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