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

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

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.

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


Propeller walk is a term used to describe the results of the bottom of the
propeller operating in a more dense environment than the top of the
propeller. The primary factor is neither of the above. Drag, due to the
propeller's interaction with a denser medium, is increased at the bottom
half of the propeller. A prop turning clockwise as the vessel proceeds
forward as seen from astern will cause the vessel's stern to move to
starboard due to increased drag. Whereas thrust in a vessel is a description
of fore and aft forces, prop walk is a description of transverse forces
caused by drag differential.

I hope this helps clarify what is a muddled discussion point in most cases -
especially where I do not deem to impart my brilliance.

Wilbur Hubbard

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

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

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


Propeller walk is a term used to describe the results of the bottom of the
propeller operating in a more dense environment than the top of the
propeller. The primary factor is neither of the above. Drag, due to the
propeller's interaction with a denser medium, is increased at the bottom
half of the propeller. A prop turning clockwise as the vessel proceeds
forward as seen from astern will cause the vessel's stern to move to
starboard due to increased drag. Whereas thrust in a vessel is a
description of fore and aft forces, prop walk is a description of
transverse forces caused by drag differential.

That is the easy answer that seems to satisfy most people but if you think
about it do you really believe that water gets measurably denser because it
is about 18" (say) deeper at the bottom of an 18"(say) propeller than the
water at the top? Of course it does not. Water is hardly compressible at
all except at very high pressures.
.. Density will only be a factor if air is being entrained from the surface.
You will find boats displaying propeller walk even if the propeller is too
deep for air entrainment to occur. My boat does.
So the question is 'why does the opposite direction of the blade at the top
not cancel the walk created by the blade at the bottom'?
I suggest that this is because as the blades rise upward towards the top the
water impinges on the hull and creates a reaction force which acts in the
same direction as the walk force created at the bottom. This is because, in
yachts especially, the blades pass very close to the hull which usually is
somewhat vee shaped and deflects the water sideways. In your example water
would be deflected to port but the reaction on the hull would be a starboard
force.

"Edgar" wrote in message
...

troll sh*t removed
That is the easy answer that seems to satisfy most people but if you think
about it do you really believe that water gets measurably denser because
it is about 18" (say) deeper at the bottom of an 18"(say) propeller than
the water at the top? Of course it does not. Water is hardly compressible
at all except at very high pressures.
. Density will only be a factor if air is being entrained from the
surface.
You will find boats displaying propeller walk even if the propeller is too
deep for air entrainment to occur. My boat does.
So the question is 'why does the opposite direction of the blade at the
top not cancel the walk created by the blade at the bottom'?
I suggest that this is because as the blades rise upward towards the top
the water impinges on the hull and creates a reaction force which acts in
the same direction as the walk force created at the bottom. This is
because, in yachts especially, the blades pass very close to the hull
which usually is somewhat vee shaped and deflects the water sideways. In
your example water would be deflected to port but the reaction on the hull
would be a starboard force.



Think hydrostatic pressure. Also, Neal is wrong.

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

"Capt. JG" wrote in message
easolutions...
"Edgar" wrote in message
...

troll sh*t removed
That is the easy answer that seems to satisfy most people but if you
think about it do you really believe that water gets measurably denser
because it is about 18" (say) deeper at the bottom of an 18"(say)
propeller than the water at the top? Of course it does not. Water is
hardly compressible at all except at very high pressures.
. Density will only be a factor if air is being entrained from the
surface.
You will find boats displaying propeller walk even if the propeller is
too deep for air entrainment to occur. My boat does.
So the question is 'why does the opposite direction of the blade at the
top not cancel the walk created by the blade at the bottom'?
I suggest that this is because as the blades rise upward towards the top
the water impinges on the hull and creates a reaction force which acts in
the same direction as the walk force created at the bottom. This is
because, in yachts especially, the blades pass very close to the hull
which usually is somewhat vee shaped and deflects the water sideways. In
your example water would be deflected to port but the reaction on the
hull would be a starboard force.



Think hydrostatic pressure. Also, Neal is wrong.

Yes, he certainly is.
I have tried your suggestion about thinking 'hydrostatic pressure' but it
is the 'static' bit that is somewhat puzzling me because propeller walk is a
dynamic phenomenon. Can you expand on this a bit?

On Dec 6, 2:02*pm, "Wilbur Hubbard"
wrote:
"Charles Momsen" wrote in message

...

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

Propeller walk is a term used to describe the results of the bottom of the
propeller operating in a more dense environment than the top of the
propeller. The primary factor is neither of the above. Drag, due to the
propeller's interaction with a denser medium, is increased at the bottom
half of the propeller. A prop turning clockwise as the vessel proceeds
forward as seen from astern will cause the vessel's stern to move to
starboard due to increased drag. Whereas thrust in a vessel is a description
of fore and aft forces, prop walk is a description of transverse forces
caused by drag differential.

I hope this helps clarify what is a muddled discussion point in most cases -
especially where I do not deem to impart my brilliance.

Wilbur Hubbard

Trust is thrust fore and aft port and stbd. Please don't muddy the
waters. To truly walk a whole boat sideways you need twin screws, 3 is
best.

The drag differentail is caused by varying thrust off the blades...
the angle of attack so to speak that thrust is transfered to the
shaft. Its up and port walk for a right hand screw, and down and
starboard for a left handed screw. Due to the weight of water it's
easier to push/slice down than up causing the pressure and thrust
difference off the fluke. It's the path of least resistance.

Joe