Steven Shelikoff wrote:

It's absolutely obvious that there's unequal thrust. If not, there
wouldn't be prop walk. The question is, where does it come from? And I
still think the answer is multiple sources all contribute, some more
than others on any given boat. I just don't think the effect you're
describing here plays as much a part on my boat as it might on others
because there's more above the prop than just a column of water and then
air.

Steve

Actually, for the most part we are in agreement.
When you go back through my post on this issue, you will always note
that I am mentioning a number of factors which will enhance and detract
from propwalk (wind, current, hull form, speed, pitch, kort nozzles,
etc.) and the fact that we are not always sure what that reaction will
be, until we experience it on a particular boat under particular conditions.
However, there has to be a "root" cause .... an initial action/reaction
which starts the process.
Can we say that the directional rotation of the prop (right or left) is
by itself the cause? I can't say with certainty that it is or isn't,
but, in my experience handling ships with CP props, when the prop is set
to zero pitch (judged to be zero due to lack of creep ahead or astern) I
generally (note, I'm saying "generally") see no side movement, yet put
in even the most minimal pitch and I will experience "walk" (loaded
condition, prop down deep with hull overhang above, and light condition,
prop close to the surface with hull overhang out of the water .... makes
no difference .... both ahead and astern ... which is why I discount
hull overhang).
What does this lead me to believe. If the rotating prop at zero pitch,
generally eg showed me no "walk" and as soon as I added pitch, I got
"walk" then I look closely at the effects of a rotating prop and it's
pitch for the root cause.
Let's now look at prop efficiency (these are MY views based on what I
see, read and feel). If we follow the rotation of a RH fixed pitch prop,
looking at it from astern starting at top dead center (ooo*) the
particular blade is pushing water to the right (and back ...always back,
but we will ignore that component for this discussion) at minimal
efficiency. this efficiency, however, is increasing as the propellor
turns towards 45* and the direction is changing to a increasingly down
direction.
As the blade reaches 45*, efficiency is close to maximum and from here
the direction is more down than to the side.
Someplace just prior to 90* the efficiency becomes maximum and as the
blade rotates toward 135* the angle of push changes to the left
(pulling/pushing the stern to stbd) the blade continues at maximum
efficiency through 180* (pushing left) but as it begins it's rotation
upwards, that efficiency, slowly begins to drop off and the direction of
push begins to angle upward/left until you reach 270*.
From this point, efficiency drops off at a marked rate and the blade is
pushing up and beginning to push slightly right. As you reach @315* you
are close to being back to minimal efficiency and pushing right/up,
which continues back to 000*.
You can see from this (my visualization) that the force pushing left
(pulling/pushing the stern to the right) occurs during that time when
the blade is pushing most efficiently, whereas the force pushing to the
right (countering that left push) occurs when the blade is pushing with
less efficiency...... propwalk.
I would love someone who designs props and is far more technically
versed in the goings on of a prop beneath the water, to critique this.
Also, so there's no misunderstanding, remember, my feeling about props
efficiency decrease on it's upward rotation is about the prop pushing
water up and into air, losing efficiency. .... and most importantly,
these are my views/visualizations .... G yours may vary.

otn