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.