What is the primary cause of propeller walk - torque or thrust?

On 5 Dec, 22:37, "Charles Momsen" wrote:
What is the primary cause of propeller walk - torque or thrust?

Who cares?

The important thing is that you know how your
boat will perform.

Regards


Donal

wrote in message
...
On 5 Dec, 22:37, "Charles Momsen" wrote:
What is the primary cause of propeller walk - torque or thrust?

Who cares?

The important thing is that you know how your
boat will perform.

Regards


Donal

Go sit in the corner with Ganz.

On Dec 5, 4:37*pm, "Charles Momsen" wrote:
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

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

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