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

How
about an explanation of those phenomena for a numerate old
thickie?

each blade the prop (in reverse) pushes water forward towards the bow of the
boat (also sides, but ignore that). Because "water flows downhill" water fills
in behind each blade as it pushed water forward

(note, if you watch an underwater prop turning in a stationary position, you
will see the level of the water surface fall behind the prop and rise forward
of the prop)

Now, the water that fills in behind the blades comes from whereever there is
water "uphill" of the blade. This is not just behind the prop, but also to the
sides and top and bottom, in more or less a hemisphere (an over simplification.
water actually comes from the "high pressure" side of the blade, the side
towards which the water is pushed.)

The flow "through the prop circle" hs the greatest velocity, with "all that
water" aft just waiting for its chance to "roll down hill". the greater the
distance from the prop, the slower the speed of the water rolling down hill
(lots of water available so it doesn't roll very fast. The speed at which the
water rolls towards the prop is inversely proportional to 4/3rd the distance
cubed (volume of sphere) is all aimed at the center of the prop.

People "think" the water flow towards the prop is straight at the prop, but it
isn't. it is from all edges of the hemisphere aft of the prop. Left, right,
up down, back. all edges.

JAXAshby wrote in message
...

The speed at which the
water rolls towards the prop is inversely proportional to 4/3rd
the distance
cubed (volume of sphere) is all aimed at the center of the
prop.

People "think" the water flow towards the prop is straight at
the prop, but it
isn't. it is from all edges of the hemisphere aft of the prop.
Left, right,
up down, back. all edges.

Lovely. I understand that. A plume with an enormous splay angle
converging into the prop. No need to do the 'rolling downhill
bit' for me.

Some of the plume being interfered with by various obstacles
(such as rudders and hulls).

At this stage you may be interested in the behaviour of my model
of smoking fag ends, bits of card and wire hinges, all mounted up
stream of the heater fan suggested by Derek Rowell.

First, there is a net force on the rudder, primarily exerted in
the direction of the fan.It has little lateral component, but
lots of fore and aft component (Those wire hinges were good for
resolving things). There's strong non-linear flow when the rudder
is deflected, but the net flow is an s bend zig-zagging around
it, going to the fan. Approach speed dropped markedly with
distance from the fan (as you comment - a cube relationship if
there are no constraints)

The rudder kicks hard over when it is allowed to pivot around its
forward vertical axis. Within the limitations of my crude
experiment, this seems to be caused mainly by the net effect of
the fore and aft component of force, not a lateral component.
This explains the rudder kick I've witnessed in astern in some
boats, and probably explains Derek Rowell's observation that the
rudder rotates when allowed to (If I understood his experiment
design right). My thanks to him for suggesting the idea of an
experiment. It's been great fun.

So I can now understand the mechanism whereby there's rudder
kick, but little or no lateral force. And I'm stuck with the
revelation that the yaw effect that my old skipper demonstrated
to me was wind, mirrors, inertia, prop walk and my hero worship.
Ah well.

But now I've got to get rid of those carpet burns before the wife
comes home.

JimB

Thanks Jim for taking the time to experiment.

Lovely. I understand that. A plume with an enormous splay angle
converging into the prop. No need to do the 'rolling downhill
bit' for me.

Some of the plume being interfered with by various obstacles
(such as rudders and hulls).

At this stage you may be interested in the behaviour of my model
of smoking fag ends, bits of card and wire hinges, all mounted up
stream of the heater fan suggested by Derek Rowell.

First, there is a net force on the rudder, primarily exerted in
the direction of the fan.It has little lateral component, but
lots of fore and aft component (Those wire hinges were good for
resolving things). There's strong non-linear flow when the rudder
is deflected, but the net flow is an s bend zig-zagging around
it, going to the fan. Approach speed dropped markedly with
distance from the fan (as you comment - a cube relationship if
there are no constraints)

The rudder kicks hard over when it is allowed to pivot around its
forward vertical axis. Within the limitations of my crude
experiment, this seems to be caused mainly by the net effect of
the fore and aft component of force, not a lateral component.
This explains the rudder kick I've witnessed in astern in some
boats, and probably explains Derek Rowell's observation that the
rudder rotates when allowed to (If I understood his experiment
design right). My thanks to him for suggesting the idea of an
experiment. It's been great fun.

So I can now understand the mechanism whereby there's rudder
kick, but little or no lateral force. And I'm stuck with the
revelation that the yaw effect that my old skipper demonstrated
to me was wind, mirrors, inertia, prop walk and my hero worship.
Ah well.

But now I've got to get rid of those carpet burns before the wife
comes home.

JimB