push vs pull vis a vis rudders
 

JAXAshby wrote in message
...
If you could demonstrate, prove or explain why water speed
should
be identical along each side of the rudder

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".

Sails/wings create lift (a force) by altering the momentum of the
air passing by.

The mechanism creating this lift is a (mean) fluid pressure
difference between one side, and the other, of the sail/wing.

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.

Interesting.

Do you disagree with the concept of conservation of energy? or do
you claim special conditions which make his equations irrelevant?

It might be a bit easier to remember that for the rudder to be
pushed one way,
it (the rudder) must push water the opposite way. If the water
is not
deflected then there is no force on the rudder.

Agree; for the rudder to create yaw, it must deflect water. It
must change the momentum of the water. Many ways of saying the
same thing. That's where I'm stuck. I see the rudder (prop in
reverse, boat static) altering the direction of the water
approaching the prop.

Now, perhaps it doesn't. Or perhaps there's an opposite effect
somewhere else which I haven't yet identified. I'm looking for
education here, not stating a flat opinion.

JimB

push vs pull vis a vis rudders
 

Derek Rowell wrote in message
news:E4o9c.108024$1p.1536914@attbi_s54...

Repeat the experiment with the "rudder" on the inlet side of
the fan
(transmission in reverse). Is there a turning effect (torque)
or not? Is
there a sideways thrust on the "rudder"?
You tell me - I just did it. The answers to all four questions
is yes.

Yes, in reverse there is a torque on the rudder. But (if I read
your hinges correctly) it may be caused by a solely fore and aft
force on the rudder. Either would certainly explain the rudder
kick I have experienced.

What we're actually looking for is a net force at right angles to
the centreline of the boat, so the proper hinge for this
experiment would be parallel to the boat centreline, above the
flow. We'd then look to see if the paper was still deflected.

Just nipping off to play with some bits of wire and card . . .

JimB

push vs pull vis a vis rudders
 

Wayne.B wrote in message
...
On 29 Mar 2004 03:27:38 GMT, (JAXAshby) wrote:

It exerts a force against the rudder,

why is that? Please explain in detail, as the physicists
disagree with you.
If you are right, you stand to make a fortune on the Nobel
prize money alone.

================================================== ==

If flow deflection takes place (rudder at angle to flow), a
force is
exerted. Old news to everyone, Nobel prize not likely.

Wayne,

I can imagine a description of flow over the rudder which would
meet Jax's flat (and rather unhelpful) statements, and also
square with the obvious deflection that must occur just before
entering the prop. Whether it's realistic or not is another
matter.

Perhaps water approaches the propeller via the deflected rudder
through an Ess bend. ie, water approaches a point about a third
of the way from the tip of the deflected rudder (lets call it the
stagnation point). From one side of this point, water idles off
at a steep angle to round the rudder tip, doing a hairpin bend to
run back to the prop. From the other side the water moves quickly
along the rudder surface to the prop.

Well, it's a thought. I'm off to play with bits of card and wire
to repeat Derek Rowells experiment, so perhaps I should add a few
burning fag ends to the picture?

JimB

push vs pull vis a vis rudders
 

******************If****************** flow deflection takes place (rudder at
angle to flow), a force is
exerted.

*IF* is the operative word. The question is why do *you* believe there is
deflection? The physicists don't believe that. Why do you?

push vs pull vis a vis rudders
 

to demonstrate a force due to suction

there is no force in nature called "suction". none.

push vs pull vis a vis rudders
 

There is a demonstration of the Feynman
sprinkler puzzle somewhere at MIT.

the link was posted last night. the guy who hijacked the professor's email
addy should have taken greater care in whose address he grabbed, for it would
seem the real professor at MIT would have long ago known of the demo that any
student -- or his mother or even little sisten in grade school -- could walk up
to and push the button to see for himself.

push vs pull vis a vis rudders
 

This is (presumably) the 'bernouili' bit you
claim is often erroneous.

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

bernouili had to do with venturi effects and "sounds" scientific to lay ears.
a 1st semester aero eng student knows that bernouili does not explain lift.

push vs pull vis a vis rudders
 

Sails/wings create lift (a force) by altering the momentum of the
air passing by.

yes.

for the rudder to create yaw, it must deflect water.

yes.

It
must change the momentum of the water.

yes.

That's where I'm stuck

yes.

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.

perhaps there's an opposite effect
somewhere else which I haven't yet identified

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

push vs pull vis a vis rudders
 

On 26 Mar 2004 21:44:34 GMT, (JAXAshby) wrote:

Intuitively, most people sense that water "pulled" over a rudder will cause a
rudder to change direction of a boat in much the same way as water "pushed"
over a rudder does. However, intuition misses some things along the way.
[...]
end) to port. However, the water drawn over the rudder's port side hits that
side and is deflected towards port. Then the rudder would push the boat
(after end) to starboard. And equal and opposite reaction. Net, net, the boat
does not turn. The pressure on each side of the rudder is equal. Nada.


Jox, since you're such an "expert" on Feynman inverse sprinkler problem
and how to misapply it to any situation, maybe you can answer a question
about it. While it's true that the sprinkler won't turn when water is
being sucked in, it's not true that no net force is generated by sucking
the water in. In fact, there is a net force generated. It's just not
in a direction that will turn the sprinkler.

In relation to your discussion about about equal and opposite, net net,
no net force, etc., how do you reconcile that with the fact that it's
not true for the inverse sprinkler problem?

Steve

push vs pull vis a vis rudders
 

On 29 Mar 2004 12:01:16 GMT, (JAXAshby) wrote:

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

Only if the rudder is parallel to the direction of flow. At an angle
to the flow, water is deflected, momentum is changed, force is
created. It's not very much force in reverse, not enough to be useful
for maneuvering, but a force nevertheless.