am not skilled at riding the board?
To lift an object out of the water, the foils need to generate lift equal to the weight of the object. This is embodied in a simple mathematical equation, Lift = wing area * coefficient of lift (an arbitrary dimensionless number that evaluates the "efficiency" of the airfoil) * fluid density * (.5 * (velocity of the flow squared)). If that number adds up to more than the weight of the object in question, you're flying, either in water or air. So, if you want to fly at a slower speed you can design a more efficient shape that has a greater coefficient of lift (although some very complex physics provides a limit to that), or build bigger wing to compensate for the slower flow. That's why airplanes that want to fly at slow speeds have big complex flaps (bigger coefficient of lift, although at the cost of drag), and have big wings compared to the airplane's weight.
In regards to your explanation as to why bubbles are not seen on the upper trailing edge of the wing, it seems like there’s a generous amount of bubbles that were available to fill that void. And there seems to be plenty of bubbles just downstream of the trailing edge that could fill the void if they were indeed moving upstream. Are you convinced that these are the accurate explanations. Thanks.
If we had to bet money on it, we’d say it’s flow separation (e.g stall) driven by an adverse pressure gradient.
how much weight can hydrofoils left?
Depends on the speed and surface area of the foil. With the right combination, then can lift huge vessels like military ships and ferries.
@@Waterlust so all i need is the right combinations to left up my dream yacht 🤗😊, thanks Patric you really is cool and super smart 💌
@@sweetagony666 Appreciate the support! Stay tuned for more videos like these coming soon!
OK. That's how we are staying above water. But how we aren't diving down? How is this jumping and pumping leg moves keeping us above water?
How is vertical jumping on the payload platform converting to horizontal movement forward?
Similar dynamics to flapping a wing. Might have to make a separate video on foil pumping!
😱 great
Could you be my physics teacher?
I love science. 😭
Science rules!
Airplane wing in water, simple as
This is very useful 😀 i am doing a scool progect on lift and this is the perfect vidieo
By the way i am in grade 5
It works on the same principle that they created the shape of an airplane wing.
There is a misconception that foils are faster than planing surfaces such as surfboards and power boats. Foils are not FASTER. Surfboards and sailboards are slower with a foil. Power boats are significantly slower with the addition of foils. Foils are, instead, more EFFICIENT in low power environments. SPEED and EFFICIENCY are two different things. Cheers
We would say that foils produce less drag than planning craft for most speed regimes. Less drag results in higher speeds when the same thrust is applied. In that sense, foils are “faster”, but really we’re saying they’re more efficient at being faster. Planning hulls, under the right conditions, can reach the highest speeds ever achieved by watercraft, but the thrust requirements are large. The current speed sailing record is held by a planning craft, however there are a number of next-generation foil designs under development that could beat it. Will be exciting to watch…
@@Waterlust Yes, you just expounded the definition of efficiency. Foils are not even close to the speed and acceleration of planing craft. They have their place, for sure, in their efficiency. Cheers
@@Waterlust When you analyze the stats for speed sailing, it's evident that foils are both the solution and the problem. Compared to land and ice sailing, foils are almost half as slow - 125mph for land sailing and 75mph for foils. This shows just how much drag the foils are producing - 50mph worth of drag. Can foils be designed to match these speeds? I doubt it. Because, in a sense, foils are the equivalent of gears, which convert top speed into efficiency at medium to lower speeds. Unless foils can be made to ever reduce in accordance with higher speed, they cannot reach those higher speeds achieved by a planing hull. Cheers again.
@@robertcain3426 agreed, foils will always have a hard limit due to the friction they produce. Unlike a planning hull that can reduce its wetted surface area at higher speeds, foils must always be submerged in order to function. In order to go faster, the foil must be smaller, higher aspect ratio, etc…the problem with this is that smaller foils are less capable at slower speeds, and every foiling craft starts from rest. We’ve seen on the AC racing some boats beings “towed up” onto the foil in light winds. Depending on how the rules are interpreted, this may be an option that opens the door for small, higher speed foils. There are also some interesting kite based record attempts in the works like the SP80 project that seem well poised to break the 500 meter speed record of 65 knots held by Sailrocket for the last 10+ years. Land and ice sailing are different beasts as they have significantly less drag, though much can be learned from their rigs with respect to high apparent wind sails.
@@Waterlust Yes. That is a conundrum; the size of the foil. I have to dissagree with your comment about the high aspect foils in relation to high speed because HAR foils are about power and efficiency. Whereas LAR foils are about speed. For some reason this is a forgotten fact about LAR foils. This is another conumdrum. So eeither foil is reduced or becomes more of a LAR to go faster.
It’s a cheat code for water 😂
That was way too simplistic
𝕡𝕣𝕠𝕞𝕠𝕤𝕞 🎉
Take fluid mechanics again!
Did he get something wrong?
@@potsmkr87 Totally! A foil doesn't work by deflecting water doenward and then pushing upward the payload as a reaction! If that were the case we would simply use flat plates! And we don't!
@@PereAndreuUbachdeFuentes flat plates should work as foils, they are just not as effective
@@potsmkr87 , by stating that, he is invoking the principle of conservation of quantity of movement. But that principle is not at play here, because the foil doesn't change its vertical velocity, and neither water's overall vertical velocity isn't changed if you measure the velocity of water of a section sufficiently upstream and sufficiently downstream from the foil. No, that's not how a foil works!
@@Waterlust I insist: conservation of momentum has nothing to do with lift! As there is no upward momentum gained by the foil, nor the fuselage nor the payload. Upward force is not the same as upward momentum!
bla bla bla
Very well explained. Learnt many things in better way. Yet I got 1 Q. How these hydrofoil surfer gets fwd motion without motors?
how about the pump foil? what is the physics behind it? how the people keep pumping can actually accelerate the foil
Great question! Pumping a foil is similar to a bird flapping a wing. The underlying physics of how the foil itself creates lift and minimizes drag is the same, but there is a lot more complexity around the pumping motion and how it affects the overall flow field. Too much to cover in a comments section, perhaps we should make a video that dives into all the fun details!
@@Waterlust yes pls do a video on it. It would definitely help