"Edgar" wrote in message
...

"Capt. JG" wrote in message
easolutions...
"Edgar" wrote in message
...

troll sh*t removed
That is the easy answer that seems to satisfy most people but if you
think about it do you really believe that water gets measurably denser
because it is about 18" (say) deeper at the bottom of an 18"(say)
propeller than the water at the top? Of course it does not. Water is
hardly compressible at all except at very high pressures.
. Density will only be a factor if air is being entrained from the
surface.
You will find boats displaying propeller walk even if the propeller is
too deep for air entrainment to occur. My boat does.
So the question is 'why does the opposite direction of the blade at the
top not cancel the walk created by the blade at the bottom'?
I suggest that this is because as the blades rise upward towards the top
the water impinges on the hull and creates a reaction force which acts
in the same direction as the walk force created at the bottom. This is
because, in yachts especially, the blades pass very close to the hull
which usually is somewhat vee shaped and deflects the water sideways. In
your example water would be deflected to port but the reaction on the
hull would be a starboard force.



Think hydrostatic pressure. Also, Neal is wrong.

Yes, he certainly is.
I have tried your suggestion about thinking 'hydrostatic pressure' but it
is the 'static' bit that is somewhat puzzling me because propeller walk is
a dynamic phenomenon. Can you expand on this a bit?


Not an engineer. However, there is a difference in the "pressure" between
the high and low of the prop. I have a hydrostatic PFD (vs. the old pill
melting) autoinflate. It's a good choice if you go offshore or sail
dinghies, and you don't want the spray to trigger the PFD. I believe it's
activated in 2' of water, but don't quote me. I had the pill melt on me once
while sailing a dinghy, and I don't want to wear the bulky PFDs.

My understanding is that it's only a small part of the effect. As most of us
know, the prop wash can be observed on the opposite side from the direction
the prop walk forces the stern (usually, the prop wash is observed most
strongly on the starboard side, thus the stern moves to port). This is much
like a fire hose that forces water out the business end and pushes against
the person holding the hose. In the case of the boat, the stern moves in the
opposite because there's nothing but some water to oppose it.

I think Donal said it best, that it really doesn't matter, since the issue
we're all concerned with is how to either use it or deal with it. I find
pulsing the engine works vs. actually putting the engine in neutral. I get
90% of the benefit with 1/2 the hassle.

--
"j" ganz @@
www.sailnow.com

"Capt. JG" wrote in message
easolutions...
"Edgar" wrote in message
...

"Capt. JG" wrote in message
easolutions...
"Edgar" wrote in message
...

troll sh*t removed
That is the easy answer that seems to satisfy most people but if you
think about it do you really believe that water gets measurably denser
because it is about 18" (say) deeper at the bottom of an 18"(say)
propeller than the water at the top? Of course it does not. Water is
hardly compressible at all except at very high pressures.
. Density will only be a factor if air is being entrained from the
surface.
You will find boats displaying propeller walk even if the propeller is
too deep for air entrainment to occur. My boat does.
So the question is 'why does the opposite direction of the blade at the
top not cancel the walk created by the blade at the bottom'?
I suggest that this is because as the blades rise upward towards the
top the water impinges on the hull and creates a reaction force which
acts in the same direction as the walk force created at the bottom.
This is because, in yachts especially, the blades pass very close to
the hull which usually is somewhat vee shaped and deflects the water
sideways. In your example water would be deflected to port but the
reaction on the hull would be a starboard force.



Think hydrostatic pressure. Also, Neal is wrong.

Yes, he certainly is.
I have tried your suggestion about thinking 'hydrostatic pressure' but
it is the 'static' bit that is somewhat puzzling me because propeller
walk is a dynamic phenomenon. Can you expand on this a bit?


Not an engineer. However, there is a difference in the "pressure" between
the high and low of the prop. I have a hydrostatic PFD (vs. the old pill
melting) autoinflate. It's a good choice if you go offshore or sail
dinghies, and you don't want the spray to trigger the PFD. I believe it's
activated in 2' of water, but don't quote me. I had the pill melt on me
once while sailing a dinghy, and I don't want to wear the bulky PFDs.

Yes, OK that is hydrostatic pressure but nothing to do with prop walk.

My understanding is that it's only a small part of the effect. As most of
us know, the prop wash can be observed on the opposite side from the
direction the prop walk forces the stern (usually, the prop wash is
observed most strongly on the starboard side, thus the stern moves to
port). This is much like a fire hose that forces water out the business
end and pushes against the person holding the hose. In the case of the
boat, the stern moves in the opposite because there's nothing but some
water to oppose it.

A hose stays in one position and certainly produces a reaction force that
you have to resist in order to hold it still.
But the prop is rotating and consequently the sideways force it generates
is also rotating.
So, as I said, one has to explain why the force to one side produced while
the blade is at the bottom is not balanced by the opposite force produced
while it is at the top. This is what I tried to explain in my first post.
Neal's explanation was quite wrong and I am quite sure he knows it..

I think Donal said it best, that it really doesn't matter, since the issue
we're all concerned with is how to either use it or deal with it. I find
pulsing the engine works vs. actually putting the engine in neutral. I get
90% of the benefit with 1/2 the hassle.

I fully agree with that. Pulsing is the best way to make use of prop walk.
Now Joe has just said that to really walk a boat you need twin screws and
that is not right either. With twin screws you are turning the boat by
regulating the thrust by going ahead on one and astern on the other as
necessary.
However, I have seen fishermen in heavy deep draught traditional boats with
big single screws that could park them perfectly alongside in a narrow gap
using prop walk while carrying on a conversation with the guy in the next
boat!
I have handled such a boat and can vouch for the usefulness of the effect
once you are used to your boat's characteristics.

My own boat is 38' and has a short(ish) central keel, no skeg and a spade
rudder. You can spin her on the proverbial dime ahead or astern once you
have steerage way but backing out of my marina slot needs great care because
prop walk when you first engage reverse from rest is a real problem
especially if a cross wind makes it worse, and I always have to be careful
that my pulpit does not gouge the side of the power boat in the next slot
before I have steerage way.

"Edgar" wrote in message
...

"Capt. JG" wrote in message
easolutions...
"Edgar" wrote in message
...

"Capt. JG" wrote in message
easolutions...
"Edgar" wrote in message
...

troll sh*t removed
That is the easy answer that seems to satisfy most people but if you
think about it do you really believe that water gets measurably denser
because it is about 18" (say) deeper at the bottom of an 18"(say)
propeller than the water at the top? Of course it does not. Water is
hardly compressible at all except at very high pressures.
. Density will only be a factor if air is being entrained from the
surface.
You will find boats displaying propeller walk even if the propeller is
too deep for air entrainment to occur. My boat does.
So the question is 'why does the opposite direction of the blade at
the top not cancel the walk created by the blade at the bottom'?
I suggest that this is because as the blades rise upward towards the
top the water impinges on the hull and creates a reaction force which
acts in the same direction as the walk force created at the bottom.
This is because, in yachts especially, the blades pass very close to
the hull which usually is somewhat vee shaped and deflects the water
sideways. In your example water would be deflected to port but the
reaction on the hull would be a starboard force.



Think hydrostatic pressure. Also, Neal is wrong.

Yes, he certainly is.
I have tried your suggestion about thinking 'hydrostatic pressure' but
it is the 'static' bit that is somewhat puzzling me because propeller
walk is a dynamic phenomenon. Can you expand on this a bit?


Not an engineer. However, there is a difference in the "pressure" between
the high and low of the prop. I have a hydrostatic PFD (vs. the old pill
melting) autoinflate. It's a good choice if you go offshore or sail
dinghies, and you don't want the spray to trigger the PFD. I believe it's
activated in 2' of water, but don't quote me. I had the pill melt on me
once while sailing a dinghy, and I don't want to wear the bulky PFDs.

Yes, OK that is hydrostatic pressure but nothing to do with prop walk.

Actually, I think it does, although prop walk is also related to Newton's
3rd law, the angle of the shaft and the pitch of the prop. There *is* a
hydrostatic pressure difference between the top and the bottom. Not being an
engineer, I don't know how to calculate the diff or the effect.

Another way to look it is, it seems to me, is to look at a limiting case.
Take the paddle wheel. The paddle wheel pushes the water to cause the boat
to go forward (or reverse, whatever) by resistance against the water (N's
3rd), but there is pressure against the paddles above the surface of the
water... minimal though it is. I wonder what the effect would be if the
paddle wheel were tilted? It seems to me that that would be similar to the
situation approaching a normal sailboat prop. I wonder if this has ever been
tried.

My understanding is that it's only a small part of the effect. As most of
us know, the prop wash can be observed on the opposite side from the
direction the prop walk forces the stern (usually, the prop wash is
observed most strongly on the starboard side, thus the stern moves to
port). This is much like a fire hose that forces water out the business
end and pushes against the person holding the hose. In the case of the
boat, the stern moves in the opposite because there's nothing but some
water to oppose it.

A hose stays in one position and certainly produces a reaction force that
you have to resist in order to hold it still.
But the prop is rotating and consequently the sideways force it generates
is also rotating.
So, as I said, one has to explain why the force to one side produced while
the blade is at the bottom is not balanced by the opposite force produced
while it is at the top. This is what I tried to explain in my first post.
Neal's explanation was quite wrong and I am quite sure he knows it..

See previous. My continued understanding it that's a combination of several
things.

I think Donal said it best, that it really doesn't matter, since the
issue we're all concerned with is how to either use it or deal with it. I
find pulsing the engine works vs. actually putting the engine in neutral.
I get 90% of the benefit with 1/2 the hassle.

I fully agree with that. Pulsing is the best way to make use of prop walk.
Now Joe has just said that to really walk a boat you need twin screws and
that is not right either. With twin screws you are turning the boat by
regulating the thrust by going ahead on one and astern on the other as
necessary.

I believe twin screws turning in opposite directions is one solution.
Another is restricting the prop blades inside a tube. Again, the limiting
case is a long, narrow tube with a prop inside. You don't get prop walk with
this arrangement, but it's expensive to make/use.

However, I have seen fishermen in heavy deep draught traditional boats
with big single screws that could park them perfectly alongside in a
narrow gap using prop walk while carrying on a conversation with the guy
in the next boat!
I have handled such a boat and can vouch for the usefulness of the effect
once you are used to your boat's characteristics.

I've seen such but have never been on one, at least not in a position to
observe the helm maneauvers.

My own boat is 38' and has a short(ish) central keel, no skeg and a spade
rudder. You can spin her on the proverbial dime ahead or astern once you
have steerage way but backing out of my marina slot needs great care
because prop walk when you first engage reverse from rest is a real
problem especially if a cross wind makes it worse, and I always have to be
careful that my pulpit does not gouge the side of the power boat in the
next slot before I have steerage way.



Yes, I have the same issue. Fortunately, I'm able to basically let her go in
reverse and use the prop walk to make a big U turn in reverse to get out. I
have my students keep the wheel hard to starboard until the bow is clear,
then gently turn back to port. The starboard wheel will keep her straight
long enough to get moving in a straight line, and I could keep backing
straight with the pulsing technique, but I'd run out of room in the fairway.

--
"j" ganz @@
www.sailnow.com

On Dec 7, 12:50*pm, "Edgar" wrote:
"Capt. JG" wrote in message

easolutions...





"Edgar" wrote in message
...

"Capt. JG" wrote in message
areasolutions...
"Edgar" wrote in message
...

troll sh*t removed
That is the easy answer that seems to satisfy most people but if you
think about it do you really believe that water gets measurably denser
because it is about 18" (say) deeper at the bottom of an 18"(say)
propeller than the water at the top? *Of course it does not. Water is
hardly compressible at all except at very high pressures.
. Density will only be a factor if air is being entrained from the
surface.
You will find boats displaying propeller walk even if the propeller is
too deep for air entrainment to occur. My boat does.
So the question is 'why does the opposite direction of the blade at the
top not cancel the walk created by the blade at the bottom'?
I suggest that this is because as the blades rise upward towards the
top the water impinges on the hull and creates a reaction force which
acts in the same direction as the walk force created at the bottom.
This is because, in yachts especially, the blades pass very close to
the hull which usually is somewhat vee shaped and deflects the water
sideways. In your example water would be deflected to port but the
reaction on the hull would be a starboard force.

Think hydrostatic pressure. Also, Neal is wrong.

Yes, he certainly is.
*I have tried your suggestion about thinking 'hydrostatic pressure' but
it is the 'static' bit that is somewhat puzzling me because propeller
walk is a dynamic phenomenon. Can you expand on this a bit?

Not an engineer. However, there is a difference in the "pressure" between
the high and low of the prop. I have a hydrostatic PFD (vs. the old pill
melting) autoinflate. It's a good choice if you go offshore or sail
dinghies, and you don't want the spray to trigger the PFD. I believe it's
activated in 2' of water, but don't quote me. I had the pill melt on me
once while sailing a dinghy, and I don't want to wear the bulky PFDs.

Yes, OK that is hydrostatic pressure but nothing to do with prop walk.



My understanding is that it's only a small part of the effect. As most of
us know, the prop wash can be observed on the opposite side from the
direction the prop walk forces the stern (usually, the prop wash is
observed most strongly on the starboard side, thus the stern moves to
port). This is much like a fire hose that forces water out the business
end and pushes against the person holding the hose. In the case of the
boat, the stern moves in the opposite because there's nothing but some
water to oppose it.

A hose stays in one position and certainly produces a reaction force that
you have to resist in order to hold it still.
*But the prop is rotating and consequently the sideways force it generates
is also rotating.
So, as I said, one has to explain why the force to one side produced while
the blade is at the bottom is not balanced by the opposite force produced
while it is at the top. This is what I tried to explain in my first post.
*Neal's explanation was quite wrong and I am quite sure he knows it..



I think Donal said it best, that it really doesn't matter, since the issue
we're all concerned with is how to either use it or deal with it. I find
pulsing the engine works vs. actually putting the engine in neutral. I get
90% of the benefit with 1/2 the hassle.

I fully agree with that. Pulsing is the best way to make use of prop walk..
Now Joe has just said that to really walk a boat you need twin screws and
that is not right either. With twin screws you are turning the boat by
regulating the thrust by going ahead on one and astern on the other as
necessary.
*However, I have seen fishermen in heavy deep draught traditional boats with
big single screws that could park them perfectly alongside in a narrow gap
using prop walk while carrying on a conversation with the guy in the next
boat!
I have handled such a boat and can vouch for the usefulness of the effect
once you are used to your boat's characteristics.

My own boat is 38' and has a short(ish) central keel, no skeg and a spade
rudder. You can spin her on the proverbial dime ahead or astern once you
have steerage way but backing out of my marina slot needs great care because
prop walk when you first engage reverse from rest is a real problem
especially if a cross wind makes it worse, and I always have to be careful
that my pulpit does not gouge the side of the power boat in the next slot
before I have steerage way.
*- Hide quoted text -

- Show quoted text -

If you put a single screw in reverse it will pull to one side or the
other depending on rotation.

Your understanding of a twin screw is pivoting a boat, not walking it,
big difference .

With a single screw you can only pivot then counter with a trust.
With a twin you can check the bow movement and move sideways.

A Captain Ron style landing is about the best you can do with a single
screw
http://www.youtube.com/watch?v=8alNxLjCBJc
i suspect this is what you refer to the fisherman doing in narrow
canals.
That is using wheel walk, but is not walking a boat.

Walking a boat is moving it sideways not having and fore and aft
movement at all ...and no turning.
this can only be done and distance or period of time with a twin screw
vessel with inboard rotating screws.

Joe

"Joe" wrote in message
...
On Dec 7, 12:50 pm, "Edgar" wrote:
"Capt. JG" wrote in message

easolutions...





"Edgar" wrote in message
...

"Capt. JG" wrote in message
areasolutions...
"Edgar" wrote in message
...

troll sh*t removed
That is the easy answer that seems to satisfy most people but if you
think about it do you really believe that water gets measurably
denser
because it is about 18" (say) deeper at the bottom of an 18"(say)
propeller than the water at the top? Of course it does not. Water is
hardly compressible at all except at very high pressures.
. Density will only be a factor if air is being entrained from the
surface.
You will find boats displaying propeller walk even if the propeller
is
too deep for air entrainment to occur. My boat does.
So the question is 'why does the opposite direction of the blade at
the
top not cancel the walk created by the blade at the bottom'?
I suggest that this is because as the blades rise upward towards the
top the water impinges on the hull and creates a reaction force which
acts in the same direction as the walk force created at the bottom.
This is because, in yachts especially, the blades pass very close to
the hull which usually is somewhat vee shaped and deflects the water
sideways. In your example water would be deflected to port but the
reaction on the hull would be a starboard force.

Think hydrostatic pressure. Also, Neal is wrong.

Yes, he certainly is.
I have tried your suggestion about thinking 'hydrostatic pressure' but
it is the 'static' bit that is somewhat puzzling me because propeller
walk is a dynamic phenomenon. Can you expand on this a bit?

Not an engineer. However, there is a difference in the "pressure"
between
the high and low of the prop. I have a hydrostatic PFD (vs. the old pill
melting) autoinflate. It's a good choice if you go offshore or sail
dinghies, and you don't want the spray to trigger the PFD. I believe
it's
activated in 2' of water, but don't quote me. I had the pill melt on me
once while sailing a dinghy, and I don't want to wear the bulky PFDs.

Yes, OK that is hydrostatic pressure but nothing to do with prop walk.



My understanding is that it's only a small part of the effect. As most
of
us know, the prop wash can be observed on the opposite side from the
direction the prop walk forces the stern (usually, the prop wash is
observed most strongly on the starboard side, thus the stern moves to
port). This is much like a fire hose that forces water out the business
end and pushes against the person holding the hose. In the case of the
boat, the stern moves in the opposite because there's nothing but some
water to oppose it.

A hose stays in one position and certainly produces a reaction force that
you have to resist in order to hold it still.
But the prop is rotating and consequently the sideways force it generates
is also rotating.
So, as I said, one has to explain why the force to one side produced while
the blade is at the bottom is not balanced by the opposite force produced
while it is at the top. This is what I tried to explain in my first post.
Neal's explanation was quite wrong and I am quite sure he knows it..



I think Donal said it best, that it really doesn't matter, since the
issue
we're all concerned with is how to either use it or deal with it. I find
pulsing the engine works vs. actually putting the engine in neutral. I
get
90% of the benefit with 1/2 the hassle.

I fully agree with that. Pulsing is the best way to make use of prop walk.
Now Joe has just said that to really walk a boat you need twin screws and
that is not right either. With twin screws you are turning the boat by
regulating the thrust by going ahead on one and astern on the other as
necessary.
However, I have seen fishermen in heavy deep draught traditional boats
with
big single screws that could park them perfectly alongside in a narrow gap
using prop walk while carrying on a conversation with the guy in the next
boat!
I have handled such a boat and can vouch for the usefulness of the effect
once you are used to your boat's characteristics.

My own boat is 38' and has a short(ish) central keel, no skeg and a spade
rudder. You can spin her on the proverbial dime ahead or astern once you
have steerage way but backing out of my marina slot needs great care
because
prop walk when you first engage reverse from rest is a real problem
especially if a cross wind makes it worse, and I always have to be careful
that my pulpit does not gouge the side of the power boat in the next slot
before I have steerage way.
- Hide quoted text -

- Show quoted text -

+If you put a single screw in reverse it will pull to one side or the
+other depending on rotation.
+
+Your understanding of a twin screw is pivoting a boat, not walking it,
+big difference .
+
+With a single screw you can only pivot then counter with a trust.
+With a twin you can check the bow movement and move sideways.
+
+A Captain Ron style landing is about the best you can do with a single
+screw
+http://www.youtube.com/watch?v=8alNxLjCBJc
+i suspect this is what you refer to the fisherman doing in narrow
+canals.
+That is using wheel walk, but is not walking a boat.
+
+Walking a boat is moving it sideways not having and fore and aft
+movement at all ...and no turning.
+this can only be done and distance or period of time with a twin screw
+vessel with inboard rotating screws.

You'll still have prop walk in a twin screw if the props are turning in the
same direction. If you have separate controls on the engines, then it isn't
an issue, since as you said, you can compensate. If you have props turning
in opposite directions without dual controls, you can also eliminate prop
walk.

The Capt. Ron video is cool of course.

Prop walk is quite useful, e.g., in narrow situations, with the back and
fill technique, where you use prop walk in reverse to both stop forward
momentum and move the bow/stern in the direction you want to complete the
turn in a small area.


--
"j" ganz @@
www.sailnow.com

You'll still have prop walk in a twin screw if the props are turning in the
same direction. If you have separate controls on the engines, then it isn't
an issue, since as you said, you can compensate.

uhhh.. Have you ever seen in your life, a twin screw that did not have
"Separate" controls?
I curious.... what was the application?

If you have props turning
in opposite directions without dual controls, you can also eliminate prop
walk.

That would be counter acting, not eliminating. To eliminate it you
need a Voith set-up.

Joe

The Capt. Ron video is cool of course.


--
"j" ganz - Hide quoted text -

- Show quoted text -

"Joe" wrote in message
...


You'll still have prop walk in a twin screw if the props are turning in
the
same direction. If you have separate controls on the engines, then it
isn't
an issue, since as you said, you can compensate.

uhhh.. Have you ever seen in your life, a twin screw that did not have
"Separate" controls?
I curious.... what was the application?

Never said I did. I said that if you had props turning in the opposite
direction, the prop walk effect would be eliminated. You can play games with
the semantics, but it's still going to be gone.


If you have props turning
in opposite directions without dual controls, you can also eliminate prop
walk.



--
"j" ganz @@
www.sailnow.com

"Joe" wrote in message
...


You'll still have prop walk in a twin screw if the props are turning in
the
same direction. If you have separate controls on the engines, then it
isn't
an issue, since as you said, you can compensate.

uhhh.. Have you ever seen in your life, a twin screw that did not have
"Separate" controls?
I curious.... what was the application?

If you have props turning
in opposite directions without dual controls, you can also eliminate prop
walk.

That would be counter acting, not eliminating. To eliminate it you
need a Voith set-up.

Joe

The Capt. Ron video is cool of course.


--
"j" ganz - Hide quoted text -

- Show quoted text -



FYI, a quick google:
http://www.battleships-cruisers.co.u...htm#HMS%20Arun


--
"j" ganz @@
www.sailnow.com

"Capt. JG" wrote in message
ons...
It's actually not a particularly "good" question.


Here's a decent link that describes the forces involved:
http://www.sailingislife.com/id28.html.

That's not a particularly "good" answer.


The bulk modulus of water is such that the density differential at 14
inches is neglible. Plus if density was a contributing factor, it occurs in
a vertical profile and is zero difference is a horizontal profile, so it
would not cause the boat to walk, but rather to pitch.

The torque of the propeller does twist the boat and the thrust lateral to
the propeller directed toward the hull and redirected by the angle of the
hull. A right hand prop leans the boat to the left and the starboard bottom
of the hull directs more thrust laterally than downward compared to the port
side and hence the walk to the port.

The angle of the driveshaft has nothing to do with walk. The asymmetry is
vertical, not horizontal, in the thrust difference.

This graph:

http://www.windows.ucar.edu/tour/lin....html&edu=high

Shows the density of water as a function of depth. Water density changes
from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in
250 ft. Since that portion of the curve is linear, one can estimate that
water density would change .0004% over the diameter (tip to tip) of a 12
inch propeller. So is a .0004% change in water density (in the vertical
plane no less) going to walk a boat sideways? Don't think so.

Now if the propeller shaft mounted off centerline reduces the walk, this
would indicate that sideways (lateral) thrust from the propeller directed
against the hull is a factor. Also with the shaft off center the boat is
less likely to "torque" against the propeller. It's always good to find
physical evidence to support a conjecture.