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