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".

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.

I mentioned Feynman because some clowns on this ng (I speak of schlackoff and
jeffies and others) go ape squat when I make a statement, absolutely insisting
that if I say it I must be making it up (I make up nothing) will argue for
weeks (like sophomores in college wasting afternoons in the student cafeteria
as they consider their fourth or fifth major) to prove because they didn't know
something prior, no one else could have either.

Feynman, a serious physicist, got sick and tired of arguing with the 4th major
sophomore types and made a movie of the situation, showing clearing exactly
what was expected. I used Feynman's name to shut up schlackoff (fat chance)
and jeffies (who became quiet once he goggled the name Feynman).

I mentioned the whole issue because I have met boaters who, when the complained
about troubles backing up their ruddered boat, had unscrupulous marinas try to
sell them a multi-thousand dollar "solution" to the problem by "moving the prop
closer to the rudder for better control". Which won't work, of course.

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

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.

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

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

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.

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.

okay, yo-yo. which WAY is the rudder deflected if it is pushed to port?

please explain your reasoning.

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.

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

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.