Propellers

[OceanObsessed]

I think the vapor the pressure of water is irrelevant in
the process.


Regards,

--Mike

It certainly has to do with vapor pressure of water.

That is almost correct.

I am not sure what part of my original comment you are disagreeing with. It sounds to me that we are saying the exact same thing but simply explaining it differently.

The only thing that I would disagree with is that the relationship between the pressure and temperature in this case are related via the Ideal Gas Law (PV = nRT) not the Integrated Gas Law (I know that is what you meant to say Bob ) and the pressure and temperature are directly (not inversely) proportional to each other. You can play with the variables in the equation PV=nRT to determine the inverse and direct relationships.

Of course there are assumtions that the water is behaving as an ideal gas. Many of the gas/fluid dynamic equations are the same they just use different densities to represent the gas or fluid. I would be interested in hearing more about what Nemrod said about there being controversy with the origin of cavitation. This certainly seems to be a contoversial thread .

 

[mrjimboalaska]

Well,
I may be way off base here, but a sub at depth, the screws are not creating "pockets of gas" right? on most surface craft, water and air is being pushed under the boat and surfaces again as the boat passes. That, and most lower end units exhaust out the blade area. Or am I just lost?

 

[Nemrod]

Yes mrjimboalaska, your lost and confused, you asked. What your describing is more closely known as ventilation--not cavitation. N

 

[victor]

Copied from Wikipedia http://en.wikipedia.org/wiki/Cavitation

When a volume of liquid is subjected to a sufficiently low pressure it may rupture and form a cavity. This phenomenon is termed cavitation inception and may occur behind the blade of a rapidly rotating propeller or on any surface vibrating underwater with sufficient amplitude and acceleration.
Other ways of generating cavitation voids involve the local deposition of energy such as an intense focussed laser pulse (optic cavitation) or with an electrical discharge through a spark.

Vapour gasses evaporate into the cavity from the surrounding medium, thus the cavity is not a perfect vacuum but has a relatively low gas pressure.

Such a low pressure cavitation bubble in a liquid will begin to collapse due to the higher pressure of the surrounding medium. As the bubble collapses, the pressure and temperature of the vapour within will increase. The bubble will eventually collapse to a minute fraction of its original size, at which point the gas within dissipates into the surrounding liquid via a rather violent mechanism, which releases a significant amount of energy in the form of an acoustic shock-wave and as visible light.
At the point of total collapse, the temperature of the vapour within the bubble may be several thousand kelvins, and the pressure several hundred atmospheres.

 

[Gilldiver]

Copied from Wikipedia http://en.wikipedia.org/wiki/Cavitation

When a volume of liquid is subjected to a sufficiently low pressure it may rupture and form a cavity. This phenomenon is termed cavitation inception and may occur behind the blade of a rapidly rotating propeller or on any surface vibrating underwater with sufficient amplitude and acceleration.
Other ways of generating cavitation voids involve the local deposition of energy such as an intense focussed laser pulse (optic cavitation) or with an electrical discharge through a spark.

Vapour gasses evaporate into the cavity from the surrounding medium, thus the cavity is not a perfect vacuum but has a relatively low gas pressure.

Such a low pressure cavitation bubble in a liquid will begin to collapse due to the higher pressure of the surrounding medium. As the bubble collapses, the pressure and temperature of the vapour within will increase. The bubble will eventually collapse to a minute fraction of its original size, at which point the gas within dissipates into the surrounding liquid via a rather violent mechanism, which releases a significant amount of energy in the form of an acoustic shock-wave and as visible light.
At the point of total collapse, the temperature of the vapour within the bubble may be several thousand kelvins, and the pressure several hundred atmospheres.

And now you know how an ultrasonic cleaner works. The shock wave breakes off the contamination. It does help the speed and efficency if you have a warm soap solution. Here is a trick, if you have an ultrasonic cleaner hang a small piece of thin aluminum foil (like from a gum wrapper) in it for a few minutes. If the cleaner is working the foil will be eaten away by the shock waves.

 

[knotical]

I think the gas formed by cavitation includes both water vapor and previously dissolved gases (air). The water vapor mostly returns to its liquid state upon collapse of the bubbles, but the air mostly bubbles off.

 

[Meng_Tze]

Well,
I may be way off base here, but a sub at depth, the screws are not creating "pockets of gas" right? on most surface craft, water and air is being pushed under the boat and surfaces again as the boat passes. That, and most lower end units exhaust out the blade area. Or am I just lost?

Huh? The hunt for the Red October and other sub footage does show bubbles coming off of the props. Heck even the caterpillar drive pushes out bubbles... there are bubbles all around us.
:mooner:

 

[String]

As well as cavitation lots of motors, outboards in particular have an exhaust that comes from the prop centre area itself which does put real gas in.

 

[PanamaMike]

It certainly has to do with vapor pressure of water.


I am not sure what part of my original comment you are disagreeing with. It sounds to me that we are saying the exact same thing but simply explaining it differently.

The only thing that I would disagree with is that the relationship between the pressure and temperature in this case are related via the Ideal Gas Law (PV = nRT) not the Integrated Gas Law (I know that is what you meant to say Bob ) and the pressure and temperature are directly (not inversely) proportional to each other. You can play with the variables in the equation PV=nRT to determine the inverse and direct relationships.

Of course there are assumtions that the water is behaving as an ideal gas. Many of the gas/fluid dynamic equations are the same they just use different densities to represent the gas or fluid. I would be interested in hearing more about what Nemrod said about there being controversy with the origin of cavitation. This certainly seems to be a contoversial thread .

The vapor pressure of water at 20C is less than
10% of an atmosphere and drops even lower with
common sea water temperatures. For the water to
boil, you then have to first pull a mechanical vacuum
and then you can get some boiling to fill the
cavity that's already formed. You would get
cavitation even without any water boiling, just
at a slightly higher rpm. So for me, the vapor
pressure of water and boiling effect is decoration
on the fact that cavitation is just the prop
pulling a vacuum and doesn't depend on boiling
or and any other material phase changes or
chemical bonds breaking.

Regards,

--Mike

 

[OceanObsessed]

The vapor pressure of water at 20C is less than
10% of an atmosphere and drops even lower with
common sea water temperatures. For the water to
boil, you then have to first pull a mechanical vacuum
and then you can get some boiling to fill the
cavity that's already formed. You would get
cavitation even without any water boiling, just
at a slightly higher rpm. So for me, the vapor
pressure of water and boiling effect is decoration
on the fact that cavitation is just the prop
pulling a vacuum and doesn't depend on boiling
or and any other material phase changes or
chemical bonds breaking.

Regards,

--Mike

Mike,
I believe that the water vaporizing because of the mechanical vaccum (in this case the negative pressure side of the prop) is the actual definition of cavitation.

Everyone,
Let me stir the pot a bit for some fun.

It is interesting to note that the negative pressure side of the prop can commonly generate more power than the positive pressure side of the prop. This means that the prop can generate more power by sucking itself forward than by pushing itself forward.

A sail on a sailboat acts as a foil (like an airplane wing) during an upwind tack. Similar to a prop, there is negative pressure on the forward side of the sail and positive pressure on the aft side of the sail. Because of this a sailboat on an upwind tack can be capable of sailing faster than the windspeed. For example, some high speed saiboats (if there is such a thing ) can maintain 12 knots in a 10 knot wind.
-Chris