On 30 Mar 2004 00:17:24 GMT, (JAXAshby) wrote:

steve, consider just where the air on the "supply side" of the fan blades come
from and consider how that differs from the air on the "demand side". the
demand side is more or less a stream that expands. the supply side is more
like a hemi-sphere of air (actually, air from the demand side passes back to
the supply side as each blade of the fan passes, i.e. tip vortices).

Consider that it doesn't really matter as long as there is flow of fluid
media over the rudder.

Consider, also, that *if* fluid drawn over a rudder by a prop have any effect
on the rudder, mariners would all know which direction the stern moved with
which rudder position. Even the guys who insist pulled water affects a rudder
don't have a clew which way the boat turns. indeed, the "good professor" was
reduced to claiming that friction in the rudder bearin made the difference.

Consider the fact that fluid drawn over a rudder by a prop may have an
effect on how the stern moves, but one that is much less then prop walk.

Steve

Consider that it doesn't really matter as long as there is flow of fluid
media over the rudder.

but it does, because the rudder and prop are hooked together. If the prop were
fixed as to direction, the rudder would turn into it until the movement
stopped.

Consider the fact that fluid drawn over a rudder by a prop may have an
effect on how the stern moves, but one that is much less then prop walk.


the "good professor" argued that without friction in the rudder bearings rudder
would move. I say it doesn't.

plainly, a shot of forward throttle with the rudder turned turns the stern, and
all (most?) mariners know in which direction the boat will turn from
experience. nobody can remember which direction a boat will turn with a shot
of reverse throttle because nobody has seen it.

On 30 Mar 2004 01:37:32 GMT, (JAXAshby) wrote:

Consider that it doesn't really matter as long as there is flow of fluid
media over the rudder.

but it does, because the rudder and prop are hooked together. If the prop were
fixed as to direction, the rudder would turn into it until the movement
stopped.

no it doesn't. It doesn't matter that they are attached. The rudder
will have an effect if there is water flowing over it. Whether that
effect is just to lessen or greaten the larger effect of prop walk, it's
still an effect.

Consider the fact that fluid drawn over a rudder by a prop may have an
effect on how the stern moves, but one that is much less then prop walk.

plainly, a shot of forward throttle with the rudder turned turns the stern, and
all (most?) mariners know in which direction the boat will turn from
experience. nobody can remember which direction a boat will turn with a shot
of reverse throttle because nobody has seen it.

Of course that doesn't mean the rudder has no effect at all, which is
what you claimed.

Steve

On Mon, 29 Mar 2004 21:00:56 -0700, Keith Hughes
wrote:



Steven Shelikoff wrote:

Consider that it doesn't really matter as long as there is flow of fluid
media over the rudder.

Again, here you're assuming laminar (or at least unidirectional)
flow. When inserted into a laminar flow stream, and angled surface,
such as a rudder, will certainly be subjected to a force related to
the mass of the fluid deflected. Fluid flow on the 'suction' side is
nowhere near laminar, and will in fact be totally non-uniform around
the rudder. All fluid will be redirected immediately upon clearing

That's ok. Fluid on the pressure side of the prop is nowhere near
laminar either and will in fact be totally non-uniform around the
rudder. Yet the rudder still has an effect on the boat's direction.

Consider the fact that fluid drawn over a rudder by a prop may have an
effect on how the stern moves,

It *may* for a brief instant until an equilibrium is reached and the
pressure equalizes on both rudder surfaces (remember, water is *not*
elastic in the way air is, so you can't create a vacuum in water
like you do in air - if you do, you cavitate and dissolve gases come
out of solution until the partial pressures equalize and/or until
water 'fills in the void' and the gases redissolve).

Sure you can create a vacuum in water, just like in air. The only
difference is that water doesn't change it's volume (as much, but it
does a small amoutn) when the pressure changes. There's still a vacuum
though.

And you can certainly create a vacuum in water without cavitation.
Cavitation only occurs if the pressure of the water drops below it's
vapor pressure. There's a whole art/science of creating props that work
without cavitation for use with submarines.

but one that is much less then prop walk.

Many orders of magnitude less IME and IMO.

Especially with an angled propshaft. But there nontheless.

Steve

Steven Shelikoff wrote:

Consider that it doesn't really matter as long as there is flow of fluid
media over the rudder.

Again, here you're assuming laminar (or at least unidirectional)
flow. When inserted into a laminar flow stream, and angled surface,
such as a rudder, will certainly be subjected to a force related to
the mass of the fluid deflected. Fluid flow on the 'suction' side is
nowhere near laminar, and will in fact be totally non-uniform around
the rudder. All fluid will be redirected immediately upon clearing
the rudder, and the resulting reaction force is parallel to the
boats centerline.


Consider, also, that *if* fluid drawn over a rudder by a prop have any effect
on the rudder, mariners would all know which direction the stern moved with
which rudder position. Even the guys who insist pulled water affects a rudder
don't have a clew which way the boat turns. indeed, the "good professor" was
reduced to claiming that friction in the rudder bearin made the difference.


Consider the fact that fluid drawn over a rudder by a prop may have an
effect on how the stern moves,

It *may* for a brief instant until an equilibrium is reached and the
pressure equalizes on both rudder surfaces (remember, water is *not*
elastic in the way air is, so you can't create a vacuum in water
like you do in air - if you do, you cavitate and dissolve gases come
out of solution until the partial pressures equalize and/or until
water 'fills in the void' and the gases redissolve).

but one that is much less then prop walk.

Many orders of magnitude less IME and IMO.

Keith Hughes

JAXAshby wrote in message
...

jim, please don't make the mistake of saying that wings lift
"because they are
round on one side". you can go to any airshow on the planet
and see aircraft
fly upside down, the round side of the wing towards the ground

Of course I won't make that mistake. What made you think I would?
I repeat the relevant part of my post:

"Any pressure change in a freely flowing fluid will be matched to
a change in local fluid speed (barring supersonics, flow
breakaway, and the trivial effects of surface viscosity) to
conserve energy. This is (presumably) the 'bernouili' bit you
claim is often erroneous."

I said this in response to your statement that pressure change
does not have to be related to a speed change in the
circumstances we're talking about. This seemed to me to violate
the laws of conservation of energy. It was you who called
Bernoulli into it, bless his cotton socks. I quote from your
post:

"water speed does not have to be equal or greater or less. This
can be a bit
confusing because "bernoulli" is often -- though erroneously --
given as the
reason sails/wings have "lift"."

You were here responding to my assumption that if there's a
(mean) pressure differential over the rudder, than there will be
an allied mean change in fluid speed. Just like an airplane wing
creating lift. The fluid speed on the low pressure side will be
faster (caveats for supersonic flow etc - we are talking boats).
I hope you don't disagree with that.

JimB

JAXAshby wrote in message
...

I see the rudder (prop in
reverse, boat static) altering the direction of the water
approaching the prop.

no, the water pressure of either side of th rudder is the same.

Now, perhaps it doesn't

it doesn't.

the water pressure on either side of a rudder is the same for
water drawn over
the rudder.

Those are statements, not explanations. That's why I'm stuck. How
about an explanation of those phenomena for a numerate old
thickie? Try third year fluid dynamics instead of first year. It
won't kill me.

JimB

JAXAshby wrote in message
...
similar to what the good professor at MIT observed with
his fan.

what the "good professor at MIT observed" was that starting
with an an empty
tube there was a tiny movement until the tube filled.


I seem to remember you damned the professor for using a metaphor
.. . .

JimB

Steven Shelikoff wrote:

That's ok. Fluid on the pressure side of the prop is nowhere near
laminar either and will in fact be totally non-uniform around the
rudder. Yet the rudder still has an effect on the boat's direction.


No, it's not laminar, it is unidirectional along one axis.
Unidirectional flow can be diverted creating a thrust vector, unlike
the non-unidirectional flow on the suction side where the rudder
provides pressure drop instead of redirection/diversion. That's the
difference.


Sure you can create a vacuum in water,

You need to check the definition of vacuum if you believe this.
"Vacuum in water" is an oxymoron.

just like in air. The only
difference is that water doesn't change it's volume (as much, but it
does a small amoutn) when the pressure changes.

The *liquid* volume does not change, that's a basic property of
liquids. Their volume is temperature dependent, not pressure
dependent. If you reduce the pressure, dissolved gases will evolve
(that *is* cavitation) but you now have bubbles suspended in a
liquid, i.e. foam.

There's still a vacuum
though.

Don't think so.

And you can certainly create a vacuum in water without cavitation.
Cavitation only occurs if the pressure of the water drops below it's
vapor pressure.

Yes, and you would create a vacuum without doing this exactly how?
Fluid is not elastic. Move it from one point too quickly (what you'd
*have* to do to create a local low pressure area) and you will
liberate dissolve gas (even gaseous water) due to the low pressure
and/or high temperature created by the shear. Water doesn't stretch.

There's a whole art/science of creating props that work
without cavitation for use with submarines.

Quite so. They do not, however, generate 'pockets of vacuum' in
doing so.

Keith Hughes

steeeeeeeeeeeeeeeeeeeeeeeve!!


That's ok. Fluid on the pressure side of the prop is nowhere near
laminar either and will in fact be totally non-uniform around the
rudder.

you are mixing mixie pixie dust with polymorphism. the words _sound_ alike,
but you described a totally different issue from the one you addressed.