On Fri, 11 Jun 2004 18:51:27 GMT, otnmbrd wrote:
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.

They're apt to have anywhere from 3-6. This view has occurred over many
years, watching many props, some on diesels some on steam turbine
(sitting on a dock watch the warm up spinning of the prop fwd and astern).
You need to remember, these are props with @ 25' diameters turning,
initially at maybe 15-30 RPM.
G Call me an oddball for standing there watching/studying this, but
then again, I like watch porpoise swim in the bow wave and seeing how
they move with it.

The problem is that you still don't know which blade the splash at any
point is from when there are many blades and the blade spacing is very
close, without much empty space between them. It would be much easier
with something like a 2 bladed sailboat prop.

On to propwalk ....

In reading the last few post, it's obvious we are now just chasing each
other in circles and resolving nothing.
A main issue is prop efficiency at various angles of rotation.
My opinion:
For a boat floating on the surface, when discussing prop efficiency we
must consider that the prop is affecting two mediums -water and air.
The prop is at maximum efficiency when it is affecting water alone and
at less than maximum efficiency when it affects air and water.
Is the depth of the hub of the prop important? To a degree, yes, but for
the boats we are discussing, the effect is constant, it's degree varies.
For instance, hub at surface half of prop out of water (Arneson drive);
blade tip at surface, hub submerged; blade totally submerged..... the
blade is more efficient, overall, in one semicircle, than the other, in
all of these conditions, in my opinion.

The problem is that for the totally submerged blade, the portion of
where it's less efficient isn't the entire arc from 270 to 90. And
there are large portions within that arc where it's more efficient than
portions within the arc from 90 to 270. For instance, consider a 2
bladed prop... the blade of interest at say 60 degrees (it's within
your semicircle of lower efficiency) is much more efficient than the
blade at 240 degrees for a net left force. But move it a little in your
semicircle and a blade at 280 degrees is less efficient than one at 100
degrees for a net left force.

During the rotation of the blade, it is my opinion that the blade is at
the most maximum efficiency between the angles 045* and 225* of rotation
because it is acting against only the single not compressible medium of
water.
However, between the angles 225* and 045* the blade is operating at LESS
than maximum efficiency because it is acting upon both water AND air.

Ah, I see you've now moved your semicircles around a little. But going
with the above and taking only your theories of column length and
leakage into account, it's pretty hard to believe that a blade at say 40
degrees (within your semicircle of lower efficiency) is less efficient
than the opposing one at 220 degrees (within your semicircle of higher
efficieny). Especially when the first one is pressing down at a fairly
good angle on "solid" water and any "leakage" would have to be off at
more than a 40 degree angle to reach the surface at all while the second
one is pressing up at a 40 degree angle against air. This situation
goes completely counter to your water column theory.

BG Until we resolve this issue between us, we are just butting
heads.....and I'm running low on paper ....

Ayup.

Steve

As a quick aside, I went into work to check on my "Valet parking"
schedule for next week and mentioned this discussion. One individual
that has owned a number of powerdriven boats, both sail and power, asked
me "What's propwalk?" . In all honesty, I was not as surprised at the
question, as were some.

Steven Shelikoff wrote:

The problem is that you still don't know which blade the splash at any
point is from when there are many blades and the blade spacing is very
close, without much empty space between them. It would be much easier
with something like a 2 bladed sailboat prop.

I'm going to have to ask you to trust me on this one, Steve. When you
watch a ship propellor start, especially when the are "spinning" a steam
turbine, you can discern the action of an individual blade. Especially
if you sit there long enough and watch/analyze..... i.e. this is not a
casual observation involving one or two times.



The problem is that for the totally submerged blade, the portion of
where it's less efficient isn't the entire arc from 270 to 90.

True, but the only part where it approaches maximum efficiency is
@045* (give or take some degrees) to 090*

And
there are large portions within that arc where it's more efficient than
portions within the arc from 90 to 270.

Name one

For instance, consider a 2
bladed prop... the blade of interest at say 60 degrees (it's within
your semicircle of lower efficiency)

Actually, no, it's within my semicircle of "maximum" efficiency.

is much more efficient than the
blade at 240 degrees for a net left force.

The overall efficiency of the blade is greater at 060* than 240*.
However, in both cases the left/right force is minimal .... well, maybe
not minimal, but equally decreasing. If I split hairs, I would say net
force -right. (at 060* prop pushing back,down,right -at 240*,prop
pushing back,up,left, in descending order of percentage).

But move it a little in your
semicircle and a blade at 280 degrees is less efficient than one at 100
degrees for a net left force.

True as to the efficiency, but if there is any discernible left/right
force still remaining, it will be pushing net right (I just realized
something. When I say "right" in this case, I mean the blade is pushing
the water to the right. It's pushing the hull to the left. I guess we
also need to be sure we are together on this.)


Ah, I see you've now moved your semicircles around a little.

I have, to maintain my true feeling as to where the prop is most
efficient, but I could easily just say 270-090/090-270 to cover the
BASIC premise.

But going
with the above and taking only your theories of column length and
leakage into account, it's pretty hard to believe that a blade at say 40
degrees (within your semicircle of lower efficiency) is less efficient
than the opposing one at 220 degrees (within your semicircle of higher
efficieny).

The main point, is overall efficiency during the entire 180* arc. You
will find some points, that may match. However, overall, because of
water column and leakage, although they may not be far apart, my guess
is that 220 may be more efficient that 040 (kinda a toss-up here).
Please don't get hung up on specific numbers. By going to
045-225/225-045 I am just trying to give an indication of what I feel is
truer numbers .... they are open to interpretation as well as variation.

Especially when the first one is pressing down at a fairly
good angle on "solid" water and any "leakage" would have to be off at
more than a 40 degree angle to reach the surface at all while the second
one is pressing up at a 40 degree angle against air. This situation
goes completely counter to your water column theory.

Once again, you're trying to hold me to specific angle numbers, to prove
a point .... don't. Look at the overall (at least I've got you comparing
opposites). Sure, the specific 040 may be more efficient than 220, as I
said above, it's a toss up because it's close to the limits of the 180*
arc..... which could also be in error +/-.


BG Until we resolve this issue between us, we are just butting
heads.....and I'm running low on paper ....


Ayup.

Steve

otn

On Sat, 12 Jun 2004 00:53:46 GMT, otnmbrd wrote:

As a quick aside, I went into work to check on my "Valet parking"
schedule for next week and mentioned this discussion. One individual
that has owned a number of powerdriven boats, both sail and power, asked
me "What's propwalk?" . In all honesty, I was not as surprised at the
question, as were some.

Steven Shelikoff wrote:

The problem is that you still don't know which blade the splash at any
point is from when there are many blades and the blade spacing is very
close, without much empty space between them. It would be much easier
with something like a 2 bladed sailboat prop.

I'm going to have to ask you to trust me on this one, Steve. When you
watch a ship propellor start, especially when the are "spinning" a steam
turbine, you can discern the action of an individual blade. Especially
if you sit there long enough and watch/analyze..... i.e. this is not a
casual observation involving one or two times.

I still think it's pretty easy to be fooled on some parts of the
rotation. I.e., if the blade you're watching is going down but you see
a "splash" coming up above it breaking the surface, it could just as
easily be "push" from the blade just behind it that's coming up.

The problem is that for the totally submerged blade, the portion of
where it's less efficient isn't the entire arc from 270 to 90.

True, but the only part where it approaches maximum efficiency is
@045* (give or take some degrees) to 090*

And
there are large portions within that arc where it's more efficient than
portions within the arc from 90 to 270.

Name one

Easy. 45 (outside the arc above) degrees is more efficient than 225
degrees (inside the arc above).

For instance, consider a 2
bladed prop... the blade of interest at say 60 degrees (it's within
your semicircle of lower efficiency)

Actually, no, it's within my semicircle of "maximum" efficiency.

Not before you changed it below. First it was the top half (270 to 90)
vs. the bottom half (90 to 270). Now you rotated it by 45 degrees. See
below.

is much more efficient than the
blade at 240 degrees for a net left force.

The overall efficiency of the blade is greater at 060* than 240*.
However, in both cases the left/right force is minimal .... well, maybe
not minimal, but equally decreasing. If I split hairs, I would say net

Actually, the left/right force is minimal right at 90 and 270 degrees.
This happens to be just where the efficiency is at a maximum and a
minimum, which makes the difference in left/right force even less a
factor.

force -right. (at 060* prop pushing back,down,right -at 240*,prop
pushing back,up,left, in descending order of percentage).

But move it a little in your
semicircle and a blade at 280 degrees is less efficient than one at 100
degrees for a net left force.

True as to the efficiency, but if there is any discernible left/right
force still remaining, it will be pushing net right (I just realized
something. When I say "right" in this case, I mean the blade is pushing
the water to the right. It's pushing the hull to the left. I guess we
also need to be sure we are together on this.)

I was talking about what direction the force is. I.e., a right moving
blade is creating a force to the left. Doesn't really matter for the
purposes of this discussion though.

Ah, I see you've now moved your semicircles around a little.

I have, to maintain my true feeling as to where the prop is most
efficient, but I could easily just say 270-090/090-270 to cover the
BASIC premise.

But going
with the above and taking only your theories of column length and
leakage into account, it's pretty hard to believe that a blade at say 40
degrees (within your semicircle of lower efficiency) is less efficient
than the opposing one at 220 degrees (within your semicircle of higher
efficieny).

The main point, is overall efficiency during the entire 180* arc. You

And I'm agreeing with you that overall there will be a net sideways
force if you look at the entire rotation. It's just that the net force
is much smaller than you're making it out to be when you say things like
the efficiency of an entire arc in one direction is more or less than
the efficiency in the other direction. It's really only a tiny area of
difference, like 8-9 degrees on each side of the rotation for a 1' prop
3' deep, that contributes to the net force.

will find some points, that may match. However, overall, because of
water column and leakage, although they may not be far apart, my guess
is that 220 may be more efficient that 040 (kinda a toss-up here).

Maybe, maybe not. Again, it all depends on the specific configuration.

Please don't get hung up on specific numbers. By going to
045-225/225-045 I am just trying to give an indication of what I feel is
truer numbers .... they are open to interpretation as well as variation.

Especially when the first one is pressing down at a fairly
good angle on "solid" water and any "leakage" would have to be off at
more than a 40 degree angle to reach the surface at all while the second
one is pressing up at a 40 degree angle against air. This situation
goes completely counter to your water column theory.

Once again, you're trying to hold me to specific angle numbers, to prove
a point .... don't. Look at the overall (at least I've got you comparing
opposites). Sure, the specific 040 may be more efficient than 220, as I
said above, it's a toss up because it's close to the limits of the 180*
arc..... which could also be in error +/-.

I'm not trying to hold you to specific angles. You're the one
mentioning specific angles and I'm just showing how those angles don't
follow the theory you're trying to support.

And I've always been comparing opposites. It's just that up till now
I've been comparing opposites in terms of the horizontal component of
the force produced by the blade because to me, that's the easiest way to
find where the forces don't balance. Above I temporarily switched to
comparing opposing sides of the prop because that seems to be easier for
you to deal with even though it much harder to come up with a correct
answer like that. That's because the horizontal forces don't balance
out that way since there's not 180 degree symmetry in the efficiency of
the blade through it's rotation.

Steve

Steven Shelikoff wrote:


I still think it's pretty easy to be fooled on some parts of the
rotation. I.e., if the blade you're watching is going down but you see
a "splash" coming up above it breaking the surface, it could just as
easily be "push" from the blade just behind it that's coming up.

Sure it is, but I'm watching that blade in "slow motion" compared to the
one on your boat, and I'm not being fooled.


The problem is that for the totally submerged blade, the portion of
where it's less efficient isn't the entire arc from 270 to 90.

True, but the only part where it approaches maximum efficiency is
@045* (give or take some degrees) to 090*

And

there are large portions within that arc where it's more efficient than
portions within the arc from 90 to 270.

Name one

Poorly posed question. (Consider a 1' dia prop with the hub submerged 2'
and we will stick with 270-090/090-270) Name a portion of the arc (in
degrees) between 270-000 where the overall efficiency of the blade
matches or exceeds the overall efficiency of the blade between 090-180,
then give it as a percentage of the total arc between 270-000/090-180
(90*). Then do the same for 000-090 versus 180-270


As for the rest, I see two main points:

1. Please explain how you can narrow down overall net sideways force to
a mere 8-9 degrees ( I would consider it closer to 135* +/-).

2. If we consider a prop (in this case, for this question, just
submerged) to be less efficient between 270-090 than it is between
090-270, how can dropping that prop (the prop is 1'dia so hub would

be @6" underwater) 2' 6" to 3', make all that much noticeable change
in the overall efficiency and arcs of efficiency, considering the
horsepower, turning it. i.e., do I think that inefficient side may
have gained some efficiency ? .... yes. Do I think I could perceive
the difference ?.... unlikely, in most cases. (9,999.99 out of 10,000
-couldn't G).

As I was starting this, I remembered I had one of those long paint
stirring thingies (propellor on a shaft) for my drill. Out to the pool!
Started just beneath the surface .... helluva splash.
Then lowered it as deep as possible (barely avoiding electrocution).
Interesting. At first, no noticeable movement, but within 2-3 sec. I
could see a moving "bulge" on the surface. To be sure I wasn't being
fooled, I tried it a number of times (whilst listening to my wife
shouting in the background about her possible need to dial 911, shortly)
with the same results.
Now I admit there is little good science here and the results are open
to discussion, but it was interesting.

otn

On Sat, 12 Jun 2004 18:24:34 GMT, otnmbrd wrote:



Steven Shelikoff wrote:


I still think it's pretty easy to be fooled on some parts of the
rotation. I.e., if the blade you're watching is going down but you see
a "splash" coming up above it breaking the surface, it could just as
easily be "push" from the blade just behind it that's coming up.

Sure it is, but I'm watching that blade in "slow motion" compared to the
one on your boat, and I'm not being fooled.

It doesn't matter how fast or slow the prop is turning. Even if it's
barely moving you could still be seeing a "push" up from the blade right
behind the one that's going down if it's close enough behind it. Oh,
and I've never see the prop on my sailboat boat turn. Many others, like
my outboard and quite a few big ships (although I haven't spent hours
studying it. but not the sailboat. The only way to see it is if I was
in the water next to it. And that's not gonna happen.

The problem is that for the totally submerged blade, the portion of
where it's less efficient isn't the entire arc from 270 to 90.

True, but the only part where it approaches maximum efficiency is
@045* (give or take some degrees) to 090*

And

there are large portions within that arc where it's more efficient than
portions within the arc from 90 to 270.

Name one

Poorly posed question. (Consider a 1' dia prop with the hub submerged 2'
and we will stick with 270-090/090-270) Name a portion of the arc (in
degrees) between 270-000 where the overall efficiency of the blade
matches or exceeds the overall efficiency of the blade between 090-180,

I don't think there is any where it's greater. There are places where
it's very close, like around the end of that arc for a deep prop.

then give it as a percentage of the total arc between 270-000/090-180

100%

(90*). Then do the same for 000-090 versus 180-270

For that configuration, all of it except for the first 7 degrees or so.
92%. But now you have to look at the imbalance of forces for that 7
degree difference to see how significant it is. Yes, I agree there's an
imbalance. Always have. It's just not as signficant as you're making
it out to be. Yes, it seems it could be a contributor to prop walk.
But not the only one and maybe not the most significant one on any
particular boat.

Of course, all that assumes that there really is an efficiency
difference on a blade going up than a blade going down. So far, that's
just a "feeling" you have and hasn't been shown to be true yet.

Looking at it another way, the loss of efficiency is due to the blade
doing some other work that does not involve moving the boat forward or
reverse. When the blade is moving up, the work is creating a pressure
wave that causes a bulge at the surface. When the blade is moving down,
the work is creating a pressure wave that causes an indent on the
bottom. Same loss of efficiency either way. So I'd love to see
something that shows the mere act of creating a bulge at the air/water
interface causes a greater loss of efficiency.


As for the rest, I see two main points:

1. Please explain how you can narrow down overall net sideways force to
a mere 8-9 degrees ( I would consider it closer to 135* +/-).

I already have in a previous post. But basically it has to do how the
forces balance for the entire rotation. Also, one thing that's true
which you haven't addressed which makes the difference much smaller than
you believe is that the "leakage" which causes you to think there's less
efficiency from 0 to 45 degrees also would occur (if it occurs at all)
from 135-180 degrees or at least a major portion of that. This *mostly*
balances out the loss of efficiency from 0 to 45 in the opposite
direction.

2. If we consider a prop (in this case, for this question, just
submerged) to be less efficient between 270-090 than it is between
090-270, how can dropping that prop (the prop is 1'dia so hub would

be @6" underwater) 2' 6" to 3', make all that much noticeable change
in the overall efficiency and arcs of efficiency, considering the
horsepower, turning it. i.e., do I think that inefficient side may
have gained some efficiency ? .... yes. Do I think I could perceive
the difference ?.... unlikely, in most cases. (9,999.99 out of 10,000
-couldn't G).

It's geometry. That's why the formula I gave before is important to
show you the effect of changing the prop/depth ratio.

As I was starting this, I remembered I had one of those long paint
stirring thingies (propellor on a shaft) for my drill. Out to the pool!
Started just beneath the surface .... helluva splash.
Then lowered it as deep as possible (barely avoiding electrocution).
Interesting. At first, no noticeable movement, but within 2-3 sec. I
could see a moving "bulge" on the surface. To be sure I wasn't being
fooled, I tried it a number of times (whilst listening to my wife
shouting in the background about her possible need to dial 911, shortly)
with the same results.
Now I admit there is little good science here and the results are open
to discussion, but it was interesting.

Now what you have to do is cut off one of the blades so you only have
one. Make the depth so that when the blade is at 0 it's just below the
surface. Then put it at the 45 degree angle and move it down and see if
there is a noticable splash up and to the right, anywhere near as
noticable as when you move it from 315 to 360.

Steve

Steven Shelikoff wrote:

It doesn't matter how fast or slow the prop is turning. Even if it's
barely moving you could still be seeing a "push" up from the blade right
behind the one that's going down if it's close enough behind it. Oh,
and I've never see the prop on my sailboat boat turn. Many others, like
my outboard and quite a few big ships (although I haven't spent hours
studying it. but not the sailboat. The only way to see it is if I was
in the water next to it. And that's not gonna happen.

I'll comment on the above, one final time.
I have conceded, and I still concede, that due to the nearness of the
next blade in rotation that one could be fooled into thinking what I was
watching was from that following blade.
HOWEVER, I have taken this into consideration, I have closely analyzed
the motion, I have closely watched the particular blade (the one
starting at 000*), I have noted how far back the next blade was in a
3,4,5 bladed configuration, I have been quite able to differentiate
between the two and the reactions from the two.
What I am talking about has nothing to do with that blade which is
following. It has EVERYTHING to do with the blade which starts at 000*.
If what I was seeing was from the proceeding blade, it would be
occurring behind the blade in question.
G In final answer to your point, NO, what I am discussing is not from
the next blade.


Poorly posed question. (Consider a 1' dia prop with the hub submerged 2'
and we will stick with 270-090/090-270) Name a portion of the arc (in
degrees) between 270-000 where the overall efficiency of the blade
matches or exceeds the overall efficiency of the blade between 090-180,


I don't think there is any where it's greater. There are places where
it's very close, like around the end of that arc for a deep prop.

At no point between 270-000 does the blade meet or exceed the efficiency
of the blade between 090-180, because it is impacting two mediums (air
and water-whether it is just submerged or down to 3') At ALL times the
efficiency of the blade from 090-180 exceeds the efficiency of the
opposite blade moving from 270-000, because it is only impacting water.


then give it as a percentage of the total arc between 270-000/090-180


100%

gwrong


(90*). Then do the same for 000-090 versus 180-270


For that configuration, all of it except for the first 7 degrees or so.
92%. But now you have to look at the imbalance of forces for that 7
degree difference to see how significant it is. Yes, I agree there's an
imbalance. Always have. It's just not as signficant as you're making
it out to be. Yes, it seems it could be a contributor to prop walk.
But not the only one and maybe not the most significant one on any
particular boat.

We can't agree. At 000-045 the efficiency starts low but increases,
whereas from 180-225, the efficiency starts at maximum but decreases.
Overall I have to give the portion 180-245, the nod as to being "more"
efficient (especially in the propwalk component) for the simple reason
of the 000-045 arc's closer proximity to working against the air/water
medium.
The 045-090/225-270 comparison is a wash, overall (I tend towards
045-090 being more efficient because 045-090 is against one medium),
but, combine the two as originally stated, I have to say 180-270, is
marginally more efficient than 000-090, because of the blades closer
proximity to air, at 000-090.
Please note, that in all cases, I have stated, that the greater overall
efficiency of the blades is occurring when the "side" force which cause
propwalk, is a portion of that arc (with the exception of 045-090).

As stated, we can't agree.

Of course, all that assumes that there really is an efficiency
difference on a blade going up than a blade going down. So far, that's
just a "feeling" you have and hasn't been shown to be true yet.

Hmmm, I thought you were the one who stated that the downward rotation
pushed up more so than the upward rotation pushed down, causing a list.
Since a blade going up is for the most part impacting against two
mediums, whereas a blade going down, for the most part is impacting
against on the one medium of water, then yes eg the up is, overall,
less efficient than the down, overall.

Looking at it another way, the loss of efficiency is due to the blade
doing some other work that does not involve moving the boat forward or
reverse. When the blade is moving up, the work is creating a pressure
wave that causes a bulge at the surface. When the blade is moving down,
the work is creating a pressure wave that causes an indent on the
bottom.

Not if the bottom is solid granite. Even if it's soft mud, if the water
depth was greater than the water column above the prop, impact, would
happen first, above.

Same loss of efficiency either way.

NO
So I'd love to see
something that shows the mere act of creating a bulge at the air/water
interface causes a greater loss of efficiency.

G Sorry, I don't have a formal Doctorates in propellor sciences, just
an informal one in Marine Sciences.



As for the rest, I see two main points:

1. Please explain how you can narrow down overall net sideways force to
a mere 8-9 degrees ( I would consider it closer to 135* +/-).


I already have in a previous post. But basically it has to do how the
forces balance for the entire rotation. Also, one thing that's true
which you haven't addressed which makes the difference much smaller than
you believe is that the "leakage" which causes you to think there's less
efficiency from 0 to 45 degrees also would occur (if it occurs at all)
from 135-180 degrees or at least a major portion of that. This *mostly*
balances out the loss of efficiency from 0 to 45 in the opposite
direction.

Absolutely, positively, incorrect. There can be NO "leakage" (except in
minute quantities, only measurable in and during lab conditions, not of
any significance related to this discussion) between 135-180 because the
blade is pushing down into only the ONE medium, on all sides ... i.e.,
even if there were, it would in no way equal or even begin to approach
the amount experienced between 000-045ish.
Sorry, no balancing out.


2. If we consider a prop (in this case, for this question, just
submerged) to be less efficient between 270-090 than it is between
090-270, how can dropping that prop (the prop is 1'dia so hub would

be @6" underwater) 2' 6" to 3', make all that much noticeable change
in the overall efficiency and arcs of efficiency, considering the
horsepower, turning it. i.e., do I think that inefficient side may
have gained some efficiency ? .... yes. Do I think I could perceive
the difference ?.... unlikely, in most cases. (9,999.99 out of 10,000
-couldn't G).


It's geometry. That's why the formula I gave before is important to
show you the effect of changing the prop/depth ratio.

It may be geometry, but to fully see it you would have to calculate it
for many, many points along the blade at many, many angles of rotation.
Hell, take the "U" (not "V" we only want direct line not "leakage") and
draw 20 vertical lines along the blade, and rotate it.... you'll see the
result more clearly and much more quickly.


As I was starting this, I remembered I had one of those long paint
stirring thingies (propellor on a shaft) for my drill. Out to the pool!
Started just beneath the surface .... helluva splash.
Then lowered it as deep as possible (barely avoiding electrocution).
Interesting. At first, no noticeable movement, but within 2-3 sec. I
could see a moving "bulge" on the surface. To be sure I wasn't being
fooled, I tried it a number of times (whilst listening to my wife
shouting in the background about her possible need to dial 911, shortly)
with the same results.
Now I admit there is little good science here and the results are open
to discussion, but it was interesting.


Now what you have to do is cut off one of the blades so you only have
one. Make the depth so that when the blade is at 0 it's just below the
surface. Then put it at the 45 degree angle and move it down and see if
there is a noticable splash up and to the right, anywhere near as
noticable as when you move it from 315 to 360.

Steve

LOL Blades way too small to give a good visual picture.... you really
need a big ship prop to see this.

otn

On Mon, 14 Jun 2004 00:17:23 GMT, otnmbrd wrote:



Steven Shelikoff wrote:

It doesn't matter how fast or slow the prop is turning. Even if it's
barely moving you could still be seeing a "push" up from the blade right
behind the one that's going down if it's close enough behind it. Oh,
and I've never see the prop on my sailboat boat turn. Many others, like
my outboard and quite a few big ships (although I haven't spent hours
studying it. but not the sailboat. The only way to see it is if I was
in the water next to it. And that's not gonna happen.

I'll comment on the above, one final time.
I have conceded, and I still concede, that due to the nearness of the
next blade in rotation that one could be fooled into thinking what I was
watching was from that following blade.
HOWEVER, I have taken this into consideration, I have closely analyzed
the motion, I have closely watched the particular blade (the one
starting at 000*), I have noted how far back the next blade was in a
3,4,5 bladed configuration, I have been quite able to differentiate
between the two and the reactions from the two.
What I am talking about has nothing to do with that blade which is
following. It has EVERYTHING to do with the blade which starts at 000*.
If what I was seeing was from the proceeding blade, it would be
occurring behind the blade in question.

See, that's the problem. If what you're seeing is off at an angle (like
it must be if the effect is at the surface and the prop is beneath the
surface) the effect would not necessarily be behind the blade in
question to be an effect from the blade behind it. If you believe that
must be true, then I can see how you may be fooled.

For instance, say we have a 5 bladed 5 foot prop (use a big one since
that's what you've described watching) where the center is 5 feet deep
and the blade of interest is at 45 degrees so the blade behind it will
be at 333 degrees. The tip of the blade of interest is off to the right
and is around 1.7 feet to the right of the shaft and you see a "splash"
a little to the right even further than that so you think it's from the
blade at 45. But now look at what the following blade is doing. It's
pushing water at a 63 degree angle which, if you follow the water column
being pushed by that blade, will cause a "bulge" and splash at the
surface around 3 feet to the right of the shaft. That's just in *front*
of the blade you're watching. I.e., you're confusing the effect from
one blade with another.

Even more interesting, consider that same ship above but now the blade
you're watching is a 0 degrees. The blade behind it is at 288 degrees
and the column of water it's pushing is at 18 degrees. The "splash"
from the blade behind the one at 0 degrees will be a little to the
*right* of the blade at 0, which can easily confuse you into thinking
that it's the blade at 0 degrees causing the splash.

G In final answer to your point, NO, what I am discussing is not from
the next blade.

I know you don't think it is. But that doesn't mean it really isn't,
especially in the case where the blades are closly spaced.

Poorly posed question. (Consider a 1' dia prop with the hub submerged 2'
and we will stick with 270-090/090-270) Name a portion of the arc (in
degrees) between 270-000 where the overall efficiency of the blade
matches or exceeds the overall efficiency of the blade between 090-180,


I don't think there is any where it's greater. There are places where
it's very close, like around the end of that arc for a deep prop.

At no point between 270-000 does the blade meet or exceed the efficiency
of the blade between 090-180, because it is impacting two mediums (air

Um, no. That's just not true. The blade is *always* impacting two
mediums. Just in one case the other medium is air and in the other case
it's not. But I'll allow that gaff and the other gaff that the arc
really doesn't start at 270 (the greater the ratio of prop diameter to
depth the further past 270 the arc you're thinking of starts) and agree
that at no point between a little past 270 to 0 does the blade exceed
(it does meet, but not exceed) the efficiency of the blade between
90-180.

and water-whether it is just submerged or down to 3') At ALL times the
efficiency of the blade from 090-180 exceeds the efficiency of the
opposite blade moving from 270-000, because it is only impacting water.

That's not quite true either. See above. But for the most part, it's
correct assuming you're theory about efficiency is correct..

then give it as a percentage of the total arc between 270-000/090-180


100%

gwrong

Lol, you're disagreeing with yourself, and so am I. Just above you say
it's 100% and when I agree with that you say wrong. Then I go and show
that it's not 100% just after I agreed that it was. Time for a break.

(90*). Then do the same for 000-090 versus 180-270


For that configuration, all of it except for the first 7 degrees or so.
92%. But now you have to look at the imbalance of forces for that 7
degree difference to see how significant it is. Yes, I agree there's an
imbalance. Always have. It's just not as signficant as you're making
it out to be. Yes, it seems it could be a contributor to prop walk.
But not the only one and maybe not the most significant one on any
particular boat.

We can't agree. At 000-045 the efficiency starts low but increases,

I agree with that. But I think it increases very rapidly and you think
it doesn't. And I think the reason you think it doesn't is because
you're not taking into account the effects from the blade behind the one
you're watching when you watch the prop turn.

whereas from 180-225, the efficiency starts at maximum but decreases.

I agree with that too.

Overall I have to give the portion 180-245, the nod as to being "more"

I think you mean 180-225 since that's 45 degrees.

efficient (especially in the propwalk component) for the simple reason
of the 000-045 arc's closer proximity to working against the air/water
medium.

Lol. Now you're disagreeing with your own theory. You're saying above
that the blade pushing against the air/water interface for 45 degrees of
it's rotation is more efficient than the one pushing down against only
water. You might need to take a step back and think a little more about
it. Maybe revise your theories some so they agree with these further
thoughts.

The 045-090/225-270 comparison is a wash, overall (I tend towards
045-090 being more efficient because 045-090 is against one medium),

Wow, that also goes against your theory since 45-90 is pushing against
only water the entire way and is at it's most efficient since it's past
the point where you think "leakage" is robbing it of efficiency. Yet
the blade from 225-270 is just around the absolute minimum efficiency
since at 270 (well, just a little past 270) the water column to the
surface is as small as it's going to get for the entire rotation.
Again, you may want to revise your theory since it doesn't agree with
these thoughts/observations.

but, combine the two as originally stated, I have to say 180-270, is
marginally more efficient than 000-090, because of the blades closer
proximity to air, at 000-090.
Please note, that in all cases, I have stated, that the greater overall
efficiency of the blades is occurring when the "side" force which cause
propwalk, is a portion of that arc (with the exception of 045-090).

As stated, we can't agree.

I agree. But I think one reason is that in defense of your theory,
now you're stating things that go against your own theory. It's much
harder to hit a moving bullseye.

Of course, all that assumes that there really is an efficiency
difference on a blade going up than a blade going down. So far, that's
just a "feeling" you have and hasn't been shown to be true yet.

Hmmm, I thought you were the one who stated that the downward rotation
pushed up more so than the upward rotation pushed down, causing a list.

Um, no. My original statement regarding list was due to the torque
effect. But I did say that if your theory is correct it would cause
list.

Since a blade going up is for the most part impacting against two
mediums, whereas a blade going down, for the most part is impacting
against on the one medium of water, then yes eg the up is, overall,
less efficient than the down, overall.

Yes, if a blade is less efficienty when it's pushing up against water
and air than when it's pushing down against water and the bottom then
there's a net up force from the prop and it's off center, which would
cause a list. One thing though, the imbalance of forces (due to your
air/water interface theory) in the up/down direction would be MUCH
greater than the imbalance of forces in the sideways direction.

Looking at it another way, the loss of efficiency is due to the blade
doing some other work that does not involve moving the boat forward or
reverse. When the blade is moving up, the work is creating a pressure
wave that causes a bulge at the surface. When the blade is moving down,
the work is creating a pressure wave that causes an indent on the
bottom.

Not if the bottom is solid granite. Even if it's soft mud, if the water

The only place I've ever seen a solid granite bottom is the pond in an
old quarry. Practically anywhere else you'll have stuff on the bottom.
Sediment in some form or another.

depth was greater than the water column above the prop, impact, would
happen first, above.

How deep are we talking here? The pressure wave travels at about 1 mile
per second in water. It doesn't really matter though since we're
talking about a continuous train of pulses, not a single pulse. So even
if the water is a mile deep, the pulse down from a second ago would
balance out the current one up.

Same loss of efficiency either way.

NO
So I'd love to see
something that shows the mere act of creating a bulge at the air/water
interface causes a greater loss of efficiency.

G Sorry, I don't have a formal Doctorates in propellor sciences, just
an informal one in Marine Sciences.

You don't need a doctorate in propellor sciences. All you need to do is
find information from someone else who is.

As for the rest, I see two main points:

1. Please explain how you can narrow down overall net sideways force to
a mere 8-9 degrees ( I would consider it closer to 135* +/-).


I already have in a previous post. But basically it has to do how the
forces balance for the entire rotation. Also, one thing that's true
which you haven't addressed which makes the difference much smaller than
you believe is that the "leakage" which causes you to think there's less
efficiency from 0 to 45 degrees also would occur (if it occurs at all)
from 135-180 degrees or at least a major portion of that. This *mostly*
balances out the loss of efficiency from 0 to 45 in the opposite
direction.

Absolutely, positively, incorrect. There can be NO "leakage" (except in
minute quantities, only measurable in and during lab conditions, not of
any significance related to this discussion) between 135-180 because the
blade is pushing down into only the ONE medium, on all sides ... i.e.,
even if there were, it would in no way equal or even begin to approach
the amount experienced between 000-045ish.
Sorry, no balancing out.

A little inconsistent here because the blade at 45 degrees is pushing
down too, but I'll let it slide.

2. If we consider a prop (in this case, for this question, just
submerged) to be less efficient between 270-090 than it is between
090-270, how can dropping that prop (the prop is 1'dia so hub would

be @6" underwater) 2' 6" to 3', make all that much noticeable change
in the overall efficiency and arcs of efficiency, considering the
horsepower, turning it. i.e., do I think that inefficient side may
have gained some efficiency ? .... yes. Do I think I could perceive
the difference ?.... unlikely, in most cases. (9,999.99 out of 10,000
-couldn't G).


It's geometry. That's why the formula I gave before is important to
show you the effect of changing the prop/depth ratio.

It may be geometry, but to fully see it you would have to calculate it
for many, many points along the blade at many, many angles of rotation.
Hell, take the "U" (not "V" we only want direct line not "leakage") and
draw 20 vertical lines along the blade, and rotate it.... you'll see the
result more clearly and much more quickly.

You can do that and the results would have to come out the same as the
geometric formula or else something's wrong. I.e., if you do what you
said above with your U and put it on a paper blade and rotate the blade,
you'd have to see the effects of the depth/diameter ratio or you're not
doing it correctly.

As I was starting this, I remembered I had one of those long paint
stirring thingies (propellor on a shaft) for my drill. Out to the pool!
Started just beneath the surface .... helluva splash.
Then lowered it as deep as possible (barely avoiding electrocution).
Interesting. At first, no noticeable movement, but within 2-3 sec. I
could see a moving "bulge" on the surface. To be sure I wasn't being
fooled, I tried it a number of times (whilst listening to my wife
shouting in the background about her possible need to dial 911, shortly)
with the same results.
Now I admit there is little good science here and the results are open
to discussion, but it was interesting.


Now what you have to do is cut off one of the blades so you only have
one. Make the depth so that when the blade is at 0 it's just below the
surface. Then put it at the 45 degree angle and move it down and see if
there is a noticable splash up and to the right, anywhere near as
noticable as when you move it from 315 to 360.

LOL Blades way too small to give a good visual picture.... you really
need a big ship prop to see this.

It's shouldn't be. These things scale very well. That's why models in
tanks are pretty good at predicting what the full scale thing will do.

Steve

Steven Shelikoff wrote:
On Mon, 14 Jun 2004 00:17:23 GMT, otnmbrd wrote:

G In final answer to your point, NO, what I am discussing is not from
the next blade.


I know you don't think it is. But that doesn't mean it really isn't,
especially in the case where the blades are closly spaced.

LOL OK, take a 25' diameter 3-bladed prop one blade is at 000, the next
blade in rotation is at 240. Please explain how what I am seeing could
be coming from blade #2 for initial rotation.
Sorry Steve, all interesting and potentially valid points, but not
applicable to what I'm saying.



Poorly posed question. (Consider a 1' dia prop with the hub submerged 2'
and we will stick with 270-090/090-270) Name a portion of the arc (in
degrees) between 270-000 where the overall efficiency of the blade
matches or exceeds the overall efficiency of the blade between 090-180,


I don't think there is any where it's greater. There are places where
it's very close, like around the end of that arc for a deep prop.

At no point between 270-000 does the blade meet or exceed the efficiency
of the blade between 090-180, because it is impacting two mediums (air


Um, no. That's just not true. The blade is *always* impacting two
mediums. Just in one case the other medium is air and in the other case
it's not.

Very true. If the second medium you are referring to is the bottom mud,
clay, gravel, etc. then, fine, we have a second medium, but there's no
way you'll convince me that mud, clay, gravel, etc. has any where near
the degree of compressibility that air has.

But I'll allow that gaff and the other gaff that the arc
really doesn't start at 270 (the greater the ratio of prop diameter to
depth the further past 270 the arc you're thinking of starts) and agree
that at no point between a little past 270 to 0 does the blade exceed
(it does meet, but not exceed) the efficiency of the blade between
90-180.

We disagree about the part in parenthesis ... it never meets.


and water-whether it is just submerged or down to 3') At ALL times the
efficiency of the blade from 090-180 exceeds the efficiency of the
opposite blade moving from 270-000, because it is only impacting water.


That's not quite true either. See above. But for the most part, it's
correct assuming you're theory about efficiency is correct..

G completely true - see above


then give it as a percentage of the total arc between 270-000/090-180


100%

gwrong


Lol, you're disagreeing with yourself, and so am I. Just above you say
it's 100% and when I agree with that you say wrong. Then I go and show
that it's not 100% just after I agreed that it was. Time for a break.

G Depends on how you apply the answer (100%). I applied it, as you
saying that overall efficiency of the blade between 270-000 matched or
exceeded the efficiency of the blade between 090-180, 100% of the time
..... room for misinterpretation here.


We can't agree. At 000-045 the efficiency starts low but increases,


I agree with that. But I think it increases very rapidly and you think
it doesn't.

If I compare it to 180-225, no matter how fast, or not so fast, the
overall efficiency of 180-225 still exceeds it.

And I think the reason you think it doesn't is because
you're not taking into account the effects from the blade behind the one
you're watching when you watch the prop turn.

Nope, see above.


Overall I have to give the portion 180-245, the nod as to being "more"


I think you mean 180-225 since that's 45 degrees.

Oops.


efficient (especially in the propwalk component) for the simple reason
of the 000-045 arc's closer proximity to working against the air/water
medium.


Lol. Now you're disagreeing with your own theory. You're saying above
that the blade pushing against the air/water interface for 45 degrees of
it's rotation is more efficient than the one pushing down against only
water. You might need to take a step back and think a little more about
it. Maybe revise your theories some so they agree with these further
thoughts.

Nope. The blade from 000-045 is always in "bad" water ( close or closer
to air/water interface ..eg including "bleed off"). Whereas the blade
from 180-225 starts out in "great" water and goes to only not so great
water ( always a greater relative distance from air water interface than
it's counterpart.) Overall, greater efficiency 180-225, than 000-045.


The 045-090/225-270 comparison is a wash, overall (I tend towards
045-090 being more efficient because 045-090 is against one medium),


Wow, that also goes against your theory since 45-90 is pushing against
only water the entire way and is at it's most efficient since it's past
the point where you think "leakage" is robbing it of efficiency. Yet
the blade from 225-270 is just around the absolute minimum efficiency
since at 270 (well, just a little past 270) the water column to the
surface is as small as it's going to get for the entire rotation.
Again, you may want to revise your theory since it doesn't agree with
these thoughts/observations.

Nope again. As stated, I tend towards 045-090 being more efficient, but
I'm calling it a wash because I'm not sure how the forces balance out
overall and I figure waddahey, there ain't much left/right component
during those arcs anyhoo.


Yes, if a blade is less efficienty when it's pushing up against water
and air than when it's pushing down against water and the bottom then
there's a net up force from the prop and it's off center, which would
cause a list. One thing though, the imbalance of forces (due to your
air/water interface theory) in the up/down direction would be MUCH
greater than the imbalance of forces in the sideways direction.

As per usual, I don't fully agree. If I called the up/down 100%, at the
least, I'd call the left/right 75% ... i.e., they're closer than YOU think.

The only place I've ever seen a solid granite bottom is the pond in an
old quarry. Practically anywhere else you'll have stuff on the bottom.
Sediment in some form or another.

And nowhere does it's compressibility compare to air, which makes this
train of thought not worth pursuing, no matter the water 5' or 5 miles deep.

Absolutely, positively, incorrect. There can be NO "leakage" (except in
minute quantities, only measurable in and during lab conditions, not of
any significance related to this discussion) between 135-180 because the
blade is pushing down into only the ONE medium, on all sides ... i.e.,
even if there were, it would in no way equal or even begin to approach
the amount experienced between 000-045ish.
Sorry, no balancing out.


A little inconsistent here because the blade at 45 degrees is pushing
down too, but I'll let it slide.

No inconsistency The blade between 000-045 is impacting air/water still.
The blade between 135-180 is impacting water only .... are you sure
you meant to compare 000-045 to 135-180?


You can do that and the results would have to come out the same as the
geometric formula or else something's wrong. I.e., if you do what you
said above with your U and put it on a paper blade and rotate the blade,
you'd have to see the effects of the depth/diameter ratio or you're not
doing it correctly.

You don't put the "U" on the blade. The "U" is drawn on tracing paper,
it's curved portion being 1/2 of the full circle of rotation of the
blade. where the circle of the "U" meets the vertical sides of the "U",
draw a line, perpendicular to the sides across the diameter of the
circular portion of the "U", and at this lines midpoint (center of the
circular portion of the "U" push a pin through as you overlay it on the
full circle of the props rotation and also push the pin through the
center of the circle. Now, from the center pushpin along that line to
the left (looking down) draw about twenty vertical lines which stop at
the water surface just above the circle of rotation, then turn the "U"
trace and watch what happens along all the lines, regarding water column.
At the tip of the blade the water column always decreases, but at the
center it always increases .... what happens in between, varies.


It's shouldn't be. These things scale very well. That's why models in
tanks are pretty good at predicting what the full scale thing will do.

Steve

A night of disagreeing. Things may scale fairly well, but the size makes
it difficult to pick up the details with the "naked eye".

Gotta go play on the water for the night .....

otn

On Tue, 15 Jun 2004 03:29:30 GMT, otnmbrd wrote:



Steven Shelikoff wrote:
On Mon, 14 Jun 2004 00:17:23 GMT, otnmbrd wrote:

G In final answer to your point, NO, what I am discussing is not from
the next blade.


I know you don't think it is. But that doesn't mean it really isn't,
especially in the case where the blades are closly spaced.

LOL OK, take a 25' diameter 3-bladed prop one blade is at 000, the next
blade in rotation is at 240. Please explain how what I am seeing could
be coming from blade #2 for initial rotation.
Sorry Steve, all interesting and potentially valid points, but not
applicable to what I'm saying.

A 3 blade? Maybe, maybe not. How wide are the blades? I.e., how much
angular space does each blade take up?

Poorly posed question. (Consider a 1' dia prop with the hub submerged 2'
and we will stick with 270-090/090-270) Name a portion of the arc (in
degrees) between 270-000 where the overall efficiency of the blade
matches or exceeds the overall efficiency of the blade between 090-180,


I don't think there is any where it's greater. There are places where
it's very close, like around the end of that arc for a deep prop.

At no point between 270-000 does the blade meet or exceed the efficiency
of the blade between 090-180, because it is impacting two mediums (air


Um, no. That's just not true. The blade is *always* impacting two
mediums. Just in one case the other medium is air and in the other case
it's not.

Very true. If the second medium you are referring to is the bottom mud,
clay, gravel, etc. then, fine, we have a second medium, but there's no
way you'll convince me that mud, clay, gravel, etc. has any where near
the degree of compressibility that air has.

I would try to. But it is compressible which takes some efficiency
away.

[...]

We can't agree. At 000-045 the efficiency starts low but increases,


I agree with that. But I think it increases very rapidly and you think
it doesn't.

If I compare it to 180-225, no matter how fast, or not so fast, the
overall efficiency of 180-225 still exceeds it.

I don't believe it. I think the blade at 45 degrees pressing down on
water and the bottom would be much more efficient than the opposing side
at 225 degrees pressing up against air, assuming your theory is correct.

And I think the reason you think it doesn't is because
you're not taking into account the effects from the blade behind the one
you're watching when you watch the prop turn.

Nope, see above.

Yup, see the part you snipped.

efficient (especially in the propwalk component) for the simple reason
of the 000-045 arc's closer proximity to working against the air/water
medium.


Lol. Now you're disagreeing with your own theory. You're saying above
that the blade pushing against the air/water interface for 45 degrees of
it's rotation is more efficient than the one pushing down against only
water. You might need to take a step back and think a little more about
it. Maybe revise your theories some so they agree with these further
thoughts.

Nope. The blade from 000-045 is always in "bad" water ( close or closer
to air/water interface ..eg including "bleed off"). Whereas the blade

The only reason it would make a difference being close to the surface is
in the situation where the top of the prop has lower water pressure and
would cavitate when the bottom of the prop doesn't. That's a tiny
envelope of operation. Try an experiment. Take the lid of a trashcan
and put it 1 foot below the surface of a pool and try and push it
straight down to 2 feet. Then move it to 3 feet deep and try and push
it straight down to 4 feet. I don't think you'll notice any difference
in how hard it is to press it down at those different depths even though
the 1 foot depth is closer to the "bad" water at the air/water interface
and "bleed off" from the side of the lid doesn't have as far to go to
get to the surface.

from 180-225 starts out in "great" water and goes to only not so great
water ( always a greater relative distance from air water interface than
it's counterpart.) Overall, greater efficiency 180-225, than 000-045.

Again, it doesn't matter how "great" the water is as long as it's not
cavitating. The fact that at 225 is pressing up against the air/water
interface is all that matters and it's efficiency is well less than at
45 degrees when the blade is pressing down..

Try another experiment with the trashcan lid in the pool. Put it at 1
foot deep and push it straight down to 2 feet. Then turn it over and
put it at 3 feet and push it straight up to 2 feet. I think you'll find
that if your theory is correct it's probably easier to do that since
it's bulging the water at the surface above it. But you'll also notice
that it's *much* more important what direction the blade is moving then
whether it's closer or further from the surface, as long as there's not
cavitation.

The 045-090/225-270 comparison is a wash, overall (I tend towards
045-090 being more efficient because 045-090 is against one medium),


Wow, that also goes against your theory since 45-90 is pushing against
only water the entire way and is at it's most efficient since it's past
the point where you think "leakage" is robbing it of efficiency. Yet
the blade from 225-270 is just around the absolute minimum efficiency
since at 270 (well, just a little past 270) the water column to the
surface is as small as it's going to get for the entire rotation.
Again, you may want to revise your theory since it doesn't agree with
these thoughts/observations.

Nope again. As stated, I tend towards 045-090 being more efficient, but

Again, if so, that goes against your theory whether you recognize it or
not.

I'm calling it a wash because I'm not sure how the forces balance out
overall and I figure waddahey, there ain't much left/right component
during those arcs anyhoo.

It all adds up.

Yes, if a blade is less efficienty when it's pushing up against water
and air than when it's pushing down against water and the bottom then
there's a net up force from the prop and it's off center, which would
cause a list. One thing though, the imbalance of forces (due to your
air/water interface theory) in the up/down direction would be MUCH
greater than the imbalance of forces in the sideways direction.

As per usual, I don't fully agree. If I called the up/down 100%, at the
least, I'd call the left/right 75% ... i.e., they're closer than YOU think.

And I think that if the/up down is 100% then the left/right is no more
than 25%, probably less, for the simple reason that from the prop to the
surface is in the up/down direction so you have by far the greatest
effect on efficiency in that direction.

The only place I've ever seen a solid granite bottom is the pond in an
old quarry. Practically anywhere else you'll have stuff on the bottom.
Sediment in some form or another.

And nowhere does it's compressibility compare to air, which makes this
train of thought not worth pursuing, no matter the water 5' or 5 miles deep.

Sure it is. You must have never walked on a bottom that was several
feet of loose mud. It's very easily compressed and bulged by the
practically non-compressible water pressure wave.

Absolutely, positively, incorrect. There can be NO "leakage" (except in
minute quantities, only measurable in and during lab conditions, not of
any significance related to this discussion) between 135-180 because the
blade is pushing down into only the ONE medium, on all sides ... i.e.,
even if there were, it would in no way equal or even begin to approach
the amount experienced between 000-045ish.
Sorry, no balancing out.


A little inconsistent here because the blade at 45 degrees is pushing
down too, but I'll let it slide.

No inconsistency The blade between 000-045 is impacting air/water still.
The blade between 135-180 is impacting water only .... are you sure
you meant to compare 000-045 to 135-180?

Yes. If you draw your "U" or "V" or whatever so that the "leakage" is
coming off the blade equally on both sides of the blade you'll see that
0-45 is very close to the leakage of 135-180 as long as the prop
diameter/depth ratio is small. For a large shallow prop they don't
balance out as much.

You can do that and the results would have to come out the same as the
geometric formula or else something's wrong. I.e., if you do what you
said above with your U and put it on a paper blade and rotate the blade,
you'd have to see the effects of the depth/diameter ratio or you're not
doing it correctly.

You don't put the "U" on the blade. The "U" is drawn on tracing paper,
it's curved portion being 1/2 of the full circle of rotation of the
blade. where the circle of the "U" meets the vertical sides of the "U",
draw a line, perpendicular to the sides across the diameter of the
circular portion of the "U", and at this lines midpoint (center of the
circular portion of the "U" push a pin through as you overlay it on the
full circle of the props rotation and also push the pin through the
center of the circle. Now, from the center pushpin along that line to
the left (looking down) draw about twenty vertical lines which stop at
the water surface just above the circle of rotation, then turn the "U"
trace and watch what happens along all the lines, regarding water column.
At the tip of the blade the water column always decreases, but at the
center it always increases .... what happens in between, varies.

Wow, hard to follow how to draw that but I think I've got it. However,
if it's drawn the way you've described above it doesn't really show you
what's going on. When you have to do is draw the prop blade and from
somewhere near the center of the blade put your "U" perpendicular to it
and just touching it. Or better yet a "V" where the point of the V is
just touching the blade near the center and the bisecting angle of the V
is perpendicular to the blade and the width of the V is how wide you
think the prop wash is off the sides of the blade. Then rotate that
around and watch what happens.

It's shouldn't be. These things scale very well. That's why models in
tanks are pretty good at predicting what the full scale thing will do.

A night of disagreeing. Things may scale fairly well, but the size makes
it difficult to pick up the details with the "naked eye".

Seems to be difficult for a big prop too when you can't separate the
blade interaction.

Gotta go play on the water for the night .....

Have fun.

Steve

Steven Shelikoff wrote:


A 3 blade? Maybe, maybe not. How wide are the blades? I.e., how much
angular space does each blade take up?

LOL I think we're at the stage when we can anticipate each other too well.
At any rate, doesn't matter. The initial direction the blade will push
the water, is up and to port, not up and to stbd. G You will find a
reason to hold your point of view, as valid, I consider everything
you've said to date, regarding the basic premise of what I'm seeing, as
invalid.


Very true. If the second medium you are referring to is the bottom mud,
clay, gravel, etc. then, fine, we have a second medium, but there's no
way you'll convince me that mud, clay, gravel, etc. has any where near
the degree of compressibility that air has.


I would try to. But it is compressible which takes some efficiency
away.

G Waste of time. If we go by your numbers and we are sitting in water
5 mi. deep, with 6" of soft mud then solid clay with a prop centered at
3' underwater, the down thrust would take 5 sec to hit the silt, then
solid clay, whereas the up thrust would take 0.00049 sec to hit air
...... not worthy of discussion, only mention, in passing.

[...]


We can't agree. At 000-045 the efficiency starts low but increases,


I agree with that. But I think it increases very rapidly and you think
it doesn't.

If I compare it to 180-225, no matter how fast, or not so fast, the
overall efficiency of 180-225 still exceeds it.


I don't believe it. I think the blade at 45 degrees pressing down on
water and the bottom would be much more efficient than the opposing side
at 225 degrees pressing up against air, assuming your theory is correct.

So would I. However, we are not talking two specific degree points, we
are discussing two overall arcs. 000-045/180-225. Again, comment worth
noting, but not valid for the overall discussion.


And I think the reason you think it doesn't is because

you're not taking into account the effects from the blade behind the one
you're watching when you watch the prop turn.

Nope, see above.


Yup, see the part you snipped.

I snipped it because it didn't apply. G

The only reason it would make a difference being close to the surface is
in the situation where the top of the prop has lower water pressure and
would cavitate when the bottom of the prop doesn't. That's a tiny
envelope of operation. Try an experiment. Take the lid of a trashcan
and put it 1 foot below the surface of a pool and try and push it
straight down to 2 feet. Then move it to 3 feet deep and try and push
it straight down to 4 feet. I don't think you'll notice any difference
in how hard it is to press it down at those different depths even though
the 1 foot depth is closer to the "bad" water at the air/water interface
and "bleed off" from the side of the lid doesn't have as far to go to
get to the surface.

All well and good, problem is we are talking 000-045, not straight
down. Take your lid, and start at 000 and push to 045, noting the
difference as you approach 045.

from 180-225 starts out in "great" water and goes to only not so great

water ( always a greater relative distance from air water interface than
it's counterpart.) Overall, greater efficiency 180-225, than 000-045.


Again, it doesn't matter how "great" the water is as long as it's not
cavitating. The fact that at 225 is pressing up against the air/water
interface is all that matters and it's efficiency is well less than at
45 degrees when the blade is pressing down..

Again, true, but we are not talking the specific 045/225 degree points.

Try another experiment with the trashcan lid in the pool. Put it at 1
foot deep and push it straight down to 2 feet. Then turn it over and
put it at 3 feet and push it straight up to 2 feet. I think you'll find
that if your theory is correct it's probably easier to do that since
it's bulging the water at the surface above it. But you'll also notice
that it's *much* more important what direction the blade is moving then
whether it's closer or further from the surface, as long as there's not
cavitation.

It's proximity to the surface, is what makes it easier.


The 045-090/225-270 comparison is a wash, overall (I tend towards
045-090 being more efficient because 045-090 is against one medium),


Wow, that also goes against your theory since 45-90 is pushing against
only water the entire way and is at it's most efficient since it's past
the point where you think "leakage" is robbing it of efficiency. Yet
the blade from 225-270 is just around the absolute minimum efficiency
since at 270 (well, just a little past 270) the water column to the
surface is as small as it's going to get for the entire rotation.
Again, you may want to revise your theory since it doesn't agree with
these thoughts/observations.

Nope again. As stated, I tend towards 045-090 being more efficient, but


Again, if so, that goes against your theory whether you recognize it or
not.

Not in the least. First off, the area 045-090/225-270 has minimal affect
on propwalk. Secondly, I'm no talking about particular degrees of
rotation, but instead, general arcs of rotation.


I'm calling it a wash because I'm not sure how the forces balance out
overall and I figure waddahey, there ain't much left/right component
during those arcs anyhoo.


It all adds up.

true, but it's overall degree of importance, varies.


Yes, if a blade is less efficienty when it's pushing up against water
and air than when it's pushing down against water and the bottom then
there's a net up force from the prop and it's off center, which would
cause a list. One thing though, the imbalance of forces (due to your
air/water interface theory) in the up/down direction would be MUCH
greater than the imbalance of forces in the sideways direction.

As per usual, I don't fully agree. If I called the up/down 100%, at the
least, I'd call the left/right 75% ... i.e., they're closer than YOU think.


And I think that if the/up down is 100% then the left/right is no more
than 25%, probably less, for the simple reason that from the prop to the
surface is in the up/down direction so you have by far the greatest
effect on efficiency in that direction.

Ok, but for the left/right component, the majority of the time the blade
is pushing water left 091-269, it's in good to not quite as good water,
whereas from 271-089, it bad through 135* of rotation .... nope, 75%.


The only place I've ever seen a solid granite bottom is the pond in an
old quarry. Practically anywhere else you'll have stuff on the bottom.
Sediment in some form or another.

And nowhere does it's compressibility compare to air, which makes this
train of thought not worth pursuing, no matter the water 5' or 5 miles deep.


Sure it is. You must have never walked on a bottom that was several
feet of loose mud. It's very easily compressed and bulged by the
practically non-compressible water pressure wave.

Bulged for how far? 6" ? before solid bottom? .... versus outer
space..... no comparison, not a valid argument.

No inconsistency The blade between 000-045 is impacting air/water still.
The blade between 135-180 is impacting water only .... are you sure
you meant to compare 000-045 to 135-180?


Yes. If you draw your "U" or "V" or whatever so that the "leakage" is
coming off the blade equally on both sides of the blade you'll see that
0-45 is very close to the leakage of 135-180 as long as the prop
diameter/depth ratio is small. For a large shallow prop they don't
balance out as much.

Negative. There is absolutely no comparison, doesn't matter if it's a
"U" or a "V". It's not about depth ratio, it's all about direction of push.

Wow, hard to follow how to draw that but I think I've got it. However,
if it's drawn the way you've described above it doesn't really show you
what's going on.

Sometimes when writing something, what's sounds clear to one is not
clear to others. Let me try again.
On one piece of paper, draw a circle. At 000 on the circle, draw a line
tangent to the circle to represent the water surface.
On a second piece of tracing paper, draw another circle of the same
diameter. Draw a line from 000-180 and 270-090. Then draw a line,
tangent to this circle at the 270 point, equal to the diameter of the
circle.
Between this line and the 000-180 line, draw 20 equally spaced lines,
parallel to the 000-180 line, starting on the 270-090 line and up, to
where they would meet a line drawn tangent to the 000 point. Now overlay
the two circles, and rotate the tracing paper circle.

The water column in front of the 270-090 line, as it rotates, will vary
in length, but not follow what your mathematical numbers stated, overall.


A night of disagreeing. Things may scale fairly well, but the size makes
it difficult to pick up the details with the "naked eye".


Seems to be difficult for a big prop too when you can't separate the
blade interaction.

BG Difference is, I can, you can't.


Gotta go play on the water for the night .....


Have fun.

Steve

Been a long two days, gotta hit the sack.

otn