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ventalation vs cavitation #275454
09/17/14 09:30 AM
09/17/14 09:30 AM

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MN3
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MN3
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someone on another forum was claiming he had cavitation issues on his prindle.

someone else suggested it's not cavitation but ventalation "your problem is VENTILATION, not cavitation, unless you're doing more than 20kt or so:"

he referenced this link ... http://books.google.com/books?id=Qp...e&q=rudder%20ventilation&f=false


is that correct?

-- Have You Seen This? --
Re: ventalation vs cavitation [Re: ] #275457
09/17/14 10:15 AM
09/17/14 10:15 AM
Joined: Jun 2001
Posts: 12,310
South Carolina
Jake Offline
Carpal Tunnel
Jake  Offline
Carpal Tunnel

Joined: Jun 2001
Posts: 12,310
South Carolina
Originally Posted by MN3
someone on another forum was claiming he had cavitation issues on his prindle.

someone else suggested it's not cavitation but ventalation "your problem is VENTILATION, not cavitation, unless you're doing more than 20kt or so:"

he referenced this link ... http://books.google.com/books?id=Qp...e&q=rudder%20ventilation&f=false


is that correct?


Yes...specifically using the terminology, "cavitation" is when the water pressure gets low enough that the water turns to vapor (steam). This can happen as water flows over a foil but it does definitely require a fair amount of speed for that to happen. It's the major limiting factor in speed for the America's Cup boats on foils. They spend a great deal of resources trying to design the foils that keep the low pressures spread out while still maintaining a high degree of efficiency. It's a massive engineering challenge. Also note that "cavitation" isn't normally something that happens suddenly - in the case of hydrofoils, it comes on gradually as speed increases and it causes both a loss of lift and efficiency. It presents itself as an ever increasing force as boat speed increases and it ultimately limits the speed of the foil born vessel. You also won't see a trail of bubbles in the water when something is cavitating. While there are some wave effects that can prolong the cavitation trail, generally speaking the small bubbles of water vapor generated from cavitation over a foil collapse immediately as soon as the pressure drops back below the vapor point. This collapse can also erode the surface of the foil in question and is a major consideration when designing large propellers.

Sucking air down from the surface along an edge of a foil is ventilation. Where the foil pierces the surface of the water, sometimes those low pressure areas can suck the air down. That air, being less dense, causes a sudden loss of lift and can be dramatic.

However, more likely at conventional catamaran speeds, the foil is just stalled and its spinning off a lot of turbulent water that causes some air mixing at the surface of the water. So, if you turn your rudders hard enough to lose traction, the lack of control you feel is unlikely to be cavitation, probably not because of ventilation, and is just a stall.

Here's a decent video on cavitation


Jake Kohl
Re: ventalation vs cavitation [Re: ] #275460
09/17/14 12:03 PM
09/17/14 12:03 PM
Joined: Jul 2001
Posts: 1,253
Columbia South Carolina, USA
dave mosley Offline
veteran
dave mosley  Offline
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Posts: 1,253
Columbia South Carolina, USA
Like the Old N20(Inter 20) rudders?


The men were amazed, and said, "What kind of a man is this, that even the winds and the sea obey Him?" Matthew 8:27





Re: ventalation vs cavitation [Re: dave mosley] #275461
09/17/14 12:51 PM
09/17/14 12:51 PM
Joined: Jun 2001
Posts: 12,310
South Carolina
Jake Offline
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Jake  Offline
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Joined: Jun 2001
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South Carolina
Originally Posted by dave mosley
Like the Old N20(Inter 20) rudders?


That's definitely a stall - yup. That was definitely related to the large amount of sail area that could easily overpower the small rudders.


Jake Kohl
Re: ventalation vs cavitation [Re: ] #275462
09/17/14 01:23 PM
09/17/14 01:23 PM

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thanks jake

Re: ventalation vs cavitation [Re: ] #275463
09/17/14 01:45 PM
09/17/14 01:45 PM
Joined: Jun 2009
Posts: 524
Petten Netherlands
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northsea junkie Offline
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In windsurfing this phenomenom is called spinout and is notorious. It always has been a little unclear if this is caused by ventilation (the board is planing only on the last centimers of its tail) or by cavitation (speed too fast).

But it always happens suddenly and sometimes leads to a fall. That sudden character is not so strange because the skeg (fin) of a windsurfboard gives the only lateral resistance to the sailforce. They are both in fragile balance, so decreasing from one will break this balance abruptly.

A Prindle cat gets its lateral resistance for a very big part from its rudderblades. With your blades high you cannot hardly sail the cat straight forward.(The "weather helm" is enormous).
So, if those blades start cavitate or to ventilate, the cat will indeed suffer a spinout.

Prindles are therefor more sensitive to this nuisance then others.

Jake, great vid about this subject.


ronald
RAIDER-15 (homebuilt)

hey boy, what did you do over there, alone far out at sea?..
"huh....., that's the only place where I'm happy, sir.
Re: ventalation vs cavitation [Re: northsea junkie] #275467
09/17/14 06:23 PM
09/17/14 06:23 PM
Joined: Jun 2001
Posts: 733
Home is where the harness is.....
Will_R Offline
old hand
Will_R  Offline
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Joined: Jun 2001
Posts: 733
Home is where the harness is.....
Excellent demo of ventilation.
https://www.youtube.com/watch?v=0J0jaMIRMR0&index=36&list=WL

compared to cavitation:
https://www.youtube.com/watch?v=DijdU0rmDdc&index=12&list=FLCqf4CrtfoxFcfAyVh0wMkg

Magnus talks about it around 11:45 and the fences they used to minimize ventilating the foil.
http://vimeo.com/72619588#

There is quite a bit behind the reasons and why/when ventilation happens. We see it on the CFR a lot b/c as a uni with a LOT of sail area, the mast is raked WAY back which of course loads the rudders more. The pressure differential is the reason that you get ventilation and it becomes much worse as you heel b/c of additional vertical flow along the foil. Those rudders are also more prone to stalling b/c they are a higher aspect shape with a thinner section. The stall can initiate a worse ventilation event at which time they sort of enter into a "chicken and the egg" situation; one can initiate the other, but they both are part and parcel to the event when you lose lift (steerage) b/c of how relatively shallow beach cat rudders are.

I think the situation that people confuse is that a stall can start at the tip of a foil and get progressively worse, or depending on the foil shape, happen along the entire foil at the same time. Most boats today have elliptical rudders (more efficient), but they will stall all at once while a square tip (old P-19 and 18-2) will stall progressively starting at the tip. This is where the L-foil boats run into a lot of trouble; those things are typically elliptical b/c of the performance envelope, so when it stalls, night night....

Like Jake said though, you make a big course correction (think bear away to duck) and you significantly increase the angle of attack while simultaneously increasing the load due to drag from AoA and changing wind angle and therefore sail pressure. There are some good crashes in the VX40's and AC45's from this exact thing happening.

Interesting enough though is that Oracle had so much trouble with cavitation that they were having to repair their stainless rudder foils at the rudder/foil intersection daily until they changed the shape mid way through the finals.

http://biekerboats.blogspot.com/2013/09/oracle-team-usa-wins-americas-cup.html


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