On Thu, 10 Jun 2004 20:16:24 GMT, otnmbrd wrote:

G And, awaaaaaaay we go .......

Steven Shelikoff wrote:


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.

I can't really argue pro or con with that, but I think/sense you are
high on your propwalk numbers versus astern.

It all depends on the prop. One with big flat paddle blades might be
pretty efficient in reverse and have much more thruse than prop walk.
Something like a fixed sailboat prop with 2 tiny blades might not be,
and the small amount of reverse thrust could be overwhelmed by prop
walk.

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.

I would picture an ever increasing spiral as you gained speed.

Me too. And how tight the spiral is dependent on how much prop walk
there is relative to reverse thrust.

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.

The pivot point has changed position. Now, rather than being @1/3rd of
the way from the bow, it's @1/3rd of the way forward of the stern.

Doesn't matter. It's still within the confines of the boat.

As a possible explanation for this .... You're DIW, single screw RH
prop. Put your rudder hard left, then put your engine ahead. Generally,
the first thing you will notice is the beginning of a turn to port.
My sense on this .... the hard left rudder with propwash going over it
can more easily overcome the mass of the boats tendency to not turn than
it's tendency to want to remain DIW, so you notice the turn first, then
see ahead motion.

But this is not true in reverse since you don't have the propwash going
over the rudder.

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.

No. With these direct drive diesels, frequently when they stop the
engine, one blade will stop at TDC. When they restart the engine, you
can watch that blade and what it does, before the effects of the one
behind it become apparent.

How many blades does the prop you're watching have? If it's one or two
then maybe you're not being fooled. If it's 3 or more, especially if
it's 4 or more, then you can't separate what you think you may be seeing
one blade doing from what the rest are doing because they are too close
to each other in angle. For instance, if you're watching a 4 blade
prop, what "splashing" you think you're seeing from the blade at TDC
starting to move over and down could just as easily be from the blade at
270 and starting to come up and over.

In nature, there are many things which take the path of least resistance
....water is one of them.
You are correct that at 000* the blade is pushing against an infinite
column of water (facing 090), and to the right of that column is solid
water toward and to the ocean floor, but .... uhoh, to the left, is a
short column and air.
If the blade pushing right into that infinite column could hold that
water in line, then the blade would be at maximum efficiency. However,
it can't. The water wants to take the path of least resistance, so it
"leaks" off, up into the air. This is why I don't feel the blade is at
maximum efficiency during the entire rotation from 000-090. My own sense
is that it doesn't start approaching maximum until it reaches @ 045* of
rotation..... until then, some of of that "push" is being lost upward.

*If* that's true, that the blade can be moving down but some of the
water it's pushing against moves up because it's the path of least
resistance, *then* it also must be true that the blade starts loosing
efficiency well before it's reaches 180 on it's way from 90 to 180 for
the same reason.

If the prop is deep enough that the blade diameter to prop depth ratio
is small, then those efficiency losses will almost exactly balance out.
It's the same effect on efficiency as what's happening from 180 to 360
but due to something else. In the 180 to 360 case it's due to the blade
directly pushing against a varying sized water column. From 0 to 180
it's from the wash of a blade taking the path of least resistance, i.e.,
to the surface, even if the blade isn't traveling in that direction.

So, if you think there is less efficiency at 0 and it doesn't reach
maximum until 45 (well beyond where I think by we'll go with it) then
it's also true that the blade starts to loose efficiency at around 135
and continues to loose it until at 180 it's about where it was at 0.

Of couse, the exact numbers depend on the shape of the blade and the
depth to diameter ratio. The effect of the wash due to shape seems hard
to predict. But the effect of depth to diameter ratio is the same as
the formula I gave for the other side of the rotation. I.e., if a 1'
blade is 3' deep then there's about 8-9 degrees more push one way then
the other way.

[...]
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?

See above


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.

Let me say this another way.
The "right" component starts just after 270* when the efficiency of the
prop is down. The right component continues through 000* (during the
entire time of which the efficiency of the blade is down. As the blade

It's down, but increasing on it's way to 0, just like it was up but
decreasing on it's way from 180 to 270.

passes 000* the right component is directly right, but because the push
can "leak" off (as I've stated above) the efficiency of the blade is
still down, but beginning to increase, just at the point that the
"right" component is beginning to decrease and head down. This reduced
efficiency continues to increase to a point @045* at the same time that
the right component is rapidly approaching zero.... i.e. the "right
component occurs during the period, the majority of which the propellor
efficiency is down.

While I can agree with that, the effect is nowhere near as much as
you're making it out to be. The efficiency during the right motion is
not constantly low. It's varying for the entire trip right. Same thing
as for it's trip left. There are many angles for the trip right where
the efficiency is higher then the efficiency of the opposing angle left.

On the other side, the left component begins just after 090* when the
prop is at maximum efficiency and continues to increase as the blade
approaches 180* (during this entire time, the blade is at maximum
efficiency). After 180, the blade is still pushing left, but it's also

Um, no. Not according to your "leakage" theory. Unless for some reason
you want to argue that the water can only follow the path of least
resistance in one direction ... Which I think you'll have a hard time
convincing anyone of.

I think you're problem is that you're looking at 2 different things at
once ("leakage" and "column length") and not applying the effects both
of them all the way around the prop rotation. It's easy to see why
there's a net force in some direction if you can, in your mind, ignore a
force that exists in some quadrants yet count it in other quadrants.

beginning to push up which means that the closer it gets to 270* the
left component is decreasing as is the efficiency.
Now, if I compare the first paragraph to the second, at no time during
the rotation, during the time the "right" component could be said to
dominate, is the propellor ALSO at maximum efficiency. Whereas at the
same time on the other side when the "left" component could be said to
be dominate (is that the word I want?) the blade is a maximum efficiency
all the way to 180* and then begins to decrease as the "left" component
decreases ....... LOL PROPWALK.

Sure, if you say it that way it's easy. But the underlying basis of the
paragraph above is wrong. While it can contribute to prop walk, the
difference between the left and right components during the entire 360
degree trip is significantly less then one might assume if they took the
paragraph above at face value.

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.

The blade/ water column does not have to break the surface, it just has
to be able to "bulge" it.

True. But the only time what you said above is true is if the blade
*is* breaking the surface. If not, then the minimum efficiency point is
beyond 270 degrees.

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.

Ouch, math. I've hated math since they made me do celestial and Great
circles, long hand .... no slide rule, even.
At any rate, I'm going to have to print this out and study it, but on
[...]
began to increase. Naturally, as you got close to the hub the column
resumed increasing sooner.
The problem with this from my standpoint is that at no point is the
column as large for the majority of the blade during this entire (for
the most part) as it is between 090-180 which would be it's opposite
balancing blade.

You have to open up your U so that it's more like a V if you want to see
the effects of prop wash taking the path of least resistance. If you
do, you'll see that the efficiency begins to drop before the blade gets
to 180, just like it wasn't at max until some angle past 0.

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.

OK let me rewrite: We are discussing a rotating machine which creates
thrust. At a minimum this is being done with 2 blades for balance, and

Actually, it can be done with 1 blade if you balance the blade with a
"blob" of heavy material on the other side. A 1 blade prop is the most
efficient since it's not in the wash of any other blades.

in this case you would have to compare what is happening at one blade
with exactly what's happening with the one 180* opposite to understand
the net results.
However, boats can have 2,3,4,5,6 bladed props, in which case you would
need to compare the total constant effect as one sum of effect for the
entire rotation, but not side by side effects.

The easiest way to do it is to just look at 1 blade and sum all of the
horizontal components for it's entire rotation. You can't just look at
what's going on in the "quadrant of interest." To do the summing of
components, it's easiest to do a comparison of equal and opposite forces
and cancel them out and see what's left that's not equal and opposite.
I.e., look at the quadrants where there's only a motion right and see
how much of an opposing force you can cancel out from a corresponding
quadrant where there's a motion left.

The quadrants that correspond most are those from 0-90 vs. 90-180 where
the vast majority of the horizontal forces (but not all) cancel out and
from 180-270 vs. 270-360, where again, the vast majority of the
horizontal forces (but not all) cancel 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?

Actually, the length of the water column is greater from 000-180 than it
is from 180 to 000, but that puts us back to the beginning and ignores

Well of course. But again, so what. You haven't broken it down far
enough to get any significant information if you look at those large
spans of angle because there's a cancelling right and left component in
both 0-180 and 180-0.

all the variables of direction, push, etc., we've introduced.

Yup.

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.

No, I'm considering the entire rotation, but for the basic cause I'm
sticking with even number blades such as two/four where you must compare
the opposites: Two bladed - compare 10-80 to 190-260, four bladed same
as two, but add, 100-170 compared to 280-350.
G now make odd number blades and my feeling is you have to compare all
at the same time for wherever they are in rotation, but obviously the
effect (net) come out the same.

No, you don't really have to compare blades at all as long as you
believe the same thing is happening to each blade. The only thing you
might have to worry about the number of blades for is where the wash of
one affects the one behind it (but again, that should be the same for
all blades as long as the prop is symmetrical, which it likely is) and
because the total prop walk will be higher the more blades you have.
That's because, as you add more blades you're not adding more efficiency
and the marginal increase in thrust from each additional blade is
smaller the more blades you have. But the prop walk effects we're
talking about here seem like they would be cumulative and not have a
lower marginal effect for each additional blade.

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.

No, because they are not happening at the same time.... do that and
that boat will bounce left and right

Yes, because we're only looking at the quadrants for a force
calculation. In trying to solve the problem, there's no difference in
the result if you look at one blade at a time and sum it's forces for
the entire rotation vs. trying to see what's happening at any given prop
angle and worrying about what's happening to all the blades. It's just
much easier to concentrate on one blade the whole way around if you want
to come up with a correct answer. It may be the fact that you're trying
to think about what's happening to all the blades simultaneously that's
leading you astray.

Oh, and if your theory is correct, the boat will feel like it's bouncing
left and right because the prop walk force will be pulsating.

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.

If you view it from your point of view, correct. However, from mine, the
net "right" never exceeds the net "left" because the net "right"
occurs when the propellor blade is at reduced efficiency while the net
"left occurs during maximum efficiency.

That may not be true once you're underlying foundation is butressed.
I.e., once you consider the decreasing efficiency as the blade travels
from 90 to 180 due to the "leakage" part of your theory. Once you
consider that the min/max efficiency points are not at 270/90 and will
change where they are at different depths.

Even just applying your theory of "leakage" and "column length" is more
complicated then you're making it out to be. And while I can agree that
there is a net sideways force from your theory, it's much smaller than
you're envisioning and does nothing to explain prop walk once a prop
gets far enough away from the surface that the depth to diameter ratio
is very large.

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.

G Sorry, but as you can see, I totally disagree with that.

I'm not sure how you can. How is it possible that, from the same point
under the water, the "solidity" is different if you go up 45 degrees to
the right vs. going up 45 degrees to the left? If you can disagree with
that underlying foundation then all bets are off and anything is
possibly.

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.

I don't think you will ever reach that balance point on a small boat
floating on the surface,or even a ship .... goes back to my question

I don't think so either. In fact, it will never reach a balance point
no matter how low you go. But it doesn't take going very deep (i.e.,
only a few feet with a 1 foot prop) for the effect to balance out enough
that the reverse thrust should easily be able to overcome it. It
doesn't though. Which means there must be some other, or many other,
causes of prop walk that all combine.

about subs. Do they experience propwalk at a depth of 1,000 feet? If
not, at what depth does it stop ... sure would confirm or deny our
points, wouldn't it?

It would help.

Steve