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Old March 26th 04, 09:44 PM
JAXAshby
 
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Default push vs pull vis a vis rudders

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.

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Old March 26th 04, 11:56 PM
Doug Dotson
 
Posts: n/a
Default push vs pull vis a vis rudders

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


Not sure what the point of steering if you are not moving would be? Forward
or reverse. If you are not moving, steering has no meaning.

Doug
s/v Callista



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Old March 27th 04, 12:15 AM
JAXAshby
 
Posts: n/a
Default push vs pull vis a vis rudders

actually, there is a difference. If you put the tranny in forward and push the
throttle forward you can turn the boat one way or the other. Because of this,
some people believe you can do the same in reverse. You can't.

In addition, I have met people who had trouble backing up their boats who in
some cases were told that if they were to spend $XX,XXX's by moving the prop
closer to the rudder they would get better turning ability on the boat. They
would not.

Indeed, in a recent thread "Thrust Vectoring" many people insisted that pulling
water over a rudder would turn a boat. When I said it would not, several
people wanted to argue about it (including one dumb cluck who claims to have a
degree in physics, really dumb because this problem in physics is a classic
taught to early semester students). A couple people asked for an explanation
as why pulling water over a rudder had no effect and all, and I said I would
write one up today.

If one wants to steer by rudder backing up, one needs to start the boat moving
backwards slowly, otherwise prop walk will twist the boat sideways. Start
slowly until the boat is moving and the rudder becomes effective, depending on
how much the boat is moving through the water.

Net, net, you wanna steer with a rudder backing up, prop forward of the

rudder,
you MUST be moving.


Not sure what the point of steering if you are not moving would be? Forward
or reverse. If you are not moving, steering has no meaning.

Doug
s/v Callista











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Old March 27th 04, 12:54 AM
Shen44
 
Posts: n/a
Default push vs pull vis a vis rudders

Subject: push vs pull vis a vis rudders
From: "Doug Dotson"


Actually, I'm not sure how the subject of steering astern got into the
discussion as it was not a consideration of the original post about "thrust
vectoring".
However, on a twin screw boat (and this was what I believe he was discussing)
you don't need to have headway or sternway to alter your heading. This can be
done with props alone, or more easily and quickly, with props and rudders.
In various maneuvering situations, this can be a great advantage, and just to
throw in a kicker, the advantage can work with single screw as well.

Shen

Net, net, you wanna steer with a rudder backing up, prop forward of the

rudder,
you MUST be moving.


Not sure what the point of steering if you are not moving would be? Forward
or reverse. If you are not moving, steering has no meaning.

Doug
s/v Callista



  #5   Report Post  
Old March 27th 04, 01:10 AM
Doug Dotson
 
Posts: n/a
Default push vs pull vis a vis rudders

This all fits with my expierence. In reverse, propwalk is dominant
until some way is acheived. Then rudder control is possible.

Doug
s/v Callista

"JAXAshby" wrote in message
...
actually, there is a difference. If you put the tranny in forward and

push the
throttle forward you can turn the boat one way or the other. Because of

this,
some people believe you can do the same in reverse. You can't.

In addition, I have met people who had trouble backing up their boats who

in
some cases were told that if they were to spend $XX,XXX's by moving the

prop
closer to the rudder they would get better turning ability on the boat.

They
would not.

Indeed, in a recent thread "Thrust Vectoring" many people insisted that

pulling
water over a rudder would turn a boat. When I said it would not, several
people wanted to argue about it (including one dumb cluck who claims to

have a
degree in physics, really dumb because this problem in physics is a

classic
taught to early semester students). A couple people asked for an

explanation
as why pulling water over a rudder had no effect and all, and I said I

would
write one up today.

If one wants to steer by rudder backing up, one needs to start the boat

moving
backwards slowly, otherwise prop walk will twist the boat sideways. Start
slowly until the boat is moving and the rudder becomes effective,

depending on
how much the boat is moving through the water.

Net, net, you wanna steer with a rudder backing up, prop forward of the

rudder,
you MUST be moving.


Not sure what the point of steering if you are not moving would be?

Forward
or reverse. If you are not moving, steering has no meaning.

Doug
s/v Callista















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Old March 27th 04, 01:14 AM
Doug Dotson
 
Posts: n/a
Default push vs pull vis a vis rudders

I responded to an original post entitled "push vs pull vis a vis rudders".
Not familiar with the thread about Thrust Vectoring.

doug
S/v Callista

"Shen44" wrote in message
...
Subject: push vs pull vis a vis rudders
From: "Doug Dotson"


Actually, I'm not sure how the subject of steering astern got into the
discussion as it was not a consideration of the original post about

"thrust
vectoring".
However, on a twin screw boat (and this was what I believe he was

discussing)
you don't need to have headway or sternway to alter your heading. This can

be
done with props alone, or more easily and quickly, with props and rudders.
In various maneuvering situations, this can be a great advantage, and just

to
throw in a kicker, the advantage can work with single screw as well.

Shen

Net, net, you wanna steer with a rudder backing up, prop forward of the

rudder,
you MUST be moving.


Not sure what the point of steering if you are not moving would be?

Forward
or reverse. If you are not moving, steering has no meaning.

Doug
s/v Callista





  #7   Report Post  
Old March 27th 04, 04:19 AM
Gould 0738
 
Posts: n/a
Default push vs pull vis a vis rudders

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.
  #8   Report Post  
Old March 27th 04, 11:18 AM
JimB
 
Posts: n/a
Default push vs pull vis a vis rudders


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



  #9   Report Post  
Old March 27th 04, 02:59 PM
JAXAshby
 
Posts: n/a
Default push vs pull vis a vis rudders

schlackoff, the rudders don't control anything in reverse, unless the boat is
also moving backwards. They can't.



However, on a twin screw boat (and this was what I believe he was discussing)
you don't need to have headway or sternway to alter your heading. This can be
done with props alone, or more easily and quickly, with props and rudders.
In various maneuvering situations, this can be a great advantage, and just to
throw in a kicker, the advantage can work with single screw as well.

Shen

Net, net, you wanna steer with a rudder backing up, prop forward of the

rudder,
you MUST be moving.


Not sure what the point of steering if you are not moving would be? Forward
or reverse. If you are not moving, steering has no meaning.

Doug
s/v Callista











  #10   Report Post  
Old March 27th 04, 03:01 PM
JAXAshby
 
Posts: n/a
Default push vs pull vis a vis rudders

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.










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