On Thu, 17 Jun 2004 06:15:25 GMT, otnmbrd wrote:



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

Yup. But even if the blade is a line with no angular width, it will
only take 30 degrees of rotation before it's pushing up and to stbd.
And remember that if it's true that the blade is a line and can only
push water straight up, none to it's left and right, then the same thing
is true for the blade at 0. This theoretical prop you've envisioned
will have 100 percent efficiency from 0 to 180.

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.

I realize that. It's probably because you actually believe you can see
the actions of individual blades and that other very nearby blades are
not affecting what you're seeing at all. It obvious I can't convince
you that's not the case, but it's not. If you just thought about it a
little more and tried to visualize exactly what's going on, you wouldn't
trust your eyes.

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.

If it took 5 minutes to hit the silt vs. 1e-100 to hit the air, the
effects should be the same as if they hit simultaneously. If they're
not, then you would notice a difference in prop walk in water, say 15
feet deep vs. 1500 vs. 15,000 feet deep. To tell the truth, I can't say
whether I have noticed that difference or not because I don't think I've
every backed up under power in *very* deep water. But if someone has,
maybe they can tell us whether there was a difference.

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.

Well if that's the case, then the efficiency from 0 to 180 is less than
that from 180 to 360. Again, looking at wide arcs like that is worth
noting but not valid for the overall discussion because it's not until
you start looking at specific angles and adding them all up that you can
realize just how much different the efficiency is between from 0-180 and
180-360.

For example, if you were to say that the efficiency from 0-45 is less
than from 180-225 for the entire arc, I would disagree with that
because according to your water column theory it's not true. But we'll
go with your thoughts for a second. To see how much more efficient
180-225 is than 0-45 you have do have to look at the specific range of
angles where one is more efficient than the other. To do it correctly,
you have to integrate the force over the range of angles where one is
more efficient than the other and subtract that from the result of
integrating the force over the range of angles where the other is more
efficient.

So now, you do agree that at 45, it's more efficient than at 225. How
about 40 vs. 220? How about 35 vs. 215? How about 30 vs. 210? And so
on...

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

I know you think it doesn't apply. That's part of the problem. I.e.,
you think you can separate the visual effects of one blade vs. another.
But you can't when they are closely spaced. Probably the only way you
can do that is with something like a thin-bladed 2 blade prop like that
on a sailboat.

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.

If the top of the lid is, say 1 foot under the water surface and you
moved it in the motion of an arc like what a prop blade will follow, I
think that once typically shaped lid passed, say 5 degrees or so, you
would notice no more loss of efficiency all the way to 180. It wouldn't
get any harder to push from 5 to 45 or anywhere else down to 180.

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.

Why, because it doesn't support your theory? Since you like talking
about arcs, then how about the arc between 35-45 vs. the arc between
215-225. Which one of those is more efficient?

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.

No no no. Because in this case, it's easier to push when it's further
from the surface. The direction is more important. If you want further
proof of that, keep the proximity to the surface the same for both
directions of travel and this time push the lid down as hard as you can
right from the surface down to 1 foot deep with the lid and note how
much work that took to do for the entire trip. Then push it as hard as
you can from 1 foot down right up to the surface and note how much work
that took for the the entire trip. You should find that the direction
going up took less work than down even though the proximity is the same.

Now vary the proximity and keep the direction the same. As long as you
start far enough away from the surface to keep the near surface effect
small (like with a prop more than a foot or two deep) you shouldn't
notice a difference.

I.e., take your trashcan lid, start from 2 feet deep and push it to 3
feet deep. Then push it from 3 feet deep to 4 feet deep. You won't
notice any difference because the direction is the same even though the
proximity to the surface is different.

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.

There is no point from 45-90 that is less efficient than from 225-270.
So obviously the entire arc from 45-90 is more efficient than from
225-270. As for how much it contributes to prop walk, that's easy to
determine. Just integrate cos(x) from 45-90 and compare that to
integrating cos(x) from 0-45.

The answer is sqrt(2)/2 vs. 1-sqrt(2)/2, or approx 0.7 vs. 0.3. 0.3,
while less than 0.7, is not "minimal." It's just over 2:1. I.e., the
first 45 degrees of rotation has a little over twice the impact on
propwalk as the second 45 degrees of rotation. Again, twice is a lot.
But 1/2 is not minimal. Ok, not quite 1/2. But it is:

1-sqrt(2)/2
--------------- x 100% = 41.4%
sqrt(2)/2


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.

It varies by a lot more than you think though. I.e., it's not "minimal"
and must be accounted for. This all goes towards showing you that
what's actually going on is, when you get down and analyze it, a lot
different than what you think is going on by thinking about it as simply
the efficiency going left is different than the efficiency going right.
And while I agree with that general statement, it's a whole lot less of
a difference than you imagine.

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%.

And it's simplistic analysis like that which is leading you astray.

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.

Except for above the prop (which is the effect of the blade coming up
when it's at 270-280 degrees or so, the bulge at the surface is well
less than 6". So it is 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.

Ah, another mistake. It's about both. The depth ratio affects what the
column of water (the U or V in this case) is hitting at certain
directions 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.
[...]

That's ok. I think I've had enough.


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.

No, you just think you can.

Steve