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