On Sat, 12 Jun 2004 00:53:46 GMT, otnmbrd wrote:

As a quick aside, I went into work to check on my "Valet parking"
schedule for next week and mentioned this discussion. One individual
that has owned a number of powerdriven boats, both sail and power, asked
me "What's propwalk?" . In all honesty, I was not as surprised at the
question, as were some.

Steven Shelikoff wrote:

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.

I'm going to have to ask you to trust me on this one, Steve. When you
watch a ship propellor start, especially when the are "spinning" a steam
turbine, you can discern the action of an individual blade. Especially
if you sit there long enough and watch/analyze..... i.e. this is not a
casual observation involving one or two times.

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.

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

Easy. 45 (outside the arc above) degrees is more efficient than 225
degrees (inside the arc above).

For instance, consider a 2
bladed prop... the blade of interest at say 60 degrees (it's within
your semicircle of lower efficiency)

Actually, no, it's within my semicircle of "maximum" efficiency.

Not before you changed it below. First it was the top half (270 to 90)
vs. the bottom half (90 to 270). Now you rotated it by 45 degrees. See
below.

is much more efficient than the
blade at 240 degrees for a net left force.

The overall efficiency of the blade is greater at 060* than 240*.
However, in both cases the left/right force is minimal .... well, maybe
not minimal, but equally decreasing. If I split hairs, I would say net

Actually, the left/right force is minimal right at 90 and 270 degrees.
This happens to be just where the efficiency is at a maximum and a
minimum, which makes the difference in left/right force even less a
factor.

force -right. (at 060* prop pushing back,down,right -at 240*,prop
pushing back,up,left, in descending order of percentage).

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.

True as to the efficiency, but if there is any discernible left/right
force still remaining, it will be pushing net right (I just realized
something. When I say "right" in this case, I mean the blade is pushing
the water to the right. It's pushing the hull to the left. I guess we
also need to be sure we are together on this.)

I was talking about what direction the force is. I.e., a right moving
blade is creating a force to the left. Doesn't really matter for the
purposes of this discussion though.

Ah, I see you've now moved your semicircles around a little.

I have, to maintain my true feeling as to where the prop is most
efficient, but I could easily just say 270-090/090-270 to cover the
BASIC premise.

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).

The main point, is overall efficiency during the entire 180* arc. You

And I'm agreeing with you that overall there will be a net sideways
force if you look at the entire rotation. It's just that the net force
is much smaller than you're making it out to be when you say things like
the efficiency of an entire arc in one direction is more or less than
the efficiency in the other direction. It's really only a tiny area of
difference, like 8-9 degrees on each side of the rotation for a 1' prop
3' deep, that contributes to the net force.

will find some points, that may match. However, overall, because of
water column and leakage, although they may not be far apart, my guess
is that 220 may be more efficient that 040 (kinda a toss-up here).

Maybe, maybe not. Again, it all depends on the specific configuration.

Please don't get hung up on specific numbers. By going to
045-225/225-045 I am just trying to give an indication of what I feel is
truer numbers .... they are open to interpretation as well as variation.

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.

Once again, you're trying to hold me to specific angle numbers, to prove
a point .... don't. Look at the overall (at least I've got you comparing
opposites). Sure, the specific 040 may be more efficient than 220, as I
said above, it's a toss up because it's close to the limits of the 180*
arc..... which could also be in error +/-.

I'm not trying to hold you to specific angles. You're the one
mentioning specific angles and I'm just showing how those angles don't
follow the theory you're trying to support.

And I've always been comparing opposites. It's just that up till now
I've been comparing opposites in terms of the horizontal component of
the force produced by the blade because to me, that's the easiest way to
find where the forces don't balance. Above I temporarily switched to
comparing opposing sides of the prop because that seems to be easier for
you to deal with even though it much harder to come up with a correct
answer like that. That's because the horizontal forces don't balance
out that way since there's not 180 degree symmetry in the efficiency of
the blade through it's rotation.

Steve