I think you missed my point. I wasn't arguing with the basics of the
discussion - just suggesting the Chapman-described reason may not be the
strongest component in this case.

JAXAshby wrote:

dave? not good.

Which part? Seems like we are in agreement more than we differ!


ex-Navy acquaintance that even ships with a horizontal
prop shaft still have prop walk

end-plate effect


But still an existing effect whatever you call it and different than
what was attributed in the mentioned Chapman's discussion?


Chapman's explanation here uses the incline of the prop shaft relative
to the flow of water past the blades.

"asymetrical thrust" it is called, though Chapmans does not use the term


"A rose by any other name"? I didn't argue that if water passes over
the prop at an angle to the shaft (by whatever cause) this effect will
occur. But if water flow is exactly parallel to the shaft (very
unlikely I agree) no SUCH effect would occur. (The other effects that
cause prop walk would then become more significant in this unlikely situation.)



the inclined shaft and the HORIZONTAL flow of water
past the prop

doesn't have to horizontal, just has to different from the axis of the prop.


Again, I agree but I was discussing the suitability of the use of
Chapman's words and he (was quoted in the proceeding discussion to have)
used the term "parallel to the water's surface and to the flow of water
past the blades" (which implies the flow of water is horizontal, because
the shaft could never be parallel to the water's surface and to the flow
of water at the same time if the flow of water he (Chapman) is referring
to here isn't parallel to the water's surface - which is assumed to be
horizontal in most simple cases!)



caused by the (forward) motion of the boat through the
water.

doesn't have to be from boat movement, just has to be water movement.

So I trust you are not adding a new "fact" to the original discussion -
that the boat is tied to the dock but there is a current flowing past
the dock and the boat? Why not add that there is a current parallel to
the dock pushing the stern to port or to starboard? I think the
original description would imply the boat and dock are in still water.
However if the "water movement" you refer to is just from the prop, I
think I covered that... (see below)


As the boat is jammed against the dock and not able to move through the
water there will be no horizontal flow of water due to forward motion.

no, the hull of the boat causes the water flow behind of the forward pushing
prop to "line up" not in line with the prop shaft, thus asym thrust.


Again, see below. You left off part of my comment on this!


The only flow past the prop will therefore probably be a flow parallel
to the shaft

no, the hul gets in the way, at least if the hull is anywhere near the prop


You ignored a significant part of my statement: "(possibly modified
slightly by hull effects)" so in fact we are in agreement here.



the water leaving the ascending blade (on the
port side of the boat) may produce more push on the port side of the
hull than does the water swirling down from the descending blade

why is this? what has "swirling" to do with it?


The water leaves the prop in a sort of corkscrew fashion - that which
leaves the descending starboard blade will tend to corkscrew downward
and back away from the hull. That which leaves the ascending port blade
will corkscrew upward and back tending to cause a net push on the aft
sections of the hull.



I have even heard the explanation that since the water is more dense at
the bottom of the rotation than at the top,

nah, the difference is virtually nothing. There is less than 1/2 psi pressure
difference per foot of water depth and water compresses
soooooooooooooooooooooooooooooo little than even several thousand psi wouldn't
make any difference.


And again aren't you just agreeing with me? ("Surely this density
difference is small so this contribution to the overall effect from this
must also be a minor part.")

All I was trying to get across is that the effects causing prop walk are
multiple and in varying degrees and in the specific setup originally
described (tied bow to dock) attributing the action to one single effect
(the mentioned Chapman explanation) may not be fully justified.

I am sure we can both agree that to most boaters knowing the Physics
behind these effects is really unnecessary. Knowing what boat will do
in each situation and being able to use it to safely and effectivly
control the boat in a tight location is the thing!

Dave