"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

"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?

"Edgar" wrote in message
...

"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?


Not an engineer. However, there is a difference in the "pressure" between
the high and low of the prop. I have a hydrostatic PFD (vs. the old pill
melting) autoinflate. It's a good choice if you go offshore or sail
dinghies, and you don't want the spray to trigger the PFD. I believe it's
activated in 2' of water, but don't quote me. I had the pill melt on me once
while sailing a dinghy, and I don't want to wear the bulky PFDs.

My understanding is that it's only a small part of the effect. As most of us
know, the prop wash can be observed on the opposite side from the direction
the prop walk forces the stern (usually, the prop wash is observed most
strongly on the starboard side, thus the stern moves to port). This is much
like a fire hose that forces water out the business end and pushes against
the person holding the hose. In the case of the boat, the stern moves in the
opposite because there's nothing but some water to oppose it.

I think Donal said it best, that it really doesn't matter, since the issue
we're all concerned with is how to either use it or deal with it. I find
pulsing the engine works vs. actually putting the engine in neutral. I get
90% of the benefit with 1/2 the hassle.

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

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