"Peter" wrote
Cavitation is caused by the prop spinning too fast for the forward
motion of the vessel.


Gilligan wrote:
Cavitation is caused by water not being able to move fast enough to fill low
pressure spaces created around such things as a propeller.

Which is caused by... ?
You're both right. Peter is talking about the cause on a
macro scale.

... A spinning disc
has no forward speed regardless of RPM yet there is no cavitation. A
propeller is a rotating wing and the low pressure side causes the water to
"boil", hencce the creation of cavitation.

But what makes the water boil? Increased temps due to
friction? Tiny devil-spirits waving their pitchforks?


.... Higher RPM gives higher
tangentail blade speeds and greater pressure differential between the faces
of the prop.


Somewhat right, tip speed above a certain linear velocity
will cause cavitation no matter the dP, and if the dP gets
high enough the tip speed matters less.

You could also mention the dread voodoo curse "viscous
shear" which means that somebody is trying to force those
lazy water molecules to move faster than they are willing.

Fluid flow does a lot of things counter-intuitive to people
familiar with normal physics. One of my favorite engineering
profs used to say that fluid molecules are like basketballs
smeared with syrup, which helps intuit the behavior.

DSK

"DSK" wrote in message
...

But what makes the water boil? Increased temps due to friction? Tiny
devil-spirits waving their pitchforks?


Lower pressure.



.... Higher RPM gives higher tangentail blade speeds and greater pressure
differential between the faces of the prop.

Somewhat right, tip speed above a certain linear velocity will cause
cavitation no matter the dP,

Then it would be the void (low pressure) behind the propeller. There is
still cavitation due to drag, which would occur with a spinning disc.


You could also mention the dread voodoo curse "viscous shear" which means
that somebody is trying to force those lazy water molecules to move faster
than they are willing



Fluid flow does a lot of things counter-intuitive to people familiar with
normal physics. One of my favorite engineering profs used to say that
fluid molecules are like basketballs smeared with syrup, which helps
intuit the behavior.

DSK

"Gilligan" wrote in message . ..
|
| "DSK" wrote in message
| ...
|
| But what makes the water boil? Increased temps due to friction? Tiny
| devil-spirits waving their pitchforks?
|
|
| Lower pressure.

Hee hee! Lower pressure never makes any liquid boil. It makes it vaporize.
Boil is a subjective term. It only refers to that temperature at atmospheric
pressure at which a liquid vaporizes. Some engineer you are Mr. Gilligan!

Paladin

--
Posted via a free Usenet account from http://www.teranews.com

http://en.wikipedia.org/wiki/Boiling



Boiling is the rapid vaporization of a liquid, which typically occurs when a
liquid is heated to a temperature such that its vapor pressure is above that
of the surroundings, such as air pressure. Thus, a liquid may also boil when
the pressure of the surrounding atmosphere is sufficiently reduced, such as
the use of a vacuum pump or at high altitudes. Boiling occurs in three
characteristic stages, which are nucleate, transition and film boiling.
These stages generally take place from low to high surface temperatures,
respectively.

Nucleate boiling is characterized by the incipience and growth of bubbles on
a heated surface, which rise from discrete points on a surface, whose
temperature is only slightly above the liquid’s saturation temperature. In
general, the number of nucleation sites are increased by an increasing
surface temperature. An irregular surface of the boiling vessel (i.e.
increased surface roughness) can create additional nucleation sites, while
an exceptionally smooth surface (such as glass) lends itself to
superheating.

When the surface temperature reaches a maximum value, the critical
superheat, vapor begins to form faster than liquid can reach the surface.
Thus, the heated surface suddenly becomes covered with a vapor layer.
Because of the vapor layer’s lower thermal conductivity, this vapor layer
insulates the surface. This condition of a vapor film insulating the surface
from the liquid characterizes film boiling.

Transition boiling may be defined as the unstable boiling, which occurs at
surface temperatures between the maximum attainable in nucleate and the
minimum attainable in film boiling.

"Gilligan" wrote in message . ..
| http://en.wikipedia.org/wiki/Boiling
|
|
|
| Boiling is the rapid vaporization of a liquid, which typically occurs when a
| liquid is heated to a temperature such that its vapor pressure is above that
| of the surroundings, such as air pressure. Thus, a liquid may also boil when
| the pressure of the surrounding atmosphere is sufficiently reduced, such as
| the use of a vacuum pump or at high altitudes. Boiling occurs in three
| characteristic stages, which are nucleate, transition and film boiling.
| These stages generally take place from low to high surface temperatures,
| respectively.
|
| Nucleate boiling is characterized by the incipience and growth of bubbles on
| a heated surface, which rise from discrete points on a surface, whose
| temperature is only slightly above the liquid’s saturation temperature. In
| general, the number of nucleation sites are increased by an increasing
| surface temperature. An irregular surface of the boiling vessel (i.e.
| increased surface roughness) can create additional nucleation sites, while
| an exceptionally smooth surface (such as glass) lends itself to
| superheating.
|
| When the surface temperature reaches a maximum value, the critical
| superheat, vapor begins to form faster than liquid can reach the surface.
| Thus, the heated surface suddenly becomes covered with a vapor layer.
| Because of the vapor layer’s lower thermal conductivity, this vapor layer
| insulates the surface. This condition of a vapor film insulating the surface
| from the liquid characterizes film boiling.
|
| Transition boiling may be defined as the unstable boiling, which occurs at
| surface temperatures between the maximum attainable in nucleate and the
| minimum attainable in film boiling.
|
|

Good info but mostly obfuscation to keeping from saying. "Oops, I
was wrong.

The facts of the matter that the verb boil means to heat to a temperature
at which a liquid vaporizes. I'm saying there is not enough heating going
on in a cavitating prop to vaporize water. Thus to say it boils is to speak
in error.

The fact of the matter is you used the word boil when you should have
corrected DSK and used vaporized instead. It's a pressure thing and
not a temperature thing,

Paladin


--
Posted via a free Usenet account from http://www.teranews.com

"Gilligan" wrote in message . ..
| http://en.wikipedia.org/wiki/Boiling
|
|
|
| Boiling is the rapid vaporization of a liquid, which typically occurs when a
| liquid is heated to a temperature such that its vapor pressure is above that
| of the surroundings, such as air pressure. Thus, a liquid may also boil when
| the pressure of the surrounding atmosphere is sufficiently reduced, such as
| the use of a vacuum pump or at high altitudes. Boiling occurs in three
| characteristic stages, which are nucleate, transition and film boiling.
| These stages generally take place from low to high surface temperatures,
| respectively.
|
| Nucleate boiling is characterized by the incipience and growth of bubbles on
| a heated surface, which rise from discrete points on a surface, whose
| temperature is only slightly above the liquid’s saturation temperature. In
| general, the number of nucleation sites are increased by an increasing
| surface temperature. An irregular surface of the boiling vessel (i.e.
| increased surface roughness) can create additional nucleation sites, while
| an exceptionally smooth surface (such as glass) lends itself to
| superheating.
|
| When the surface temperature reaches a maximum value, the critical
| superheat, vapor begins to form faster than liquid can reach the surface.
| Thus, the heated surface suddenly becomes covered with a vapor layer.
| Because of the vapor layer’s lower thermal conductivity, this vapor layer
| insulates the surface. This condition of a vapor film insulating the surface
| from the liquid characterizes film boiling.
|
| Transition boiling may be defined as the unstable boiling, which occurs at
| surface temperatures between the maximum attainable in nucleate and the
| minimum attainable in film boiling.
|
|

Good info but mostly obfuscation to keeping from saying. "Oops, I
was wrong.

The facts of the matter that the verb boil means to heat to a temperature
at which a liquid vaporizes. I'm saying there is not enough heating going
on in a cavitating prop to vaporize water. Thus to say it boils is to speak
in error.

The fact of the matter is you used the word boil when you should have
corrected DSK and used vaporized instead. It's a pressure thing and
not a temperature thing,

Paladin


--
Posted via a free Usenet account from http://www.teranews.com

http://encarta.msn.com/media_4615415...for_Water.html

"Gilligan" wrote in message . ..
|
| http://encarta.msn.com/media_4615415...for_Water.html
|
|
|

The diagram proves my point. Since there is no significant temperature
change involved with a propeller but there is a significant pressure change
then the water does not vaporize because it boils. Rather it vaporizes because
of the pressure change.

I'm just so brilliant. You can't even manage to misdirect me.

Paladin

--
Posted via a free Usenet account from http://www.teranews.com

"Gilligan" wrote in message
. ..
http://en.wikipedia.org/wiki/Boiling



Boiling is the rapid vaporization of a liquid, which typically occurs when
a liquid is heated to a temperature such that its vapor pressure is above
that of the surroundings, such as air pressure. Thus, a liquid may also
boil when the pressure of the surrounding atmosphere is sufficiently
reduced, such as the use of a vacuum pump or at high altitudes. Boiling
occurs in three characteristic stages, which are nucleate, transition and
film boiling. These stages generally take place from low to high surface
temperatures, respectively.

Nucleate boiling is characterized by the incipience and growth of bubbles
on a heated surface, which rise from discrete points on a surface, whose
temperature is only slightly above the liquid's saturation temperature. In
general, the number of nucleation sites are increased by an increasing
surface temperature. An irregular surface of the boiling vessel (i.e.
increased surface roughness) can create additional nucleation sites, while
an exceptionally smooth surface (such as glass) lends itself to
superheating.

When the surface temperature reaches a maximum value, the critical
superheat, vapor begins to form faster than liquid can reach the surface.
Thus, the heated surface suddenly becomes covered with a vapor layer.
Because of the vapor layer's lower thermal conductivity, this vapor layer
insulates the surface. This condition of a vapor film insulating the
surface from the liquid characterizes film boiling.

Transition boiling may be defined as the unstable boiling, which occurs at
surface temperatures between the maximum attainable in nucleate and the
minimum attainable in film boiling.

The propeller is the thing that makes the boat go, innit?

Max