"Marty" wrote in message
...
Wilbur Hubbard wrote:
"Marty" wrote in message
...
Wilbur Hubbard 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

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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
Wrong! Pitch angle doesn't matter. My outboard motor on the back of my
bluewater yacht can be put in reverse on a calm day at about 1/8
throttle while I'm tied to my mooring. My yacht will turn slow circles
caused by prop walk. The propeller is totally horizontal.

Wrong yourself. Pitch angle contributes. In your case, the vortex is
propelled upward on the starboard side, impinging the starboard side of
your boat, on the port side the vortex tends be more downward. Pitch
angle just adds to the effect.

Cheers
Martin

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.

I do hope you're not being deliberately obtuse. I specifically mentioned
the pitch of the propeller blades and the angle of the shaft in the same
post so as to make it clear.

Anyway if you like, change my 'Pitch Angle" and yours, to 'Shaft Angle',
and all is well. If the shaft is horizontal, blade pitch still causes the
vortex to rise on the starboard side, inclining the shaft just adds to the
effect

Cheers
Martin

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.

Wilbur Hubbard