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.

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

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

"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.

"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

Oh Bull **** Pete

You posted; "Reduction gearing has EVERYTHING to do with it."

It seems you've elected to ignore, Prop Pitch, Prop Size, and the Medium
it is operating in. You have also neglected to recognize throttle
control




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

"Thom Stewart" wrote in message ...
| Oh Bull **** Pete
|
| You posted; "Reduction gearing has EVERYTHING to do with it."
|
| It seems you've elected to ignore, Prop Pitch, Prop Size, and the Medium
| it is operating in. You have also neglected to recognize throttle
| control
|

Other things being equal a larger prop with more pitch and lower
gearing is more efficient and delivers more drive per BTU of fuel
used. A one bladed prop would be most efficient but there is
a problem balancing such a thing. Three-bladed props seem to be
a very good compromise for powering sailing yachts with their
inherent slow velocity through the water.

Paladin

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

Pal(?)

Once again you've neglected to consider the drag created when not using
the engine.

Please look at the Subject heading. Then try to remember what kind of
Prop was being referred two AND in what kind of sea condition were being
described.

You can "Color me Gone" with this reply.




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