JALhazmat

Cavitation, super cavitation, base ventilating foils and other crap that doesn’t need putting in every topic

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I thought about starting this thread, but was not fast enough.

For reference from the INEOS thread:

14 hours ago, mako23 said:

You need a foil that can change its shape depending on speed. Once cavitation starts occurring,  the foil changes into a different shape. You could have a foil with half the wing is the flap. At cavitation speed the flap moves inside  the front half of the wing. Now your left with a wedge shape which is more likely to do super cavitation. I’m sure the rules prevent this, also the front half of the wing has to be hollow to allow the flap to move inside it. The strength of this material would have to be a super alloy 

 

9 hours ago, P Flados said:

I am pretty sure the required material for the above is that good old unobtainium we occasion hear about.

If it were not for the symmetry requirements, I could see having a canted Y foil with the outboard element as a flapped "normal" foil and the inboard element as a non-flapped supercavitating foil.  When you are in "normal mode" you use the outboard element and the arm to provide all required lift so that the supercavitating foil is not trying to produce lift and thereby minimize drag.  Above some threshold (say 40 knots), you use your flap to unload the normal element and let the supercavitating element provide most if not all of your required lift.  Altitude control in supercavitating mode would be tricky but not impossible.  Possibly similar to the Altitude control provided by the Hydroptere configuration.    

 

3 hours ago, mako23 said:

very interesting P Flados 
Yes you could use a flap so the outer foil produces less lift, but how does that stop cavitation ?

maybe you could have this side sticking out of the water

 

 

14 minutes ago, P Flados said:

Reducing lift is part of how you can go faster before the onset of cavitation with traditional foil technology.  Thin sections also help.  So if you just want a little more speed you can keep the tradition foil in the water and use the flap for control.

And yes the other choice is to increase boat altitude and end up with nothing but the supercavitating element in the water.  This is where the Hydroptere style of altitude control would be required.

Either of the above would probably make supercavitating work for the main foil on an AC75 platform.  The actual top speed gain would then probably be limited by the rudder.  Balancing the boat to minimize rudder loading (both vertical and horizontal) at speed would become more important than it currently is. 

 

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From the Emirates thread:

16 hours ago, MaxHugen said:

A paper I read about ventilation on V foils had a diagram of a surface piercing foil at around 45°, which showed water pushing up on the downward side, and air being sucked down on the upward side.   Most images I've seen of actual fences do seem larger... except perhaps those of Hydroptere's:

image.png.447acaba80a0490a28210d7a3b1749a1.png

 

16 hours ago, mako23 said:


image.png.4e4c093db2773d904ef610084831d8c7.png

This amazing boat is widely different than an AC75 in more ways than obvious ways. The facts that these foil arm don’t have a bulbed wing at the bottom must make them work completely different. These wings are more  likely to suffer from ventilation. The fact that there at 45 degrees must also change there ventilation profile. However my knowledge of ventilation makes me concerned in making any definitive statements. 
 

ps your photo might be showing fences considering how their repeated 

 

 

13 hours ago, Lakrass said:

Maybe wrong term as the foil is not actually skipping but here are some illustration from below and sailing, front float is skipping the surface.

Sailrocket.jpg

hqdefault.jpg

 

12 hours ago, MaxHugen said:

The foil is supercavitating as mentioned on their website: http://www.sailrocket.com/node/288.  Interesting diagram showing how the foil, besides providing lift, counters the wing:

image.png.a5b3944e6b357e07feacc733fb73d67c.png

 

9 hours ago, MaxHugen said:

This pic shows the flat aft section on the V shaped foil.

image.png.d004af715fac8630eb745ba8e522c770.png

This site has some other good pics too, if interested: http://www.yachtingworld.com/blogs/elaine-bunting/half-plane-half-boat-6339

 

 

4 hours ago, nav said:

It was.

5:38 

but this is the AC, not a one tack, one AWA, one TWS, one place in the world, one boat thing..... so not that relevant, right?

 

4 hours ago, rh3000 said:

I think this is an attempt at a hybrid approach to the foil giving good lift when starting out, but shedding drag and unnecessary lift when at high speed. 

Basically, keep the entire foil submerged at lower speeds, so both sides of the wing on either side provide max lift (albeit with drag).

Once up to a certain higher speed, only one side of the wing is needed to provide enough lift, so cant the arm up further, let the outboard side of the wing break the surface and ventilate down to the foil arm joint section. This will drop drag significantly, and reduce lift, but again, the inboard side has enough lift, and with the drag reduction, speed will increase further, affording even more lift to the inboard wing.

If the rules allowed for assymetrical wings on a foil, you could maybe see the inboard wing having a different shape that might be higher drag at slower speeds but that addressed cavitation, whereas the outboard wing would have profile more suited to slower speed take-off. Get going with both, and once up to a good speed, ventilate the outboard wing, and carry on with the inboard over 50 knots. But given the wind range, and limits on foil choices, this idea is probably not feasible inside the AC36 constraints.

 

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From the Team NYYC thread

On 9/28/2020 at 1:25 AM, MaxHugen said:

Finally found an excellent video - by "the Italian guys" Vittorio and Pietro - specifically on supercavitating foils etc :

They have a whole channel on AC75 design... many good videos for the technically minded!

 

  On 9/28/2020 at 12:36 AM, MaxHugen said:

Would you happen to know of anywhere that shows the design/theory of Larson's foils?   It seems Google is NOT my friend today!

On 10/3/2020 at 1:15 AM, Basiliscus said:

About the only thing I've seen about them is a statement that they were base ventilated foils.  Here is some speculation as to what such an animal might look like: https://www.boatdesign.net/threads/sailrocket-2-set-to-launch.36825/page-15#post-598266.  However, this is purely speculation and not verified in any way by the Sailrocket folk or their public releases.

 

On 10/3/2020 at 11:32 AM, P Flados said:

I was very interested in the Sail Rocket project from the start and followed it closely. 

When the SR2 foil subject came up, I remembered that there was minimal details shared by the team at the time.  I very much appreciated the "reverse engineering" effort by Speer & Drela in the BDN thread (see post 8883 above).

I also found another SR2 item from December of 2012.  It is not nearly as deep as the BDN thread, but it has good pictures.  Similar to the post 8883 disclaimer, I think this item was never confirmed to be accurate by anyone connected to the SR team.     https://chevaliertaglang.blogspot.com/2012/12/clearing-50-knot-gate-hydrofoil.html

 

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2 hours ago, JALhazmat said:

Can we keep it all here? 

Yes I agree 

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2 hours ago, JALhazmat said:

Can we keep it all here? 

Lol, you're new here :rolleyes:

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Ok, now for new discussion.

Lets just say that Ben wins.  I know this is not believable to anyone here.  So lets add that the win occurs after an AM fan wearing a MAGA hat breaks quarantine, sneaks into the ETNZ compound and infects the entire team with a new super contagious and debilitating Covid strain. 

Ben hires Paul Larson and announces that the next round will  be faster "by a large margin" than any sailboat racing ever seen before.  He goes on to say that the only significant changes to the boat rules will be unrestricted foil development.

Now what are your ideas for the changes to the foils and what are your ideas for changes to the racing needed to showcase the higher speed potential.

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1 hour ago, rh3000 said:

Uptips?

sorry, but wasn't it UpTiPs, or UpTips, or UptiPs ?? one's gotta be careful with his verbiage, punctuation and whatnot around here.

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1 hour ago, P Flados said:

Ok, now for new discussion.

Lets just say that Ben wins.  I know this is not believable to anyone here.  So lets add that the win occurs after an AM fan wearing a MAGA hat breaks quarantine, sneaks into the ETNZ compound and infects the entire team with a new super contagious and debilitating Covid strain. 

Ben hires Paul Larson and announces that the next round will  be faster "by a large margin" than any sailboat racing ever seen before.  He goes on to say that the only significant changes to the boat rules will be unrestricted foil development.

Now what are your ideas for the changes to the foils and what are your ideas for changes to the racing needed to showcase the higher speed potential.

Ah...so this doesn't involve any turning of corners, or upwind sailing? Just follow the beach downhill, right? ;-)

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Straight line only would be no fun at all. 

How about an equal side triangle course with one leg as a straight up wind run.

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2 hours ago, P Flados said:

Ok, now for new discussion.

Lets just say that Ben wins.  I know this is not believable to anyone here.  So lets add that the win occurs after an AM fan wearing a MAGA hat breaks quarantine, sneaks into the ETNZ compound and infects the entire team with a new super contagious and debilitating Covid strain. 

Ben hires Paul Larson and announces that the next round will  be faster "by a large margin" than any sailboat racing ever seen before.  He goes on to say that the only significant changes to the boat rules will be unrestricted foil development.

Now what are your ideas for the changes to the foils and what are your ideas for changes to the racing needed to showcase the higher speed potential.

I sense a Democrat lurking in there, either way it’s non of my business.  As a concept Ben Larson boat is interesting but taking this technology to boats sailing on an America’s cup course were there are real waves unlikely. In an attempt for the speed record so many design decisions were made, that calling Sailrocket a yacht is a stretch. It’s wind powered yes....a yacht no. A yacht should be able to sail upwind and downwind and tack. 
 

Funding a rudder that doesn’t have cavitation , steers and provides lift upwards and controls trim is a big ask. 

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4 hours ago, P Flados said:

Paul Larson interview with cool new details and Paul waving around chunks of foil.

https://sailinganarchy.com/2020/10/09/rocket-man-2/

 

Thanks, that is quite a fascinating video. Very long, at 2hrs, but well worth watching it all, even if Paul does "ramble" a bit at times. Around the 1 hour mark, he gets into the foil design, covering cavitation and ventilation issues, and shows the foil section etc.

One particular point of interest, if I interpreted it correctly, is that the foil is used for "negative" lift, as the wing tries to lift SR into the air!

Another interesting take-away was that after breaking the record, he confessed that he couldn't say exactly how what they did actually worked!

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1 hour ago, MaxHugen said:

Thanks, that is quite a fascinating video. Very long, at 2hrs, but well worth watching it all, even if Paul does "ramble" a bit at times. Around the 1 hour mark, he gets into the foil design, covering cavitation and ventilation issues, and shows the foil section etc.

One particular point of interest, if I interpreted it correctly, is that the foil is used for "negative" lift, as the wing tries to lift SR into the air!

Another interesting take-away was that after breaking the record, he confessed that he couldn't say exactly how what they did actually worked!

Look total credit to this Aussie. What he did with mates is the essence of Australian Digger attitude. Nothing but admiration to him. With a budget that I imagine was very small, he came up with a machine that warps one sense of vision. Wearing a T shirt speedos and flip flops he nonchalantly sailed his machine to 65 mph. I’m pretty sure that’s quite dangerous. Forget crocodile Dundee this guy couldn’t be more old time Aussie if you tried.  
I imagine in Australia like NZ such gems are now rare.  Old farts like me can remember this type better. Irrelevant they may be to our modern world. 

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1 hour ago, MaxHugen said:

Thanks, that is quite a fascinating video. Very long, at 2hrs, but well worth watching it all, even if Paul does "ramble" a bit at times. Around the 1 hour mark, he gets into the foil design, covering cavitation and ventilation issues, and shows the foil section etc.

One particular point of interest, if I interpreted it correctly, is that the foil is used for "negative" lift, as the wing tries to lift SR into the air!

Another interesting take-away was that after breaking the record, he confessed that he couldn't say exactly how what they did actually worked!

As all the diagrams pics etc showed the foil was used to grip the water opposing the force in the rig. 
 

he showed A foil section not THE foil 

and yes the admission (for the foil) as to still not 100% sure how, explains why there aren’t papers and detailed images.

it just worked

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16 hours ago, P Flados said:

Straight line only would be no fun at all. 

How about an equal side triangle course with one leg as a straight up wind run.

On second thought, it would probably be better to have a more traditional sized upwind - downwind course with couple of markers on each side and a requirement to slalom downhill on a broad reach going around the side markers.  This would give the second place team the option to split at the top and then we would see crosses at the middle of the course.  If one boat was slower uphill and one was faster downhill, you could get multiple lead changes and every cross at the middle on both legs would be great for spectator value (even for non-sailors).

For what it is worth, I seem to remember SR2 was running closer to a broad reach than to a beam reach at Walvis bay. 

Although this would be true around the cans racing, there would be a good possibility that one or more teams would "get over the hump" as discussed recently by Paul Larson and find a foil section that works well in the 60+ knot range.  If so, there is a chance they would approach or exceed the SR2 record while racing. 

I admit that the chances of the AC going this way would be slim to none. 

There is another possibility that might be less far fetched. 

The above train of thought could open the door way for LE / RC to spend some cash to try and make the F50 boats the undisputed "fastest" racing sailboats.  The test prototype effort would be to fit a boat with a "canted Y main foil with base ventilated inboard section", use a small rig and explore what it could do in say 25 knots of wind.  The fact that the F50 is set up to use computer controlled main foil raking for altitude control would be a big plus.  For the prototype effort, water ballast and/or relocation of batteries and other fixed weight items would be easy and would reduce the drag penalty of using the windward rudder for downforce.

Hey, someone please tell Russell to give Paul a call.  I am sure he would consider a new gig.  With Paul onboard, the publicity value of just trying would be worth the spend.

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Yeah speed sailing actually gets done on a broad reach.. why would you think they would be beam reaching? 

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5 minutes ago, JALhazmat said:

Yeah speed sailing actually gets done on a broad reach.. why would you think they would be beam reaching? 

I was just making a point that mixing in some "speed sailing" into "around the cans sailing" is not that far fetched and showcasing max speed does not need to be the boring "drag racing" type of events that some have tried.

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I have talked to Paul quite a bit about the Sail Rocket adventure.  When you try to get to the nitty gritty, he gets kind of vague, I think because while he is very good technically, he doesn’t really have the detail expertise of the designers.  Paul builds it and figures out how to manage and sail it. He reports back to Malcolm Beardsley what he learns amid they collaboratively refine the machine and each iteration.  

Paul’s description of the design philosophy wears pretty simple:  we designed a wing that could drag a foil ( capable of generating enough side force to resist side force of the wing) at 60 knots.  You can calculate the lift to drag ratio of the wing and the lift to drag ratio of the hydrofoil and get a first order approximation of what is required.  But there are many tiger pits and walls in between that first assessment and the goal.  Paul is the dedicated, talented guy to hammer away at those problems for 10 years and get it done.  He isn’t necessarily the guy who can tell you the exact cause and effect of every phenomenon, with mathematical modeling to prove it.

For example, there were many times when the team went home and built the next piece that was going to break clear.  Funds were always tight, so these things were not done Willy Nilly, or as often as they might be.  Often they improved performance but didn’t solve the big problem.  The final foil was just one of these things.  The boat was still stuck in the 40’s.  After enough trials to see that it really wasn’t just a matter of set up or technique, they added a leading edge fence about the size of a playing card to the foil. This was done carefully, but not with 1000 hours of super computer simulation.  Paul in a container doing it carefully, not in a clean room with laser guided CNC robots.  Suddenly the boat started doing what it was supposed to do and kicked down all the trash can on the block.

Talking to Paul,  I speculated that the fence delayed the onset of ventilation; until it was securely at the speed when was a net gain. He contradicted that and said the foil wasn’t designed for cavitation, but to delay cavitation as long as possible.  The fence just delayed the onset of cavitation a bit longer.    Having gained a bit more experience with hydrofoil struts since then, I  know that the design of the strut is far more complex than it seems. What was happening on the strut at the water surface was quite different from what was happening where the work was getting done.

Sail Rocket’s main foil operates 180 degrees differently than any of the Hydrofoils on this page.  The low pressure side is inclined way from the surface and the force vector is oriented down and to Windward instead of up and to leeward.  As such there is much less tendency to gulp air (ventilate) or lose efficiency to near surface effect.    On a regular hydrofoil, there is a point where there isn’t enough water over the top of the foil to carry the load. It’s like the bubble just pops.  Because the water to Windward and below Sail Rocket’s foil goes all the way to the bottom, this is less of a problem.  At least that how I think it works.

 

SHC

 

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Given the topic of this thread, I thought some might find this relevant and interesting.

Spencer Lisenby has been developing RC gliders that are close to breaking the speed of sound. Watch this clip and try reminding yourself the footage hasn't been sped up.

Spencer gave what I found to be a pretty engrossing talk about the discovery of dynamic soaring effect, and the journey he has been on to explore that is possible. There is plenty of interesting information relating to drag, forces, and dealing with the effect of transonic speeds that are relevant to what is being explored in marine foiling, and cavitation.

 

 

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^ Wow. Amazing stuff. Thanks for posting, rh.

 

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22 hours ago, Albatros said:

sorry, but wasn't it UpTiPs, or UpTips, or UptiPs ?? one's gotta be careful with his verbiage, punctuation and whatnot around here.

Don’t say it too loudly, he might just appear. 

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2 hours ago, Sailbydate said:

^ Wow. Amazing stuff. Thanks for posting, rh.

 

Yeah I agree amazing stuff

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3 hours ago, Steve Clark said:

I have talked to Paul quite a bit about the Sail Rocket adventure.  When you try to get to the nitty gritty, he gets kind of vague, I think because while he is very good technically, he doesn’t really have the detail expertise of the designers.  Paul builds it and figures out how to manage and sail it. He reports back to Malcolm Beardsley what he learns amid they collaboratively refine the machine and each iteration.  

Talking to Paul,  I speculated that the fence delayed the onset of ventilation; until it was securely at the speed when was a net gain. He contradicted that and said the foil wasn’t designed for cavitation, but to delay cavitation as long as possible.  The fence just delayed the onset of cavitation a bit longer.    Having gained a bit more experience with hydrofoil struts since then, I  know that the design of the strut is far more complex than it seems. What was happening on the strut at the water surface was quite different from what was happening where the work was getting done.

Hmm very interesting and insightful without coming across condescending. So in regards to AC75 can we say

we can delay ventilation with fences and we can delay cavitation up to 65 mph . That’s in theory....reality might be different 

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It’s not that easy.  The inherent problem is that the foils you want to go 60 are so draggy at low speed that you can’t get to the high speed.  I racing sailboat design, the game isn’t usually played for top end speed, but for improvements at the lower end of the speed range.  Falling off the foils in marginal conditions is far more disastrous than being a quarter knot slower a full chat.  

Sail Rocket simplified the problem by restricting the wind speeds they sailed in,  so they had enough wellie to just bulldoze some of the speed bumps.  None of the AC designs have tis luxury. So getting the blend right is going to be a huge challenge.

SHC

 

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4 minutes ago, Steve Clark said:

It’s not that easy.  The inherent problem is that the foils you want to go 60 are so draggy at low speed that you can’t get to the high speed.  I racing sailboat design, the game isn’t usually played for top end speed, but for improvements at the lower end of the speed range.  Falling off the foils in marginal conditions is far more disastrous than being a quarter knot slower a full chat.  

Sail Rocket simplified the problem by restricting the wind speeds they sailed in,  so they had enough wellie to just bulldoze some of the speed bumps.  None of the AC designs have tis luxury. So getting the blend right is going to be a huge challenge.

SHC

Maybe we will finish up with something like a paddle steamer, multiple foils graduated from high lift low speed to low lift high speed .. :)

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7 minutes ago, Terry Hollis said:

Maybe we will finish up with something like a paddle steamer, multiple foils graduated from high lift low speed to low lift high speed .. :)

now yer talking the next cup will be grinder powered paddle craft.

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2 hours ago, mako23 said:

 

Hmm very interesting and insightful without coming across condescending. So in regards to AC75 can we say

we can delay ventilation with fences and we can delay cavitation up to 65 mph . That’s in theory....reality might be different 

I'm not sure the Sailrocket numbers are very relevant to AC75.  A record-breaking boat only needs to work well at maximum speed, because if it's not sailing that fast it's not breaking the record.  Everything else about its design is aimed at getting to that speed.

For the AC, the boat has to work in a range of wind conditions and speeds.  It has to be efficient at sailing upwind with higher side force and lower speed compared to sailing downwind.  For the last two matches, it has to be fast on the reach from the start and to the finish (although I don't know if reaching will be important for the coming match because, IIRC, they will be using a windward start).  A base ventilated section has less drag than a supercavitated section, but it's a lot more than a subcavitating section that is operating below its onset of cavitation.  

Fences add drag, too, which is why you typically see them as an add-on that is only used after problems have been experienced without them.

You can delay cavitation to as high a speed as the structural designers will let you go.  Higher cavitation onset requires foils with a low thickness/chord ratio.  If a certain minimum thickness is needed for structure, the the thickness ratio can be had by making the foil wider.  But that adds wetted area and drag.  The top design speed becomes a negotiation between the hydro designers, the structural designers, and the sailors to set the race strategy. 

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2 hours ago, The_Alchemist said:

If you want to throw the ball as far as you can, they should use some reverse foils like the X-29

No problem as long as the helmsman can make 40 corrections a second. 

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3 hours ago, Steve Clark said:

It’s not that easy.  The inherent problem is that the foils you want to go 60 are so draggy at low speed that you can’t get to the high speed.  I racing sailboat design, the game isn’t usually played for top end speed, but for improvements at the lower end of the speed range.  Falling off the foils in marginal conditions is far more disastrous than being a quarter knot slower a full chat.  

Sail Rocket simplified the problem by restricting the wind speeds they sailed in,  so they had enough wellie to just bulldoze some of the speed bumps.  None of the AC designs have tis luxury. So getting the blend right is going to be a huge challenge.

I see that in one of the videos about gliders posted above, that they could delay flutter at higher speed. Yes I know that cavitation and flutter are different. But I’m hoping that cavitation barrier could be moved forwards by  clever foil design. Even if you moved it forward by 5 knots it could be the difference between winning and losing. I hear that that thinner wings help, but no one has said by how much speed you can gain before cavitation 

??

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11 hours ago, Steve Clark said:

Sail Rocket’s main foil operates 180 degrees differently than any of the Hydrofoils on this page.  The low pressure side is inclined way from the surface and the force vector is oriented down and to Windward instead of up and to leeward.

Hmmm... on the AC75, when the foil is canted, aren't the forces up and to windward?

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Mr Clark, are you aware of any experiments to delay the onset of cavitation on a "regular" foil profile, by allowing some fluid to "bleed" from the high pressure side, to the low pressure side?

In theory, this would keep the low pressure slightly higher than that required for cavitation, perhaps regulated by a low-pressure valve so it doesn't adversely affect low speed foil performance.

Although I suspect that the additional drag incurred may dismiss this as impractical?

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27 minutes ago, MaxHugen said:

Hmmm... on the AC75, when the foil is canted, aren't the forces up and to windward?

Yes they are but in SR the rig is to leeward. So they create opposing forces that stabilise 

 

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VSR2 foil chopping was being discussed in another thread.  I looked up the details.

The first base ventilating foil (called the wedge or the Mk1) was the one that was chopped down with final adjustment on December 9, 2011.  

http://www.sailrocket.com/blogs?page=4

The August 12, 2012 entry discusses the design of the final foil and also gives 45 cm as the amount chopped off of the Mk1.

http://www.sailrocket.com/blogs?page=3

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1 hour ago, P Flados said:

VSR2 foil chopping was being discussed in another thread.  I looked up the details.

The first base ventilating foil (called the wedge or the Mk1) was the one that was chopped down with final adjustment on December 9, 2011.  

http://www.sailrocket.com/blogs?page=4

The August 12, 2012 entry discusses the design of the final foil and also gives 45 cm as the amount chopped off of the Mk1.

http://www.sailrocket.com/blogs?page=3

Thanks for the links!  Very interesting reading, and a relief from other "discussions" going on in the forums. ;)

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19 hours ago, mako23 said:

 

Hmm very interesting and insightful without coming across condescending. So in regards to AC75 can we say

we can delay ventilation with fences and we can delay cavitation up to 65 mph . That’s in theory....reality might be different 

Knots not mph.....keep studying.

And can you explain why do you insist this is AC relevant when it has been pointed out time and again why it is not?

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14 minutes ago, nav said:

Knots not mph.....keep studying.

And can you explain why do you insist this is AC relevant when it has been pointed out time and again why it is not?

See post #10.  Speculation for a faster next AC round. 

I proposed that

  • the next round will be faster "by a large margin" than any sailboat racing ever seen before

Provided that the winner of this round goes with

  • the only significant changes to the boat rules will be unrestricted foil development
  • the downhill runs will be a broad reaching slalom course with the opportunity for the second place boat to split at the top.

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22 minutes ago, nav said:

Knots not mph.....keep studying.

And can you explain why do you insist this is AC relevant when it has been pointed out time and again why it is not?

Read the name of the topic ........this topic was created for exactly this type of conversation. Not everything in here is strictly AC75 related 

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The topic was created to give people that can’t be bothered to listen somewhere to Twitter on about stuff that’s got bugger all to do with the AC.. despite this being the AC section of Sailing Anarchy. 

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3 minutes ago, JALhazmat said:

The topic was created to give people that can’t be bothered to listen somewhere to Twitter on about stuff that’s got bugger all to do with the AC.. despite this being the AC section of Sailing Anarchy. 

That’s twice I’ve agreed with JAlhazmat in 24 hours 

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I started the bloody thread out of utter boredom of watching you running around like an over excited toddler shouting out irrelevant rubbish on all the team threads. 
 

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16 hours ago, P Flados said:

VSR2 foil chopping was being discussed in another thread.  I looked up the details.

The first base ventilating foil (called the wedge or the Mk1) was the one that was chopped down with final adjustment on December 9, 2011.  

http://www.sailrocket.com/blogs?page=4

The August 12, 2012 entry discusses the design of the final foil and also gives 45 cm as the amount chopped off of the Mk1.

http://www.sailrocket.com/blogs?page=3

The stock was chopped by 45mm, the lower foil section was never touched.

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While discussing the "new design" of the final foil, Paul provided the discussion below of sailing with the MK1 foil.  Elsewhere he expressed his fear of having the foil so short that it would pull out of the water at speed.  Given his VSR1 experience with a main foil coming out of the water, his fear was well founded.

The fact that she got up and going at all with 45 cm removed was pretty impressive. Mind you, we were sailing in top end conditions and we hope we don't have to do that again.

From Paul's December 9, 2011 post:

I had seen enough. This foil wasn't letting go so it was time to chop another 15 cm off. This was now 45cm in total and over half of the main foil. It's not the ideal way of getting a smaller foil. It's pretty brutal in fact but this foil isn't working as is and drastic measures needed to be taken in order to find this ventilated flow. The foil now only had 30 cm until the radius of the bend. there wasn't going to be much left in the water. Maybe it was a step too far.

IMG_6438_1023x800.jpg

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I’m talking about the last foil made that broke the record. ;)

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I've noodled around a bit with Xfoil to follow on from the BoatDesign.net discussion, based on Paul Larsen's video talking about thick base-ventilated foils.  My objective was to see how far I could push the incipient cavitation speed while still maintaining a substantial thickness.  The separated wake is pushing the assumptions behind Xfoil pretty far, so some of the results, especially the drag, have to be taken with a big grain of salt, but hey - the price is right.  (FWIW, the boundary layer settings I used were Reynolds number = 8e6, Ncrit=1, tripped at 5% chord from the stagnation point, KDL=2.5)

High speed doesn't require a lot of lift so the section is almost symmetrical.  In order to stay under the threshold of cavitation, the pressure distribution on the suction side has to be essentially constant, right at the threshold.  I made the pressure side essentially constant pressure in order to get the required lift.  If parts of the pressure side had a higher pressure, then for the same area between the curves (to get the required lift), the velocity on the pressure side would need to be higher, possibly leading to cavitation.  

The incipient cavitation speed based on the point of minimum pressure is a little conservative, in that cavitation is guaranteed not to occur below that speed.  In practice, the speed where cavitation is actually observed tends to be a few knots higher.

With a 12% thick section, I was able to get the incipient cavitation speed up to 63.7 kt, which would probably work out to 65 kt in practice.  This would be consistent with Sailrocket designing for a 65 kt speed and reports that they weren't observing cavitation with the latest foil.  

On the cavitation diagram, the black lines correspond to different foil loadings.  These show the locus of operating points that produce the same amount of lift at different speeds and are independent of the shape of the section.  I've arbitrarily picked 30,000 N/m^2 as a target loading.  The blue line shows the onset of cavitation for the BV05 section.  If the foil is operating to the left of the blue line, then there will be no cavitation anywhere on the section.  The red target loading intercepts the blue line at two points.  The low speed/high lift point corresponds to leading edge cavitation, caused by a pressure peak at the leading edge. 

At high speed and low lift the minimum pressure occurs at the trailing edge of the suction side.  As the foil goes faster, the onset of cavitation will move forward.  However, because the pressure will be decreasing toward the trailing edge, the vapor bubbles won't collapse near the surface, but will pass into the wake.  So there won't be any erosion of the surface from bubbles passing from low to higher pressure and collapsing next to the surface. 

However, atmospheric pressure is higher than water vapor pressure (otherwise the oceans would boil), so I'm not sure just what will be happening near the trailing edge.  The viscous Xfoil results have the pressure in the wake being close to the suction side pressure.  The pressure coefficient on both sides is negative at the trailing edge, so the pressure there is lower than ambient pressure (Cp=0).  Which for the foil operating at the surface would be atmospheric pressure.  

I'll run different thicknesses to see what the tradeoffs look like, with the caveat that especially with regard to drag Xfoil's results will be questionable.

 

plot_BV05_r8e6n1fix5e-2Kdl25e-1_Page_1.png

plot_BV05_r8e6n1fix5e-2Kdl25e-1_Page_2.png

plot_BV05_r8e6n1fix5e-2Kdl25e-1_Page_3.png

BV05_Cavitation.png

BV05.dat

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Thanks again.  

Paul showed us that there is a way to go past the limit that frustrated speed sailing teams for so long.  

I hope to see someone follow up and open the door even wider into the next level of performance.

The F50 and AC75 are certainly at a level of performance I never expected with use of subcavitating foils. 

After I saw AM running really fast with a canted Y and the outboard element coming out of the water, it just hit me that there was a way to swap from using a subcavitating foil for required lift to using a higher speed foil without having insert / retract anything.  I do not expect to see such boat, but life would be boring without a little dreaming.

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For general info, if anyone is interested:

Calculating Apparent Wind Speed and Angle, and VMG

Rather than using complex trigonometry equations, etc, I found a handy online calculator at www.calculator.net/triangle-calculator.html !

For an example, let’s say yacht A is sailing upwind, with TWS (true wind speed) = 10 knots, TWA (true wind angle) = 45°, and SOG (speed over ground) = 20 knots. Leeway is ignored but can be added to TWA.

Draw a parallelogram as shown, ABCD (see below).  A rough sketch is sufficient, as values will be calculated. Line CA represents the apparent wind speed.  The angle between lines AB and BC is 180° less the angle between DA and AB (because it’s a parallelogram) which we set to 45°, so angle ABC = 135°.

Entering lines AB and BC values, and angle ABC, the calculator gives us:

                Apparent Wind Speed   = 27.979 (knots)
                Apparent Wind Angle    =
14.639°

To get VMG, let’s assume that the next marker is directly upwind, so add line AE perpendicular to DA (true wind direction),  and vertical line EB perpendicular to AE, ie at right angles. EB is the VMG (velocity made good). As angle DAB = 45°, angle BAE also = 45°.  Since it’s a right angle triangle, angle ABE too is 45°. Enter the two 45° angles and the 20 kn “length” into the calculator, and we see that:

                Velocity Made Good (EB) = 14.142 knots

How does that compare to yacht B that is sailing slightly lower, but a bit faster? Say, 3° lower, and 1 knot faster. The results:

                Apparent Wind Speed   = 28.671 (knots)
                Apparent Wind Angle    =
15.022°
                Velocity Made Good      = 14.052 knots

In this case, although yacht B is sailing 1 knot faster (SOG), its VMG is 0.09 knots slower. Doesn’t sound like much, but Yacht A is sailing 0.64% faster... over, say the upwind half of a 20 mile course, that equates to a winning lead of some 340 feet.

                  image.png.a35382736ca9c34c18139634b145e174.png

Anyway, the online calculator makes trying out some scenarios very easy. Hope someone finds it useful.

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There's an easier way to try out scenarios - look at all of them!  The wind triangle does have some interesting implications.

Start off by defining the apparent wind angle, beta, as the angle between the apparent wind and the boat's course through the water.  Let the aero drag angle be arctan(Drag_aero/Lift_aero) and the hydrodynamic drag angle be arctan(Drag_hydro/Lift_hydro).  The hydrodynamic lift is set by the requirement that the hydrodynamic side force equal the aerodynamic side force applied by the sail rig.  The aerodynamic side force, of course, is under the control of the crew.

It turns out the apparent wind angle, beta, is equal to the sum of the aerodynamic drag angle and the hydrodynamic drag angle.  This is not an approximation - it is exact and based on the definitions of aerodynamic and hydrodynamic lift and drag.  So the apparent wind angle is directly related the lift/drag ratios.  The more efficient the boat is, the lower beta will be.

A high performance boat when it is hooked up tends to operate with a near-constant lift drag ratio as the speed or true wind angle changes.  So the lines on a polar for beta=constant are similar to the boatspeed polar.

The power to a sailboat isn't fixed, but depends on the apparent wind speed.  So the apparent wind speed/boat speed ratio is also an important factor.

It turns out that if you plot lines of constant apparent wind angle on a polar diagram, they turn out to be circles.  The centers of the circles lie on a line that is perpendicular to the true wind direction and on the polar diagram intercept the Y axis at half the true wind speed downwind.  All the circles go through the origin (zero speed) and the downwind at true wind speed point.

The lines of constant apparent wind speed/boat speed ratio are also circles.  Their centers lie on the Y axis.  The amazing thing is these circles meet the constant beta circles at right angles everywhere!

So the boat's performance can be plotted using three orthogonal coordinate systems.  The first coordinate system is Cartesian and consists of the velocity made good upwind/downwind, and the cross wind velocity.  The second is a polar coordinate system that consists of the course sailed to the true wind and the boat's speed through the water.  The third coordinate system is the apparent wind angle and the apparent wind speed/boat speed ratio.  If you know the boat's performance in any one of these coordinate systems, you can calculate the equivalent values for the other two coordinate systems.  This is what your instrument system does onboard the boat.  It measures apparent wind direction, apparent wind speed, and boat speed. This gives it the coordinates in the third coordinate system, and the system computes the velocity made good from the first coordinate system.

The three coordinate systems are shown overlaid in the first illustration below.  The Cartesian coordinate system is the solid black lines and the polar coordinate system is the dotted black lines.  The lines of constant apparent wind angle are in blue and the lines of constant apparent wind speed/boat speed ratio are in green.  The red line is the polar for the trimaran USA17 in 8 kt of wind, which is approximately the conditions under which it won the America's Cup in 2010.  The red dots show the points for best Vmg to windward and leeward and the maximum speed.  Upwind, it sailed at less than 15 degrees apparent wind angle, and it had a 20 degree apparent wind angle at maximum speed.  You can see how the constant beta circles are not a terrible approximation of the polar at these points, especially for the maximum speed point.

The second diagram is for 20 kt true wind speed.  This would be typical of a speed sailing attempt.  Say the boat is to be the first sailing craft on water to hit 70 kt.  The apparent wind angle will need to be on the order of 17 degrees, and it will probably be sailing around 110 degrees off the wind.  The apparent wind speed will be lower than the boat speed, but not by a lot.  Knowing the apparent wind angle the designer can divvy that up into the hydrodynamic and aerodynamic drag angles to come up with a drag budget for each aspect of the boat.  

 

 

 

 

PolarCircles_USA17.png

PolarCircles20kt.png

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10 hours ago, P Flados said:

...After I saw AM running really fast with a canted Y and the outboard element coming out of the water, it just hit me that there was a way to swap from using a subcavitating foil for required lift to using a higher speed foil without having insert / retract anything.  I do not expect to see such boat, but life would be boring without a little dreaming.

The Mk105 minesweeping sled has hydrofoils that work a bit like that.  It has ladder foils forward, and I believe the lower foil has a section designed for high speed and the upper foil designed for lower speed.

050811-n-1467r-066-gulf-of-mexico-aug-11

What's interesting is the upper foils appear to have been modified to be base ventilated.  A hollow trailing edge is one way to ensure air gets down to the bottom of the foil!

It's towed by a helicopter:

US_Navy_071112-N-1465K-005_An_MH-53E_Sea

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31 minutes ago, Basiliscus said:

The Mk105 minesweeping sled has hydrofoils that work a bit like that.  It has ladder foils forward, and I believe the lower foil has a section designed for high speed and the upper foil designed for lower speed.

050811-n-1467r-066-gulf-of-mexico-aug-11

What's interesting is the upper foils appear to have been modified to be base ventilated.  A hollow trailing edge is one way to ensure air gets down to the bottom of the foil!

It's towed by a helicopter:

US_Navy_071112-N-1465K-005_An_MH-53E_Sea

Fuck me - that takes some balls!

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I've always thought of kite boarding as wake boarding towed by a chopper, that you're driving. 

Thx again for the thought provoking posts B!!

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1 hour ago, barfy said:

I've always thought of kite boarding as wake boarding towed by a chopper, that you're driving. 

Thx again for the thought provoking posts B!!

It is. 

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B,

I try to "add value" and/or provide "interesting tidbits" when I can. 

You overachieve in both of the above.  Thanks again.   

Now that would make a great "foil test platform".   

On the flip side, whoever is responsible for the foils on that thing, might already have some interesting test data.

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Quite different requirements for the mine-hunding sled, there is a much greater refinement needed on the AC boats. 

In fact I suspect this round will push along foil technology quite a way generally. 

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11 minutes ago, P Flados said:

Although some amount of improvement in foil technology is likely, they are probably still focused on extending the useful range of subcavitating.  For the most part, they know that they can just drive for more VMG as they approach cavitation.

Until the rules provide a significant advantage for outright speed over VMG, there is not a lot of need to "push through the glass ceiling" (using Paul Larson's words). 

All it would take to give them this incentive would be to choose the right "race course".  With a course that has legs that alternate from up/down to 45 degree slalom runs plus a beam reach start and finish, you could make the teams deal with equal distances in all 8 points of the typical sailing compass.

And it would do so in a way that has lots of "exciting" crosses for the spectators. 

From that angle, going to at least a few courses that include some 45 degree slalom runs with crossing is a better fit for the F50s.  Showing off speed and providing "exciting" spectating (for non-sailors) is much more of a thing for SailGP.  The fact that it would push them into the cavitation region and increase required foil maintenance on their current foils would just be "part of the cost" of doing business.  

 

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The first absolute speed record over 50 kt was a kiteboard in 2008, and Hydroptere did over 51 kt in 2009.  Today, we have course-racing America's Cup yachts that can do 50 kt in actual racing.  So I think it's worth asking the question, "What comes after the AC75?" and using this thread to consider future AC matches that would be sailed at today's record speeds. (How's that for bringing a thread back on topic for the forum?)

One way to extend the onset of cavitation is to sweep the foils.  Simple sweep theory says the cavitation speed goes up as 1/cosine(sweep angle), and experimental results show sweep up to 45 degrees is effective.  A 45 degree sweep angle would get a 50 kt foil to 70 kt.  That opens up a lot of possibilities for foil design.

NACA-RM-L52J10_SweptHydrofoils.pdf

AIAA-48104-766_EffectsOfSweep.pdf

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23 hours ago, P Flados said:

The F50 and AC75 are certainly at a level of performance I never expected with use of subcavitating foils. 

After I saw AM running really fast with a canted Y and the outboard element coming out of the water, it just hit me that there was a way to swap from using a subcavitating foil for required lift to using a higher speed foil without having insert / retract anything.  I do not expect to see such boat, but life would be boring without a little dreaming.

If they're foiling on the subcavitating foil, the cavitating foil would be creating a disastrous amount of drag.  Assuming they could even get out of displacement mode. Paul Larsen had issues getting SR up to the speed required for the cavitating foil to work, due to drag. NZ may yet get some higher performance with their "BFB" (blended foil bulb) design... the foils have a much narrower chord, and may also be relatively thin, as from their AC35 design. Such a foil profile delays cavitation. The bulb area may be a problem in this regard though.

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I do not think of cavitation as just a hard wall where everything is good up to a certain point followed by terrible behavior at some magic speed.  I think performance starts declining before you get to the point to where some huge drag increase prevents any speed increase (aka "the glass ceiling").  

And there must be details that can change when a given boat hits the glass ceiling.  Russel Long hit the trifoiler glass ceiling at 43 - 44 knots.  The boat would accelerate at a good pace and then speed would absolutely flatline.  It did this over a good range of wind speeds.  Their description of this is when I first became aware of the glass ceiling. 

I believe that the AC72s, AC50s, F50s and AC75s have all pushed into the region where minor cavitation is occurring and minor surface damage has been noted.  I also think that the glass ceiling for these boats is higher than I would have expected 10 years ago.

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3 minutes ago, P Flados said:

I believe that the AC72s, AC50s, F50s and AC75s have all pushed into the region where minor cavitation is occurring and minor surface damage has been noted.  I also think that the glass ceiling for these boats is higher than I would have expected 10 years ago.

Agreed, cavitation takes many forms, such as stream, sheet, etc. Some are relatively minor, and may or may not cause pitting damage. It's generally within a narrow speed band... but a band nevertheless. Temperature also affects cavitation, the colder the water, the higher the cavitation speed. It can also begin at different points of the chord... the mid section, leading edge, and trailing area. The thin sections that ETNZ have a lot of experience with should delay mid-chord and trailing area cavitation, and very precise control of AoA is needed to also delay leading edge cavitation - which I think the Kiwis are adept at.

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Looking back, I see that my previous post may have missed your point.   

My concept is to have one section of a canted Y foil as capable of pushing through the glass ceiling.  At lower speeds, other sections would supply most or all required lift and the higher speed section would be adjusted for an AOA that gave minimal excess drag.  This would "increase drag" as compared to an all subcavitating foil, but it would not necessarily prevent the boat from having a decent low speed performance.  At higher speeds, the subcativating foil sections would be above the waterline and/or adjusted for low drag.

Paul recently noted that on one occasion, he saw what I will call "better than expected" low speed performance from one of his "higher speed" foils.  How good a higher speed foil can be made to perform at lower speeds is also a huge question.  A base ventilated foil with air supplied through passageways inside of the foil structure might perform better than you think at "normal" racing speeds.   It is a question that would get lots of attention if an AC were set up to where it mattered.  

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8 minutes ago, P Flados said:

Looking back, I see that my previous post may have missed your point.   

My concept is to have one section of a canted Y foil as capable of pushing through the glass ceiling.  At lower speeds, other sections would supply most or all required lift and the higher speed section would be adjusted for an AOA that gave minimal excess drag.  This would "increase drag" as compared to an all subcavitating foil, but it would not necessarily prevent the boat from having a decent low speed performance.  At higher speeds, the subcativating foil sections would be above the waterline and/or adjusted for low drag.

Paul recently noted that on one occasion, he saw what I will call "better than expected" low speed performance from one of his "higher speed" foils.  How good a higher speed foil can be made to perform at lower speeds is also a huge question.  A base ventilated foil with air supplied through passageways inside of the foil structure might perform better than you think at "normal" racing speeds.   It is a question that would get lots of attention if an AC were set up to where it mattered.  

I have seen a profile - I think it was a base cavitating foil - that added a sort of V shaped "trailing" section aft of the foil base that was completely within the cavitation bubble, but was supposed to help with sub-cavitation performance. I don't know if was actually tested though.

If I understood Paul correctly, he actually avoided cavitation, by ventilating the low pressure bubble. The higher air pressure introduced into this area drastically reduced drag.

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Max, are you are talking about the one at around 6:48 in the video by the French Guys (see post 7 above).  That foil has a wedge for the front, not what I normally think of as base ventilating.  

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Yes, but the diagram I saw showed no cavitation on the upper surface... maybe an AoA factor. Note that the yellow part has a "step" in from both the upper and lower surfaces of the foil, which are the points where cavitation was caused... in the article I read.

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