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

On 08 Jun 2004 03:14:52 GMT, (Shen44) wrote:


That was a good explanation and I got what you're trying to say. The
only problem with it is the overhang. My prop is about 3 ft down. The
clearance to the hull above it is around 3 to 4 inches. There's still
about 5 or 6 feet of waterline behind the prop. So on the upward
stroke, the blade is pushing the water column against the hull.

First, what is the shape of your hull? Secondly, look at the pitch of your prop
- the main push is back, the "UP" angle is only slight and not directly "up"
against you hull, even WITH (I'll bet) 5-6' of overhang.
Even in reverse, with any degree of deadrise, the push to the side and up will
not be mainly impacting on your hull for the full revolution, if at all, and

The area directly above the prop is fairly flat then as you go sideways
it flares out after it gets a few feet from the prop on either side.
The prop is only a few inches below the fairly flat part. As you go
back a few feet it starts to form a pronounced V that gets pretty sharp
where it meets the rudder. But since we're talking about reverse, it's
more important to go forward from the prop. That way, there's plenty of
boat hull on either side of any prop wash.

I'm looking at pictures of it right now and I'd say there's only maybe
15 degrees of rotation in the upward direction off to the right (it's a
LH prop so it comes up on the right side) where it only has air behind
the column of water, and maybe only 10 degrees of the 90 up and to the
left part of the rotation where it's pressing against air.

These are all just estimates from looking at a bunch of pictures of the
boat on the haulout crane. It's a LH prop and follow a blade around the
forward rotation from 0 degrees (straight up) down to 270 degrees (off
to the left) down to 180 degrees (straight down) and then it starts up.
It'll go for another 10-15 degrees with the column of water backed by
air off to the right, but then from around 190-195 all the way back to
355 degrees the column of water is backed by the hull. Then from around
350 to 360 it's backed by air off to the left side of the boat.

So basically, for around 15 degrees of the 360 rotation you have a
column of water backed by air in one direction and about 10 degrees the
column is backed by air in the other direction. The entire rest of the
rotation you have either the botom of the sea or hull.

I just don't think a tiny minute loss of efficiency during that small
amount of rotation is enough to be the main contributing factor of the
prop walk. It certainly is a factor though, among many others. And
because there is an overhang, it's a factor *more* than if there was no
overhang... see below.

even if you have no deadrise, the water will shortly leave the confines of your
hull and be allowed to push up into the air.

Not
only is there no noticable bulge of water behind the boat from the prop
but there isn't even a sign on the surface that there's a prop turning
at all. I just don't get that stream of bubbles behind me or a bulge in
the water that powerboats get or anything other than the same sort of
wake I get when sailing. And yet there's pronounced prop walk when
reversing.

My feeling on this is that the visual effects of prop thrust are not always
readily apparent, but this does not in and of itself, alter what is occurring,
i.e., you don't need a big white water wash, astern or ahead of you to be, in
reality, experiencing a lessor degree of efficiency from the prop during half

It's true that the effect can be very subtle, barely noticable. For
instance, one of the ways to find a moving submerged submarine is that
there is a pressure wave it creates as it moves through the water which
creates a wake-like bulge on the sea surface. It's tiny, but it's there
and theoretically could be picked up with a very sensitive radar looking
at the sea surface. You're not gonna see it with your eyes though. It
shows up as a minute wakelike bulge in the average sea level.

(maybe a little less, maybe a little more) of it's rotation, which is causing
the unequal pull to stbd or port, depending on it's rotation.

But in my case, it's nowhere near half the rotation, it's only around 15
degrees one way and 10 the other. It can never be more than half the
rotation. But the main problem is with your theory that you have an
almost equal amount of inefficency in either direction. I.e., think of
a completely unshrouded prop with no overhang. As it's coming up, for
90 degrees of the 180 upward travel it is less efficient in one
direction and the other 90 degrees it's less efficient in the other
direction.

So here's why I don't buy your theory for an unshrouded prop. When I
say efficiency below, I mean strictly the efficiency from the column of
water being backed by something solid vs. backed by air.

Going through the rotation of an unshrouded RH prop in forward, from 0
to 180 it's pushing against solid water so it has good efficiency. But
from 0 to 90, it's pushing the stern to the left with good efficiency
and from 90 to 180 it pushing the stern to the right with good
efficiency, the same equal and opposite efficiency as from 0 to 90. Now
on the whole way up from 180 to 360 the prop is pushing against water
that is backed by air, so you get the bulge and some loss of efficiency.
From 180 to 270 degrees it's pushing the stern to the left with poorer
efficiency and from 270 to 360 it's pushing the stern to the right with
poorer efficiency, the same equal but opposite efficiency it was pushing
to the left with. As you can see, that all cancels out.

Now, add an overhang and visualize the angles of rotation that the prop
will go through where it has neither the bottom of the ocean or the hull
of the boat to back up the column of water. You'll see that because of
the overhang and the fact that the blade of the prop is off to the side
of the centerline during most of it's upward travel, the amount of
rotation with only air behind the column is different for the 180 to 270
degree push to the right than it is for the 270 to 360 degree push to
the left.

This is where I think your theory can contribute to prop walk, but only
by a small amount. i.e., only by the amount that an overhang causes the
amount of rotation with air behind it to be different between the 180 to
270 degree part of the rotation and the 270 to 360 degree part of the
rotation.

If you followed all that, I'll be amazed. Hopefully I can somehow
make it clearer if you didn't.

Steve

On Tue, 08 Jun 2004 02:45:24 GMT, otnmbrd wrote:



Steven Shelikoff wrote:
On Mon, 07 Jun 2004 16:00:18 GMT, otnmbrd wrote:

This is one of those explanations which I feel may have an effect on
propwalk amount, but is not the "root" cause of propwalk.
Reason ... you get propwalk ahead and astern. When going ahead, the wash
does not push against the hull.


I don't think there is a "root" cause of prop walk. It's just a sum of
many factors, some of which even counteract eachother which is why you
can't be sure of the direction of prop walk just by which way the prop
turns.

G I disagree. My feeling is rotation in a water medium is the root
cause.

Um, yeah. That's kind of obvious. Not really an explanation of why
rotation in a water medium causes prop walk though. It's sort of the
same thing as saying the root cause of prop walk is a different amount
of force pushing the stern left vs. right. You'd be correct, but it
doesn't really tell you anything you don't already know.

Steve

I have often wondered if the phenomenon is partly caused by the same
forces that act on a helicopter rotor (or any other rotating mass) ... a
control input force to the rotor is applied 90 degrees ahead of the
desired output force. Applying that to the marine propeller means that
the right hand prop applying a force (better "bite") at 6 o'clock would
generate a resultant force at 9 o'clock in ahead rotation and 3 o'clock
astern. The force at 3 o'clock would result in the stern moving to port
when backing.

Rick

On Tue, 08 Jun 2004 01:28:20 GMT, (Steven
Shelikoff) wrote:
My prop is about 3 ft down. The
clearance to the hull above it is around 3 to 4 inches. There's still
about 5 or 6 feet of waterline behind the prop. So on the upward
stroke, the blade is pushing the water column against the hull.

======================================

Charles Low conjectured, and I think he may be right, that it is this
force of the prop wash hitting the hull that causes the "walk". It's
the only explanation that seems to hold up to scrutiny with different
hull and prop configurations.

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

Steven Shelikoff wrote:
On Tue, 08 Jun 2004 02:45:24 GMT, otnmbrd wrote:



Steven Shelikoff wrote:

On Mon, 07 Jun 2004 16:00:18 GMT, otnmbrd wrote:


This is one of those explanations which I feel may have an effect on
propwalk amount, but is not the "root" cause of propwalk.
Reason ... you get propwalk ahead and astern. When going ahead, the wash
does not push against the hull.


I don't think there is a "root" cause of prop walk. It's just a sum of
many factors, some of which even counteract eachother which is why you
can't be sure of the direction of prop walk just by which way the prop
turns.

G I disagree. My feeling is rotation in a water medium is the root
cause.


Um, yeah. That's kind of obvious. Not really an explanation of why
rotation in a water medium causes prop walk though. It's sort of the
same thing as saying the root cause of prop walk is a different amount
of force pushing the stern left vs. right. You'd be correct, but it
doesn't really tell you anything you don't already know.

Steve

BG see my other response .... no way I could write that again.

otn

On Tue, 08 Jun 2004 12:08:33 -0400, Wayne.B
wrote:

On Tue, 08 Jun 2004 01:28:20 GMT, (Steven
Shelikoff) wrote:
My prop is about 3 ft down. The
clearance to the hull above it is around 3 to 4 inches. There's still
about 5 or 6 feet of waterline behind the prop. So on the upward
stroke, the blade is pushing the water column against the hull.

======================================

Charles Low conjectured, and I think he may be right, that it is this
force of the prop wash hitting the hull that causes the "walk". It's
the only explanation that seems to hold up to scrutiny with different
hull and prop configurations.

It could be a contributing factor but not the only one, and maybe not
even the most important for some boats. You can have propwalk even with
no prop wash hitting the hull, like when you first start out in forward
from a standstill with a boat that has the prop beyond the stern of the
boat (an outboard or sterndrive.) It's just not as noticable because
you can control it better in forward.

Steve

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