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