It's actually not a particularly "good" question.

Both forces are at work, along with water density, angle of drive shaft,
differences in drag in forward vs. reverse, etc. For a RH (clockwise) prop
movement in forward, the front of the boat tends to turn to port, all things
being equal. In reverse, the stern turn to port. On my Sabre, they designed
the engine (and shaft) to be mounted off center (shaft exists the boat
slightly to the port) in order to reduce/eliminate forward prop walk. From
the manual:

OFFSET PROPELLERS: Sabre yachts are built with propeller shafts offset to
port or starboard depending on engine rotation. Engines with clockwise
rotation (right hand) have shafts offset to port.
After extensive research it was found that for this hull type an offset
propeller benefited engine performance and handling characteristics.

Sabre rudders are partially balanced with the rudder shaft about 15% aft of
its leading edge. This feature reduces the load on the helm and makes it
more comfortable to hold while under sail. The powerful wash of a
centerline propeller causes the helm to veer to one side or the other after
a turn is initiated. The offset propeller wash to flow to the side of the
rudder. A second benefit is that the angle between the shaft and the fore
and aft axis of the boat is less than would be possible with a centerline
installed shaft.

Here's a decent link that describes the forces involved:
http://www.sailingislife.com/id28.html.

"Joe" wrote in message
...
What is the primary cause of propeller walk - torque or thrust?

Good question. I would think thrust. Touque would only tend to roll
the boat or cause it to tip.
A cavitating prop will not walk.

Joe



--
"j" ganz @@
www.sailnow.com

"Capt. JG" wrote in message
ons...
It's actually not a particularly "good" question.


Here's a decent link that describes the forces involved:
http://www.sailingislife.com/id28.html.

That's not a particularly "good" answer.


The bulk modulus of water is such that the density differential at 14
inches is neglible. Plus if density was a contributing factor, it occurs in
a vertical profile and is zero difference is a horizontal profile, so it
would not cause the boat to walk, but rather to pitch.

The torque of the propeller does twist the boat and the thrust lateral to
the propeller directed toward the hull and redirected by the angle of the
hull. A right hand prop leans the boat to the left and the starboard bottom
of the hull directs more thrust laterally than downward compared to the port
side and hence the walk to the port.

The angle of the driveshaft has nothing to do with walk. The asymmetry is
vertical, not horizontal, in the thrust difference.

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.

Now if the propeller shaft mounted off centerline reduces the walk, this
would indicate that sideways (lateral) thrust from the propeller directed
against the hull is a factor. Also with the shaft off center the boat is
less likely to "torque" against the propeller. It's always good to find
physical evidence to support a conjecture.

"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

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

"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


Wouldn't lift increase in the same proportion as drag?

1 foot of depth gives a pressure differential of .06 atmospheres or .98 psi.

http://www.youtube.com/watch?v=u5KKcBQen-Y

"Charles Momsen" wrote in message
...

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

"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


Wouldn't lift increase in the same proportion as drag?

Don't think so. Drag increases to a greater extent than lift. If this
weren't the case your average airplane or boat propeller would never
cavitate.


1 foot of depth gives a pressure differential of .06 atmospheres or .98
psi.

http://www.youtube.com/watch?v=u5KKcBQen-Y

Excellent! That is one static psi difference in a foot. Take a three-bladed
prop with a surface area of about 600 square inches and you're dragging the
blades of the damned thing through 600 psi on the bottom half? Correct?

What kind of pressure do you have on the top half? 500PSI? Makes your mere
..98 psi seem suddenly more significant, doesn't it?

Wilbur Hubbard

So you drag the top of the prop through

"Charles Momsen" wrote in
:


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

"Charles Momsen" wrote in message
...
This graph:


http://www.windows.ucar.edu/tour/lin...ity.html&edu=h
igh

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


Wouldn't lift increase in the same proportion as drag?


Yes, you are absolutely correct.

blondie

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" 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.

It was only with the great minds combined here on ASA that such previous
unexplained hydrodynamic effects have come to full understanding for the
greater benefit of mankind. Yes, ASA is slowly rising from the dismal swamp
it had once become to shining paragon of truth, justice and the American
way! Good work to all, and let's keep our noses to the grindstone!

Admiral Momsen

"Charles Momsen" wrote in message
...

"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.

Already a false assumption. The vortex, like that from a tip of an airplane
wing will angle upwards. Why? because water pressure lessens towards the
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.

I resent these remarks. As I have shown density and pressure have everything
to do with prop walk.


It was only with the great minds combined here on ASA that such previous
unexplained hydrodynamic effects have come to full understanding for the
greater benefit of mankind. Yes, ASA is slowly rising from the dismal
swamp it had once become to shining paragon of truth, justice and the
American way! Good work to all, and let's keep our noses to the
grindstone!

Admiral Momsen

It will only rise from the abyss when Pansy Ganzy throws in the towel.

Wilbur Hubbard