So glad it was helpful for you! More ocean science videos like this coming, stay tuned!

You’re very welcome, more educational content like this coming, stay tuned!

Right on, glad you enjoyed it!

Glad you enjoyed it! The interplay between pressure and velocity in a fluid is super tricky to visualize, glad the metaphor helped!!

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!

Glad you enjoyed it!!

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!

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 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!

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.

We’d say the text we cite at the end of the video by Doug McLean is the best resource for general readers.

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!!

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”.

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.

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.

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!

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.

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.

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.

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.

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 🤔

Oh just seen it, thanks!

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.

If we had to bet money on it, we’d say it’s flow separation (e.g stall) driven by an adverse pressure gradient.

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!

Similar dynamics to flapping a wing. Might have to make a separate video on foil pumping!

Science rules!

By the way i am in grade 5

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.

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!

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