"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