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

JAXAshby wrote in message
...

Referring to Derek Rowell:

candidly, when I first saw his post and noted his email
address, I figured some
yo-yo hijacked his address. He claimed professional expertise
in fluid flow,
but his website make no mention of such, though it does promote
his "expertise"
in control systems for things such as MRI's.

It is sad, though, that you chose to libel him rather than argue
the case. Weakens your credibility.

JimB

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

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

Asking for explanations from experimental rigs is the royal road to
progress. Congratulations!
Couple of your observations bear talking about.

If a hinge surface is hinged more than about 1/4 aft of its present
leading edge it is unstable in the fluid flow. ('rudder kick')

If a surface *is* hinged about 1/4 from the leading edge, it can
still break into oscillations which are quickly destructive, unless
the mass is balanced closer to the hinge line.

If a FLAT surface is inclined slightly ( 20 degrees) to the fluid
flow, the flow over the 'upper' surface is faster and provides lower
pressure than the flow over the lower surface. The streamlines do not
follow the (flat) surface of the test article (of course!), they kick
up in a smooth curve over the top. This applies to an airfoil flown
upside down too. The streamlines look similar to the streamlines
over a right way up foil, but less efficient and with lower pressure
difference from top/bottom.

It is not necessary for a lump of fluid dividing past the foil to
join up again after it has passed..
When providing lift, the lump of fluid does not join up again, in
fact.

Brian Whatcott

sherr tells us the more experience he has the more he don't know which way his
boat is gonna go thusly:

yeah, sure. rudder right, tranny forward, throttle forward and what happens
....???

Most times you go right. Sometimes you go left. If you've never gone
left when doing the above, you need some more experience.

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

Jim, I limited time for frauds. in this case rowell lied straight out to us,
saying that professionally he was an expert in fluid flow. Yet, even his own
website drumming up business for his consulting services fails to mention fluid
flow experience, let alone expertise.

He choice of words right from the get go indicated his fraudulant
underpinnings. In the end you reduced himself to arguing that friction in the
rudder bearings were the reason reverse flow showed no effects on lateral
movement of the rudder.

Referring to Derek Rowell:

candidly, when I first saw his post and noted his email
address, I figured some
yo-yo hijacked his address. He claimed professional expertise
in fluid flow,
but his website make no mention of such, though it does promote
his "expertise"
in control systems for things such as MRI's.

It is sad, though, that you chose to libel him rather than argue
the case. Weakens your credibility.

JimB

sorry about the mistyping.

In the end you reduced himself to arguing that friction in the
rudder bearings were the reason

should be "he reduced himself"

That's a blatant lie, jaxie, you should be ashamed of yourself. Its one thing
to be stupid, that is your right, one which you exercise quite frequently. But
to lie so blatantly after you libel someone in unconscionable. It was clear
that the last thing you wanted was a professor of mechanical engineering
criticizing you logic, so you chased him away.

Your behavior was tantamount to admitting that you really don't know what you're
talking and were terrified of a rational discussion.



"JAXAshby" wrote in message
...
Jim, I limited time for frauds. in this case rowell lied straight out to us,
saying that professionally he was an expert in fluid flow. Yet, even his own
website drumming up business for his consulting services fails to mention
fluid
flow experience, let alone expertise.

He choice of words right from the get go indicated his fraudulant
underpinnings. In the end you reduced himself to arguing that friction in the
rudder bearings were the reason reverse flow showed no effects on lateral
movement of the rudder.

Referring to Derek Rowell:

candidly, when I first saw his post and noted his email
address, I figured some
yo-yo hijacked his address. He claimed professional expertise
in fluid flow,
but his website make no mention of such, though it does promote
his "expertise"
in control systems for things such as MRI's.

It is sad, though, that you chose to libel him rather than argue
the case. Weakens your credibility.

JimB