On Tue, 08 Jun 2004 17:29:47 GMT, otnmbrd wrote:



Steven Shelikoff wrote:

It's absolutely obvious that there's unequal thrust. If not, there
wouldn't be prop walk. The question is, where does it come from? And I
still think the answer is multiple sources all contribute, some more
than others on any given boat. I just don't think the effect you're
describing here plays as much a part on my boat as it might on others
because there's more above the prop than just a column of water and then
air.

Steve

Actually, for the most part we are in agreement.
When you go back through my post on this issue, you will always note
that I am mentioning a number of factors which will enhance and detract
from propwalk (wind, current, hull form, speed, pitch, kort nozzles,
etc.) and the fact that we are not always sure what that reaction will
be, until we experience it on a particular boat under particular conditions.
However, there has to be a "root" cause .... an initial action/reaction
which starts the process.

*The* root cause is a net force sideways.

[...]
Let's now look at prop efficiency (these are MY views based on what I
see, read and feel). If we follow the rotation of a RH fixed pitch prop,
looking at it from astern starting at top dead center (ooo*) the
particular blade is pushing water to the right (and back ...always back,
but we will ignore that component for this discussion) at minimal
efficiency. this efficiency, however, is increasing as the propellor
turns towards 45* and the direction is changing to a increasingly down
direction.
As the blade reaches 45*, efficiency is close to maximum and from here
the direction is more down than to the side.
Someplace just prior to 90* the efficiency becomes maximum and as the
blade rotates toward 135* the angle of push changes to the left

You haven't explained why the prop is less efficient at 0 and gains
efficiency on it's way from 0 to 90. I can think of some reasons why
that may be correct. But the reason given having to do with a column of
water only backed by air and a bulge at the surface isn't it. That's
because the whole way from 0 to 180 degrees there is either an infinite
column of water (right a 0 and 180) or the column of water is supported
by the sea floor (everywhere else between 0 and 180)

[...]
Also, so there's no misunderstanding, remember, my feeling about props
efficiency decrease on it's upward rotation is about the prop pushing
water up and into air, losing efficiency. .... and most importantly,
these are my views/visualizations .... G yours may vary.

I realize that. It just doesn't support your discussion about what
happens to the prop efficiency from 0 to 180 degrees. All it does is
explain why the prop is less efficient on the upward part of it's trip
vs. the downward part of it's trip which creates a net upward force off
the centerline, which lifts the stern and lists the boat. It does
nothing to explain why there's a net sideways force (if there's no
overhang, which I think we've agree is the root cause of prop walk
that must be explained.

Steve

Steven Shelikoff wrote:



*The* root cause is a net force sideways.

G read the next part carefully

[...]

Let's now look at prop efficiency (these are MY views based on what I
see, read and feel). If we follow the rotation of a RH fixed pitch prop,
looking at it from astern starting at top dead center (ooo*) the
particular blade is pushing water to the right (and back ...always back,
but we will ignore that component for this discussion) at minimal
efficiency. this efficiency, however, is increasing as the propellor
turns towards 45* and the direction is changing to a increasingly down
direction.
As the blade reaches 45*, efficiency is close to maximum and from here
the direction is more down than to the side.
Someplace just prior to 90* the efficiency becomes maximum and as the
blade rotates toward 135* the angle of push changes to the left





You haven't explained why the prop is less efficient at 0 and gains
efficiency on it's way from 0 to 90.

Imagine the blade is just beneath the surface
Again, ignoring the after component, up until the blade reached o,
coming from 315, the pitch of the blade was not pushing towards a solid
wall of water, it was pushing the water up and to the right into air ...
it was less efficient, compared to it's opposite blade which was
rotating from 135 to 180 which was pushing down and to the left against
solid incompressible water.
As the blade starts rotating to 090 it is pushing to the right and as it
rotates, also begins to push down(into more solid water) , so that it's
efficiency begins to increase as the angle it's pushing down, increases
and it stops pushing to the right.

I can think of some reasons why
that may be correct. But the reason given having to do with a column of
water only backed by air and a bulge at the surface isn't it.
How do you know that?

That's
because the whole way from 0 to 180 degrees there is either an infinite
column of water (right a 0 and 180) or the column of water is supported
by the sea floor (everywhere else between 0 and 180)

Not initially, but shortly after 000* it begins to be and increases.
Why aren't you considering 180 - 000? Looking at the prop just beneath
the surface, are you saying that the pitched blade is pushing against a
solid column of water as it goes from 180* and approaches the surface at
000*?

[...]

Also, so there's no misunderstanding, remember, my feeling about props
efficiency decrease on it's upward rotation is about the prop pushing
water up and into air, losing efficiency. .... and most importantly,
these are my views/visualizations .... G yours may vary.


I realize that. It just doesn't support your discussion about what
happens to the prop efficiency from 0 to 180 degrees. All it does is
explain why the prop is less efficient on the upward part of it's trip
vs. the downward part of it's trip which creates a net upward force off
the centerline, which lifts the stern and lists the boat. It does
nothing to explain why there's a net sideways force (if there's no
overhang, which I think we've agree is the root cause of prop walk
that must be explained.

If you can see a net upwards force, I'm halfway there.
You agree that the blade is more efficient from 000-180 than from 180-000?
If so, forget those numbers, consider the prop just beneath the surface
(to help the visualization) and look at the blade rotation from 090-270
and 270-090, considering a 12 inch dia prop on a boat in 3,000 feet of
water.
At 090 the blade is pushing directly down. as the blade rotates past
090, it continues to push down, but also begins to push to the left.
As the blade rotates toward 180 the downward push decreases as the
sideways push (to the left) increases,until you reach 180 where the
blade is pushing directly left. During this time, the blade has been
pushing against a solid column of water, 3,000 feet deep (maximum
efficiency).
As the blade passes 180, it continues to push left, but also begins to
push up (against a 12" column of water), efficiency decreases and as the
blade approaches 270, the upward component increases as the left
component decreases until you reach 270* where the blade is now pushing
directly up against 6" of water. From 090 to 180 the blade was pushing
at maximum efficiency in an ever increasing left component. From 180 to
270 the efficiency was decreasing at the same time as the left component
was decreasing.
Now, at 270, the blade is pushing directly up against 6" of water and as
soon as it passes 270 begins to push to the right in a decreasing column
of water. As before, as the right component increases the up component
decreases, until it disappears at 000* in zero inches of water....the
prop is relatively inefficient during this whole period or arc of
rotation, compared to it's opposite 090-180
As the blade passes 000* it is pushing directly right and as it rotates
past 000* begins to push downward (and decrease pushing right) and
consequently begins to increase in efficiency as it gets further down
and into more solid water, until we once again reach 090*.
If we consider that we've got a 2 bladed prop, blade A passing between
090 and 270 started out at maximum efficiency and continued at that to
180* where it's efficiency began to decrease. Blade B passing between
270 and 090, on the other hand started out at poor efficiency, which it
maintained until 000, where it started to pick up efficiency, going to
maximum at 090.
The net greater push is to the left (hull goes right) .... propwalk G

I've exaggerated numbers, for clarity (at least, for my attempt at it)
and don't really know how I could explain this in another way to make
the point I'm trying to get across, clearer.

otn

On Wed, 09 Jun 2004 04:56:51 GMT, otnmbrd wrote:



Steven Shelikoff wrote:



*The* root cause is a net force sideways.

G read the next part carefully

[...]

Let's now look at prop efficiency (these are MY views based on what I
see, read and feel). If we follow the rotation of a RH fixed pitch prop,
looking at it from astern starting at top dead center (ooo*) the
particular blade is pushing water to the right (and back ...always back,
but we will ignore that component for this discussion) at minimal
efficiency. this efficiency, however, is increasing as the propellor
turns towards 45* and the direction is changing to a increasingly down
direction.
As the blade reaches 45*, efficiency is close to maximum and from here
the direction is more down than to the side.
Someplace just prior to 90* the efficiency becomes maximum and as the
blade rotates toward 135* the angle of push changes to the left





You haven't explained why the prop is less efficient at 0 and gains
efficiency on it's way from 0 to 90.

Imagine the blade is just beneath the surface
Again, ignoring the after component, up until the blade reached o,
coming from 315, the pitch of the blade was not pushing towards a solid
wall of water, it was pushing the water up and to the right into air ...
it was less efficient, compared to it's opposite blade which was
rotating from 135 to 180 which was pushing down and to the left against
solid incompressible water.
As the blade starts rotating to 090 it is pushing to the right and as it
rotates, also begins to push down(into more solid water) , so that it's
efficiency begins to increase as the angle it's pushing down, increases
and it stops pushing to the right.

There's no such thing as "more solid water" unless you're talking about
ice. Once the blade goes past 0 degrees, all the way from there until
180 degrees it's pushing against the same thing. The water may be
minutely more dense and under a tiny bit of less pressure for the 0 to
90 part of the trip than from the 90 to 180 part of the trip. But that
has absolutely nothing to do with your theory of the water column being
backed by air vs. something else. Your theory gives no reason why
there's less efficiency for the 0 to 90 part of the rototation than for
the 90 to 180 part of the rotation. It also gives no reason why there's
a difference in efficiency between the 180 to 270 part of the rotation
than from the 270 to 360 part since it's backed by air for that entire
trip (unless there's hull overhang). To explain that, you have to start
looking at the amount of water before you reach air.

There's also one more thing you haven't thought about and that's the
fact that we're dealing with 2 interfaces on the way down as well as on
the way up. On the way up, the water column being pushed by the prop
can bulge the air at the surface. On the way down, the water column
being pushed by the prop can bulge the bottom, especially if the bottom
is made of something like soft mud. It may provide more support than
air at the surface, but the difference isn't as large as if there truly
was a single interface on the way down.

I can think of some reasons why
that may be correct. But the reason given having to do with a column of
water only backed by air and a bulge at the surface isn't it.
How do you know that?

Because there's no difference in the "backing" of the column of water
for the entire travel of the prop from 0 to 180 degrees. It's not
backed by air for any portion of that rotation. It's backed by
practically non-compressable water against the bottom of the sea for
that entire period of the rotation. So that theory provides no
explanation for why there would be a difference in efficiency from 0 to
90 degrees vs. 90 to 180 degrees.


That's
because the whole way from 0 to 180 degrees there is either an infinite
column of water (right a 0 and 180) or the column of water is supported
by the sea floor (everywhere else between 0 and 180)

Not initially, but shortly after 000* it begins to be and increases.

Yes, initially and for the entire trip it doesn't change. It doesn't
increase at all beyond 0. It's the same the whole way to 180.

Why aren't you considering 180 - 000? Looking at the prop just beneath
the surface, are you saying that the pitched blade is pushing against a
solid column of water as it goes from 180* and approaches the surface at
000*?

Your theory that the difference in efficiency is due to the column of
water being backed by air vs. not backed by air doesn't explain any
difference during the way up either since it' backed by air the whole
way from 180 up to 0. You have to start looking at the amount of water
before you get to air.


[...]

Also, so there's no misunderstanding, remember, my feeling about props
efficiency decrease on it's upward rotation is about the prop pushing
water up and into air, losing efficiency. .... and most importantly,
these are my views/visualizations .... G yours may vary.


I realize that. It just doesn't support your discussion about what
happens to the prop efficiency from 0 to 180 degrees. All it does is
explain why the prop is less efficient on the upward part of it's trip
vs. the downward part of it's trip which creates a net upward force off
the centerline, which lifts the stern and lists the boat. It does
nothing to explain why there's a net sideways force (if there's no
overhang, which I think we've agree is the root cause of prop walk
that must be explained.

If you can see a net upwards force, I'm halfway there.

I'm agreeing that if your theory is correct, i.e., that there is a
difference in efficiency whether the column of water being pushed by the
prop is backed up by air vs. something other than air, then there will
be a net upwards force.

You agree that the blade is more efficient from 000-180 than from 180-000?
If so, forget those numbers, consider the prop just beneath the surface
(to help the visualization) and look at the blade rotation from 090-270
and 270-090, considering a 12 inch dia prop on a boat in 3,000 feet of
water.
At 090 the blade is pushing directly down. as the blade rotates past
090, it continues to push down, but also begins to push to the left.
As the blade rotates toward 180 the downward push decreases as the
sideways push (to the left) increases,until you reach 180 where the
blade is pushing directly left. During this time, the blade has been
pushing against a solid column of water, 3,000 feet deep (maximum
efficiency).
As the blade passes 180, it continues to push left, but also begins to
push up (against a 12" column of water), efficiency decreases and as the
blade approaches 270, the upward component increases as the left
component decreases until you reach 270* where the blade is now pushing
directly up against 6" of water. From 090 to 180 the blade was pushing
at maximum efficiency in an ever increasing left component. From 180 to
270 the efficiency was decreasing at the same time as the left component
was decreasing.
Now, at 270, the blade is pushing directly up against 6" of water and as
soon as it passes 270 begins to push to the right in a decreasing column
of water. As before, as the right component increases the up component
decreases, until it disappears at 000* in zero inches of water....the

Move the blade down in the water a few feet and this is no longer true.
I.e., if the blade is 12" and it's 3 feet below the surface, it's
pushing against the smallest column of water just past 270 degrees. As
it rotates from just beyond 270 until it reaches 0 degrees, the size of
the water column before it reaches air is ever increasing, thus
increasing it's efficiency during the trip from 270 back to 0.

prop is relatively inefficient during this whole period or arc of
rotation, compared to it's opposite 090-180
As the blade passes 000* it is pushing directly right and as it rotates
past 000* begins to push downward (and decrease pushing right) and
consequently begins to increase in efficiency as it gets further down
and into more solid water, until we once again reach 090*.

Again, not true. Once it passes 0 degrees, there's no difference in
what it's pushing against all the way until 180 degrees. So you can
forget that portion of the trip giving any net force in the sideways
direction.

If we consider that we've got a 2 bladed prop, blade A passing between
090 and 270 started out at maximum efficiency and continued at that to
180* where it's efficiency began to decrease. Blade B passing between
270 and 090, on the other hand started out at poor efficiency, which it
maintained until 000, where it started to pick up efficiency, going to
maximum at 090.

No. Actually, if the blade is at the surface, the efficiency decreased
from 270 to 0 (if we're now talking about the amount of water before
air) and then got to maxumum right at 0 and stayed there until 180. If
the blade is a few feet down, it was at minimum efficiency just past 270
and then gains efficiency on it's way to 0 degrees.

The net greater push is to the left (hull goes right) .... propwalk G

I've exaggerated numbers, for clarity (at least, for my attempt at it)
and don't really know how I could explain this in another way to make
the point I'm trying to get across, clearer.

The problem is that geometry doesn't support what you're trying to show.
To support your theory, you're stating assumptions about what the prop
is doing during it's rotation and for several parts of the rotation,
those assumptions are just wrong.

Also, you're assuming more than just a difference in efficiency in water
being backed by air vs. non-air. You're also assuming that the
efficiency changes due to how much water there is before you get to air.
Except for a very tiny amount of water (i.e., right at the surface)
that's not much of a factor. That's because right at the surface, the
size of the prop is huge relative to the depth of the prop. So there's
a large relative difference between the amount of water above the blade
when it's at 180 degrees vs. 0 degrees. But move a small prop down a
few feet and that becomes less and less of a factor.

Steve

Sheesh, Steve, I just printed your last, out, so I could read it in peace.
G You and I are a couple of "long winded" writers, ain't we?
Back in a bit

otn

I've always wondered if a submarine, 1,000 feet underwater, experiences
"propwalk" .... I'm betting it doesn't G.
At any rate, to save some unnecessary reading, by the numbers on some
points at issue:

1. We are not talking about some highly visible, easily measured,
variations in efficiency. These are very subtle, but sufficient to cause
the condition.

2. Due to it's relative proximity to the surface, I do not feel that a
blade is at maximum efficiency at this angle of rotation. My
sense/guess/feel, is that isn't doesn't reach maximum until about 045*.
Why? Sit on a tug, tied to the stern of a ship and watch "propwash" for
awhile. The wash is pushed to the side, breaking the surface during the
initial part of it's rotation. However ....

3. Ignore the quadrants 000-090, and 180-270 ( My sense - the prop
efficiency is in transition during these portions of rotation. In one
the efficiency is increasing and the other it is decreasing ... 0% net
difference.)the blades are still pushing back, but there is no net
effect (arguably) which we can readily apply to "propwalk". Instead .....

4. Concentrate on the quadrants 090-180 and 270-000. From 090 to 180 the
blade is pushing back against a relatively solid column of water, down
against a relatively solid column of water and increasingly LEFT against
a relatively solid column of water. During this entire quadrant of
rotation, the blade is at maximum efficiency...... BUT, from 270-000 the
blade is pushing back relatively nearer the surface, up toward the
surface, and RIGHT towards and relatively close to the surface, where it
can and does break the surface or at least bulge the surface. So.....

5. My sense from this. The blade, in these two all important quadrants,
is more efficient between 090-180, than it is between 270-000. The
differences if you add in depth of the hub of the prop, may diminish,
but for a boat floating on the water surface, the efficiency will never
be equal...... VBG ..... propwalk.

I disagree that there is no such thing as a "more solid" column of water.
I disagree that a propellor is at maximum efficiency at 000* (on a boat
floating on the surface of the water).
I disagree that by moving the blade a few feet down in the water, you
will totally negate the effects or differences in blade efficiency.

I think we've probably hashed, thrashed, and rehashed this enough in the
NG, Steve. However, feel free to continue via E-mail if you wish.

otn

On Wed, 09 Jun 2004 17:57:52 GMT, otnmbrd wrote:

I've always wondered if a submarine, 1,000 feet underwater, experiences
"propwalk" .... I'm betting it doesn't G.
At any rate, to save some unnecessary reading, by the numbers on some
points at issue:

1. We are not talking about some highly visible, easily measured,
variations in efficiency. These are very subtle, but sufficient to cause
the condition.

But the force generated is actually pretty high. For instance, due to
the full keel of my boat it's much easier to move it fore and aft vs.
spin it sideways. Yet when I throw the thing in reverse, all that
happens at first is prop walk. By keeping the rudder fully to port I
can, by alternating forward and reverse power every few seconds, spin my
boat completely around in very little more than it's LOA. I use that
"feature" to get into very tight spaces.

2. Due to it's relative proximity to the surface, I do not feel that a
blade is at maximum efficiency at this angle of rotation. My
sense/guess/feel, is that isn't doesn't reach maximum until about 045*.
Why? Sit on a tug, tied to the stern of a ship and watch "propwash" for
awhile. The wash is pushed to the side, breaking the surface during the
initial part of it's rotation. However ....

No, that wash is coming from the upward and to the right (for a right
handed prop). For example, say the prop is 2 feet deep. When the blade
is at 315 degrees, that's when it's pushing water out 45 degrees to the
right, which works out to be 2 feet to the right. By the time the blade
goes past 0 degrees, it's only pushing water sideways and then down.
The upward wash to the left of the boat is from the 180 to 270 part of
the rotation and the upward wash to the right is from the 270 to 0 part
of the rotation.

3. Ignore the quadrants 000-090, and 180-270 ( My sense - the prop
efficiency is in transition during these portions of rotation. In one
the efficiency is increasing and the other it is decreasing ... 0% net

Separating out all the other effects and only discussing the efficiency
of the effect we're talking about here, it's not increasing from 0 to
90. It's at maximum at 0 and stays there until 180. It's decreasing as
you go from 180 to 270 and then increasing again as you go from 270 to
360. For a prop that's not right at the surface the decreasing force to
the right as it goes from 180 to 270 balances out the increasing force
to the left as it goes from 270 to 360 because the balance point (i.e.,
where the force is at a minimum due to minimum efficiency) is right at
or very near 270 degrees. I.e., the size of the water column when it's
at 280 degrees is the same as when it's at 350, just in the other
direction. 290=340, etc.

But for a prop that's very near or at the surface, those forces don't
balance out. That's because as the prop gets closer to the surface the
balance point (where the force is at a minimum due to minimum efficiency
because of the smallest water column before you get to air) move further
around the rotation. In this case, the force at 280 /= the force at 350
and you have a net sideways force.

difference.)the blades are still pushing back, but there is no net
effect (arguably) which we can readily apply to "propwalk". Instead .....

4. Concentrate on the quadrants 090-180 and 270-000. From 090 to 180 the
blade is pushing back against a relatively solid column of water, down
against a relatively solid column of water and increasingly LEFT against
a relatively solid column of water. During this entire quadrant of
rotation, the blade is at maximum efficiency...... BUT, from 270-000 the
blade is pushing back relatively nearer the surface, up toward the
surface, and RIGHT towards and relatively close to the surface, where it
can and does break the surface or at least bulge the surface. So.....

And the quadrant from 0 to 90 exactly balances out the quadrant from 90
to 180. And the quadrant from 180 to 270 "almost" exactly balances out
the quadrant from 270 to 0. The smaller the ratio of prop size/prop
depth, the closer those quadrants (180-270 and 270-0) will balance out.

Of course, all this assumes no hull overhang.

5. My sense from this. The blade, in these two all important quadrants,
is more efficient between 090-180, than it is between 270-000. The

Most definitely. But you're ignoring the other two all important
quadrants.

differences if you add in depth of the hub of the prop, may diminish,
but for a boat floating on the water surface, the efficiency will never
be equal...... VBG ..... propwalk.

I disagree that there is no such thing as a "more solid" column of water.

That doesn't really matter since it's not important as long as you
realize that the "solidity" of the column of water (if there is such a
thing) is the same for the same angle to the right vs. to the left of
the prop.

I disagree that a propellor is at maximum efficiency at 000* (on a boat
floating on the surface of the water).

If it's not then there's something else at work then the effect we're
talking about here. Because at 0 degrees the blade is pushing against
an infinite column of water directly to the right (for a RH prop).
Well, maybe not infinite because of the curvature of the earth and
because there may be a shoreline before the curvature of the earth comes
into play.

I disagree that by moving the blade a few feet down in the water, you
will totally negate the effects or differences in blade efficiency.

Not totally. But it doesn't take going very far down before everything
*nearly* balances out left and right due to the effect we're talking
about.

I think we've probably hashed, thrashed, and rehashed this enough in the
NG, Steve. However, feel free to continue via E-mail if you wish.

Nah. I'd rather continue it here. Finally a civil boating related
thread in the midst of a sea of bickering and political crap.

Steve

Steven Shelikoff wrote:

But the force generated is actually pretty high.

Not really. If it was, it would overcome the ahead or astern component.
For instance, due to
the full keel of my boat it's much easier to move it fore and aft vs.
spin it sideways. Yet when I throw the thing in reverse, all that
happens at first is prop walk.

Because of propwalk and the fact that your prop is in the stern and
everything forward of that follows your prop, much like the "tail of a dog".

By keeping the rudder fully to port I
can, by alternating forward and reverse power every few seconds, spin my
boat completely around in very little more than it's LOA. I use that
"feature" to get into very tight spaces.

If I remember correctly you have a LH prop which means this is totally
understandable and expected.

No, that wash is coming from the upward and to the right (for a right
handed prop).

No, only some of it. G it's amazing what you can see and watch over a
period of years. Because of the proximity to the surface, the blade,
starting at 000*, pushes water right, then down, BUT, again, because of
the proximity to the surface (no real column of water above that
directly in line with the pitch of the blade) it pushes water up into
the air, thus not having full efficiency. Once again, my feeling is that
it doesn't reach full efficiency until @ 045*, and moving the prop
further underwater doesn't totally negate this fact/effect, just reduce it.

For example, say the prop is 2 feet deep. When the blade
is at 315 degrees, that's when it's pushing water out 45 degrees to the
right,
Maybe, depends on the pitch, but for our discussion, ok. (as long as you
realize the direction is also UP toward the surface .....

which works out to be 2 feet to the right. By the time the blade
goes past 0 degrees, it's only pushing water sideways and then down.

Understood, but I feel the column of water above this is not "solid" ...
when you push on something, if it can't go straight, it goes to the
side, in this case up and to the surface .... path of least
resistance..... less efficiency.

The upward wash to the left of the boat is from the 180 to 270 part of
the rotation and the upward wash to the right is from the 270 to 0 part
of the rotation.

The main left component is from 090* to 180* during maximum prop
efficiency. From 180* - 270* the "left" component starts at maximum and
constantly decreases, as the upward component increases (nothing happens
individually, everything happens concurrently).




3. Ignore the quadrants 000-090, and 180-270 ( My sense - the prop
efficiency is in transition during these portions of rotation. In one
the efficiency is increasing and the other it is decreasing ... 0% net


Separating out all the other effects and only discussing the efficiency
of the effect we're talking about here, it's not increasing from 0 to
90. It's at maximum at 0 and stays there until 180.

NO

It's decreasing as
you go from 180 to 270 and then increasing again as you go from 270 to
360.

No. The "left" component is decreasing as you go from 180-270, but the
right component (from 270-000) is staying relatively low (compared to
the left component from 090 t0 180) because the direction is up toward
the surface at the same time as it is to the right, rather than
(090-180) down toward "solid" water and to the left.

For a prop that's not right at the surface the decreasing force to
the right as it goes from 180 to 270 balances out the increasing force
to the left as it goes from 270 to 360 because the balance point (i.e.,
where the force is at a minimum due to minimum efficiency) is right at
or very near 270 degrees. I.e., the size of the water column when it's
at 280 degrees is the same as when it's at 350, just in the other
direction. 290=340, etc.

Rather than going into detail .... NO. You cannot compare a force from
180-270, to a force from 270-360. you MUST compare a force from 180-270,
to 000-090, and a force from 270-360, to 090-180.
The "length" of the water column from 090-180, is far greater than the
"length" of the water column from 270-360. The body (propellor pitch) is
constantly changing direction of "push".


But for a prop that's very near or at the surface, those forces don't
balance out. That's because as the prop gets closer to the surface the
balance point (where the force is at a minimum due to minimum efficiency
because of the smallest water column before you get to air) move further
around the rotation. In this case, the force at 280 /= the force at 350
and you have a net sideways force.

True, but that sideways force does NOT equal the force at 100-170,
because the column of water above it is less than the column of water
below 100-170.


difference.)the blades are still pushing back, but there is no net
effect (arguably) which we can readily apply to "propwalk". Instead .....

4. Concentrate on the quadrants 090-180 and 270-000. From 090 to 180 the
blade is pushing back against a relatively solid column of water, down
against a relatively solid column of water and increasingly LEFT against
a relatively solid column of water. During this entire quadrant of
rotation, the blade is at maximum efficiency...... BUT, from 270-000 the
blade is pushing back relatively nearer the surface, up toward the
surface, and RIGHT towards and relatively close to the surface, where it
can and does break the surface or at least bulge the surface. So.....


And the quadrant from 0 to 90 exactly balances out the quadrant from 90
to 180. And the quadrant from 180 to 270 "almost" exactly balances out
the quadrant from 270 to 0. The smaller the ratio of prop size/prop
depth, the closer those quadrants (180-270 and 270-0) will balance out.

NO,NO,NO You cannot compare 000-090, to 090-180 or 180-270, to 270-360.
You Must compare opposites !! 000-090 and 180-270 or 090-180 and 270-360.

Of course, all this assumes no hull overhang.
It may be a factor, but not a cause EG as explained from my point
of view earlier.


5. My sense from this. The blade, in these two all important quadrants,
is more efficient between 090-180, than it is between 270-000. The


Most definitely. But you're ignoring the other two all important
quadrants.

Nope, I'm narrowing down the important quadrants of push, germane to the
discussion.


differences if you add in depth of the hub of the prop, may diminish,
but for a boat floating on the water surface, the efficiency will never
be equal...... VBG ..... propwalk.

I disagree that there is no such thing as a "more solid" column of water.


That doesn't really matter since it's not important as long as you
realize that the "solidity" of the column of water (if there is such a
thing) is the same for the same angle to the right vs. to the left of
the prop.

No. The column of water is important to the "net" right and left forces.


I disagree that a propellor is at maximum efficiency at 000* (on a boat
floating on the surface of the water).


If it's not then there's something else at work then the effect we're
talking about here. Because at 0 degrees the blade is pushing against
an infinite column of water directly to the right (for a RH prop).

No. This would only be true if the column of water was contained within
a pipe. It's not, so the water above the column can escape above the
surface, reducing efficiency.

Well, maybe not infinite because of the curvature of the earth and
because there may be a shoreline before the curvature of the earth comes
into play.


I disagree that by moving the blade a few feet down in the water, you
will totally negate the effects or differences in blade efficiency.


Not totally. But it doesn't take going very far down before everything
*nearly* balances out left and right due to the effect we're talking
about.

No, if that were the case, then your boat wouldn't experience propwalk.


I think we've probably hashed, thrashed, and rehashed this enough in the
NG, Steve. However, feel free to continue via E-mail if you wish.


Nah. I'd rather continue it here. Finally a civil boating related
thread in the midst of a sea of bickering and political crap.

Steve

LOL Well, it seems no one is really complaining, so have at it.

otn

On Thu, 10 Jun 2004 01:45:59 GMT, otnmbrd wrote:



Steven Shelikoff wrote:

But the force generated is actually pretty high.

Not really. If it was, it would overcome the ahead or astern component.

I guess it's all relative. I think it's pretty high because the side
force from the prop walk is greater than what I can do by pushing the
stern sideways from the dock. It's not as high as the ahead component
because the prop is pretty efficient in that direction. However, I
wouldn't be surprised if, for some props, it were somewhere near as high
as the astern thrust.

For instance, say there was no way of controlling the direction of the
boat. No rudder, no keel, the form of the hull in the water is such
that it can move equally well in all directions and pivot just as easily
also. I.e., if the same force applied in any direction at the stern
will move the stern in that direction by the same amount. Something
like a beachball with a prop sticking out of one side.

I wouldn't be surprised if, for some props, when you put the prop in
reverse the "stern" of the boat (meaningless wrt hull form, but in this
case the stern is simply where the prop is sticking off of) moves more
sideways than backward, spinning the boat more than it pulls it
backwards.

For instance, due to
the full keel of my boat it's much easier to move it fore and aft vs.
spin it sideways. Yet when I throw the thing in reverse, all that
happens at first is prop walk.

Because of propwalk and the fact that your prop is in the stern and
everything forward of that follows your prop, much like the "tail of a dog".

Or course the boat is all attached to itself and what the stern does the
rest has to also. But what I'm talking about is that the stern moves
sideways and the boat pivots around a point that is somewhere within the
outline of the boat. That's different than the stern pulling the boat
backwards and the rest of it following.

By keeping the rudder fully to port I
can, by alternating forward and reverse power every few seconds, spin my
boat completely around in very little more than it's LOA. I use that
"feature" to get into very tight spaces.

If I remember correctly you have a LH prop which means this is totally
understandable and expected.

No, that wash is coming from the upward and to the right (for a right
handed prop).

No, only some of it. G it's amazing what you can see and watch over a
period of years. Because of the proximity to the surface, the blade,
starting at 000*, pushes water right, then down, BUT, again, because of
the proximity to the surface (no real column of water above that
directly in line with the pitch of the blade) it pushes water up into
the air, thus not having full efficiency.

You're being fooled by what you see though. Because the water being
pushed up into the air on the right side of the prop is actually due to
the blades coming up on the left side as the blade travels to the right
on it's way from somewhere past 270 up to 360.

Once again, my feeling is that
it doesn't reach full efficiency until @ 045*, and moving the prop
further underwater doesn't totally negate this fact/effect, just reduce it.

For example, say the prop is 2 feet deep. When the blade
is at 315 degrees, that's when it's pushing water out 45 degrees to the
right,
Maybe, depends on the pitch, but for our discussion, ok. (as long as you
realize the direction is also UP toward the surface .....

Of course. That's the point I'm making, that when the blade is at 315
degrees it's pushing water UP toward the surface at a 45 degree angle
from horizontal (or vertical for that matter) to the *right*. The water
being thrown up in the air on the *right* side of the prop is actually
from the blade on the left side of the shaft coming up and over to the
right.

which works out to be 2 feet to the right. By the time the blade
goes past 0 degrees, it's only pushing water sideways and then down.

Understood, but I feel the column of water above this is not "solid" ...
when you push on something, if it can't go straight, it goes to the
side, in this case up and to the surface .... path of least
resistance..... less efficiency.

Ok, the water above is not solid. But the prop is only pushing up
against this non-solid water when there is an upward component to it's
travel around the circle. Once the prop reaches 0 degrees and until it
reaches 180 degrees, there is absolutly no upward component to the
travel of the blade. The vertical component is all down. So how is a
blade with only a downward component to it's motion going to push the
column of water up?

The upward wash to the left of the boat is from the 180 to 270 part of
the rotation and the upward wash to the right is from the 270 to 0 part
of the rotation.

The main left component is from 090* to 180* during maximum prop
efficiency. From 180* - 270* the "left" component starts at maximum and
constantly decreases, as the upward component increases (nothing happens
individually, everything happens concurrently).

Exactly. And the main right component is from 0 to 90 during maximum
prop efficiency. From 270 to 0 the right component starts at minimum
and constantly increases (if the prop is not right at the surface). The
closer it is to the surface, the further around the clock the minimum
efficiency point is, both for the decreasing and increasing part.

3. Ignore the quadrants 000-090, and 180-270 ( My sense - the prop
efficiency is in transition during these portions of rotation. In one
the efficiency is increasing and the other it is decreasing ... 0% net


Separating out all the other effects and only discussing the efficiency
of the effect we're talking about here, it's not increasing from 0 to
90. It's at maximum at 0 and stays there until 180.

NO

According to your theory, YES it is. Because you have yet to explain
how a prop blade with absolutely no upward component (true all the way
from 0 to 180 degrees) can push the water column up.

It's decreasing as
you go from 180 to 270 and then increasing again as you go from 270 to
360.

No. The "left" component is decreasing as you go from 180-270, but the
right component (from 270-000) is staying relatively low (compared to
the left component from 090 t0 180) because the direction is up toward
the surface at the same time as it is to the right, rather than
(090-180) down toward "solid" water and to the left.

That's just not true if the blade is not breaking the surface. The
minimum efficiency occurs when the water column the blade is pushing
against is at a minimum. The length of the water column is that of a
line drawn perpendicular to the blade from the blade to the surface.

Just for the hell of it, I came up with a formula for the length of the
column of water the blade is pushing against when given the angle of
it's rotation during the portion from 270 to 360 degrees. It's:

column length = (D-0.5B*sin(theta-270))/cos(theta-270)

where
theta = the angle of rotation from 270 to 360 degrees.
D = depth of the center of the prop,
B = length of the blade. I'm drawing the perpendicular line from the
middle of the blade, which is where the 0.5 multiplier for B comes from.
If you want to draw it from the tip of the blade, take out the 0.5.

I'll leave it to you to verify that the formula is correct. Now, what
you have to do is put in the constants D and B and find the value of
theta that gives the minimum column and you'll see where the efficiency
reaches the minimum value on the way from 180 to theta and then start to
gain efficiency on the way from theta to back 0.

As an example, I'll use a prop depth of 3 feet and a prop diameter of 1
foot. So, D (depth) = 3 feet, B (blade length) = 0.5 feet and solve for
the theta that gives the minimum column. The answer is 274.7 degrees.
So for that configuration, the efficiency decreases from 180 to 274.7
degrees where it reaches a minimum. Then it starts to rise on it's way
from 274.7 degrees up to 360 where it's at a maximum again, and equal to
what it was at 180 but in the opposite direction.

Using the same blade diameter, if the prop is 1 foot deep, the
efficiency reaches a minumum at 284.4 degrees. If the prop is 10 feet
deep, the efficiency reaches a minimum at 271.4 degrees.

For a prop that's not right at the surface the decreasing force to
the right as it goes from 180 to 270 balances out the increasing force
to the left as it goes from 270 to 360 because the balance point (i.e.,
where the force is at a minimum due to minimum efficiency) is right at
or very near 270 degrees. I.e., the size of the water column when it's
at 280 degrees is the same as when it's at 350, just in the other
direction. 290=340, etc.

Rather than going into detail .... NO. You cannot compare a force from
180-270, to a force from 270-360. you MUST compare a force from 180-270,
to 000-090, and a force from 270-360, to 090-180.

Um, yes you can. In fact, you MUST. You cannot just look at certain
parts of the rotation while ignoring other parts. You have to look at
the balancing forces the entire way around. You MUST balance all of the
horizontal components against eachother and see what the resulting
horizontal force component is. There are horizontal components all the
way around except for when the blade is exactly at 90 or 270 degrees.
So you must consider what's happening all the way around and find what
portions balance out the other portions and what's left over that
doesn't balance out.

The "length" of the water column from 090-180, is far greater than the
"length" of the water column from 270-360. The body (propellor pitch) is
constantly changing direction of "push".

While that's true, it's also true that the "length" of the water column
from 0-90 is far greater than the "length of the water column from 270
to 0. So what?

But for a prop that's very near or at the surface, those forces don't
balance out. That's because as the prop gets closer to the surface the
balance point (where the force is at a minimum due to minimum efficiency
because of the smallest water column before you get to air) move further
around the rotation. In this case, the force at 280 /= the force at 350
and you have a net sideways force.

True, but that sideways force does NOT equal the force at 100-170,
because the column of water above it is less than the column of water
below 100-170.

Which is completely irrevelant because the sideways force from 100-170
is balanced by the equal and opposite sideways force from 10-80. Again,
you're not considering the entire rotation and what parts of it balance
out what other parts and what's left over after the balancing act.

difference.)the blades are still pushing back, but there is no net
effect (arguably) which we can readily apply to "propwalk". Instead .....

4. Concentrate on the quadrants 090-180 and 270-000. From 090 to 180 the
blade is pushing back against a relatively solid column of water, down
against a relatively solid column of water and increasingly LEFT against
a relatively solid column of water. During this entire quadrant of
rotation, the blade is at maximum efficiency...... BUT, from 270-000 the
blade is pushing back relatively nearer the surface, up toward the
surface, and RIGHT towards and relatively close to the surface, where it
can and does break the surface or at least bulge the surface. So.....


And the quadrant from 0 to 90 exactly balances out the quadrant from 90
to 180. And the quadrant from 180 to 270 "almost" exactly balances out
the quadrant from 270 to 0. The smaller the ratio of prop size/prop
depth, the closer those quadrants (180-270 and 270-0) will balance out.

NO,NO,NO You cannot compare 000-090, to 090-180 or 180-270, to 270-360.
You Must compare opposites !! 000-090 and 180-270 or 090-180 and 270-360.

YES, YES, YES you can compare 0-90 to 90-180 because they ARE opposites
to each other in the sideways direction. In fact, they completely
cancel each other in the sideways direction. The easiest way to balance
the forces is to just exactly what you think you cannot do. So,
covering the angle of rotation from 0 to 180, the net sideways force is
ZERO. Now, you have left to find the net sideways force from 180 to
360. This net force is NOT zero *because* of what I described above,
i.e., the minimum force is not right at 270 degres. And the closer to
the surface you get, the more the sideways force is out of balance. So,
for the first example above (3' depth 1' prop) the in forward with a RH
prop, for the portion of the trip from 180-360, the blade is pushing
right for 94.7 degrees but is pushing left for only 85.3 degrees. THERE
is the out of balance force that your "water column" theory says will
cause prop walk.

5. My sense from this. The blade, in these two all important quadrants,
is more efficient between 090-180, than it is between 270-000. The


Most definitely. But you're ignoring the other two all important
quadrants.

Nope, I'm narrowing down the important quadrants of push, germane to the
discussion.

But you can't do that and still come up with a correct answer. You have
to balance out the sideways force for the entire rotation. And the
easiest way to do that is to first take out the parts that completely
balance each other and then look at what's left.

differences if you add in depth of the hub of the prop, may diminish,
but for a boat floating on the water surface, the efficiency will never
be equal...... VBG ..... propwalk.

I disagree that there is no such thing as a "more solid" column of water.


That doesn't really matter since it's not important as long as you
realize that the "solidity" of the column of water (if there is such a
thing) is the same for the same angle to the right vs. to the left of
the prop.

No. The column of water is important to the "net" right and left forces.

That's not what I said. What I said is that what you have to realize is
that the column of water is the same for the same angle right vs. left
of the prop.

[...]
I disagree that by moving the blade a few feet down in the water, you
will totally negate the effects or differences in blade efficiency.


Not totally. But it doesn't take going very far down before everything
*nearly* balances out left and right due to the effect we're talking
about.

No, if that were the case, then your boat wouldn't experience propwalk.

Yes it would. Just not totally from the effect you're postulating.
But it would contribute.

Steve

On Thu, 10 Jun 2004 00:05:11 GMT, (Steven Shelikoff) wrote:

On Wed, 09 Jun 2004 17:57:52 GMT, otnmbrd wrote:

I've always wondered if a submarine, 1,000 feet underwater, experiences
"propwalk" .... I'm betting it doesn't G.
At any rate, to save some unnecessary reading, by the numbers on some
points at issue:

1. We are not talking about some highly visible, easily measured,
variations in efficiency. These are very subtle, but sufficient to cause
the condition.

But the force generated is actually pretty high. For instance, due to
the full keel of my boat it's much easier to move it fore and aft vs.
spin it sideways. Yet when I throw the thing in reverse, all that
happens at first is prop walk. By keeping the rudder fully to port I
can, by alternating forward and reverse power every few seconds, spin my
boat completely around in very little more than it's LOA. I use that
"feature" to get into very tight spaces.

2. Due to it's relative proximity to the surface, I do not feel that a
blade is at maximum efficiency at this angle of rotation. My
sense/guess/feel, is that isn't doesn't reach maximum until about 045*.
Why? Sit on a tug, tied to the stern of a ship and watch "propwash" for
awhile. The wash is pushed to the side, breaking the surface during the
initial part of it's rotation. However ....

No, that wash is coming from the upward and to the right (for a right
handed prop). For example, say the prop is 2 feet deep. When the blade
is at 315 degrees, that's when it's pushing water out 45 degrees to the
right, which works out to be 2 feet to the right. By the time the blade
goes past 0 degrees, it's only pushing water sideways and then down.
The upward wash to the left of the boat is from the 180 to 270 part of
the rotation and the upward wash to the right is from the 270 to 0 part
of the rotation.

3. Ignore the quadrants 000-090, and 180-270 ( My sense - the prop
efficiency is in transition during these portions of rotation. In one
the efficiency is increasing and the other it is decreasing ... 0% net

Separating out all the other effects and only discussing the efficiency
of the effect we're talking about here, it's not increasing from 0 to
90. It's at maximum at 0 and stays there until 180. It's decreasing as
you go from 180 to 270 and then increasing again as you go from 270 to
360. For a prop that's not right at the surface the decreasing force to
the right as it goes from 180 to 270 balances out the increasing force
to the left as it goes from 270 to 360 because the balance point (i.e.,
where the force is at a minimum due to minimum efficiency) is right at
or very near 270 degrees. I.e., the size of the water column when it's
at 280 degrees is the same as when it's at 350, just in the other
direction. 290=340, etc.

But for a prop that's very near or at the surface, those forces don't
balance out. That's because as the prop gets closer to the surface the
balance point (where the force is at a minimum due to minimum efficiency
because of the smallest water column before you get to air) move further
around the rotation. In this case, the force at 280 /= the force at 350
and you have a net sideways force.

difference.)the blades are still pushing back, but there is no net
effect (arguably) which we can readily apply to "propwalk". Instead .....

4. Concentrate on the quadrants 090-180 and 270-000. From 090 to 180 the
blade is pushing back against a relatively solid column of water, down
against a relatively solid column of water and increasingly LEFT against
a relatively solid column of water. During this entire quadrant of
rotation, the blade is at maximum efficiency...... BUT, from 270-000 the
blade is pushing back relatively nearer the surface, up toward the
surface, and RIGHT towards and relatively close to the surface, where it
can and does break the surface or at least bulge the surface. So.....

And the quadrant from 0 to 90 exactly balances out the quadrant from 90
to 180. And the quadrant from 180 to 270 "almost" exactly balances out
the quadrant from 270 to 0. The smaller the ratio of prop size/prop
depth, the closer those quadrants (180-270 and 270-0) will balance out.

Of course, all this assumes no hull overhang.

5. My sense from this. The blade, in these two all important quadrants,
is more efficient between 090-180, than it is between 270-000. The

Most definitely. But you're ignoring the other two all important
quadrants.

differences if you add in depth of the hub of the prop, may diminish,
but for a boat floating on the water surface, the efficiency will never
be equal...... VBG ..... propwalk.

I disagree that there is no such thing as a "more solid" column of water.

That doesn't really matter since it's not important as long as you
realize that the "solidity" of the column of water (if there is such a
thing) is the same for the same angle to the right vs. to the left of
the prop.

I disagree that a propellor is at maximum efficiency at 000* (on a boat
floating on the surface of the water).

If it's not then there's something else at work then the effect we're
talking about here. Because at 0 degrees the blade is pushing against
an infinite column of water directly to the right (for a RH prop).
Well, maybe not infinite because of the curvature of the earth and
because there may be a shoreline before the curvature of the earth comes
into play.

I disagree that by moving the blade a few feet down in the water, you
will totally negate the effects or differences in blade efficiency.

Not totally. But it doesn't take going very far down before everything
*nearly* balances out left and right due to the effect we're talking
about.

I think we've probably hashed, thrashed, and rehashed this enough in the
NG, Steve. However, feel free to continue via E-mail if you wish.

Nah. I'd rather continue it here. Finally a civil boating related
thread in the midst of a sea of bickering and political crap.

Steve

Please continue it here! It's quite interesting, although I think you're going
to end up with something like, "God created prop walk to **** off people too
cheap to buy two engines."

John H

On the 'Poco Loco' out of Deale, MD
on the beautiful Chesapeake Bay!

John H wrote:
On Thu, 10 Jun 2004 00:05:11 GMT, (Steven Shelikoff) wrote:


On Wed, 09 Jun 2004 17:57:52 GMT, otnmbrd wrote:




I think we've probably hashed, thrashed, and rehashed this enough in the
NG, Steve. However, feel free to continue via E-mail if you wish.

Nah. I'd rather continue it here. Finally a civil boating related
thread in the midst of a sea of bickering and political crap.

Steve


Please continue it here! It's quite interesting, although I think you're going
to end up with something like, "God created prop walk to **** off people too
cheap to buy two engines."

John H

On the 'Poco Loco' out of Deale, MD
on the beautiful Chesapeake Bay!

EG Actually, the effects of "propwalk" are just as important, useful
and noticeable, with two engines, as they are with one.

otn