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