push vs pull vis a vis rudders
 

steeeeeeeeeeeeeeeeeeeeeeve???

Sure you can create a vacuum in water,

??? using maybe that age-old force in nature called "suction" maybe?

try again.

push vs pull vis a vis rudders
 

think it through, steve. think it through.

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.

so, which way does the stern move if the rudder is to port?

push vs pull vis a vis rudders
 

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

you can't see it, you can't measure it, you can't detect that it is there at
all, but it still exists? How is that?

And which way does the stern move with the rudder one way or the other?


Steve

push vs pull vis a vis rudders
 

How
about an explanation of those phenomena for a numerate old
thickie?

each blade the prop (in reverse) pushes water forward towards the bow of the
boat (also sides, but ignore that). Because "water flows downhill" water fills
in behind each blade as it pushed water forward

(note, if you watch an underwater prop turning in a stationary position, you
will see the level of the water surface fall behind the prop and rise forward
of the prop)

Now, the water that fills in behind the blades comes from whereever there is
water "uphill" of the blade. This is not just behind the prop, but also to the
sides and top and bottom, in more or less a hemisphere (an over simplification.
water actually comes from the "high pressure" side of the blade, the side
towards which the water is pushed.)

The flow "through the prop circle" hs the greatest velocity, with "all that
water" aft just waiting for its chance to "roll down hill". the greater the
distance from the prop, the slower the speed of the water rolling down hill
(lots of water available so it doesn't roll very fast. The speed at which the
water rolls towards the prop is inversely proportional to 4/3rd the distance
cubed (volume of sphere) is all aimed at the center of the prop.

People "think" the water flow towards the prop is straight at the prop, but it
isn't. it is from all edges of the hemisphere aft of the prop. Left, right,
up down, back. all edges.

push vs pull vis a vis rudders
 

jim, airspeed over a wing does not have to faster than airspeed below a wing
for a wing to have lift. "bernoulli" sounds conventiently scientific to
explain lift, but it ain't real.

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

push vs pull vis a vis rudders
 

he is a yo-yo.

candidly, when I first saw his post and noted his email address, I figured some
yo-yo hijacked his address. He claimed professional expertise in fluid flow,
but his website make no mention of such, though it does promote his "expertise"
in control systems for things such as MRI's.

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

push vs pull vis a vis rudders
 

On 30 Mar 2004 19:53:18 GMT, (JAXAshby) wrote:

think it through, steve. think it through.

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.

so, which way does the stern move if the rudder is to port?

forward or reverse? rh prop or lh prop? how much prop walk?

Steve

push vs pull vis a vis rudders
 

On 30 Mar 2004 19:54:48 GMT, (JAXAshby) wrote:

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

you can't see it, you can't measure it, you can't detect that it is there at
all, but it still exists? How is that?

You can measure it. You can detect if it's there or not. All you need
are stress sensors on the rudder post to measure what the rudder is
doing.

And which way does the stern move with the rudder one way or the other?

That's a big "it depends". Of course that doesn't mean the rudder has
no effect. The same thing can be said for the engine in forward. I.e.,
you can't predict which way the boat will turn when you turn the rudder
one way or another if other factors, like the wind, have a greater
effect than the rudder.

Steve

push vs pull vis a vis rudders
 

On Tue, 30 Mar 2004 08:04:41 -0700, Keith Hughes
wrote:



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.

Actually, it's not that either.

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.

Yes, you can create a thrust vector by diverting non-unidirectional flow
as long as the the sum of the non-unidirectional flow is not 0, which it
is not for the case we are talking about because if it was, the boat
would not move backwards when the engine is put in reverse.

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.

You're probably thinking of an absolute vacuum. In that case, a "vacuum
in air" is an oxymoron also. But since you mentioned a vacuum in air,
here we must talking about a relative vacuum which is simply an area
where the pressure is lower than another area. That is easily created
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.

The *liquid* volume does not change, that's a basic property of
liquids. Their volume is temperature dependent, not pressure

Actually, the liquid volume can change when the pressure changes.
However, it's a minute amount only measurable for drastic pressure
changes. But that's outside the scope of this thread, where we can
treat the liquid volume as constant when the pressure changes.

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.

That depends on how much you reduce the pressure. Is it your contention
that anytime you reduce the pressure of a liquid by any amount that you
must have cavitation? If so, you are plainly wrong.

There's still a vacuum
though.

Don't think so.

You think wrong ... if we're talking about a relative vacuum and not an
absolute vacuum, which is obvious we are from your previous statement:
"so you can't create a vacuum in water like you do in air." A vacuum in
air is also an oxymoron unless you're *not* talking about an absolute
vacuum.

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.

But it does flow from higher pressure areas to lower pressure areas.
The lower pressure areas are the vacuum in this case, just like air.
And it does not have to cavitate in the areas under lower pressure.

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.

Sure they do. The area in front of the prop blade is at a lower
pressure than the area behind the prop. i.e., one definition of a
vacuum.

You can measure a vacuum in water yourself if you want. Just put a
vacuum gauge behind a water pump and you will measure the vacuum of the
pump sucking water through it. I have several of them on my boat for
measuring the condition of fuel filters.

Steve

push vs pull vis a vis rudders
 

think it through, steve. think it through.

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.

so, which way does the stern move if the rudder is to port?

forward or reverse? rh prop or lh prop? how much prop walk?

reverse (that was the discussion), and your choice (neither affects the
outcome) and your choice (no value affects the outcome).


Steve