"Wilbur Hubbard" wrote in message
anews.com...


What is pitch?
Propeller pitch is the theoretical forward movement of a propeller for one
revolution, assuming that there is no prop slip. For example, a 21 pitch
propeller will theoretically move 21 inches for every revolution. Propeller
slip occurs with every propeller, but the amount of slip varies depending on
propeller design. More aggressively and efficiently designed propellers will
slip less.

When selecting a propeller pitch for your boat, it is important that the
propeller runs at the upper end of your engines wide-open-throttle RPM
range. If you want your RPM's to increase, go down in pitch. To decrease
RPM's, go up in pitch. As a general guide, for every 2" of pitch, RPM's will
change approximately 400 RPM's.

For water sports or extra people on board, you should generally drop 2" of
pitch to help compensate for the added weight and drag on your boat. It
makes a noticeable difference in your boat's hole shot, fuel efficiency,
RPM's, and overall performance. You should ALWAYS carry a spare propeller on
board, and if you're into water sports or occasionally load the boat with
extra people, a spare prop with a lesser pitch is a good idea. When thinking
a propeller pitch, compare it to a gear on car - lower gear, higher RPM's.

The same pitch from different manufacturers will run slightly different
RPM's due to a difference in blade design. Speed differences among the same
pitch from various manufacturers will vary even more. For example a 17 pitch
from one brand could run up to 7 mph faster than a 17 pitch from another
brand.

Pitch is the 2nd two digits that are listed in a propeller item description
(14-1/4 x 19, 19 is the pitch, 14-1/4 is the diameter)



--------------------------------------------------------------------------------

What is diameter?
Propeller diameter is distance across the imaginary circle that a spinning
propeller makes. It can be easily determined by measuring the distance from
the center of the hub to the tip of one of the blades and multiplying that
number by 2.

Diameters between different propeller manufacturers may vary slightly. For
example for a V6 outboard application, Turning Point uses a 14-1/4" diameter
where a different manufacturer may use a 14-1/2" diameter. This small
variance does not affect your performance as much as the pitch and overall
design. However, larger diameter propellers - 15" or 16" diameter - are
designed for larger boat applications - 23 ft +, and are not ideal for 15" -
22" foot run-a-bouts. These larger boats need more blade area to push more
water.



--------------------------------------------------------------------------------

What is rake?
Propeller rake is the degree that a propeller blade is angled in relation to
the hub. Props with higher rake typically have better speeds and greater
lift. The better performing propellers typically have between 20 - 30 degree
rake angles.




--------------------------------------------------------------------------------

What is cupping?
Propeller cupping is the curved lip at the trailing edge and/or tip of the
propeller. Cupping helps the propeller to get a better grip in the water for
better holding at higher trim and on turns. Cupping also increases the
efficiency of a propeller and can result in higher top end speeds when
properly designed.



--------------------------------------------------------------------------------

What are vent holes?
Vent holes are holes behind each blade designed to aid in the hole shot of a
propeller. When accelerating from neutral, these holes allow some exhaust
bubbles to flow through and flood the blades. The prop is then spinning
through more turbulent water, thus gaining RPM's and speed more quickly.
After approximately 1000 RPM's, the exhaust flows rapidly enough that is
will bypass the holes and flow through the hub, eliminating any further
slippage from the holes.

This performance feature can only be found in stainless steel propellers.
However Turning Point is the only one to offer this feature in aluminum
propellers as well.

"jlrogers±³©" wrote in message
...

"Wilbur Hubbard" wrote in message
anews.com...


What is pitch?
Propeller pitch is the theoretical forward movement of a propeller for one
revolution, assuming that there is no prop slip. For example, a 21 pitch
propeller will theoretically move 21 inches for every revolution.
Propeller slip occurs with every propeller, but the amount of slip varies
depending on propeller design. More aggressively and efficiently designed
propellers will slip less.

When selecting a propeller pitch for your boat, it is important that the
propeller runs at the upper end of your engines wide-open-throttle RPM
range. If you want your RPM's to increase, go down in pitch. To decrease
RPM's, go up in pitch. As a general guide, for every 2" of pitch, RPM's
will change approximately 400 RPM's.

For water sports or extra people on board, you should generally drop 2" of
pitch to help compensate for the added weight and drag on your boat. It
makes a noticeable difference in your boat's hole shot, fuel efficiency,
RPM's, and overall performance. You should ALWAYS carry a spare propeller
on board, and if you're into water sports or occasionally load the boat
with extra people, a spare prop with a lesser pitch is a good idea. When
thinking a propeller pitch, compare it to a gear on car - lower gear,
higher RPM's.

The same pitch from different manufacturers will run slightly different
RPM's due to a difference in blade design. Speed differences among the
same pitch from various manufacturers will vary even more. For example a
17 pitch from one brand could run up to 7 mph faster than a 17 pitch from
another brand.

Pitch is the 2nd two digits that are listed in a propeller item
description (14-1/4 x 19, 19 is the pitch, 14-1/4 is the diameter)



--------------------------------------------------------------------------------

What is diameter?
Propeller diameter is distance across the imaginary circle that a spinning
propeller makes. It can be easily determined by measuring the distance
from the center of the hub to the tip of one of the blades and multiplying
that number by 2.

Diameters between different propeller manufacturers may vary slightly. For
example for a V6 outboard application, Turning Point uses a 14-1/4"
diameter where a different manufacturer may use a 14-1/2" diameter. This
small variance does not affect your performance as much as the pitch and
overall design. However, larger diameter propellers - 15" or 16"
diameter - are designed for larger boat applications - 23 ft +, and are
not ideal for 15" - 22" foot run-a-bouts. These larger boats need more
blade area to push more water.



--------------------------------------------------------------------------------

What is rake?
Propeller rake is the degree that a propeller blade is angled in relation
to the hub. Props with higher rake typically have better speeds and
greater lift. The better performing propellers typically have between 20 -
30 degree rake angles.




--------------------------------------------------------------------------------

What is cupping?
Propeller cupping is the curved lip at the trailing edge and/or tip of the
propeller. Cupping helps the propeller to get a better grip in the water
for better holding at higher trim and on turns. Cupping also increases the
efficiency of a propeller and can result in higher top end speeds when
properly designed.



--------------------------------------------------------------------------------

What are vent holes?
Vent holes are holes behind each blade designed to aid in the hole shot of
a propeller. When accelerating from neutral, these holes allow some
exhaust bubbles to flow through and flood the blades. The prop is then
spinning through more turbulent water, thus gaining RPM's and speed more
quickly. After approximately 1000 RPM's, the exhaust flows rapidly enough
that is will bypass the holes and flow through the hub, eliminating any
further slippage from the holes.

This performance feature can only be found in stainless steel propellers.
However Turning Point is the only one to offer this feature in aluminum
propellers as well.


Thanks, but I guess I'm more of a sailor than motor head like most of the
people posting here. To a real sailor pitch is something that happens when
the bow and stern of a boat goes up and down at an angle. Propeller pitch,
while referring to the "set angle" of the blades can also mean the angle of
the centerline of the shaft from the horizontal. See defintion 8 below.

2pitch vb 1 : to erect and fix firmly in place ~ a tent 2 : throw, fling 3
: to deliver a baseball to a batter 4 : to toss (as coins) toward a mark 5 :
to set at a particular level ~ the voice low 6 : to fall headlong 7 : to
have the front end (as of a ship) alternately plunge and rise 8 : to incline
downward : slope


Wilbur Hubbard

"Marty" wrote in message
...
Edgar wrote:
"Wilbur Hubbard" wrote in message
anews.com...

So you're talking about pitch of the prop as inches per revolution and
not pitch as angle of the prop shaft?
That's going off on a tangent as those talking about pitch angle in this
thread are talking about the angle of the prop shaft from the horizontal
or at least that's the impression I got.

Wilbur Hubbard

You are confused then. Propeller pitch is the distance a prop would screw
itself forward in one revolution if there was no slip.
I noticed from your earlier post that your 'bluewater yacht' has an
outboard on the stern. ROFL


I thought it odd also Edgar. Every mariner worth his salt knows that
propeller pitch refers to the distance the prop would travel through the
water under ideal conditions, usually expressed in inches per revolution,
the use of degrees is only for academic purposes as the angle of the blade
is not uniform from hub to tip in normal applications. Shaft angle should
be rather self explanatory.

Cheers
Martin

See definition 8 please:

2pitch vb 1 : to erect and fix firmly in place ~ a tent 2 : throw, fling 3
: to deliver a baseball to a batter 4 : to toss (as coins) toward a mark 5 :
to set at a particular level ~ the voice low 6 : to fall headlong 7 : to
have the front end (as of a ship) alternately plunge and rise 8 : to incline
downward : slope

Wilbur Hubbard

"Marty" wrote in message
...
Charles Momsen wrote:
"Marty" wrote in message
...
Charles Momsen wrote:
"Joe" wrote in message
...
On Dec 11, 12:00 pm, "Wilbur Hubbard"
wrote:
"Charles Momsen" wrote in message

...

This graph:
http://www.windows.ucar.edu/tour/lin....html&edu=high
Shows the density of water as a function of depth. Water density
changes
from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of
0.1%
in 250 ft. Since that portion of the curve is linear, one can
estimate
that water density would change .0004% over the diameter (tip to tip)
of
a 12 inch propeller. So is a .0004% change in water density (in the
vertical plane no less) going to walk a boat sideways? Don't think
so.
Think, Momsen, think! There are very large forces at work when a
propeller
is turning at speed. You are stuck on static in your thinking. Picture
it
this way. Let's say you were riding a bicycle at 1mph and you had a
ten mile
per hour headwind. You would experience an 11mph head wind. Now, if
you
aren't a girly-man you should be able to sprint up to 35mph. You would
then
experience a 45mph headwind. Suddenly your inconsequential wind has
great
consequence.

It's the same way with a propeller and the lift vs.drag coefficient.
Even a
very small density difference results in a significant drag difference
between the top half of the prop and the bottom half of the prop. But
there
is another thing that has a greater effect than density causing
density to
be only part of the equation. Water density does not vary greatly due
to the
fact that it doesn't compress easily. What does change significantly
with
depth is water pressure (divers say 1 atmosphere for every 15 feet?)
The
more pressure = the more drag for the propeller. I hope this helps.

Wilbur Hubbard
Let me use examples you may understand Neal.

Ever mix paint in a 5 gallon bucket with a paint mixing propellor on a
drill?
Why is the propellor in the paint pulled off center?

A dairy has huge tanks to store milk. They keep the creme mixed in
the milk with propellors on long shafts.
They hang straight down , he shaft is vertical. When you turn them on
the long shafts bend some in the direction of wheel walk.
Are you saying that it is pressure difference when the prop is
horizional causing the walk?

When a propellor flys off an airplane they never go straight, they
spin off in the direction of walk.

Think path of least resistance to the face of the fluke

Hope this helps.

Joe

************************************************** ************************************

************************************************** ************************************


Joe,

Thanks to your input, that of Wilbur and others I believe I have come
up with the most plausible explanation for propeller walk. This
explanation may even impress Blondie!

The cause of the prop walk is due to 2 effects, namely the Magnus
Effect and the Coanda Effect. They can be found he

http://lpmpjogja.diknas.go.id/kc/a/air/airplane.htm

The spinning prop creates a vortex of water that is moving relative to
water surrounding it, especially if the prop is angled down relative to
the water's surface. A right hand prop on a forward moving boat would
create a downward angling vortex that had higher relative velocity to
the surrounding moving water on the starboard side and lower on the
port. Viewing the vortex as a rotating cylinder moving through a fluid,
the lift would be generated to port, as is observed. The Coanda Effect
would explain the draggging and leakage of water laterally by the prop.
I believe these explanations are the simplest and consistent with all
observed effects, including paint stirrers in Joe's buckets.

I searched and could find no explanation of prop walk using the Magnus
or Coanda Effect. Mostly what I found was the same false pablam of
water density and other voodoo science mindlessly regurgitated by
babbling non-thinking parrots.
Read Chapman, he explains it in exactly the same terms, (vortexes) but
leaves out calling it either Magnus or Coanda. It's a nice simple
explanation with a little drawing explaining why the port side blade on
a right hand prop has a lower angle of attack than the right side. If
you imagine perhaps the worst case: A prop has a pitch angle of 10
degrees (I know this doesn't happen with a real prop), it's on a shaft
inclined by 10 degrees. In this case the port side blade vertical at it
ascends or descends and generates no thrust, forward or aft. All the
thrust comes from the starboard side blade angled at 20 degrees, you can
clearly see there is an unbalance here. Now put it reverse and the prop
throws a rising vortex of water at the starboard side of the hull,
kicking it to port, while at the same time pulling the prop shaft
sternwards from the starboard side only, tending to skew the boat to
port. Put both together and you've got port prop walk.

Cheers
Martin

If it's the explanation I'm thinking of it involves the total path length
traveled by the blade tip on either ascent or descent (with the boat
moving forward). I could see the argument there but could not resolve if
a longer path resulted in less thrust force or more and the same with the
short path length. This same argument applies to retreating blade stall
in helicopters. I'll look through Chapman and see what he says.

The angle of attack argument only holds if the velocity of water is
significant compared to the tangential velocity of the blade. If the
water velocity is zero then the angle of attack is the same regardless of
orientation. If the blade is turning 500 rpm and is 12 inch diameter the
tip is moving at 314 inches/sec or 214 mph.


214 mph?

500rpm * (pi *12)in/rev = 1.885E4 inches/min

1.885E4 in/min * 60 min/hr = 1.131E6 inches/hour

1.131E4 in/hr / 63360 in/mi = 17.85 mile/hr

18 mph is a tad bit slower than 214.....

Back to remedial slide-rule class for you!




214/18 = 12. I did the calculation in radians and then forgot to divide
inches by 12 then 5280, I did just 5280. Good work on checking the math
errors, this is the 2nd one I've done here in recent times. At 18 mph the
angle of attack argument begins to hold water.


Cheers
Martin

Wilbur Hubbard wrote:


I see the logic in your thinking. So, in my previous example, what if were
to take the motor off the transom and attach it to the end of a hundred foot
long pole that was firmly affixed to the stern of the yacht and did the same
thing with the motor in reverse. I bet the boat would still turn lazy
circles even though no water stream was hitting the hull.


I feel not.

Cheers
Martin

Wilbur Hubbard wrote:
"Marty" wrote in message
...
Edgar wrote:
"Wilbur Hubbard" wrote in message
anews.com...

So you're talking about pitch of the prop as inches per revolution and
not pitch as angle of the prop shaft?
That's going off on a tangent as those talking about pitch angle in this
thread are talking about the angle of the prop shaft from the horizontal
or at least that's the impression I got.

Wilbur Hubbard
You are confused then. Propeller pitch is the distance a prop would screw
itself forward in one revolution if there was no slip.
I noticed from your earlier post that your 'bluewater yacht' has an
outboard on the stern. ROFL

I thought it odd also Edgar. Every mariner worth his salt knows that
propeller pitch refers to the distance the prop would travel through the
water under ideal conditions, usually expressed in inches per revolution,
the use of degrees is only for academic purposes as the angle of the blade
is not uniform from hub to tip in normal applications. Shaft angle should
be rather self explanatory.

Cheers
Martin

See definition 8 please:

2pitch vb 1 : to erect and fix firmly in place ~ a tent 2 : throw, fling 3
: to deliver a baseball to a batter 4 : to toss (as coins) toward a mark 5 :
to set at a particular level ~ the voice low 6 : to fall headlong 7 : to
have the front end (as of a ship) alternately plunge and rise 8 : to incline
downward : slope

Now I know you're being obtuse!

Cheers
Martin

Charles Momsen wrote:




214/18 = 12. I did the calculation in radians and then forgot to divide
inches by 12 then 5280, I did just 5280. Good work on checking the math
errors, this is the 2nd one I've done here in recent times. At 18 mph the
angle of attack argument begins to hold water.


Freakin 'lecrical engineers, always thinking of omega.

Cheers
Martin

"Marty" wrote in message
news
Wilbur Hubbard wrote:


I see the logic in your thinking. So, in my previous example, what if
were to take the motor off the transom and attach it to the end of a
hundred foot long pole that was firmly affixed to the stern of the yacht
and did the same thing with the motor in reverse. I bet the boat would
still turn lazy circles even though no water stream was hitting the hull.


I feel not.

Cheers
Martin

You feel wrong!

Wilbur Hubbard

"Marty" wrote in message
news
Wilbur Hubbard wrote:
"Marty" wrote in message
...
Edgar wrote:
"Wilbur Hubbard" wrote in message
anews.com...

So you're talking about pitch of the prop as inches per revolution and
not pitch as angle of the prop shaft?
That's going off on a tangent as those talking about pitch angle in
this thread are talking about the angle of the prop shaft from the
horizontal or at least that's the impression I got.

Wilbur Hubbard
You are confused then. Propeller pitch is the distance a prop would
screw itself forward in one revolution if there was no slip.
I noticed from your earlier post that your 'bluewater yacht' has an
outboard on the stern. ROFL

I thought it odd also Edgar. Every mariner worth his salt knows that
propeller pitch refers to the distance the prop would travel through the
water under ideal conditions, usually expressed in inches per
revolution, the use of degrees is only for academic purposes as the
angle of the blade is not uniform from hub to tip in normal
applications. Shaft angle should be rather self explanatory.

Cheers
Martin

See definition 8 please:

2pitch vb 1 : to erect and fix firmly in place ~ a tent 2 : throw,
fling 3 : to deliver a baseball to a batter 4 : to toss (as coins) toward
a mark 5 : to set at a particular level ~ the voice low 6 : to fall
headlong 7 : to have the front end (as of a ship) alternately plunge and
rise 8 : to incline downward : slope

Now I know you're being obtuse!

Cheers
Martin

So I'm being obtuse because I understand and use the English language better
than you do?

Wilbur Hubbard

Wilbur Hubbard wrote:
"Marty" wrote in message
news
Wilbur Hubbard wrote:
"Marty" wrote in message
...
Edgar wrote:
"Wilbur Hubbard" wrote in message
anews.com...

So you're talking about pitch of the prop as inches per revolution and
not pitch as angle of the prop shaft?
That's going off on a tangent as those talking about pitch angle in
this thread are talking about the angle of the prop shaft from the
horizontal or at least that's the impression I got.

Wilbur Hubbard
You are confused then. Propeller pitch is the distance a prop would
screw itself forward in one revolution if there was no slip.
I noticed from your earlier post that your 'bluewater yacht' has an
outboard on the stern. ROFL
I thought it odd also Edgar. Every mariner worth his salt knows that
propeller pitch refers to the distance the prop would travel through the
water under ideal conditions, usually expressed in inches per
revolution, the use of degrees is only for academic purposes as the
angle of the blade is not uniform from hub to tip in normal
applications. Shaft angle should be rather self explanatory.

Cheers
Martin
See definition 8 please:

2pitch vb 1 : to erect and fix firmly in place ~ a tent 2 : throw,
fling 3 : to deliver a baseball to a batter 4 : to toss (as coins) toward
a mark 5 : to set at a particular level ~ the voice low 6 : to fall
headlong 7 : to have the front end (as of a ship) alternately plunge and
rise 8 : to incline downward : slope
Now I know you're being obtuse!

Cheers
Martin

So I'm being obtuse because I understand and use the English language better
than you do?

Wilbur Hubbard



In the immortal words of Oz, BWAAAAAAAAAAAAAAAAAAA!

Thanks for the chuckle

Cheers
Martin