Useless propeller
 

After some use I've decided those two bladed folding props are useless in
any rough weather. I'm getting rid of mine and putting on a four bladed prop
with a gear reduction.

Bob Crantz

Useless propeller
 

Some Thing Is missing in your sailing.;---? Why in the hell are you
worrying about a Prop in Bad Weather? You have more than enough Sail
Power and if reefed properly, a hell of a lot smoother ride.

My 2 Blade, Folder would stay open at 200 RPM but cavitation did worry
me. I don't think a 4 Blade fixed is going to help cavitation.




http://community.webtv.net/tassail/ThomPage

Useless propeller
 

The gear reduction reduces the propeller RPM and reduces cavitation.

A nice big 4 blade does the job.

One should worry the most about propellers in bad weather.

Useless propeller
 

BC,

Cavitation is caused be the Prop not having enough water in Choppy Sea.
Reduction Gearing has nothing to do with it.

You're a Pretty Poor sailer if you are depending on a small Aux, Engine
to replace Sails in a Seaway. Even Trawlers hoist a steadying Sail in a
Seaway to improve ride quality




http://community.webtv.net/tassail/ThomPage

Useless propeller
 

Thom Stewart wrote:
BC,

Cavitation is caused be the Prop not having enough water in Choppy Sea.

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

Reduction Gearing has nothing to do with it.

Reduction gearing has EVERYTHING to do with it.

PDW

Useless propeller
 

"Peter" wrote in message
oups.com...


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

Cavitation is caused by water not being able to move fast enough to fill low
pressure spaces created around such things as a propeller. 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. Higher RPM gives higher
tangentail blade speeds and greater pressure differential between the faces
of the prop.

Useless propeller
 

"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

Useless propeller
 

"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

Useless propeller
 

"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

Useless propeller
 

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.