If we turn the rudder to port,
the water being pushed back by the prop strides the port side of the rudder
(and NOT the starboard side) and the boat moves starboard.

Except when examined with a high powered light and magnifying glass, that
statement is just plain wrong.

One *could* make a case that the stern moves to starboard when the rudder is
hard aport, but the "boat" itself will move to
port because of the headway.

you are correct, the "back of the boat" (i.e. rudder) moves to starboard.

If we turn the rudder to port,
the water being pushed back by the prop strides the port side of the rudder
(and NOT the starboard side) and the boat moves starboard.

Except when examined with a high powered light and magnifying glass, that
statement is just plain wrong.

One *could* make a case that the stern moves to starboard when the rudder is
hard aport, but the "boat" itself will move to
port because of the headway.

JAXAshby wrote in message
...
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.

First, let's take a boat sitting in the water, not moving the
prop not turning.
The water pressure on each side of the rudder is the same, so
turning the
rudder one way or the other does not cause the boat to turn at
all.

Now, let's put the transmission in forward and turn the prop.
The prop pushes
water aft. With the rudder centered, the water moving
backward passes by the
rudder with the pressure the same on each side. If we turn the
rudder to port,
the water being pushed back by the prop strides the port side
of the rudder
(and NOT the starboard side) and the boat moves starboard.

To remove possible confusion -
Actually, the *stern* moves to starboard and (until the boat is
moving forward) this causes:

a. The boat to yaw port and
b. the Cof G to move starboard

Once you gather way the boat will move to port due to keel lift.
These points don't affect your argument though.

Because the impact (pressure) of the water (molecules) on the
port side of the
rudder was greater than the impact (pressure) on the starboard
side. What
happened was that the water flowing past the rudder was
*diverted* from its
path and the energy in the water was used to *divert* the
rudder the other
direction. Remember the law of physics, "For each and every
action there is an
equal and opposite reaction". The water went to port, rudder
went to
starboard.

Absolutely neccessary for the rudder to force the back of the
boat to starboard
is that the rudder forced water (from the prop stream) to port.
"Equal and
opposite"

Now, let's take the same boat sitting in still water and put
the transmission
in reverse and turn the prop. What happens? Well, the prop
pushes water
forward. Where does it get "new" water from? Aft.

Now, here is the part where intuition comes apart. so, let's
going slowly.

the water fills into the prop from aft because it is under
pressure

More correctly, it accelerates under differential pressure.
There's quite a strong drop in pressure on the input side of each
prop blade, and the whole volume of water on the input side is
characterised by a pressure gradient, low by the prop, ambient at
an infinite distance. You could calculate the pressure at any
point if you knew the speed of the water relative to ambient -
conservation of energy. You could calculate the water speed at
any point if you knew the shape (cross sectional area) of this
input 'plume' and it's gradients. There's a nice equation hiding
here.

(i.e. water
pressure, or "water runs down hill"). the closer to the prop,
the faster the
water fills.

As you say . . .

YET -- and here is the big part -- at all points aft and the
same
distance from the prop have the same pressure pushing water
towards the
spinning prop.

We start to part company. You're implying that the pressure
gradient varies directly with distance from prop, irrespective of
obstacles to the water flow . . . now this may be true, but you
haven't yet persuaded me.

THAT means that the pressure on one side of the rudder **is the
same** as the pressure on the other side. net, net, you can
turn the rudder
any way you wish, but nothing happens because the pressure is
the same on each
side, just as it is when the prop is not turning and the boat
is not moving.

Still have a hard time with that? Well, let's look at it from
another view.

The prop is in reverse and is drawing water into its circle and
pushing that
water forward. Let's turn the rudder to port and see what
happens as the water
streams by the rudder. Water hits the now aft side (former
starboars side) of
the rudder? Kinda, but lets assume that it does.

Bit rash. The water will flow along the rudder surface in the
direction of whatever pressure gradient exists, starting at the
tail of the rudder with the same input conditions as the water
travelling on the other side. Exit pressures (therefore
velocities) would be the same too, except that the pressure
gradient now calls for a sharp left turn into the prop. This
change in momentum has to be caused by a force.

My thesis is that this force is created because the water
travelling around the starboard side of the rudder has to travel
a longer distance (ie, faster) round the bend. And if it's going
faster, it's at a lower pressure (back to conservation of
energy). As an aerofoil.

Your thesis implies that the starboard side water actually
travels slower, unlike flow around an aerofoil. This is, of
course, possible, but I don't see the mechanism at the moment.

Which way is the water
stream deflected? Towards starboard? Then the rudder would
push the boat (aft
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.

JimB

jim, the explanation was dirt simple and without the mathematical and physical
nuances to gladden the hearts of physicists. It is, however, accurate. zero
rudder control going backwards until the boat is actually going backwards. the
prop affects the rudder not at all in reverse. it can't. Feynman the
physicist had so many people argue so hard with his statement he actually made
a movie of the "under water lawn sprinkler" to show that drawing in water the
sprinkler head moved not at all.

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.

First, let's take a boat sitting in the water, not moving the
prop not turning.
The water pressure on each side of the rudder is the same, so
turning the
rudder one way or the other does not cause the boat to turn at
all.

Now, let's put the transmission in forward and turn the prop.
The prop pushes
water aft. With the rudder centered, the water moving
backward passes by the
rudder with the pressure the same on each side. If we turn the
rudder to port,
the water being pushed back by the prop strides the port side
of the rudder
(and NOT the starboard side) and the boat moves starboard.

To remove possible confusion -
Actually, the *stern* moves to starboard and (until the boat is
moving forward) this causes:

a. The boat to yaw port and
b. the Cof G to move starboard

Once you gather way the boat will move to port due to keel lift.
These points don't affect your argument though.

Because the impact (pressure) of the water (molecules) on the
port side of the
rudder was greater than the impact (pressure) on the starboard
side. What
happened was that the water flowing past the rudder was
*diverted* from its
path and the energy in the water was used to *divert* the
rudder the other
direction. Remember the law of physics, "For each and every
action there is an
equal and opposite reaction". The water went to port, rudder
went to
starboard.

Absolutely neccessary for the rudder to force the back of the
boat to starboard
is that the rudder forced water (from the prop stream) to port.
"Equal and
opposite"

Now, let's take the same boat sitting in still water and put
the transmission
in reverse and turn the prop. What happens? Well, the prop
pushes water
forward. Where does it get "new" water from? Aft.

Now, here is the part where intuition comes apart. so, let's
going slowly.

the water fills into the prop from aft because it is under
pressure

More correctly, it accelerates under differential pressure.
There's quite a strong drop in pressure on the input side of each
prop blade, and the whole volume of water on the input side is
characterised by a pressure gradient, low by the prop, ambient at
an infinite distance. You could calculate the pressure at any
point if you knew the speed of the water relative to ambient -
conservation of energy. You could calculate the water speed at
any point if you knew the shape (cross sectional area) of this
input 'plume' and it's gradients. There's a nice equation hiding
here.

(i.e. water
pressure, or "water runs down hill"). the closer to the prop,
the faster the
water fills.

As you say . . .

YET -- and here is the big part -- at all points aft and the
same
distance from the prop have the same pressure pushing water
towards the
spinning prop.

We start to part company. You're implying that the pressure
gradient varies directly with distance from prop, irrespective of
obstacles to the water flow . . . now this may be true, but you
haven't yet persuaded me.

THAT means that the pressure on one side of the rudder **is the
same** as the pressure on the other side. net, net, you can
turn the rudder
any way you wish, but nothing happens because the pressure is
the same on each
side, just as it is when the prop is not turning and the boat
is not moving.

Still have a hard time with that? Well, let's look at it from
another view.

The prop is in reverse and is drawing water into its circle and
pushing that
water forward. Let's turn the rudder to port and see what
happens as the water
streams by the rudder. Water hits the now aft side (former
starboars side) of
the rudder? Kinda, but lets assume that it does.

Bit rash. The water will flow along the rudder surface in the
direction of whatever pressure gradient exists, starting at the
tail of the rudder with the same input conditions as the water
travelling on the other side. Exit pressures (therefore
velocities) would be the same too, except that the pressure
gradient now calls for a sharp left turn into the prop. This
change in momentum has to be caused by a force.

My thesis is that this force is created because the water
travelling around the starboard side of the rudder has to travel
a longer distance (ie, faster) round the bend. And if it's going
faster, it's at a lower pressure (back to conservation of
energy). As an aerofoil.

Your thesis implies that the starboard side water actually
travels slower, unlike flow around an aerofoil. This is, of
course, possible, but I don't see the mechanism at the moment.

Which way is the water
stream deflected? Towards starboard? Then the rudder would
push the boat (aft
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.

JimB

"Surely You Must Be Joking, Dr. Feynman". More folks should read
his stuff. Maybe some Buckmeister Fuller as well but he make my
brain hurt

Doug
s/v Callista
"JAXAshby" wrote in message
...
jim, the explanation was dirt simple and without the mathematical and
physical
nuances to gladden the hearts of physicists. It is, however, accurate.
zero
rudder control going backwards until the boat is actually going backwards.
the
prop affects the rudder not at all in reverse. it can't. Feynman the
physicist had so many people argue so hard with his statement he actually
made
a movie of the "under water lawn sprinkler" to show that drawing in water
the
sprinkler head moved not at all.

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.

First, let's take a boat sitting in the water, not moving the
prop not turning.
The water pressure on each side of the rudder is the same, so
turning the
rudder one way or the other does not cause the boat to turn at
all.

Now, let's put the transmission in forward and turn the prop.
The prop pushes
water aft. With the rudder centered, the water moving
backward passes by the
rudder with the pressure the same on each side. If we turn the
rudder to port,
the water being pushed back by the prop strides the port side
of the rudder
(and NOT the starboard side) and the boat moves starboard.

To remove possible confusion -
Actually, the *stern* moves to starboard and (until the boat is
moving forward) this causes:

a. The boat to yaw port and
b. the Cof G to move starboard

Once you gather way the boat will move to port due to keel lift.
These points don't affect your argument though.

Because the impact (pressure) of the water (molecules) on the
port side of the
rudder was greater than the impact (pressure) on the starboard
side. What
happened was that the water flowing past the rudder was
*diverted* from its
path and the energy in the water was used to *divert* the
rudder the other
direction. Remember the law of physics, "For each and every
action there is an
equal and opposite reaction". The water went to port, rudder
went to
starboard.

Absolutely neccessary for the rudder to force the back of the
boat to starboard
is that the rudder forced water (from the prop stream) to port.
"Equal and
opposite"

Now, let's take the same boat sitting in still water and put
the transmission
in reverse and turn the prop. What happens? Well, the prop
pushes water
forward. Where does it get "new" water from? Aft.

Now, here is the part where intuition comes apart. so, let's
going slowly.

the water fills into the prop from aft because it is under
pressure

More correctly, it accelerates under differential pressure.
There's quite a strong drop in pressure on the input side of each
prop blade, and the whole volume of water on the input side is
characterised by a pressure gradient, low by the prop, ambient at
an infinite distance. You could calculate the pressure at any
point if you knew the speed of the water relative to ambient -
conservation of energy. You could calculate the water speed at
any point if you knew the shape (cross sectional area) of this
input 'plume' and it's gradients. There's a nice equation hiding
here.

(i.e. water
pressure, or "water runs down hill"). the closer to the prop,
the faster the
water fills.

As you say . . .

YET -- and here is the big part -- at all points aft and the
same
distance from the prop have the same pressure pushing water
towards the
spinning prop.

We start to part company. You're implying that the pressure
gradient varies directly with distance from prop, irrespective of
obstacles to the water flow . . . now this may be true, but you
haven't yet persuaded me.

THAT means that the pressure on one side of the rudder **is the
same** as the pressure on the other side. net, net, you can
turn the rudder
any way you wish, but nothing happens because the pressure is
the same on each
side, just as it is when the prop is not turning and the boat
is not moving.

Still have a hard time with that? Well, let's look at it from
another view.

The prop is in reverse and is drawing water into its circle and
pushing that
water forward. Let's turn the rudder to port and see what
happens as the water
streams by the rudder. Water hits the now aft side (former
starboars side) of
the rudder? Kinda, but lets assume that it does.

Bit rash. The water will flow along the rudder surface in the
direction of whatever pressure gradient exists, starting at the
tail of the rudder with the same input conditions as the water
travelling on the other side. Exit pressures (therefore
velocities) would be the same too, except that the pressure
gradient now calls for a sharp left turn into the prop. This
change in momentum has to be caused by a force.

My thesis is that this force is created because the water
travelling around the starboard side of the rudder has to travel
a longer distance (ie, faster) round the bend. And if it's going
faster, it's at a lower pressure (back to conservation of
energy). As an aerofoil.

Your thesis implies that the starboard side water actually
travels slower, unlike flow around an aerofoil. This is, of
course, possible, but I don't see the mechanism at the moment.

Which way is the water
stream deflected? Towards starboard? Then the rudder would
push the boat (aft
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.

JimB

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

Would you all agree that in areas of dispute the truth may be revealed by an
experiment? Please try the following:
Take a fan, say a large house cooling fan (that's your propellor). Take a
flat surface, for example a stiff lightweight book (thats the rudder).
Turn the fan on and hold the rudder at an angle on the outflow side
(transmission in forward). Does the flow exert a torque (turning effect) on
the rudder? Let go one corner and see. Is there a sideways thrust that
you have to oppose to keep the rudder in position?
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.

Aero/hydrodynamic lift/drag is determined by the flow patterns over surfaces
(Bernoulli effects, etc), not by the simple minded pseudo-science that is
being thrown around here. It's a VERY complex situation. We all agree
that in practice the effect is much, much weaker in reverse but it is still
present. (The reason that it is weaker is that only a small fraction of
the in-flow to the propellor actually passes over the rudder in reverse.)

"JAXAshby" wrote in message
...
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.

First, let's take a boat sitting in the water, not moving the prop not
turning.
The water pressure on each side of the rudder is the same, so turning the
rudder one way or the other does not cause the boat to turn at all.

Now, let's put the transmission in forward and turn the prop. The prop
pushes
water aft. With the rudder centered, the water moving backward passes by
the
rudder with the pressure the same on each side. If we turn the rudder to
port,
the water being pushed back by the prop strides the port side of the
rudder
(and NOT the starboard side) and the boat moves starboard.

Why?

Because the impact (pressure) of the water (molecules) on the port side of
the
rudder was greater than the impact (pressure) on the starboard side. What
happened was that the water flowing past the rudder was *diverted* from
its
path and the energy in the water was used to *divert* the rudder the
other
direction. Remember the law of physics, "For each and every action there
is an
equal and opposite reaction". The water went to port, rudder went to
starboard.

Absolutely neccessary for the rudder to force the back of the boat to
starboard
is that the rudder forced water (from the prop stream) to port. "Equal
and
opposite"

Now, let's take the same boat sitting in still water and put the
transmission
in reverse and turn the prop. What happens? Well, the prop pushes water
forward. Where does it get "new" water from? Aft.

Now, here is the part where intuition comes apart. so, let's going
slowly.

the water fills into the prop from aft because it is under pressure (i.e.
water
pressure, or "water runs down hill"). the closer to the prop, the faster
the
water fills. YET -- and here is the big part -- at all points aft and the
same
distance from the prop have the same pressure pushing water towards the
spinning prop. THAT means that the pressure on one side of the rudder
**is the
same** as the pressure on the other side. net, net, you can turn the
rudder
any way you wish, but nothing happens because the pressure is the same on
each
side, just as it is when the prop is not turning and the boat is not
moving.

Still have a hard time with that? Well, let's look at it from another
view.

The prop is in reverse and is drawing water into its circle and pushing
that
water forward. Let's turn the rudder to port and see what happens as the
water
streams by the rudder. Water hits the now aft side (former starboars
side) of
the rudder? Kinda, but lets assume that it does. Which way is the water
stream deflected? Towards starboard? Then the rudder would push the boat
(aft
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.

Net, net, you wanna steer with a rudder backing up, prop forward of the
rudder,
you MUST be moving.

dude, don't try to metaphor the answer. metaphor is metaphor, not science.

Feynman made a movie of the exact issue people were arguing with him about.
The movie showed, as it would, nothing happens when fluid is pulled past a
rudder/lawn sprinkler.

If you don't know what Feynman did for a living, do a google on his name.

Would you all agree that in areas of dispute the truth may be revealed by an
experiment? Please try the following:
Take a fan, say a large house cooling fan (that's your propellor). Take a
flat surface, for example a stiff lightweight book (thats the rudder).
Turn the fan on and hold the rudder at an angle on the outflow side
(transmission in forward). Does the flow exert a torque (turning effect) on
the rudder? Let go one corner and see. Is there a sideways thrust that
you have to oppose to keep the rudder in position?
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.

Aero/hydrodynamic lift/drag is determined by the flow patterns over surfaces
(Bernoulli effects, etc), not by the simple minded pseudo-science that is
being thrown around here. It's a VERY complex situation. We all agree
that in practice the effect is much, much weaker in reverse but it is still
present. (The reason that it is weaker is that only a small fraction of
the in-flow to the propellor actually passes over the rudder in reverse.)

"JAXAshby" wrote in message
...
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.

First, let's take a boat sitting in the water, not moving the prop not
turning.
The water pressure on each side of the rudder is the same, so turning the
rudder one way or the other does not cause the boat to turn at all.

Now, let's put the transmission in forward and turn the prop. The prop
pushes
water aft. With the rudder centered, the water moving backward passes by
the
rudder with the pressure the same on each side. If we turn the rudder to
port,
the water being pushed back by the prop strides the port side of the
rudder
(and NOT the starboard side) and the boat moves starboard.

Why?

Because the impact (pressure) of the water (molecules) on the port side of
the
rudder was greater than the impact (pressure) on the starboard side. What
happened was that the water flowing past the rudder was *diverted* from
its
path and the energy in the water was used to *divert* the rudder the
other
direction. Remember the law of physics, "For each and every action there
is an
equal and opposite reaction". The water went to port, rudder went to
starboard.

Absolutely neccessary for the rudder to force the back of the boat to
starboard
is that the rudder forced water (from the prop stream) to port. "Equal
and
opposite"

Now, let's take the same boat sitting in still water and put the
transmission
in reverse and turn the prop. What happens? Well, the prop pushes water
forward. Where does it get "new" water from? Aft.

Now, here is the part where intuition comes apart. so, let's going
slowly.

the water fills into the prop from aft because it is under pressure (i.e.
water
pressure, or "water runs down hill"). the closer to the prop, the faster
the
water fills. YET -- and here is the big part -- at all points aft and the
same
distance from the prop have the same pressure pushing water towards the
spinning prop. THAT means that the pressure on one side of the rudder
**is the
same** as the pressure on the other side. net, net, you can turn the
rudder
any way you wish, but nothing happens because the pressure is the same on
each
side, just as it is when the prop is not turning and the boat is not
moving.

Still have a hard time with that? Well, let's look at it from another
view.

The prop is in reverse and is drawing water into its circle and pushing
that
water forward. Let's turn the rudder to port and see what happens as the
water
streams by the rudder. Water hits the now aft side (former starboars
side) of
the rudder? Kinda, but lets assume that it does. Which way is the water
stream deflected? Towards starboard? Then the rudder would push the boat
(aft
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.

Net, net, you wanna steer with a rudder backing up, prop forward of the
rudder,
you MUST be moving.