"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
************************************************** ************************************
************************************************** ************************************
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
If it's the explanation I'm thinking of it involves the total path length
traveled by the blade tip on either ascent or descent (with the boat moving
forward). I could see the argument there but could not resolve if a longer
path resulted in less thrust force or more and the same with the short path
length. This same argument applies to retreating blade stall in helicopters.
I'll look through Chapman and see what he says.
The angle of attack argument only holds if the velocity of water is
significant compared to the tangential velocity of the blade. If the water
velocity is zero then the angle of attack is the same regardless of
orientation. If the blade is turning 500 rpm and is 12 inch diameter the tip
is moving at 314 inches/sec or 214 mph.