You STILL trying to figure that one out, Jax?

SV


which one is that?

dave? not good.

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

end-plate effect

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

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.

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.

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.

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

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?

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.

no, jim, what you see was quick typing on my part. "prop walk" is always to
port (on my boat, a rh prop) because I only worry about it when backing up.
Going forward I use the rudder to counter, something I can not do when backing
up until the boat is moving. I typed the tranny in forward and just typed prop
walk port. In forward, the prop walk is starboard.

quick typing, that's all. asym thrust is asym thrust. someone even quoted
Chapmans on the issue.

you are right, gould. prop walk in forward would be starboard
because the prop
is turning cw as viewed from behind.

btw, Chapman's is correct as far as asym thrust is concerned,
but end-plate
effect would add to the total lateral force.

In your language - a flip flop?

I seem to remember about a week ago that I challenged your
explanation of prop walk (the aeronautical simile) with a bit of
trig to point out that the asymmetric thrust effect is
insignificant compared to hull interference (you call it end
plate effect?).

What I missed, in fact, is that it is even in the opposite sense!

JimB

"JAXAshby" wrote in message
...


Q: same question, rudder centered, but tranny in reverse (bow of boat tied
to
dock)?

A: the stern will move to port due to prop walk


I would have said to starboard, reason being that I would expect it to be
the opposite of the prop walk direction when in forward gear.

I have heard several different theories for "prop walk".

"Asymmetrical thrust" is certainly one, and obviously the one being debated
the most right now.

Cavitation is another. Consider that both surfaces of the prop are being
used. The "push" side of the prop can generate unlimited PSI, constrained
only by what can be delivered down the shaft and the strength of the prop
itself. The "Pull" side of the prop can only develop the pressure of the
ambient.

Consider a prop that is spinning just under the surface of the water. As
the tip of the blade passes over the top, the total pressure that can be
developed on the "pull" side is 14.7 PSI. (at the tip). As the tip of the
blade passes down the bottom side it is in deeper water, and hence will have
more ambient pressure. If the prop is 12 inches in diameter then it would
have a foots worth of water more pressure, or approximately another 1/2 PSI
or so. This difference in pressure top to bottom gives the bottom of the
prop just a little more bite, and the prop wants to walk.

There can be other reasons why a given boat kicks one way or the other.
Some inboards I have seen have the rudder mounted just off center, which
allows removal of the prop shaft without having to remove the rudder (which
it would hit if the rudder was on center). I would expect an off center
rudder to have some effect on prop walk.

What is the dominant effect? I have no clue. I don't care. I do know that
my single engine inboard is impossible to steer reliably in reverse. It
usually (but not always) kicks to port in reverse. Which is why when you see
me going down a tight fairway in the marina I will be hugging the left side
as I can turn right much better than I can turn left.

Rod McInnis

On Tue, 30 Mar 2004 18:09:21 -0800, "Rod McInnis"
wrote:


"JAXAshby" wrote in message
...


Q: same question, rudder centered, but tranny in reverse (bow of boat tied
to
dock)?

A: the stern will move to port due to prop walk


I would have said to starboard, reason being that I would expect it to be
the opposite of the prop walk direction when in forward gear.

I have heard several different theories for "prop walk".

"Asymmetrical thrust" is certainly one, and obviously the one being debated
the most right now.

Cavitation is another. Consider that both surfaces of the prop are being
used. The "push" side of the prop can generate unlimited PSI, constrained
only by what can be delivered down the shaft and the strength of the prop
itself. The "Pull" side of the prop can only develop the pressure of the
ambient.

Consider a prop that is spinning just under the surface of the water. As
the tip of the blade passes over the top, the total pressure that can be
developed on the "pull" side is 14.7 PSI. (at the tip). As the tip of the
blade passes down the bottom side it is in deeper water, and hence will have
more ambient pressure. If the prop is 12 inches in diameter then it would
have a foots worth of water more pressure, or approximately another 1/2 PSI
or so. This difference in pressure top to bottom gives the bottom of the
prop just a little more bite, and the prop wants to walk.

There can be other reasons why a given boat kicks one way or the other.
Some inboards I have seen have the rudder mounted just off center, which
allows removal of the prop shaft without having to remove the rudder (which
it would hit if the rudder was on center). I would expect an off center
rudder to have some effect on prop walk.

What is the dominant effect? I have no clue. I don't care. I do know that
my single engine inboard is impossible to steer reliably in reverse. It
usually (but not always) kicks to port in reverse. Which is why when you see
me going down a tight fairway in the marina I will be hugging the left side
as I can turn right much better than I can turn left.


Our boat exhibits no prop walk that I can detect. It also can't steer
from propwash in forward. It must be moving for any steering to occur.
The prop (18-inch Martec) is 12 feet forward of the spade rudder and
fairly close to the keel, so it has little lever arm to turn the boat.

A friend has a boat (Bristol 35.5) that walks horribly. It has a
Maxprop that is much further aft than ours in a boat about the same
length. I suspect the Maxprop is set for excessive pitch, because it
doesn't produce much thrust, but walks like crazy.



Rodney Myrvaagnes NYC J36 Gjo/a

"WooWooism lives" Anon grafitto on the base of the Cuttyhunk breakwater light

Comments interspersed:

Rod McInnis wrote:
"JAXAshby" wrote in message
...



Q: same question, rudder centered, but tranny in reverse (bow of boat tied

to

dock)?

A: the stern will move to port due to prop walk



I would have said to starboard, reason being that I would expect it to be
the opposite of the prop walk direction when in forward gear.

I believe he was talking about a RH, fixed pitch prop, no wind or
current.....stern should go to port in reverse, to stbd when ahead.

I have heard several different theories for "prop walk".

G Same here .... My opinion FWIW, prop walk is a function of the prop
and pitch. The angle of the shaft may increase it, but is not the basic
cause as will some hull shapes and prop locations (tunnels or nozzles),
increase or decrease it.....




There can be other reasons why a given boat kicks one way or the other.
Some inboards I have seen have the rudder mounted just off center, which
allows removal of the prop shaft without having to remove the rudder (which
it would hit if the rudder was on center). I would expect an off center
rudder to have some effect on prop walk.

My first impression is that this would have minimal effect if any, G
would need some test, and although I've worked some boats with various
rudders types and locations, I can't say that I've ever noticed a
difference that I could attribute to the rudder.


What is the dominant effect? I have no clue. I don't care. I do know that
my single engine inboard is impossible to steer reliably in reverse. It
usually (but not always) kicks to port in reverse.

Going to assume that you have a RH fixed pitch prop. This is the norm,
however, wind and/or current can make a boat which normally backs to
port because of the prop, back to stbd from the "get go".
Steering your boat "reliably" in reverse, would involve many possible
solutions, including "your boat just won't steer reliably in reverse".

Which is why when you see
me going down a tight fairway in the marina I will be hugging the left side
as I can turn right much better than I can turn left.

Interesting. Normally a single screw RH fixed pitch prop will steer more
easily to port than to stbd, unless you are talking a back and fill
turn, in which case you want to turn to stbd.

otn

Rod,

Actually, the asymmetric response to forward and reverse can be used
to do some neat tricks (which I use all the time). Try this, next
time you're out in your boat with some time to spend:

Motor out to a bouy (navigation, crab pot, whatever) in open water,
where there's no current. The bouy doesn't have to be very close --
it's only to gauge forward motion.

Stop the boat, put the rudder hard over to starboard, put the motor in
forward, and gun it. The first reaction will be that the stern kicks
to port. The next will be that the boat starts to move forward.

Immediately put the motor in reverse, LEAVING THE RUDDER HARD TO
STARBOARD. Apply moderate juice. The stern will continue to move to
port, and eventually the boat will come to a stop. Keep reversing,
and the stern will continue to move to port, due to the effects that
have been described already in this thread. Note where the boat is
with respect to the buoy, and continue backing until you've canceled
any forward motion and she's back where she started.

Now put the motor in forward and repeat the whole operation. With a
little practice, you can almost eliminate all the forward and aft
motions, and simply turn the boat inside her own length, by simply
pushing the stick forward and back, all the while holding the rudder
hard over. When you need to do this in close quarters (say, in a
marina with only a foot or two of room between your bow and the boat
ahead of you, and between your stern and the boat astern), you will
draw applause from onlookers.

Now, suppose some day you need to turn the boat in the opposite
direction, say, 90 degrees. Is there a similar way to turn her inside
her length, but in the opposite direction? No. So, simply turn her
the 270 degrees the way she wants to go (clockwise) until you've
accomplished the turn you need! Big ships do this all the time, and
it works great.

So no need ever again to hug the port side of your channel!

"Rod McInnis" wrote in message ...
"JAXAshby" wrote in message
...


Q: same question, rudder centered, but tranny in reverse (bow of boat tied
to
dock)?

A: the stern will move to port due to prop walk


I would have said to starboard, reason being that I would expect it to be
the opposite of the prop walk direction when in forward gear.

I have heard several different theories for "prop walk".

"Asymmetrical thrust" is certainly one, and obviously the one being debated
the most right now.

Cavitation is another. Consider that both surfaces of the prop are being
used. The "push" side of the prop can generate unlimited PSI, constrained
only by what can be delivered down the shaft and the strength of the prop
itself. The "Pull" side of the prop can only develop the pressure of the
ambient.

Consider a prop that is spinning just under the surface of the water. As
the tip of the blade passes over the top, the total pressure that can be
developed on the "pull" side is 14.7 PSI. (at the tip). As the tip of the
blade passes down the bottom side it is in deeper water, and hence will have
more ambient pressure. If the prop is 12 inches in diameter then it would
have a foots worth of water more pressure, or approximately another 1/2 PSI
or so. This difference in pressure top to bottom gives the bottom of the
prop just a little more bite, and the prop wants to walk.

There can be other reasons why a given boat kicks one way or the other.
Some inboards I have seen have the rudder mounted just off center, which
allows removal of the prop shaft without having to remove the rudder (which
it would hit if the rudder was on center). I would expect an off center
rudder to have some effect on prop walk.

What is the dominant effect? I have no clue. I don't care. I do know that
my single engine inboard is impossible to steer reliably in reverse. It
usually (but not always) kicks to port in reverse. Which is why when you see
me going down a tight fairway in the marina I will be hugging the left side
as I can turn right much better than I can turn left.

Rod McInnis

Our boat exhibits no prop walk that I can detect. It also can't steer
from propwash in forward. It must be moving for any steering to occur.
The prop (18-inch Martec) is 12 feet forward of the spade rudder and
fairly close to the keel, so it has little lever arm to turn the boat.

yeah, 12 feet doesn't leave much "push" still in the prop stream, and too a
prop at about the pivot point can't do much twisting of the boat.

My opinion FWIW, prop walk is a function of the prop
and pitch.

owners of AutoProps report limited prop walk as compared to their prior fixed
blade props. AutoProp blades continuously change pitch during a rotation so
that (more or less) total thrust on each side of the prop rev is the same.

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