The physics behind this tip regarding speed, is that you want to have a layer of water molecules sticking to the fin or foil reducing the hydrodynamic drag of the water as it pushes around the fin. Water forms chains of molecules and drag if there is a thin layer of water molecules already on the fin/foil the water slips by without sticking, increasing speed - The same thing can be achieved by carefully sanding the bottom of your board. This technique was first used with America's Cup boats (at least that is where I learned about it). 😊👍
Very good tip! Water sticking will indeed reduce the risk of ventilation. Just one remark, this effect is called ventilation, not cavitation. Cavitation is water evaporating due to very low pressure (only happens on speed fins), while ventilation is sucking air from the surface.
true submarines can cavitate their props hundreds of feet down where the pressure is tremendous and there is no air, obviously. But it seems everyone i know refers to spinning out as cavitating , semantics really.
Another hint, use a soft sanding block rather than bare hands. Something like a flat piece of polystyrene or similar to support the paper. The job will take less time and create a more even surface. Speaking from too many years experience in the yacht racing arena...
I'm going to move off topic. Let's start with your new association with Starboard. Starboard has a new evolution foil series. Basically a new set of fuselages and fuselage to wing connection for 2022. There is very little internet content on the new evolution foil supercrusier series. It looks like good stuff! I suggest it be promoted perhaps with a video! Have a great day!
Nico, Concerning the hydrophobic surface of carbon I made similar experiences with GASOIL fins. Well, I didn't smash my boom as beautifully as you. 😉 The best product to sand carbon surfaces are Micro-Mesh Soft Touch Pads. With this kind of sanding pad you get the best surface possible. Kind regards, Seb
The comments here are a great forum to discuss and gather new information. Some engineers here in the comments and many different theories. Just goes to show it's not all that simple. My background is not physics but testing on the water and find ways to solve problems in the gear. So be sure to read through the comments. Also, I just learned that this effect on foils, caused by the surface, is called ventilation. You can see the effect in this video: czcams.com/video/NefnpuxS2uM/video.html
Wow, that is a quick bubble over the entire length. I guess that explains what looks like the board spins out a little. I’ve had that happen sometimes on IQFoil gear, but only if I ride the board really flat, and I’m not going slalom speeds.
I think one thing people are missing is the massive drag increase from the mast ventalting and that drag being so low down from the center of effort of the sail, would be enough too cause catapult on its own. There is also the loss of sideways grip, and when you start to slide sidewards you in turn get a loss of pressure from the sail, causing you too fall windward rail in
Hi Nico, I remember the principle of cavitation back from my days in the German Navy. Cavitation happens at the propellers and might even destroy the surface of a bronze, copper-nickel or steel propeller.
The stall is happening on the front left-hand part of the foil and it's caused by the rotational movement of the board. It's what's called a 'tip stall'. Explanation: Just before the stall happens the right-hand edge of the board drops and you counter by forcing the left-hand side back down. This means the left-hand tip of the foil is moving downwards through the water giving it a higher angle of attack (it's the angle of attack going too high that causes a stall). At the same time this is happening you are pitching the board up to bring it back out of the water (further increasing the angle of attack) and rotating horizontally toward the left (meaning the left-hand side of the foil is going slower than the rest, making it more vulnerable to a stall). Once the tip of the foil stalls due to the movement, the lift decreases on that side and worsens the rotation causing the stall to develop more and more. It's interesting that you noticed that not sanding the foil mast caused the problem. My guess is the increased friction by having the surface too smooth either increased the amount of rotation the board was experiencing or caused flow issues around the root of the foil which meant you had to rely more on the lift from the foil tips. There's a secret target roughness each team has for America's Cup foils in order to decrease friction and improve flow for the same reasons. I hope my explanation was easy enough to understand/ not too long winded and hopefully it helps you to prevent it happening again.
@@ManOfSauce It wouldn't necessarily help unless you were planning to stay on one tack and even then I would say you're unlikely to notice much of a difference. Even on the scale of an AC yacht, where each foil is only used in one direction, we don't really worry too much about it. It's more important to reduce the friction on both sides as much as possible. There's a sweet spot for roughness that will give the best reduction in friction, but you'll need to do a bit of trial and error (or talk to other windsurfers/ sailors) to work out what it is.
@@mommomom6501 Haha I know. It's good practice between cup cycles though. During the cup we're constantly doing this sort of analysis to work out what the other teams are doing and what we can change in order to be faster.
I wonder if this applies to fins as well? Very interesting, it makes perfect sense thinking a little more about it. Thanks for the update, I was so curious as to what caused the catapults.
It's a tip that is based on sound science. The sanding creates lots of microscopic scratches which are water channels. It's the same science behind sharkskin and sharkskin swim suits. Those suits have been banned in swimming now due to the technology advantage. If it has performance benefits for a slow ass swimmer. It will definitely make a difference for a pro windsurfer which is why you guys sand all ur gear. Maybe one for starboard to innovate some sharkskin finish on boards and fins to take advantage of the tech
Wow an amazing interesting video, anything to do with lift and flight is just so cool and geeky, my favourite is the Coanda effect, makes a Dyson air wrap cool.
The demo with the water-hose is quite illustrative. It at least proves the water 'wets' the surface more after the sanding, so it has indeed become more hydrophylic. Did you actualy sail with that mast since you sanded it? Could you tell the difference? Do you know of colleagues who also sand their masts/wings/fins? I've read discussions on our dutch windsurf-forum sanding slalom-fins and bottoms of boards, it became almost a religious battle :) . Difficult to separate the science from the religion. If it really is about surface-tension, washing the mast (or wings/masts) with a strong soap should also work.
Very Interesting subject, I remember some saying about sanding the Bottom of the boards with fine wet and dry after spraying as you do not want a smooth surface as in a slick gloss wax finish where the water is rebelled, one’s brain would think the slicker Equals less drag, when you spray a boards bottom using car paint rattle can, once dried it comes out with a rough bobbly surface when you rub your hand over it, using wet and dry 2500 grit with water leaves the surface feeling lovely and smooth but without the water repellent effect. I was also told that you need some drag from bottom of board to water contact to go faster, feel free to comment to get to the bottom of this 🤣
Is there any reason why manufacturers do hydrophobic surfaces in the first place? I know that for example The Board Lady (Eva-M. Hollman) recommends sanding the bottom of the board in very similar manner to increase speed as well.
The hydrophobic property most likely comes from how smooth the carbon is when it comes out of the mould. America's Cup foils have the same issues, they're intentionally roughened afterwards to improve the hydrodynamics. When it's slightly rougher there's a boundary layer of water that sticks to the surface and reduces the overall friction.
Carbon wings. mast and most fins come from a negative mould. The surface of these moulds is prepared with a release-agent like a wax or something else that prevents the epoxy (or paint or gel-coat or whatever was applied first in the mold) from sticking to the surface. With this, the part would stay stuck in the mold or the mould's surface would get damaged. Some of this release agent transfers to the surface of the part (in this case: the mast). Especially a new mast would show this behavior (it wears of during use). But you can also cause it for example by handling your fin with greasy fingers, basically making the surface of your fin hydrofobic with the grease of your finger's skin. In theory, I think washing the mast with a strong soap, like a washing-up liquid, should also do the trick.
When sanding, you want to create a water layer on the foil so that water glides on water instead of on the foil itself (laminar flow). When sanding too fine (2000 grain), this could hamper the laminar flow. However, Nico Goyard also sands his foil with 2000er grain, so it should not be (too) wrong. In some cases, laminar flow can create more drag than turbulent flow (see the surface of golf balls) though this is probably not the case for foils in water. I've no idea if sanding the mast will really solve the problem that was most likely caused by a stall (Strömungsabriss) at the rear wing. However, taking off the warning sticker that Starboard sticks at one side of the base of the mast and that creates one-sided turbulences should improve the situation. You did not have this sticker on your former mast, if I saw right. And please tell Starboard to use another way to make the brand logo visible; these stickers on the mast have been annoying for a long time... And thanks so much for creating this community with your videos; more than 100 technical comments below your video are spectacular.
Laminar flow always have less drag than turbulent flow. Turbulence is energy and energy is drag. Golfball phenomenon is not correctly described. I.e. the phenomenon is that with the dimples you create less turbulence because it created little turbulent layer where the laminar flow can flow longer around the ball therefore the diameter of turblence is smaller, i.e. the dissipative force less.
Very cool video. Kind of not the same thing, but I've had a handful of crashes that seemed to act this way. They were at lower speeds (IQFoil gear not PWA slalom gear). I think in those I was riding the board more flat, without banking it windward, which likely caused a mast stall? I'm not sure if it got ventilated like this, but it's possible. Since I've been giving the board more heel, except in downwind, those particular types of crashes seem to have mostly gone away for me.
I sand flat all the letterings,stickers and graphics on my foil gear (they usually stick out a little), i also clean the dried salt&crap before sailing.The effect of sanding exists but having a clean,smooth surface is an even bigger factor IMHO.
For me in the beginning of studying "Material Science and Engineering" a very interesting Video. Seeing what impact such simple things can have. Maybe once i will be developing new stuff wich than not has not to be sanded anymore... Something like that would be my future dream :)
Recently got a new 725 front wing. Whistles a lot. Make sense to sand it. Didnt want to sand my new wings but guessing I will now. That's really good info...cheers
Hi .good subject. it is really a key point for foiling. But a very old sailing discussion started perhaps 100 years ago. On the net you could find a lot of scientific studies and experiments on that. Up to now the best surface to reduce the friction for reaching high speed is a "shark skin" type to create a thin layer of water. The shark skin is not smooth at all.do not worry too much with 500 1000 or less sand paper. But at high speed around 25 30 knts sure you enter in the cavitation danger zone. Cavitation not ventilation. And very litlle will initiate it .in particular quick angular variations or even dust in the water. It will limit the top speed. It is unavoidable. The propose solution is to create and maintain and control the cavitation .the supercavitation. .it will be in this case not only a thin layer of water but a thin layer of vaporized water. Used for military weapons. Alex caizergue team will try to use it for the speed record.check syroco project.
I was sure it was ventilation due to wing tip breach. But never expected the mast. This is new info, really thank you for this, i will go and sand my mast now😂👍
If sanding the mast improves the performance, why doesn’t Starboard sands it as part of the production process and provides it ready for flight to the customer? Industrial sanding could be much better and avoid horrible home-made mistakes... Cheers from the Swiss fun club😎
I suspect this is not actually cavitation but air getting sucked from the surface down along the mast. The air then gets sucked behind the stabilizer and disrupts it's lift which in turn suddenly disrupts the attack angle of the main wing. When you go fast enough... there is no overall positive angle of attack and the near vertical or even worse forward leaning foil mast sucks the air in. Alpine-foil make curved masts to prevent this for their kite-foils. The trick is to deliberately redirect the flow from the foil mast bottom to the top preventing the suction effect.
@@Nico_GER7 No need to apologize 😉 It's great that you reported your findings! It helps everybody to be safer. And the sanding trick is super interesting! Do you know the approximate speed it happened at? Do you think the wing was to large for the particular conditions. Was it the same wing every time? BTW I'm not an expert, just sharing my thoughts 😁 Thanks for the great video!
If sanding the surface is meant to help on this issue by leaving tiny scratches that get filled with water, so the surrounding water floats on water - then your sanding direction is wrong. the effect is at max when u sand it 90° to the direction of the water flow - for the foil mast that would be along its long side.
Wenn man sich ein Foil mit Finite Elemente Simulation ansieht, wird man merken, dass es an ein paar Kanten solche Unterdrucke geben kann, dass Kavitation entsteht. Diese Bereiche sind aber sehr sehr klein. Das kostet Geschwindigkeit, weil Energie verbraten wird, aber es führt nicht zu den Spin outs. Die kommen wie schon erwähnt von der Ventilation. Man kann das auch mit Sandpapier reduzieren - allerdings indem man die Kanten schärfer macht. Das ist eines der Physik Details, die bei Foils oft falsch verstanden werden. Der häufigste Irrglaube ist aber, dass das Foil so schnell ist, weil es die Oberfläche des Unterwasserschiffs reduziert. Die Fläche wird in der Realität sogar größer gemacht. Foils haben fast an jeder Stelle eine optimierte Spindelform mit CW Werten im Bereich < 0,08 (Mast bis zu 0,02). Das reduziert die Kräfte so.
hola nico, gracias por la explicacion, he marcado las alas del foil con las rocas, es necesario lijarlas? cual seria el efecto si las dejo asi rayadas?
Thank you for the tip and great video, as usual. Since the hydrophobic coating causes cavitation (or ventilation), why do manufacturers put this coating on foils?
So you’re saying that Starboard and other companies are selling DIY carbon foil parts instead of finished products ready and safe for sailing at all speeds?
No, I’m saying that you can tune your foil. Just like you can tune your board and your fin. Everything has its limits and it doesn’t happen with every new Mast. I had some where I had no problem. All I’m saying is if you have this problem, this could possibly be a solution.
Have you had an opportunity to try out the now sanded problem foil in similar conditions? Just a free rider. Never reaching your foiling speed. Occasionally do hear a "gurgling" sound.
Thx for sharing...maybe should be noticed by producer or made directly from production? Though we are adults and this is great to know ::) Thx for sharing!
Not sure on this BUT could it make sense to sand with different grades on different parts of the mast/fins etc? No idea but that just popped into my head.
Hi Nico. Great tip! Glad you investigated and found out the reason for the strange behaviour of the equipment. This reminds me of one of your earlier videos on the spinout behaviour (czcams.com/video/ya1vCfzpvS0/video.html&lc=UgyTqezfW9EQlDApuvN4AaABAg). I had commented that I had a feeling that the spinout is often caused by an air bubble caught around the fin, therefore reducing its effectiveness. When the fin doesn't have the right amount of friction (i.e. resistance), the whole gear acts as if there's no fin - or maybe as if there's a soft/fat fin in the shape of an air bubble! This problem with the foil mast is similar in that sense; that when water doesn't have the right contact with the mast, the function of the mast suffers. In the case of the foil of course, there's no time to correct and recover, hence the catapult!
Hi Nico. Would you recommand using 2000 send paper for the fuselage, wings, fins and bottom of boards. Thanks for the tips. Please do more videos like this one. Always great to learn from the pros (maybe including your teammates in other disciplines to know what we should to get better)....
It's been said that 2000 is too fine of a finish for most water sports. It might be good for higher speed America Cup boats. (speed and water temp dependent) 400 to 800 should be good for 20-30 knots. Faster speeds finer the sand paper. Also it's said to sand perpendicular to flow. jus t what I've read over the years.
Does it help to sand carbon fins, too? I've already learned that slalom fins handle the way I load them where as free-ride fins just don't but even the former spin out, for me, from time to time, and I'd really like to learn any trick to make them stick more reliably.
Nico i am following your channel, obviously i am very pleased with your windsurfing content. A pro windsurfer is not a scientist, giving those without the proper background could backfire really hard. Imagine people following your tip and sand the wrong way items costing 3K, or not having the expected result or seeing your video after an accident with serious injuries. If i was a Starboard manager i would be very sensitive with some of the comments implying that it is a product problem. Next thing, someone will start a legal case using your video against Starboard. Reputation is far more important than legal fines for corporations! Finally let me give you my point of view regarding our topic i.e. catapulting @ Defi and not Cavitation or Ventilation (any comment from a scientist windsurfer will only add credits to this discussion). Your video @ 2:08 has a slo mo showing a movement to the left of your tail followed immediately by a movement on the right (over correction) that changed your body balance, resulting in nose diving. What i am missing in the slo mo is the sudden movement of the tail on the right just before the sequence shown @ 2:08. Check @ 2:02, tail goes slightly on the right (rider error?), then on the left where slo mo starts (over-correction?) and finally on the right + nose dive. All this in one second, on high speeds, during a race on choppy water. I am not an expert and it would be very awkward to advice a pro, but from father to a son train more on high wind foiling before looking for dinosaurs in the closet :) Keep up with the good windsurfing footage!
And by the way, I didn't catapult once at the PWA in Israel during nine races. That was also on foil, but much more wind. I'm quite used to foiling in strong wind, which is why I can comfortably look at possible issues outside my skills.
For Aluminium mast mine is very noisy and I saw some video that I should sand it to solve it. For which reason you don’t recommend to sand a Alu mast foil ?
How does the cavitation on the mast extend to the foils? If the mast starts a cavitation you should experience a spin out instead of a catapult. I think that the problem here is the leading edge of the front wing and the joints of the foil to the fuselage that create spots for cavitation. If the front wing stalls the board will suddenly nose dive and therefore catapult you. In addition to that, sailing at these speeds could twist the mast and vary the angle of the attack of the mast itself to the point that cavitation happens on the mast. Changing the roughness of the mast surface will increase the boundary layer and simply delay a little the problem.
i think just the massive drag increase from the mast ventalting is so low down from the center of effort of the sail it would be enough too cause catapult on its own
Please don't sand it just with your fingers! You'll get an uneven surface and scratches under your fingertips. Use a foam block or anything similar to get an evenly surface and a perfect result.
I have one idea to the Starboard team..Why not using seagull wings one the front wing of the foil....It will make turns easyes and will not lose lift on curves thius..being more stable.....It will be a sucecess....!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!..If i want to reward me later....Don´t need,, just help the world to be a better one
isnt it that just your angle of attack was causing it... Also it seems that you where a lot higher above the water. cavitation happens only with higher speeds and knowing that foiling is still not going over 40kts, and guesing you where there doing perhaps 35kts there cannot be the case of cavitation.
As a researcher in fluid dynamics and more specifically fluid-structure-interaction I can tell you that your explanation seems quite unlikely. The mast does not provide much lateral thrust due to direct fluid action, therefore it's not the flow around the mast which is cavitating. The mast can instead provide a preferential way for air to be sucked down along the mast towards the foil/stabilizer. That would happen when you fly very high, but it's not a destructive process, should be a very gentle loss of lift. I would rather think you have a problem with stabilizer pitch. It's worth sticking a camera under The board in front of the mast, pointing at both foil wings and stabilizer.
Thanks Pietro. I can exclude that assumption of the stabilizer as this is the set up I've been using a lot and never had this problem. I mentioned that the mast is the only thing I changed and therefore is what I looked at.
This is interesting, thanks for sharing. I recently began having a problem with my foil, where I would be foiling perfectly fine around 21-23 kts, then all of a sudden it felt like a slight spinout would happen (I also thought I may have been hitting a cluster of seaweed) and it would cause me to quickly lose stability and usually forced me onto the leeward rail, sending me abruptly downwind. It most often resulted in a catapult - I actually broke two mast bases because of it. The strange thing is, it would only happen on port tack and never starboard tack. The loss of control was very quick. To diagnose my issue, I noticed I had some sticky residue (leftover marine grease) on the fuselage from when I moved my Slingshot switchfuse setup from position B into position C. I removed the grease with goo-gone, which hopefully resolves the problem. I haven't been foiling since, but I am hoping that the stickiness on the fuselage was the source of my problem. I wonder if any sort of hydrophobic/hydrophilic variance on water passing along any of the foil parts (even aluminum parts) is going to cause stability issues, even at my modest amateur speeds 😀.
Also try heeling the board a little bit more windward. The i76 also gets a bit harder to ride somewhere around it's top speed (which you seem to be very close to)
@@Nico_GER7 Years ago, I heard you should de-grease your fin and board, as your "get's worse" comment would support. I am a long way from pro speeds, but I first wash my gear with dish soap (leaves no film) then wet sand my gear. I was told there may always be a slight oil slick on water due to many factors like outboard motor oil in exhaust (less with 4 stroke outboards taking over and not so likely in open seas), spills, settling out of the air or ??? Ever do that? Maybe on your next new part, try washing first before sanding to see if the water sticks. Mold release compounds? Suntan lotion from hands/handling?
First things first: I am a physisist. And I have to say, that I don't know of any theory that backs up your assumption. Cavitation is a myth that lasts in windsurfing since day 1. But this effect which in very unlikely to happen in windsurfing compared to propellers in shipping boats has nothing to do with it. It is likely that the foils stalls but that it has to do with the mast and sanding the mast doesn't make any sense to me. We all know stalling from fins, i.e. spin out, but since my 40 years windsurfing I have not heard that stalling in fins can be avoided with sanding it. There is to much myth about sanding the hull and gain any measurable differences wrt to the friction as well. I can not judge really what is your problem and why you say it has gone away with sanding, but to me it sounds inconsistent. BTW, did you do again runs within similar! conditions with your setup after you sanded it and can you for sure say that your problems now do not happen again. Which is the minimum, which need to be done to back up your assumptions. Besides that, I really enjoy your channel. :-)
Interesting, thanks for the detail. I know that shark skin is more hydrodynamic than human skin because it is so rough. Some swimmers wear shark skin like suits to make them faster!
in other words, this is a manufacturer s bug, right? can one claim his money back? I mean, this is abnormal behavior and dangerous for the rider. imagine you are in the middle of the sea and your boom brakes bacause you catapulted.
Like Nico's boom broke at the Defi in offshore winds. Maybe Starboard should supply some sand paper and instructions on how to finish their product off.
Isn't windsurfing dangerous for the rider? We are talking about 28-30 knots + speed, which is not achievable by most non professional riders. Most of us will not event feel this kind of behaviour of the foil. The ones who get to these speeds know that you have to tune the equipment and water sanding the foil mast and wings is part of the tuning. Kite foilers do it all the time. During the defi all the pros were also doing it
@@thibault2173 this is not an argument. with the same logic, Porsche should not sell cars to not pro drivers. I'm trying to say that, if it only for pro equipment, then you have to inform the buyer. otherwise, they fool people
@@aisimeon please read again my comment. I said that amateurs who would get this feeling know how to use the equipment. Cavitation happens in kite foil as well as I said. My brother had no issues with his foil until 35 knots. Then the foil started cavitating. He had to sand paper it. Done. Is the supplier responsible ? I don't think so. Most people are happy with the foil they have and did not find any issue. In the case of starboard it is the same. They don't fool people as to cavitate it means that you use the equipment as the edge of what it is designed for. If you spin out and broke your ankle, are you going to say that the supplier of the fin fooled you because it is softer than what it should be? or the board supplier because the paint of the bottom is not as it should be to avoid spin outs? I don't think so.
Thank you for the tip and great video, as usual. Since the hydrophobic coating causes cavitation (or ventilation), why do manufacturers put this coating on foils?
The physics behind this tip regarding speed, is that you want to have a layer of water molecules sticking to the fin or foil reducing the hydrodynamic drag of the water as it pushes around the fin. Water forms chains of molecules and drag if there is a thin layer of water molecules already on the fin/foil the water slips by without sticking, increasing speed - The same thing can be achieved by carefully sanding the bottom of your board. This technique was first used with America's Cup boats (at least that is where I learned about it). 😊👍
Very good tip! Water sticking will indeed reduce the risk of ventilation. Just one remark, this effect is called ventilation, not cavitation. Cavitation is water evaporating due to very low pressure (only happens on speed fins), while ventilation is sucking air from the surface.
You're right, I already corrected that with my pinned comment 😊
@@Nico_GER7 great content! Keep it coming 😉
true submarines can cavitate their props hundreds of feet down where the pressure is tremendous and there is no air, obviously. But it seems everyone i know refers to spinning out as cavitating , semantics really.
Another hint, use a soft sanding block rather than bare hands. Something like a flat piece of polystyrene or similar to support the paper. The job will take less time and create a more even surface. Speaking from too many years experience in the yacht racing arena...
Yes, this is a good one! Important not to use a stiff block.
I'm going to move off topic. Let's start with your new association with Starboard. Starboard has a new evolution foil series. Basically a new set of fuselages and fuselage to wing connection for 2022. There is very little internet content on the new evolution foil supercrusier series. It looks like good stuff! I suggest it be promoted perhaps with a video! Have a great day!
Nico,
Concerning the hydrophobic surface of carbon I made similar experiences with GASOIL fins. Well, I didn't smash my boom as beautifully as you. 😉
The best product to sand carbon surfaces are Micro-Mesh Soft Touch Pads. With this kind of sanding pad you get the best surface possible.
Kind regards,
Seb
The comments here are a great forum to discuss and gather new information. Some engineers here in the comments and many different theories.
Just goes to show it's not all that simple. My background is not physics but testing on the water and find ways to solve problems in the gear. So be sure to read through the comments.
Also, I just learned that this effect on foils, caused by the surface, is called ventilation.
You can see the effect in this video: czcams.com/video/NefnpuxS2uM/video.html
Wow, that is a quick bubble over the entire length. I guess that explains what looks like the board spins out a little. I’ve had that happen sometimes on IQFoil gear, but only if I ride the board really flat, and I’m not going slalom speeds.
I think one thing people are missing is the massive drag increase from the mast ventalting and that drag being so low down from the center of effort of the sail, would be enough too cause catapult on its own.
There is also the loss of sideways grip, and when you start to slide sidewards you in turn get a loss of pressure from the sail, causing you too fall windward rail in
Hi Nico, I remember the principle of cavitation back from my days in the German Navy. Cavitation happens at the propellers and might even destroy the surface of a bronze, copper-nickel or steel propeller.
Indeed, and this is not cavitation that takes place on a foil but "ventilation". The speed is simply not high enough to generate cavitation.
Didn’t know Germany had a navy
The stall is happening on the front left-hand part of the foil and it's caused by the rotational movement of the board.
It's what's called a 'tip stall'.
Explanation:
Just before the stall happens the right-hand edge of the board drops and you counter by forcing the left-hand side back down. This means the left-hand tip of the foil is moving downwards through the water giving it a higher angle of attack (it's the angle of attack going too high that causes a stall).
At the same time this is happening you are pitching the board up to bring it back out of the water (further increasing the angle of attack) and rotating horizontally toward the left (meaning the left-hand side of the foil is going slower than the rest, making it more vulnerable to a stall).
Once the tip of the foil stalls due to the movement, the lift decreases on that side and worsens the rotation causing the stall to develop more and more.
It's interesting that you noticed that not sanding the foil mast caused the problem. My guess is the increased friction by having the surface too smooth either increased the amount of rotation the board was experiencing or caused flow issues around the root of the foil which meant you had to rely more on the lift from the foil tips. There's a secret target roughness each team has for America's Cup foils in order to decrease friction and improve flow for the same reasons.
I hope my explanation was easy enough to understand/ not too long winded and hopefully it helps you to prevent it happening again.
Thanks 🤙
You have too much free time
So
Would you suggest sanding to a different grade on one side of a foil mast to the other in order to counter such things?
@@ManOfSauce It wouldn't necessarily help unless you were planning to stay on one tack and even then I would say you're unlikely to notice much of a difference. Even on the scale of an AC yacht, where each foil is only used in one direction, we don't really worry too much about it. It's more important to reduce the friction on both sides as much as possible. There's a sweet spot for roughness that will give the best reduction in friction, but you'll need to do a bit of trial and error (or talk to other windsurfers/ sailors) to work out what it is.
@@mommomom6501 Haha I know. It's good practice between cup cycles though. During the cup we're constantly doing this sort of analysis to work out what the other teams are doing and what we can change in order to be faster.
I wonder if this applies to fins as well? Very interesting, it makes perfect sense thinking a little more about it. Thanks for the update, I was so curious as to what caused the catapults.
Yes, but with the fin having a much smaller surface it doesn’t make that big of a difference.
It's a tip that is based on sound science. The sanding creates lots of microscopic scratches which are water channels. It's the same science behind sharkskin and sharkskin swim suits. Those suits have been banned in swimming now due to the technology advantage. If it has performance benefits for a slow ass swimmer. It will definitely make a difference for a pro windsurfer which is why you guys sand all ur gear. Maybe one for starboard to innovate some sharkskin finish on boards and fins to take advantage of the tech
Wow an amazing interesting video, anything to do with lift and flight is just so cool and geeky, my favourite is the Coanda effect, makes a Dyson air wrap cool.
The demo with the water-hose is quite illustrative. It at least proves the water 'wets' the surface more after the sanding, so it has indeed become more hydrophylic.
Did you actualy sail with that mast since you sanded it? Could you tell the difference? Do you know of colleagues who also sand their masts/wings/fins? I've read discussions on our dutch windsurf-forum sanding slalom-fins and bottoms of boards, it became almost a religious battle :) . Difficult to separate the science from the religion. If it really is about surface-tension, washing the mast (or wings/masts) with a strong soap should also work.
Very Interesting subject, I remember some saying about sanding the Bottom of the boards with fine wet and dry after spraying as you do not want a smooth surface as in a slick gloss wax finish where the water is rebelled, one’s brain would think the slicker Equals less drag, when you spray a boards bottom using car paint rattle can, once dried it comes out with a rough bobbly surface when you rub your hand over it, using wet and dry 2500 grit with water leaves the surface feeling lovely and smooth but without the water repellent effect. I was also told that you need some drag from bottom of board to water contact to go faster, feel free to comment to get to the bottom of this 🤣
Is there any reason why manufacturers do hydrophobic surfaces in the first place? I know that for example The Board Lady (Eva-M. Hollman) recommends sanding the bottom of the board in very similar manner to increase speed as well.
The hydrophobic property most likely comes from how smooth the carbon is when it comes out of the mould. America's Cup foils have the same issues, they're intentionally roughened afterwards to improve the hydrodynamics. When it's slightly rougher there's a boundary layer of water that sticks to the surface and reduces the overall friction.
Often they are finished with some sort of a gel coat or varnish to protect the carbon or fill out pores.
Carbon wings. mast and most fins come from a negative mould. The surface of these moulds is prepared with a release-agent like a wax or something else that prevents the epoxy (or paint or gel-coat or whatever was applied first in the mold) from sticking to the surface. With this, the part would stay stuck in the mold or the mould's surface would get damaged. Some of this release agent transfers to the surface of the part (in this case: the mast). Especially a new mast would show this behavior (it wears of during use). But you can also cause it for example by handling your fin with greasy fingers, basically making the surface of your fin hydrofobic with the grease of your finger's skin. In theory, I think washing the mast with a strong soap, like a washing-up liquid, should also do the trick.
So useful and clear. Thanks Nico for shareing all these advice and keep going 💪
When sanding, you want to create a water layer on the foil so that water glides on water instead of on the foil itself (laminar flow). When sanding too fine (2000 grain), this could hamper the laminar flow. However, Nico Goyard also sands his foil with 2000er grain, so it should not be (too) wrong. In some cases, laminar flow can create more drag than turbulent flow (see the surface of golf balls) though this is probably not the case for foils in water.
I've no idea if sanding the mast will really solve the problem that was most likely caused by a stall (Strömungsabriss) at the rear wing. However, taking off the warning sticker that Starboard sticks at one side of the base of the mast and that creates one-sided turbulences should improve the situation. You did not have this sticker on your former mast, if I saw right. And please tell Starboard to use another way to make the brand logo visible; these stickers on the mast have been annoying for a long time...
And thanks so much for creating this community with your videos; more than 100 technical comments below your video are spectacular.
That sticker has helped us non IQ foilers. Why did you have to give away the secret :).
Laminar flow always have less drag than turbulent flow. Turbulence is energy and energy is drag. Golfball phenomenon is not correctly described. I.e. the phenomenon is that with the dimples you create less turbulence because it created little turbulent layer where the laminar flow can flow longer around the ball therefore the diameter of turblence is smaller, i.e. the dissipative force less.
Very cool video.
Kind of not the same thing, but I've had a handful of crashes that seemed to act this way. They were at lower speeds (IQFoil gear not PWA slalom gear). I think in those I was riding the board more flat, without banking it windward, which likely caused a mast stall? I'm not sure if it got ventilated like this, but it's possible.
Since I've been giving the board more heel, except in downwind, those particular types of crashes seem to have mostly gone away for me.
So you basically made a spin out with a foil? 😅
I sand flat all the letterings,stickers and graphics on my foil gear (they usually stick out a little), i also clean the dried salt&crap before sailing.The effect of sanding exists but having a clean,smooth surface is an even bigger factor IMHO.
So in case you don't want to send it you have to sand it, sweet 😉
For me in the beginning of studying "Material Science and Engineering" a very interesting Video. Seeing what impact such simple things can have. Maybe once i will be developing new stuff wich than not has not to be sanded anymore... Something like that would be my future dream :)
Recently got a new 725 front wing. Whistles a lot. Make sense to sand it. Didnt want to sand my new wings but guessing I will now. That's really good info...cheers
Hi .good subject.
it is really a key point for foiling.
But a very old sailing discussion started perhaps 100 years ago.
On the net you could find a lot of scientific studies and experiments on that. Up to now the best surface to reduce the friction for reaching high speed is a "shark skin" type to create a thin layer of water. The shark skin is not smooth at all.do not worry too much with 500 1000 or less sand paper.
But at high speed around 25 30 knts sure you enter in the cavitation danger zone. Cavitation not ventilation. And very litlle will initiate it .in particular quick angular variations or even dust in the water. It will limit the top speed.
It is unavoidable.
The propose solution is to create and maintain and control the cavitation .the supercavitation. .it will be in this case not only a thin layer of water but a thin layer of vaporized water. Used for military weapons.
Alex caizergue team will try to use it for the speed record.check syroco project.
I was sure it was ventilation due to wing tip breach. But never expected the mast. This is new info, really thank you for this, i will go and sand my mast now😂👍
If sanding the mast improves the performance, why doesn’t Starboard sands it as part of the production process and provides it ready for flight to the customer? Industrial sanding could be much better and avoid horrible home-made mistakes... Cheers from the Swiss fun club😎
This is quite a discovery, wow, amazing.
I suspect this is not actually cavitation but air getting sucked from the surface down along the mast. The air then gets sucked behind the stabilizer and disrupts it's lift which in turn suddenly disrupts the attack angle of the main wing. When you go fast enough... there is no overall positive angle of attack and the near vertical or even worse forward leaning foil mast sucks the air in. Alpine-foil make curved masts to prevent this for their kite-foils. The trick is to deliberately redirect the flow from the foil mast bottom to the top preventing the suction effect.
yes, sorry it's ventilation then.
@@Nico_GER7 No need to apologize 😉 It's great that you reported your findings! It helps everybody to be safer. And the sanding trick is super interesting! Do you know the approximate speed it happened at? Do you think the wing was to large for the particular conditions. Was it the same wing every time? BTW I'm not an expert, just sharing my thoughts 😁 Thanks for the great video!
Don’t foil just ride a slalom board, but this is super interesting
If sanding the surface is meant to help on this issue by leaving tiny scratches that get filled with water, so the surrounding water floats on water - then your sanding direction is wrong. the effect is at max when u sand it 90° to the direction of the water flow - for the foil mast that would be along its long side.
Latest rumors from iQFOiL scene is 45° in both directions. I guess every naval architect has their own interpretation.
Wenn man sich ein Foil mit Finite Elemente Simulation ansieht, wird man merken, dass es an ein paar Kanten solche Unterdrucke geben kann, dass Kavitation entsteht. Diese Bereiche sind aber sehr sehr klein. Das kostet Geschwindigkeit, weil Energie verbraten wird, aber es führt nicht zu den Spin outs. Die kommen wie schon erwähnt von der Ventilation. Man kann das auch mit Sandpapier reduzieren - allerdings indem man die Kanten schärfer macht.
Das ist eines der Physik Details, die bei Foils oft falsch verstanden werden. Der häufigste Irrglaube ist aber, dass das Foil so schnell ist, weil es die Oberfläche des Unterwasserschiffs reduziert. Die Fläche wird in der Realität sogar größer gemacht. Foils haben fast an jeder Stelle eine optimierte Spindelform mit CW Werten im Bereich < 0,08 (Mast bis zu 0,02). Das reduziert die Kräfte so.
hola nico, gracias por la explicacion, he marcado las alas del foil con las rocas, es necesario lijarlas? cual seria el efecto si las dejo asi rayadas?
Thank you for the tip and great video, as usual. Since the hydrophobic coating causes cavitation (or ventilation), why do manufacturers put this coating on foils?
He Nico, thanks. Now i sand all Foil parts but what do u say about the Board? Hypopheobic would release the Board better of the Water...?
'No matter where you are in the world, don't forget to saaaaand iiiit'
Thanks, this helps a lot. I normally would be quite reluctant to sand a brand new mast. I guess the same applies to fins?
So you’re saying that Starboard and other companies are selling DIY carbon foil parts instead of finished products ready and safe for sailing at all speeds?
No, I’m saying that you can tune your foil. Just like you can tune your board and your fin.
Everything has its limits and it doesn’t happen with every new Mast. I had some where I had no problem. All I’m
saying is if you have this problem, this could possibly be a solution.
Sehr Interessant! Danke Nico!
I have a question to everybody.Does anyone have an opinion on Unifiber Maverick Sails?
Have you had an opportunity to try out the now sanded problem foil in similar conditions? Just a free rider. Never reaching your foiling speed. Occasionally do hear a "gurgling" sound.
Thx for sharing...maybe should be noticed by producer or made directly from production? Though we are adults and this is great to know ::) Thx for sharing!
Not sure on this BUT could it make sense to sand with different grades on different parts of the mast/fins etc? No idea but that just popped into my head.
Hi Nico. Great tip! Glad you investigated and found out the reason for the strange behaviour of the equipment. This reminds me of one of your earlier videos on the spinout behaviour (czcams.com/video/ya1vCfzpvS0/video.html&lc=UgyTqezfW9EQlDApuvN4AaABAg). I had commented that I had a feeling that the spinout is often caused by an air bubble caught around the fin, therefore reducing its effectiveness. When the fin doesn't have the right amount of friction (i.e. resistance), the whole gear acts as if there's no fin - or maybe as if there's a soft/fat fin in the shape of an air bubble! This problem with the foil mast is similar in that sense; that when water doesn't have the right contact with the mast, the function of the mast suffers. In the case of the foil of course, there's no time to correct and recover, hence the catapult!
Ist das auf einem Parkplatz in Schönberg? :)
Hi Nico. Would you recommand using 2000 send paper for the fuselage, wings, fins and bottom of boards. Thanks for the tips. Please do more videos like this one. Always great to learn from the pros (maybe including your teammates in other disciplines to know what we should to get better)....
Well this explains my random catapults whilst pushing for speed, as consequence I had had some huge cracks on my IQ board..
Only problem is I don't think sanding of anything is allowed for iQ competition.
Hi Nico, good information. Do you think it might also be worth wiht fin's?
It's been said that 2000 is too fine of a finish for most water sports. It might be good for higher speed America Cup boats. (speed and water temp dependent)
400 to 800 should be good for 20-30 knots. Faster speeds finer the sand paper. Also it's said to sand perpendicular to flow. jus t what I've read over the years.
Yes, I came to similar conclusions now
Does It Works for fins Also?
Nico, does this sanding theory also apply to fins?
Can you do that on your slalom fin?
Does it help to sand carbon fins, too? I've already learned that slalom fins handle the way I load them where as free-ride fins just don't but even the former spin out, for me, from time to time, and I'd really like to learn any trick to make them stick more reliably.
In principle, yes. But I would start with just wshing them with a strong soap before going at them with sand-paper.
Thanks for a good content
Are you allowed to sand the mast in IQ foil competition?
Nico , what are the specs of that proto in your van … the blue one
Nico i am following your channel, obviously i am very pleased with your windsurfing content. A pro windsurfer is not a scientist, giving those without the proper background could backfire really hard.
Imagine people following your tip and sand the wrong way items costing 3K, or not having the expected result or seeing your video after an accident with serious injuries. If i was a Starboard manager i would be very sensitive with some of the comments implying that it is a product problem. Next thing, someone will start a legal case using your video against Starboard. Reputation is far more important than legal fines for corporations!
Finally let me give you my point of view regarding our topic i.e. catapulting @ Defi and not Cavitation or Ventilation (any comment from a scientist windsurfer will only add credits to this discussion).
Your video @ 2:08 has a slo mo showing a movement to the left of your tail followed immediately by a movement on the right (over correction) that changed your body balance, resulting in nose diving. What i am missing in the slo mo is the sudden movement of the tail on the right just before the sequence shown @ 2:08. Check @ 2:02, tail goes slightly on the right (rider error?), then on the left where slo mo starts (over-correction?) and finally on the right + nose dive. All this in one second, on high speeds, during a race on choppy water. I am not an expert and it would be very awkward to advice a pro, but from father to a son train more on high wind foiling before looking for dinosaurs in the closet :)
Keep up with the good windsurfing footage!
Hi Demetris, I'm not managing Starboardfoils. It's a different company.
And by the way, I didn't catapult once at the PWA in Israel during nine races.
That was also on foil, but much more wind. I'm quite used to foiling in strong wind, which is why I can comfortably look at possible issues outside my skills.
For Aluminium mast mine is very noisy and I saw some video that I should sand it to solve it. For which reason you don’t recommend to sand a Alu mast foil ?
I think it recommends the trailing edge? If you sand the whole surface, you might get corrosion.
@@Nico_GER7 yes right different issues same solution but not on the whole surface
Nico, what's your guess about how many of the top race foilers sand their masts? I'd be especially interested if the other Nico (Goyard) does it.
He does. He and his brother are some of the best stuff tuners in the world.
For me foiling is completely different from fin.to keep contrôle we need much concentration and much training
Interesting video again!
Can you do it on a fin aswell ?
sure!
My axis carbon mast ventilates, I wonder whether this would help. Has anyone tried?
Maybe we should give it a try. Let me know about your results 😊
Where is your next big event at?
Dafür hast Du also die Dusche vom Vantourer?
How does the cavitation on the mast extend to the foils? If the mast starts a cavitation you should experience a spin out instead of a catapult. I think that the problem here is the leading edge of the front wing and the joints of the foil to the fuselage that create spots for cavitation. If the front wing stalls the board will suddenly nose dive and therefore catapult you. In addition to that, sailing at these speeds could twist the mast and vary the angle of the attack of the mast itself to the point that cavitation happens on the mast. Changing the roughness of the mast surface will increase the boundary layer and simply delay a little the problem.
You can see that the cavitation on the mast happens first.
i think just the massive drag increase from the mast ventalting is so low down from the center of effort of the sail it would be enough too cause catapult on its own
Please don't sand it just with your fingers! You'll get an uneven surface and scratches under your fingertips. Use a foam block or anything similar to get an evenly surface and a perfect result.
Where are you in North Germany ?
I have one idea to the Starboard team..Why not using seagull wings one the front wing of the foil....It will make turns easyes and will not lose lift on curves thius..being more stable.....It will be a sucecess....!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!..If i want to reward me later....Don´t need,, just help the world to be a better one
Great
thx for the tip 🤙🏻🤙🏻🤙🏻🤙🏻🤙🏻
It may be easier to find a hydrophilic treatment to accomplish the same and not possibly damage carbon fiber parts.
You don't damage the carbon fibers, because you're only sanding the coating.
Wow!
isnt it that just your angle of attack was causing it... Also it seems that you where a lot higher above the water.
cavitation happens only with higher speeds and knowing that foiling is still not going over 40kts, and guesing you where there doing perhaps 35kts there cannot be the case of cavitation.
That would mean my angle of attack would have been too high, which was certainly not the case.
As a researcher in fluid dynamics and more specifically fluid-structure-interaction I can tell you that your explanation seems quite unlikely. The mast does not provide much lateral thrust due to direct fluid action, therefore it's not the flow around the mast which is cavitating. The mast can instead provide a preferential way for air to be sucked down along the mast towards the foil/stabilizer. That would happen when you fly very high, but it's not a destructive process, should be a very gentle loss of lift. I would rather think you have a problem with stabilizer pitch. It's worth sticking a camera under The board in front of the mast, pointing at both foil wings and stabilizer.
Thanks Pietro. I can exclude that assumption of the stabilizer as this is the set up I've been using a lot and never had this problem.
I mentioned that the mast is the only thing I changed and therefore is what I looked at.
This is interesting, thanks for sharing. I recently began having a problem with my foil, where I would be foiling perfectly fine around 21-23 kts, then all of a sudden it felt like a slight spinout would happen (I also thought I may have been hitting a cluster of seaweed) and it would cause me to quickly lose stability and usually forced me onto the leeward rail, sending me abruptly downwind. It most often resulted in a catapult - I actually broke two mast bases because of it. The strange thing is, it would only happen on port tack and never starboard tack. The loss of control was very quick.
To diagnose my issue, I noticed I had some sticky residue (leftover marine grease) on the fuselage from when I moved my Slingshot switchfuse setup from position B into position C. I removed the grease with goo-gone, which hopefully resolves the problem. I haven't been foiling since, but I am hoping that the stickiness on the fuselage was the source of my problem. I wonder if any sort of hydrophobic/hydrophilic variance on water passing along any of the foil parts (even aluminum parts) is going to cause stability issues, even at my modest amateur speeds 😀.
Also try heeling the board a little bit more windward. The i76 also gets a bit harder to ride somewhere around it's top speed (which you seem to be very close to)
Insane! So you think if I sand my carbon fin with 2000grit sand paper, I can increase my speed?
Try to oil the foil and see what will happen
It gets worse 😂
@@Nico_GER7 Years ago, I heard you should de-grease your fin and board, as your "get's worse" comment would support. I am a long way from pro speeds, but I first wash my gear with dish soap (leaves no film) then wet sand my gear. I was told there may always be a slight oil slick on water due to many factors like outboard motor oil in exhaust (less with 4 stroke outboards taking over and not so likely in open seas), spills, settling out of the air or ??? Ever do that? Maybe on your next new part, try washing first before sanding to see if the water sticks. Mold release compounds? Suntan lotion from hands/handling?
First things first: I am a physisist. And I have to say, that I don't know of any theory that backs up your assumption. Cavitation is a myth that lasts in windsurfing since day 1. But this effect which in very unlikely to happen in windsurfing compared to propellers in shipping boats has nothing to do with it. It is likely that the foils stalls but that it has to do with the mast and sanding the mast doesn't make any sense to me. We all know stalling from fins, i.e. spin out, but since my 40 years windsurfing I have not heard that stalling in fins can be avoided with sanding it. There is to much myth about sanding the hull and gain any measurable differences wrt to the friction as well. I can not judge really what is your problem and why you say it has gone away with sanding, but to me it sounds inconsistent. BTW, did you do again runs within similar! conditions with your setup after you sanded it and can you for sure say that your problems now do not happen again. Which is the minimum, which need to be done to back up your assumptions. Besides that, I really enjoy your channel. :-)
Interesting, thanks for the detail. I know that shark skin is more hydrodynamic than human skin because it is so rough. Some swimmers wear shark skin like suits to make them faster!
LOL
in other words, this is a manufacturer s bug, right? can one claim his money back? I mean, this is abnormal behavior and dangerous for the rider. imagine you are in the middle of the sea and your boom brakes bacause you catapulted.
Like Nico's boom broke at the Defi in offshore winds. Maybe Starboard should supply some sand paper and instructions on how to finish their product off.
Isn't windsurfing dangerous for the rider?
We are talking about 28-30 knots + speed, which is not achievable by most non professional riders. Most of us will not event feel this kind of behaviour of the foil.
The ones who get to these speeds know that you have to tune the equipment and water sanding the foil mast and wings is part of the tuning.
Kite foilers do it all the time. During the defi all the pros were also doing it
@@thibault2173 this is not an argument. with the same logic, Porsche should not sell cars to not pro drivers. I'm trying to say that, if it only for pro equipment, then you have to inform the buyer. otherwise, they fool people
@@aisimeon please read again my comment. I said that amateurs who would get this feeling know how to use the equipment.
Cavitation happens in kite foil as well as I said. My brother had no issues with his foil until 35 knots. Then the foil started cavitating. He had to sand paper it. Done. Is the supplier responsible ? I don't think so. Most people are happy with the foil they have and did not find any issue.
In the case of starboard it is the same. They don't fool people as to cavitate it means that you use the equipment as the edge of what it is designed for.
If you spin out and broke your ankle, are you going to say that the supplier of the fin fooled you because it is softer than what it should be? or the board supplier because the paint of the bottom is not as it should be to avoid spin outs?
I don't think so.
Smells like...................excuses 🤔
Yep, when something doesn't work, always give your best to find excuses. Or solutions... you can see it however you want.
Some of this concept could transfer to surfboard surfacing 🤔 water molecular bonding is an exceptional science that shall improve performance 💯%👌
Thank you for the tip and great video, as usual. Since the hydrophobic coating causes cavitation (or ventilation), why do manufacturers put this coating on foils?