I've heard that foils have a max speed of about 50knts due to cavitation, so did a bit of googling:

"Foils tend to cavitate around 50 knots because at that speed, the low pressure created on the foil due to its design becomes so extreme that it reaches the vapor pressure of water, causing the water to essentially "boil" and form bubbles, effectively reducing the foil's ability to generate lift and increasing drag; this phenomenon is known as cavitation, and is a significant limiting factor for high-speed sailing vessels using foils"

I am wondering is this not also an issue of fins? By my (very limited) understanding a fin is essentially a foil. Given the current record is about 53(?) knts have we topped out?

Quite possibly, but I'm not game to say yes, you never know what some smart bugger has up their sleeve round the corner.

Americas Cup boats do 55 knots on foils so your research may be a bit flawed. Windsurfers have peaked at just under 56 knots. But to throw a cat amongst the pigeons SailRocket did over 68 knots on a supercavitating foil. If you use the science anything is possible.

I'm also going to say a foil has a much larger pressure differential- it's lifting what, 100kg easily at low speeds? a fin doesn't have 100kg acting laterally surely so my guess is there is a massive difference in threshold for 'problems' of any kind caused by pressure

but the title of this thread does exclude me

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Mark _australia said..
I'm also going to say a foil has a much larger pressure differential- it's lifting what, 100kg easily at low speeds? a fin doesn't have 100kg acting laterally surely so my guess is there is a massive difference in threshold for 'problems' of any kind caused by pressure

but the title of this thread does exclude me

Cavitation can occur without any pressure differential between sides.
The low pressure at the CL is enough to do it on both sides of a symmetric foil at zero angle of attack.
But I feel when this happens, varies with the foil

I've said this before, they should put fluoride in the water to stop this cavitation.

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PhilUK said..
I've said this before, they should put fluoride in the water to stop this cavitation.

What if we made fins from chalk ?

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Panno said..
"Foils tend to cavitate around 50 knots because at that speed, the low pressure created on the foil due to its design becomes so extreme that it reaches the vapor pressure of water, causing the water to essentially "boil" and form bubbles, effectively reducing the foil's ability to generate lift and increasing drag; this phenomenon is known as cavitation, and is a significant limiting factor for high-speed sailing vessels using foils"

I am wondering is this not also an issue of fins? By my (very limited) understanding a fin is essentially a foil. Given the current record is about 53(?) knts have we topped out?

Fins are foils, but the "around" is important. The pressure differential that causes cavitation depends not only on speed, but also on angle of attack and foil thickness. So speeds can be pushed past 50 knots by using thinner foils (and fins), and very broad reaches as in Luderitz allow for lower angles of attack. There is a limit, though, since speed typically comes in squared into foil force calculations.

For windsurfing, I doubt that cavitation is currently the limiting factor. That's simply based on the observation (or rather, guess) that "catastrophic" fin failures, which cavitation at 50+ knots would be, seem to be quite rare, even in runs with top speeds above 50 knots. If we really were so close to the absolute maximum, I'd expect to see more spinout-like crashes caused by cavitation.

There's an interesting analysis at boards.co.uk/how-to/how-fast-can-we-go-the-science-of-speed.html that looks at forces and drag components in speedsurfing. It's not perfect; for example, it basically assumes that the maximum sail force is limited to 40% of the body weight, and seems to ignore the effect of body tension. But the general analysis of the relation between forward thrust, side forces, drag, and apparent wind angle is quite useful. Very simplified, to go 10 knots faster than the current speed limit would require a lot more wind, too - not just 10 knots more. Given the crazy conditions needed for the current record speeds, it's hard to imaging how someone would start windsurfing a speed board in, say, 20 more knots of wind.

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boardsurfr said.. That's simply based on the observation (or rather, guess) that "catastrophic" fin failures, which cavitation at 50+ knots would be, seem to be quite rare, even in runs with top speeds above 50 knots. If we really were so close to the absolute maximum, I'd expect to see more spinout-like crashes caused by cavitation.

There's an interesting analysis at boards.co.uk/how-to/how-fast-can-we-go-the-science-of-speed.html that looks at forces and drag components in speedsurfing. It's not perfect; for example, it basically assumes that the maximum sail force is limited to 40% of the body weight, and seems to ignore the effect of body tension. But the general analysis of the relation between forward thrust, side forces, drag, and apparent wind angle is quite useful. Very simplified, to go 10 knots faster than the current speed limit would require a lot more wind, too - not just 10 knots more. Given the crazy conditions needed for the current record speeds, it's hard to imaging how someone would start windsurfing a speed board in, say, 20 more knots of wind.

I don't quite agree here Peter.
Firstly we mustn't get ventilation and cavitation mixed up, they are 2 separate things.

When it's said cavitation begins at 50 knots, it's just talking about a few small vapor burbles forming on the fin. This doesn't necessarily lead to spin out but does reduce lift and increase drag. This will have a limiting effect on speed, but not be an absolute barrier that suddenly appears at 50 knots. It's an increasing resistance as speed builds up,

I also don't think any pressure differential has much to do with it. It's when the pressure on the fin reduces below the vapor pressure of water.

If vapour pressure is important, then speed sailors should find a nice flat spot behind an iceberg

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Mr Milk said..
If vapour pressure is important, then speed sailors should find a nice flat spot behind an iceberg

We did talk about this and the importance of the cold dense air awhile back with beer and brainstorming crazy ideas. Might have moved all the furniture so we could rig a sail in the lounge room during Speedfortnight days

What's the water temp at Luderitz?????

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decrepit said..
Firstly we mustn't get ventilation and cavitation mixed up, they are 2 separate things.

When it's said cavitation begins at 50 knots, it's just talking about a few small vapor burbles forming on the fin. This doesn't necessarily lead to spin out but does reduce lift and increase drag. This will have a limiting effect on speed, but not be an absolute barrier that suddenly appears at 50 knots. It's an increasing resistance as speed builds up,

I am quite aware of the difference between cavitation and ventilation. But were we disagree is about what causes spinouts at high speeds - or, more specifically, if cavitation would cause spinouts, in particular at high speed.

There are two ways of getting spinouts at high speed (which are not mutually exclusive): going over chop, and load changes on the fin. Going over chop seems to point towards ventilation, where air is sucked down from the surface. There is plenty of evidence that ventilation plays a big role in many spinouts. But that does not mean that cavitation does not play a role when we are talking about very high speeds.

The lift the fin produces keeps it from siding sideways. If we assume the scenario you describe, where cavitation happens gradually and reduces lift, then the reduced lift would mean the fin starts sliding sideways. If this happens in a controlled way, then this leads to a higher angle of attack, which increases lift, so the side way slide stops, and we reach a new steady state at a higher (and slower) angle of attack. That's the scenario you describe. This is known to be the case for some foiling boats, where foils can show damage from cavitation bubbles.

But in the case of vertical lift, like we have in foiling boats, the worst-case scenario of very bad cavitation is that the boat drops of the foil. Racing catamarans usually handle this well.

The scenario is very different for speedsurfing, though, where the lift of the foil is horizontal, not vertical. One problem is that a higher angle of attack also reduces the pressure on the low-pressure side of the fin further, which then increases cavitation. In other words, we have a positive feedback loop here. The much smaller size of the speed fins compared to foils on race boats also means that the relative effect of the first cavitation bubbles that form will be a lot larger. It seems quite possible, or indeed likely, that the cavitation at the fin would produce a full-scale spinout.

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decrepit said..
I also don't think any pressure differential has much to do with it. It's when the pressure on the fin reduces below the vapor pressure of water.

Well, how low the pressure will go is directly proportional to the pressure differential. Higher pressure on one side is balanced by lower pressure on the other side (although you may have to think in a 3D integration way for this). So the higher the pressure differential, the more likely cavitation becomes. "The greater the speed and/or the angle of incidence, the lower the pressure on the extrados becomes. When this pressure gets low enough, cavitation (vapor formation) happens." (from syro.co/en/news/cavitation-dummies/). The "angle of incidence" or effective angle of attack is also determined by the foil profile, so a thinner speed foil would cavitate at higher speed than a thicker freeride foil (but also produce less lift at identical speeds).

So I just had my first go at chatgpt.
Yes, cavitation can occur on a symmetric foil at zero angle of attack, but it depends on the flow conditions and the foil's shape. Here's how:
1. Flow-Induced Pressure Drop

Even at zero angle of attack, a symmetric foil can still experience cavitation if:

The flow velocity is high enough to create a low-pressure region on the foil.
The ambient static pressure is low, such as at high altitudes or in deep-sea conditions.
The foil is designed with curvature or features that cause localized acceleration of flow.

2. Types of Cavitation on a Symmetric Foil

Bubble Cavitation: If the pressure anywhere on the foil drops below the vapor pressure of water, vapor bubbles form.
Sheet Cavitation: A stable vapor layer can form along the foil's surface.
Leading-Edge Cavitation: If the foil has a sharp leading edge, it can cause a pressure drop even without an angle of attack.

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boardsurfr said..
It seems quite possible, or indeed likely, that the cavitation at the fin would produce a full-scale spinout.).

Certainly, but indirectly I think, an increased angle of attack increases the chance of stalling and ventilation.

So yes cavitation can induce ventilation. and spinout.
Trying to ascertain if cavitation alone can cause spinout without ventilation occurring, would be rather difficult I think.

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decrepit said..
So I just had my first go at chatgpt.
Yes, cavitation can occur on a symmetric foil at zero angle of attack, but it depends on the flow conditions and the foil's shape. Here's how:
1. Flow-Induced Pressure Drop

Even at zero angle of attack, a symmetric foil can still experience cavitation if:

The flow velocity is high enough to create a low-pressure region on the foil.
The ambient static pressure is low, such as at high altitudes or in deep-sea conditions.
The foil is designed with curvature or features that cause localized acceleration of flow.

This is quite an amusing answer from chatgpt. If you ask it why the ambient pressure us low in deep-see conditions, it might have seconds thoughts. And who would have even thunken about putting curvature in a foil?

It's worth looking at what slowie has to say: www.tribalwindsurfing.com/pages/cavitation_tech
He seems quite convinced that cavitation is limiting windsurfing speeds. He also mentions that cavitation can induce ventilation and thus spinouts, but pretty much says that's not always the case. His theory that cavitation is collection air bubbles in the water, rather than water vapor from reduced pressure (www.myengineeringtools.com/Data_Diagrams/Water_Boiling_Point_Vs_Pressure.html), seems off, though.

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boardsurfr said..

Panno said..
"Foils tend to cavitate around 50 knots because....

...

There's an interesting analysis at boards.co.uk/how-to/how-fast-can-we-go-the-science-of-speed.html that looks at forces and drag components...

Interesting article!

Now I wonder what happens if you design a sail with a lower center of effort. Like 1.8m instead of 2.4m like in their calculation.
Max sail force reaches 53% of bodyweight with the same body, boom and board geometry.

The article is indeed interesting, but it has quite a few flaws. Here's the diagram the analysis is based on:


Here's a screen shot from the canal:

Things to note:
1. The sailor is not just hanging out to the side, but also backwards. Possibly more backwards than to the side, although that's tough to tell from the picture.
2. The front leg is almost horizontal.
3. The harness lines are more horizontal then vertical, which is the opposite of the diagram. The harness line orientation is an indication that the sail's center of effort is lower than in the diagram.

The bigger issue, though, is that the diagram ignores forces from the sailor's body tension. Anyone who has ever done the pulling exercise in an ABK camp, where you lean backwards in a figure 7 stance and pull the person holding you by just tensioning your calf muscles, will know what I mean. For those who have not done this exercise, here's a different example from a strong man contest:




This is from a truck pulling competition. If he's just stand there like this, the truck would never move. To get the truck to move, he needs to use a lot of muscle power. Note that the position is somewhat similar to a speedsurfing stance. Here's the video the screen shot is from:

Bottom line is that the forces in play during 50+ knot runs are likely to be a lot higher than the article estimates. If so, that also casts doubt on the various drag estimates, which are just estimates, anyway,

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Nicko29 said..

boardsurfr said..

Panno said..
"Foils tend to cavitate around 50 knots because....

...

There's an interesting analysis at boards.co.uk/how-to/how-fast-can-we-go-the-science-of-speed.html that looks at forces and drag components...

Interesting article!

Now I wonder what happens if you design a sail with a lower center of effort. Like 1.8m instead of 2.4m like in their calculation.
Max sail force reaches 53% of bodyweight with the same body, boom and board geometry.

I think lower centre would be better. However , you have to get over the strangeness that happens at water level.

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Panno said..
I've heard that foils have a max speed of about 50knts due to cavitation, so did a bit of googling:

"Foils tend to cavitate around 50 knots because at that speed, the low pressure created on the foil due to its design becomes so extreme that it reaches the vapor pressure of water, causing the water to essentially "boil" and form bubbles, effectively reducing the foil's ability to generate lift and increasing drag; this phenomenon is known as cavitation, and is a significant limiting factor for high-speed sailing vessels using foils"

I am wondering is this not also an issue of fins? By my (very limited) understanding a fin is essentially a foil. Given the current record is about 53(?) knts have we topped out?

OMG, of course it is an issue for fins as well. The question only is, at what speed the issue of low pressure / i.e. boiling water arrises. I don't think, that the windsurfing industry has the resources to research that like the sailing industry has.

All you need to read:

www.yacht.de/en/regatta/vendee-globe/technology-there-is-no-such-thing-as-the-perfect-foil-how-airfoils-work/

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Panno said..
I've heard that foils have a max speed of about 50knts due to cavitation, so did a bit of googling:

"Foils tend to cavitate around 50 knots because at that speed, the low pressure created on the foil due to its design becomes so extreme that it reaches the vapor pressure of water, causing the water to essentially "boil" and form bubbles, effectively reducing the foil's ability to generate lift and increasing drag; this phenomenon is known as cavitation, and is a significant limiting factor for high-speed sailing vessels using foils"

I am wondering is this not also an issue of fins? By my (very limited) understanding a fin is essentially a foil. Given the current record is about 53(?) knts have we topped out?

OMG, of course it is an issue for fins as well. The question only is, at what speed the issue of low pressure / i.e. boiling water arrises. I don't think, that the windsurfing industry has the resources to research that like the sailing industry has.

All you need to read:

www.yacht.de/en/regatta/vendee-globe/technology-there-is-no-such-thing-as-the-perfect-foil-how-airfoils-work/

and in case you want to see super kavitation happing. A pupils experiment. Gr8 to watch...

My work experience is related to high-output underwater acoustic devices (think along the lines of sonar), and I've generally found that initial "cavitation" isn't true cavitation, but a combination of dissolved gas coming out of solution and/or expansion of tiny air bubbles in low pressure regions to be visible. This happens at pressure significantly higher than the theoretical vapor pressure of water at whatever ambient temperature.

Once water is forcibly de-gassed through low pressure action, it does not immediately reabsorb the gas bubbles, creating a cloud that looks like cavitation during system operation. The same behavior will certainly apply to the low pressure regions of fins and foils. This will act to add volume to the low pressure side and reduce overall pressure differential achieved for a given AoA, effectively reducing lift and increasing drag. I suppose it's also possible for the problem to be severe enough to be indistinguishable from ventilation, but that strikes me as a more probable issue in the wave-sailing environment rather than speed-sailing.

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ozzimark said..
My work experience is related to high-output underwater acoustic devices (think along the lines of sonar), and I've generally found that initial "cavitation" isn't true cavitation, but a combination of dissolved gas coming out of solution and/or expansion of tiny air bubbles in low pressure regions to be visible. This happens at pressure significantly higher than the theoretical vapor pressure of water at whatever ambient temperature.

That's an interesting detail to add to this, thank you.

So, it sounds like for the ultimate in performance we will need to develop foils with an internal cooling system in order to go faster. I imagine that could look super trick, with a cool anodised compressor humming away between the rear footstraps and a badass radiator grille. Nice

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ozzimark said..
My work experience is related to high-output underwater acoustic devices (think along the lines of sonar), and I've generally found that initial "cavitation" isn't true cavitation, but a combination of dissolved gas coming out of solution and/or expansion of tiny air bubbles in low pressure regions to be visible. This happens at pressure significantly higher than the theoretical vapor pressure of water at whatever ambient temperature.

Interesting. This is exactly what Chris said in the video about cavitation. For true cavitation from water transitioning from liquid to gas, pressure has to drop about 40-fold, depending on temperature etc. But if temperature drops only 10-fold, existing bubbles should increase in volume 10-fold. It's also easy to see that this is a gradual process.

If existing bubbles and/or dissolved gas in the water are the main cause of the "cavitation speed wall" in speedsurfing, then anything reducing chop should improve speeds. I guess Luderitz needs the chop-killing surfactants that the weed in Lake George produces.

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boardsurfr said..

ozzimark said..
My work experience is related to high-output underwater acoustic devices (think along the lines of sonar), and I've generally found that initial "cavitation" isn't true cavitation, but a combination of dissolved gas coming out of solution and/or expansion of tiny air bubbles in low pressure regions to be visible. This happens at pressure significantly higher than the theoretical vapor pressure of water at whatever ambient temperature.

Interesting. This is exactly what Chris said in the video about cavitation. For true cavitation from water transitioning from liquid to gas, pressure has to drop about 40-fold, depending on temperature etc. But if temperature drops only 10-fold, existing bubbles should increase in volume 10-fold. It's also easy to see that this is a gradual process.

If existing bubbles and/or dissolved gas in the water are the main cause of the "cavitation speed wall" in speedsurfing, then anything reducing chop should improve speeds. I guess Luderitz needs the chop-killing surfactants that the weed in Lake George produces.

I wonder how much drag shade balls would cause.
?si=ElSVs6YyMq0szyob

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choco said..

boardsurfr said..

ozzimark said..
My work experience is related to high-output underwater acoustic devices (think along the lines of sonar), and I've generally found that initial "cavitation" isn't true cavitation, but a combination of dissolved gas coming out of solution and/or expansion of tiny air bubbles in low pressure regions to be visible. This happens at pressure significantly higher than the theoretical vapor pressure of water at whatever ambient temperature.

Interesting. This is exactly what Chris said in the video about cavitation. For true cavitation from water transitioning from liquid to gas, pressure has to drop about 40-fold, depending on temperature etc. But if temperature drops only 10-fold, existing bubbles should increase in volume 10-fold. It's also easy to see that this is a gradual process.

If existing bubbles and/or dissolved gas in the water are the main cause of the "cavitation speed wall" in speedsurfing, then anything reducing chop should improve speeds. I guess Luderitz needs the chop-killing surfactants that the weed in Lake George produces.

I wonder how much drag shade balls would cause.

?si=ElSVs6YyMq0szyob

They are so cute! Do they mate and have little shade-balls children? That's why they are so many?