On Mon, 29 Mar 2004 21:00:56 -0700, Keith Hughes
wrote:



Steven Shelikoff wrote:

Consider that it doesn't really matter as long as there is flow of fluid
media over the rudder.

Again, here you're assuming laminar (or at least unidirectional)
flow. When inserted into a laminar flow stream, and angled surface,
such as a rudder, will certainly be subjected to a force related to
the mass of the fluid deflected. Fluid flow on the 'suction' side is
nowhere near laminar, and will in fact be totally non-uniform around
the rudder. All fluid will be redirected immediately upon clearing

That's ok. Fluid on the pressure side of the prop is nowhere near
laminar either and will in fact be totally non-uniform around the
rudder. Yet the rudder still has an effect on the boat's direction.

Consider the fact that fluid drawn over a rudder by a prop may have an
effect on how the stern moves,

It *may* for a brief instant until an equilibrium is reached and the
pressure equalizes on both rudder surfaces (remember, water is *not*
elastic in the way air is, so you can't create a vacuum in water
like you do in air - if you do, you cavitate and dissolve gases come
out of solution until the partial pressures equalize and/or until
water 'fills in the void' and the gases redissolve).

Sure you can create a vacuum in water, just like in air. The only
difference is that water doesn't change it's volume (as much, but it
does a small amoutn) when the pressure changes. There's still a vacuum
though.

And you can certainly create a vacuum in water without cavitation.
Cavitation only occurs if the pressure of the water drops below it's
vapor pressure. There's a whole art/science of creating props that work
without cavitation for use with submarines.

but one that is much less then prop walk.

Many orders of magnitude less IME and IMO.

Especially with an angled propshaft. But there nontheless.

Steve

Steven Shelikoff wrote:

That's ok. Fluid on the pressure side of the prop is nowhere near
laminar either and will in fact be totally non-uniform around the
rudder. Yet the rudder still has an effect on the boat's direction.


No, it's not laminar, it is unidirectional along one axis.
Unidirectional flow can be diverted creating a thrust vector, unlike
the non-unidirectional flow on the suction side where the rudder
provides pressure drop instead of redirection/diversion. That's the
difference.


Sure you can create a vacuum in water,

You need to check the definition of vacuum if you believe this.
"Vacuum in water" is an oxymoron.

just like in air. The only
difference is that water doesn't change it's volume (as much, but it
does a small amoutn) when the pressure changes.

The *liquid* volume does not change, that's a basic property of
liquids. Their volume is temperature dependent, not pressure
dependent. If you reduce the pressure, dissolved gases will evolve
(that *is* cavitation) but you now have bubbles suspended in a
liquid, i.e. foam.

There's still a vacuum
though.

Don't think so.

And you can certainly create a vacuum in water without cavitation.
Cavitation only occurs if the pressure of the water drops below it's
vapor pressure.

Yes, and you would create a vacuum without doing this exactly how?
Fluid is not elastic. Move it from one point too quickly (what you'd
*have* to do to create a local low pressure area) and you will
liberate dissolve gas (even gaseous water) due to the low pressure
and/or high temperature created by the shear. Water doesn't stretch.

There's a whole art/science of creating props that work
without cavitation for use with submarines.

Quite so. They do not, however, generate 'pockets of vacuum' in
doing so.

Keith Hughes

On Tue, 30 Mar 2004 08:04:41 -0700, Keith Hughes
wrote:



Steven Shelikoff wrote:

That's ok. Fluid on the pressure side of the prop is nowhere near
laminar either and will in fact be totally non-uniform around the
rudder. Yet the rudder still has an effect on the boat's direction.


No, it's not laminar, it is unidirectional along one axis.

Actually, it's not that either.

Unidirectional flow can be diverted creating a thrust vector, unlike
the non-unidirectional flow on the suction side where the rudder
provides pressure drop instead of redirection/diversion. That's the
difference.

Yes, you can create a thrust vector by diverting non-unidirectional flow
as long as the the sum of the non-unidirectional flow is not 0, which it
is not for the case we are talking about because if it was, the boat
would not move backwards when the engine is put in reverse.

Sure you can create a vacuum in water,

You need to check the definition of vacuum if you believe this.
"Vacuum in water" is an oxymoron.

You're probably thinking of an absolute vacuum. In that case, a "vacuum
in air" is an oxymoron also. But since you mentioned a vacuum in air,
here we must talking about a relative vacuum which is simply an area
where the pressure is lower than another area. That is easily created
in water.

just like in air. The only
difference is that water doesn't change it's volume (as much, but it
does a small amoutn) when the pressure changes.

The *liquid* volume does not change, that's a basic property of
liquids. Their volume is temperature dependent, not pressure

Actually, the liquid volume can change when the pressure changes.
However, it's a minute amount only measurable for drastic pressure
changes. But that's outside the scope of this thread, where we can
treat the liquid volume as constant when the pressure changes.

dependent. If you reduce the pressure, dissolved gases will evolve
(that *is* cavitation) but you now have bubbles suspended in a
liquid, i.e. foam.

That depends on how much you reduce the pressure. Is it your contention
that anytime you reduce the pressure of a liquid by any amount that you
must have cavitation? If so, you are plainly wrong.

There's still a vacuum
though.

Don't think so.

You think wrong ... if we're talking about a relative vacuum and not an
absolute vacuum, which is obvious we are from your previous statement:
"so you can't create a vacuum in water like you do in air." A vacuum in
air is also an oxymoron unless you're *not* talking about an absolute
vacuum.

And you can certainly create a vacuum in water without cavitation.
Cavitation only occurs if the pressure of the water drops below it's
vapor pressure.

Yes, and you would create a vacuum without doing this exactly how?
Fluid is not elastic. Move it from one point too quickly (what you'd
*have* to do to create a local low pressure area) and you will
liberate dissolve gas (even gaseous water) due to the low pressure
and/or high temperature created by the shear. Water doesn't stretch.

But it does flow from higher pressure areas to lower pressure areas.
The lower pressure areas are the vacuum in this case, just like air.
And it does not have to cavitate in the areas under lower pressure.

There's a whole art/science of creating props that work
without cavitation for use with submarines.

Quite so. They do not, however, generate 'pockets of vacuum' in
doing so.

Sure they do. The area in front of the prop blade is at a lower
pressure than the area behind the prop. i.e., one definition of a
vacuum.

You can measure a vacuum in water yourself if you want. Just put a
vacuum gauge behind a water pump and you will measure the vacuum of the
pump sucking water through it. I have several of them on my boat for
measuring the condition of fuel filters.

Steve

schlackoff, don't drink so much before you post. tomorrow you read your post
below and you are going to one embarrassed dude.

you can create a thrust vector by diverting non-unidirectional flow
as long as the the sum of the non-unidirectional flow is not 0, which it
is not for the case we are talking about because if it was, the boat
would not move backwards when the engine is put in reverse.


you tipped a bit more and wrote this:

You're probably thinking of an absolute vacuum. In that case, a "vacuum
in air" is an oxymoron also. But since you mentioned a vacuum in air,
here we must talking about a relative vacuum which is simply an area
where the pressure is lower than another area. That is easily created
in water.


you tipped even more and wrote this embarrassing drivel


Actually, the liquid volume can change when the pressure changes.
However, it's a minute amount only measurable for drastic pressure
changes. But that's outside the scope of this thread, where we can
treat the liquid volume as constant when the pressure changes.

onward you went with this:

That depends on how much you reduce the pressure. Is it your contention
that anytime you reduce the pressure of a liquid by any amount that you
must have cavitation? If so, you are plainly wrong.

you wrote the following which makes no sense at all.


You think wrong ... if we're talking about a relative vacuum and not an
absolute vacuum, which is obvious we are from your previous statement:
"so you can't create a vacuum in water like you do in air." A vacuum in
air is also an oxymoron unless you're *not* talking about an absolute
vacuum.

more confusiong with:

But it does flow from higher pressure areas to lower pressure areas.
The lower pressure areas are the vacuum in this case, just like air.
And it does not have to cavitate in the areas under lower pressure.


huh??

Sure they do. The area in front of the prop blade is at a lower
pressure than the area behind the prop. i.e., one definition of a
vacuum.

You can measure a vacuum in water yourself if you want. Just put a
vacuum gauge behind a water pump and you will measure the vacuum of the
pump sucking water through it. I have several of them on my boat for
measuring the condition of fuel filters.

Steve

Jox, adults are having a conversation. Come back when you have
something of value to post.

Steve

yuk yuk. schaloff make a funny.

Jox, adults are having a conversation. Come back when you have
something of value to post.

Steve

Steven Shelikoff wrote in message
...
On Tue, 30 Mar 2004 08:04:41 -0700, Keith Hughes
wrote:

enormous snip

Sorry to interrupt this thread, but you may remember a little
earlier I said I'd go away and play with fans, bits of card, bits
of wire and smoking fag ends to get my brain around this. The
idea was suggested by a very reasonable post from Derek Rowell.

I'm trying to square Jax's flat 'nada' with rudder kick I've
observed, and an impression that the rudder direction affects
boat yaw when in reverse and not moving,

I fixed a card to a bit of wire which I could bend at various
angles to the card (rudder angle). I then hung the card upstream
of the fan so that it could pivot only along the fore and aft
axis (above the fan) and again so it could pilot only along the
lateral axis, and again so that the rudder could rotate around
the vertical axis of its front post. Smoking fag ends came later,
with a rigid mount. Fan was turned on.

Forces were observed by noting the degree of card deflection
around the relevant hinge. There was a net force on the rudder,
primarily exerted towards the fan. It has little lateral
component, but lots of fore and aft component.

Smoking fag ends showed strong non-linear flow when the rudder
was deflected, but the net flow is an s bend zig-zagging around
the rudder towards the fan and two carpet burns. Smoke speed
dropped markedly with distance away from the fan.

The rudder kicked hard over (either way) when allowed to pivot
around its forward vertical axis. Within the limitations of my
crude experiment, rudder kick is probably caused by the net
effect of the fore and aft component of force, not a lateral
component.

I think this explains the rudder kick I've witnessed in astern in
some boats engaging astern gear, and probably explains Derek
Rowell's observation that the rudder rotates when allowed to (If
I understood his experiment design right).

However, the zig-zagging airflow proves to my satisfaction that
the rudder may not create a net lateral force, so 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

On Wed, 31 Mar 2004 10:20:59 +0100, "JimB"
wrote:


Steven Shelikoff wrote in message
...
On Tue, 30 Mar 2004 08:04:41 -0700, Keith Hughes
wrote:

enormous snip

Sorry to interrupt this thread, but you may remember a little
earlier I said I'd go away and play with fans, bits of card, bits
of wire and smoking fag ends to get my brain around this. The
idea was suggested by a very reasonable post from Derek Rowell.

Just to settle it for myself, once and for all, I just did my own
experiment. I have a fan in the living room. It's about 12" in
diameter. I got a light plastic spatula from the kitchen. I turned the
fan on high and hung the spatula blade in front of the fan free to swing
in all directions while I was controlling the angle of the blade to the
fan. As expected, when I rotate the blade left, the spatula swings
forward and to the left. Rotate right, it swings forward and to the
right.

So I hung the spatula just behind the fan. Lo and behold, the same
thing happens but just a little less. When I rotate the spatula to the
left, there is a noticable *left* motion to the blade... i.e., it's not
only drawn forward into the blade but it also moved to the left from
where it was when the spatula blade was perpendicular to the fan. When
I turn it to the right, the spatula swings to the right.

That proves to my satisfaction that if the rudder is close enough to the
prop, it's direction will have some effect on the motion of the boat
when you throw it in reverse even before the boat starts making sterway.

Steve

Steven Shelikoff wrote in message
...
On Wed, 31 Mar 2004 10:20:59 +0100, "JimB"

So I hung the spatula just behind the fan. Lo and behold, the
same
thing happens but just a little less. When I rotate the
spatula to the
left, there is a noticable *left* motion to the blade... i.e.,
it's not
only drawn forward into the blade but it also moved to the left
from
where it was when the spatula blade was perpendicular to the
fan. When
I turn it to the right, the spatula swings to the right.

Steve, that was the experiment I first did. Then I realised that,
to yaw the boat, I had to look solely at lateral force. To do
this I had to constrain the card so that it could only hinge
laterally (no fore and aft motion permitted). This is where the
bits of wire came in. The card had a bit of wire attached rigidy
to the top, sticking at 45 deg horizontal angle to the card. The
card end of the wire bent down to stop the card swinging around
the wrong end of the wire. I hung the card (your spatula I
guess!) through two loops (hinges) first mounted parallel to the
centre line of the fan, then at right angles.

This gave a different result, very little lateral swing, lots of
fore and aft swing. Of course (a weakness in the experiment) it
didn't check for any lateral force effects on the fan of changes
in airflow, nor was it a very good representaion of relative
sizes of prop and rudder.

That proves to my satisfaction that if the rudder is close
enough to the
prop, it's direction will have some effect on the motion of the
boat
when you throw it in reverse even before the boat starts making
sterway.

My initial conclusion too, until I changed the hinging
arrangement.

JimB

On Thu, 1 Apr 2004 11:45:50 +0100, "JimB"
wrote:


Steven Shelikoff wrote in message
...
On Wed, 31 Mar 2004 10:20:59 +0100, "JimB"

So I hung the spatula just behind the fan. Lo and behold, the
same
thing happens but just a little less. When I rotate the
spatula to the
left, there is a noticable *left* motion to the blade... i.e.,
it's not
only drawn forward into the blade but it also moved to the left
from
where it was when the spatula blade was perpendicular to the
fan. When
I turn it to the right, the spatula swings to the right.

Steve, that was the experiment I first did. Then I realised that,
to yaw the boat, I had to look solely at lateral force. To do
this I had to constrain the card so that it could only hinge
laterally (no fore and aft motion permitted). This is where the
bits of wire came in. The card had a bit of wire attached rigidy
to the top, sticking at 45 deg horizontal angle to the card. The
card end of the wire bent down to stop the card swinging around
the wrong end of the wire. I hung the card (your spatula I
guess!) through two loops (hinges) first mounted parallel to the
centre line of the fan, then at right angles.

This gave a different result, very little lateral swing, lots of
fore and aft swing. Of course (a weakness in the experiment) it

Your experiment seems to be flawed if you're trying to look solely at
lateral force with no fore and aft motion permitted and yet you get a
lot of for and aft swing.

To prove to myself again that there is a lateral force even with no fore
and aft movement, I put a string around the bottom end of the spatula
which would allow it to swing laterally but hold it from being moved
toward the fan. So, we have a plastic spatula hung by the little
hanging hole at the top from a hook which allows it to swing in all
directions like a pendulum but I can firmly control the angle of the
blade by turning the hook. And there is a string looped around the
handle just above the blade which I can hold to prevent the blade from
moving towards the fan so there's no fore and aft motion.

Result: same thing. When it's behind the fan and you turn the blade so
that it's not perpendicular to the fan, the spatula swings *only*
laterally since there's a string keeping it from moving toward the fan.

My initial conclusion has only been reinforced.

Steve