Steven Shelikoff wrote:



That theory sounds compelling at explaining a difference in the
efficiency of the blade during it's downward vs. upward travel (if
there's no overhang. This part of your explanation would create an
upward force. It would also create a listing torque force since the
upward force of your theory above is applied off the center of the prop.
But since the column of water is the same in both the sideways
directions, where does the sideways force come from to create prop walk?

Steve

The part of his explanation which is not being stressed enough, is that
you must follow the blade through it's complete revolution.
The blade starts pushing down/back (pitch of the blade) but as it
rotates it begins to push to the side/back (RH prop going ahead),
pulling/pushing the stern to stbd.
Again, the blade is more efficient during the lower portion of this arc
(solid water) than it is in the upper portion (water being lifted into
air) which causes the propwalk.
The next time you get to look at a ship in ballast or riding light, with
it's prop just beneath the surface, watch the "wash" from the prop at
the surface. You'll see it being "thrown" up/back and to the side/back.
As for the "overhang" of the hull,issue, look again at the pitch of the
blade. When the blade is pushing up/back, it's not straight up, it's G
BACK/up, then look at the wash astern of your boat .... you'll see it
breaking the surface astern of you.

otn

On Mon, 07 Jun 2004 15:52:25 GMT, otnmbrd wrote:

As for the "overhang" of the hull,issue, look again at the pitch of the
blade. When the blade is pushing up/back, it's not straight up, it's G
BACK/up, then look at the wash astern of your boat .... you'll see it
breaking the surface astern of you.

Not on my boat. There's still maybe 5 feet of boat hull in the water
above the prop behind where the prop exits. There's no prop wash at all
breaking the surface astern of me. And since we're talking about
backing up from standing still, there really is nothing of the sort
you've described above. And yet I still get prop walk.

Steve

Steven Shelikoff wrote:
On Mon, 07 Jun 2004 15:52:25 GMT, otnmbrd wrote:


As for the "overhang" of the hull,issue, look again at the pitch of the
blade. When the blade is pushing up/back, it's not straight up, it's G
BACK/up, then look at the wash astern of your boat .... you'll see it
breaking the surface astern of you.


Not on my boat. There's still maybe 5 feet of boat hull in the water
above the prop behind where the prop exits. There's no prop wash at all
breaking the surface astern of me. And since we're talking about
backing up from standing still, there really is nothing of the sort
you've described above. And yet I still get prop walk.

Steve

G There's always the boat which doesn't appear to show the obvious
reaction.
Considering the angle of pitch of the prop, when ahead, 5 feet is
relatively nothing..... also, what is the shape of your hull aft of the
prop?
I've seen the same results, but my feeling is it takes very little
"lift" of the wash from the prop blade to create that unequal thrust
back and to the side, we know as "propwalk".
You are saying there's nothing of the sort that I've described. I'm
saying it's not always readily apparent, but it IS there.
Back a boat from a dead start .... you won't immediately see the wash.
Back a boat that has headway .... it will be even longer before you see
the wash .... but .... the unequal thrust WILL be occurring.
If you have a low power to weight ratio, the visual results will be
greatly lessened. However, this does not mean they aren't occurring.
Again, BG I'm no scientist, engineer, Naval architect, or prop
designer .... my opinions have developed over a good many years of
talking to individuals, handling all kinds of boats, watching wakes and
reactions .... the "root" cause is prop rotation .... everything else is
a variable, adding or detracting from the mix.

otn

On Tue, 08 Jun 2004 02:33:44 GMT, otnmbrd wrote:



Steven Shelikoff wrote:
On Mon, 07 Jun 2004 15:52:25 GMT, otnmbrd wrote:


As for the "overhang" of the hull,issue, look again at the pitch of the
blade. When the blade is pushing up/back, it's not straight up, it's G
BACK/up, then look at the wash astern of your boat .... you'll see it
breaking the surface astern of you.


Not on my boat. There's still maybe 5 feet of boat hull in the water
above the prop behind where the prop exits. There's no prop wash at all
breaking the surface astern of me. And since we're talking about
backing up from standing still, there really is nothing of the sort
you've described above. And yet I still get prop walk.

Steve

G There's always the boat which doesn't appear to show the obvious
reaction.
Considering the angle of pitch of the prop, when ahead, 5 feet is
relatively nothing..... also, what is the shape of your hull aft of the
prop?
I've seen the same results, but my feeling is it takes very little
"lift" of the wash from the prop blade to create that unequal thrust
back and to the side, we know as "propwalk".
You are saying there's nothing of the sort that I've described. I'm
saying it's not always readily apparent, but it IS there.
Back a boat from a dead start .... you won't immediately see the wash.
Back a boat that has headway .... it will be even longer before you see
the wash .... but .... the unequal thrust WILL be occurring.

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

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.
Can we say that the directional rotation of the prop (right or left) is
by itself the cause? I can't say with certainty that it is or isn't,
but, in my experience handling ships with CP props, when the prop is set
to zero pitch (judged to be zero due to lack of creep ahead or astern) I
generally (note, I'm saying "generally") see no side movement, yet put
in even the most minimal pitch and I will experience "walk" (loaded
condition, prop down deep with hull overhang above, and light condition,
prop close to the surface with hull overhang out of the water .... makes
no difference .... both ahead and astern ... which is why I discount
hull overhang).
What does this lead me to believe. If the rotating prop at zero pitch,
generally eg showed me no "walk" and as soon as I added pitch, I got
"walk" then I look closely at the effects of a rotating prop and it's
pitch for the root cause.
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
(pulling/pushing the stern to stbd) the blade continues at maximum
efficiency through 180* (pushing left) but as it begins it's rotation
upwards, that efficiency, slowly begins to drop off and the direction of
push begins to angle upward/left until you reach 270*.
From this point, efficiency drops off at a marked rate and the blade is
pushing up and beginning to push slightly right. As you reach @315* you
are close to being back to minimal efficiency and pushing right/up,
which continues back to 000*.
You can see from this (my visualization) that the force pushing left
(pulling/pushing the stern to the right) occurs during that time when
the blade is pushing most efficiently, whereas the force pushing to the
right (countering that left push) occurs when the blade is pushing with
less efficiency...... propwalk.
I would love someone who designs props and is far more technically
versed in the goings on of a prop beneath the water, to critique this.
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.

otn

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

Read Chapman's piloting book.
G
"otnmbrd" wrote in message
k.net...


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.
Can we say that the directional rotation of the prop (right or left) is
by itself the cause? I can't say with certainty that it is or isn't,
but, in my experience handling ships with CP props, when the prop is set
to zero pitch (judged to be zero due to lack of creep ahead or astern) I
generally (note, I'm saying "generally") see no side movement, yet put
in even the most minimal pitch and I will experience "walk" (loaded
condition, prop down deep with hull overhang above, and light condition,
prop close to the surface with hull overhang out of the water .... makes
no difference .... both ahead and astern ... which is why I discount
hull overhang).
What does this lead me to believe. If the rotating prop at zero pitch,
generally eg showed me no "walk" and as soon as I added pitch, I got
"walk" then I look closely at the effects of a rotating prop and it's
pitch for the root cause.
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
(pulling/pushing the stern to stbd) the blade continues at maximum
efficiency through 180* (pushing left) but as it begins it's rotation
upwards, that efficiency, slowly begins to drop off and the direction of
push begins to angle upward/left until you reach 270*.
From this point, efficiency drops off at a marked rate and the blade is
pushing up and beginning to push slightly right. As you reach @315* you
are close to being back to minimal efficiency and pushing right/up,
which continues back to 000*.
You can see from this (my visualization) that the force pushing left
(pulling/pushing the stern to the right) occurs during that time when
the blade is pushing most efficiently, whereas the force pushing to the
right (countering that left push) occurs when the blade is pushing with
less efficiency...... propwalk.
I would love someone who designs props and is far more technically
versed in the goings on of a prop beneath the water, to critique this.
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.

otn

Gordon wrote:
Read Chapman's piloting book.
G

Sorry Gordon, don't have one and at this stage of my life, too cheap to
buy one.
Maybe you could quote the relevant sections from that book?

otn

On Wed, 9 Jun 2004 08:30:12 -0700, "Gordon" wrote:

Read Chapman's piloting book.
G

I have it but it's on the boat, not available right now. I read it but
don't remember a discussion about what exactly causes prop walk. What
does it say?

Steve