On Fri, 11 Jun 2004 18:51:27 GMT, otnmbrd wrote:
How many blades does the prop you're watching have? If it's one or two
then maybe you're not being fooled. If it's 3 or more, especially if
it's 4 or more, then you can't separate what you think you may be seeing
one blade doing from what the rest are doing because they are too close
to each other in angle. For instance, if you're watching a 4 blade
prop, what "splashing" you think you're seeing from the blade at TDC
starting to move over and down could just as easily be from the blade at
270 and starting to come up and over.

They're apt to have anywhere from 3-6. This view has occurred over many
years, watching many props, some on diesels some on steam turbine
(sitting on a dock watch the warm up spinning of the prop fwd and astern).
You need to remember, these are props with @ 25' diameters turning,
initially at maybe 15-30 RPM.
G Call me an oddball for standing there watching/studying this, but
then again, I like watch porpoise swim in the bow wave and seeing how
they move with it.

The problem is that you still don't know which blade the splash at any
point is from when there are many blades and the blade spacing is very
close, without much empty space between them. It would be much easier
with something like a 2 bladed sailboat prop.

On to propwalk ....

In reading the last few post, it's obvious we are now just chasing each
other in circles and resolving nothing.
A main issue is prop efficiency at various angles of rotation.
My opinion:
For a boat floating on the surface, when discussing prop efficiency we
must consider that the prop is affecting two mediums -water and air.
The prop is at maximum efficiency when it is affecting water alone and
at less than maximum efficiency when it affects air and water.
Is the depth of the hub of the prop important? To a degree, yes, but for
the boats we are discussing, the effect is constant, it's degree varies.
For instance, hub at surface half of prop out of water (Arneson drive);
blade tip at surface, hub submerged; blade totally submerged..... the
blade is more efficient, overall, in one semicircle, than the other, in
all of these conditions, in my opinion.

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. And
there are large portions within that arc where it's more efficient than
portions within the arc from 90 to 270. For instance, consider a 2
bladed prop... the blade of interest at say 60 degrees (it's within
your semicircle of lower efficiency) is much more efficient than the
blade at 240 degrees for a net left force. But move it a little in your
semicircle and a blade at 280 degrees is less efficient than one at 100
degrees for a net left force.

During the rotation of the blade, it is my opinion that the blade is at
the most maximum efficiency between the angles 045* and 225* of rotation
because it is acting against only the single not compressible medium of
water.
However, between the angles 225* and 045* the blade is operating at LESS
than maximum efficiency because it is acting upon both water AND air.

Ah, I see you've now moved your semicircles around a little. But going
with the above and taking only your theories of column length and
leakage into account, it's pretty hard to believe that a blade at say 40
degrees (within your semicircle of lower efficiency) is less efficient
than the opposing one at 220 degrees (within your semicircle of higher
efficieny). Especially when the first one is pressing down at a fairly
good angle on "solid" water and any "leakage" would have to be off at
more than a 40 degree angle to reach the surface at all while the second
one is pressing up at a 40 degree angle against air. This situation
goes completely counter to your water column theory.

BG Until we resolve this issue between us, we are just butting
heads.....and I'm running low on paper ....

Ayup.

Steve