Jax,

I accept that 'suction' will not create a force. Any forces come
from the new exit momentum of a fluid (point the hosepipe where
you will . . . .) presumably this was Feynman's case. I haven't
met the guy, Fine. The effect is easy to observe on VTOL
aircraft. They suck from forward, but don't have to lean their
jet output forward to cancel any 'sucking' force when hovering
static. Here we agree.

I'm looking for an explanation of the phenomenon I thought I had
seen on a very old tug, also on an old trawler, neither of which
had any significant prop walk in astern, though both had big
props. The phenomenon was that rudder deflection with engine in
reverse (boat static) could be used to create a little yaw. The
explanation given to me was that 'flow over the rudder' created
this effect. I rationalised this explanation (perhaps wrongly) by
assuming the rudder changed the momentum of the water ingested by
the prop. ie, water speed along the inside of the rudder is
faster than water speed over the backside of the rudder; a very
simple 'hydrofoil in free stream' effect.

As a result of this thread I am re-examining this assumption and
my observations.

Now, it could be that my observations were wrong, and the
phenomenon did not occur. I was, perhaps, seeing yaw caused by
another effect - inertia due to a previous action maybe. And
perhaps my observations were clouded by the pre-conception
planted in my mind that it worked. But I'm afraid your
explanation (paraphrased very crudely) 'you're wrong because
Feynman says so' doesn't help me. Also, Derek Rowell's experiment
shows that there is some effect which needs explanation - rather
than dismissal.

If you could demonstrate, prove or explain why water speed should
be identical along each side of the rudder (which I assume would
porve that pressure on each side is identical), irrespective of
rudder deflection, when the boat is static with engine in
reverse, I'd happily accept your thesis that rudder has no
effect. As in many of these cases, it may help to explain this
for an extreme case; a balanced spade rudder at 70degrees
deflection close to the prop?

If the rudder suffers some net pressure, then I'd like to
understand what mechanism cancels it.

Can you help without appealing to higher authorities?

JimB

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