Simple question.

Q: Which way does the stern of a boat t-boned to a dock turn when, with the
rudder turned to port and the tranny in forward, you give a 2 second burst of
the engine?

A: I think we all agree the stern will move starboard.



Q: same question but rudder turned to starboard?

A: the stern will move to port.



Q: same question but rudder centered?

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



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



Q: same question (tranny in reverse) but rudder to port?

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



Q: same question (tranny in reverse) but rudder to starboard?

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



Anybody but sherr doubt that? (forget the "good professor". he uses the word
"prop" when he means "rudder" and he wants to argue that friction in the rudder
bearing makes the difference)

"T-boned to a dock"?

Assumptions:

1. You are describing a boat with a rh prop.

2. "T-boned to a dock" means the stem is jammed against the dock to prevent
forward movement.......(until the stern swings far enough that the angle is no
longer sufficient to brake the boat).

With those assumptions...........

Q: Which way does the stern of a boat t-boned to a dock turn when, with the
rudder turned to port and the tranny in forward, you give a 2 second burst of
the engine?

A: I think we all agree the stern will move starboard.

(you mean a two second application of forward gear, as the engine remains at
least idling, but otherwise......)

Correct.




Q: same question but rudder turned to starboard?

A: the stern will move to port.

Correct


Q: same question but rudder centered?

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

Incorrect. Here are some excerpts from my old copy of Chapman's. We can
probably assume that propellers still behave the way they did 20 years ago.

"Another factor also effects a boat's readtion to propeller rotation. While
this factgor is sometimes referred to as 'sideways blade pressure' it is more
properly an 'unequal blade thrust', exerted by the ascending and descending
blades of the propeller. Figure 709, top.

Here we are looking at the starboard side of a propeller shaft, inclined, as
most shafts are, at a significant angle to the water's surface and the the flow
of water past the blades. The actual pitch of the blades as manufactured, of
course, is the same, but the water flows diagonally across the plane in which
the blades revolve.

Figure 709 shows clearly how the effectof this is to increase the pitch of the
descending starboard blade, (right hand propeller) as compared with the
ascending port blade, when considered relative to the direction of water flow
past the propeller.

The importance of this factor is reduced as the shaft angle is decreased, and
naval architects sometimes take pains to have the engine installed as low as
possible to keep the shaft nearly parallel to the water's surface and to the
flow of water past the blades. This contributes to greater propeller eficiency,
and is a factor worth considering if it is consistent with other design
requirements. Once a boat is built, shaft angle is difficult, usually
impossible, to modify.

The relatively greater blade pitch on the starboard side creates a stronger
thrust on this side, causing the bow to turn to port.
As far as this single factor is concerned, THE STERN OF A SINGLE-SCREW BOAT
WITH A RIGHT-HAND PROPELLER
THUS NATURALLY TENDS TO GO TO STARBOARD WHEN THE PROPELLER IS GOING AHEAD, AND
TO PORT WHEN IT IS REVERSING."


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


Correct.





Q: same question (tranny in reverse) but rudder to port?

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

Correct, particularly when the bow is secured to the dock and the vessel cannot
develop sternway.


Q: same question (tranny in reverse) but rudder to starboard?

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




Anybody but sherr doubt that?

Chapman's, for one.
According to an indpendent and recognized authority, you only missed one. That
gives you what, a strong B? :-)

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.

assumptions as given, yes.


"T-boned to a dock"?

Assumptions:

1. You are describing a boat with a rh prop.

2. "T-boned to a dock" means the stem is jammed against the dock to prevent
forward movement.......(until the stern swings far enough that the angle is
no
longer sufficient to brake the boat).

With those assumptions...........

Q: Which way does the stern of a boat t-boned to a dock turn when, with the
rudder turned to port and the tranny in forward, you give a 2 second burst
of
the engine?

A: I think we all agree the stern will move starboard.

(you mean a two second application of forward gear, as the engine remains at
least idling, but otherwise......)

Correct.




Q: same question but rudder turned to starboard?

A: the stern will move to port.

Correct


Q: same question but rudder centered?

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

Incorrect. Here are some excerpts from my old copy of Chapman's. We can
probably assume that propellers still behave the way they did 20 years ago.

"Another factor also effects a boat's readtion to propeller rotation. While
this factgor is sometimes referred to as 'sideways blade pressure' it is
more
properly an 'unequal blade thrust', exerted by the ascending and descending
blades of the propeller. Figure 709, top.

Here we are looking at the starboard side of a propeller shaft, inclined, as
most shafts are, at a significant angle to the water's surface and the the
flow
of water past the blades. The actual pitch of the blades as manufactured, of
course, is the same, but the water flows diagonally across the plane in which
the blades revolve.

Figure 709 shows clearly how the effectof this is to increase the pitch of
the
descending starboard blade, (right hand propeller) as compared with the
ascending port blade, when considered relative to the direction of water flow
past the propeller.

The importance of this factor is reduced as the shaft angle is decreased, and
naval architects sometimes take pains to have the engine installed as low as
possible to keep the shaft nearly parallel to the water's surface and to the
flow of water past the blades. This contributes to greater propeller
eficiency,
and is a factor worth considering if it is consistent with other design
requirements. Once a boat is built, shaft angle is difficult, usually
impossible, to modify.

The relatively greater blade pitch on the starboard side creates a stronger
thrust on this side, causing the bow to turn to port.
As far as this single factor is concerned, THE STERN OF A SINGLE-SCREW BOAT
WITH A RIGHT-HAND PROPELLER
THUS NATURALLY TENDS TO GO TO STARBOARD WHEN THE PROPELLER IS GOING AHEAD,
AND
TO PORT WHEN IT IS REVERSING."


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


Correct.





Q: same question (tranny in reverse) but rudder to port?

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

Correct, particularly when the bow is secured to the dock and the vessel
cannot
develop sternway.


Q: same question (tranny in reverse) but rudder to starboard?

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




Anybody but sherr doubt that?

Chapman's, for one.
According to an indpendent and recognized authority, you only missed one.
That
gives you what, a strong B? :-)

JAXAshby wrote in message
...
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

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

Perhaps a bit more food for thought?...

There are several effects that all contribute to prop walk (and I am
assured by an ex-Navy acquaintance that even ships with a horizontal
prop shaft still have prop walk). The one noted below (from Chapman)
may not be the best to use for THIS case.

Chapman's explanation here uses the incline of the prop shaft relative
to the flow of water past the blades. I believe this is meant to be
the angle between the inclined shaft and the HORIZONTAL flow of water
past the prop caused by the (forward) motion of the boat through the
water. (And this is confirmed by his later paragraph that explains why
engines are mounted lower to reduce this angle between the shaft and
(horizontal) flow of water past the prop.)

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.
The only flow past the prop will therefore probably be a flow parallel
to the shaft caused by the blades pushing the water in that direction
(possibly modified slightly by hull effects) and therefore the angle of
the ascending blade and the descending blade relative to this flow is
more nearly equal. In this case the difference in thrust from an
"ascending" vs a "descending" blade is probably minor and in fact the
concept of "ascending" and "descending" loses meaning if not referenced
to the horizontal in this case perhaps.

However the rotation of the prop is causing a swirling effect of the
water leaving the prop and 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 (on the
starboard side) creating a net push on the port aft end of the boat,
adding to the turning effect to starboard. Note that if a single rudder
is mounted directly behind the prop the water hitting each side is
probably pretty equal - rather it is the water hitting whateve hull is
left in the way that probably has the greater effect and the water
corkscrewing away from the descending blade on the starboard side misses
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, the blade gets more "bight"
at the bottom of its swing and "rolls" the stern to starboard. Surely
this density difference is small so this contribution to the overall
effect from this must also be a minor part.

Dave

Gould 0738 wrote:

"T-boned to a dock"?

Assumptions:

1. You are describing a boat with a rh prop.

2. "T-boned to a dock" means the stem is jammed against the dock to prevent
forward movement.......(until the stern swings far enough that the angle is no
longer sufficient to brake the boat).

With those assumptions...........



Q: same question but rudder centered?

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

Incorrect. Here are some excerpts from my old copy of Chapman's. We can
probably assume that propellers still behave the way they did 20 years ago.

"Another factor also effects a boat's readtion to propeller rotation. While
this factgor is sometimes referred to as 'sideways blade pressure' it is more
properly an 'unequal blade thrust', exerted by the ascending and descending
blades of the propeller. Figure 709, top.

Here we are looking at the starboard side of a propeller shaft, inclined, as
most shafts are, at a significant angle to the water's surface and the the flow
of water past the blades. The actual pitch of the blades as manufactured, of
course, is the same, but the water flows diagonally across the plane in which
the blades revolve.

Figure 709 shows clearly how the effectof this is to increase the pitch of the
descending starboard blade, (right hand propeller) as compared with the
ascending port blade, when considered relative to the direction of water flow
past the propeller.

The importance of this factor is reduced as the shaft angle is decreased, and
naval architects sometimes take pains to have the engine installed as low as
possible to keep the shaft nearly parallel to the water's surface and to the
flow of water past the blades. This contributes to greater propeller eficiency,
and is a factor worth considering if it is consistent with other design
requirements. Once a boat is built, shaft angle is difficult, usually
impossible, to modify.

The relatively greater blade pitch on the starboard side creates a stronger
thrust on this side, causing the bow to turn to port.
As far as this single factor is concerned, THE STERN OF A SINGLE-SCREW BOAT
WITH A RIGHT-HAND PROPELLER
THUS NATURALLY TENDS TO GO TO STARBOARD WHEN THE PROPELLER IS GOING AHEAD, AND
TO PORT WHEN IT IS REVERSING."

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.

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

comments intersperced

end-plate effect


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

yes, different. end-plate effect means the thrust is greater for the blade tip
coming "close" ("close" is a variable term) to an end plate, which constrains
the flow.

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


"A rose by any other name"?

it is a common term, though Chapmans didn't use it.

But if water flow is exactly parallel to the shaft (very
unlikely I agree) no SUCH effect would occur.

true.

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!)

Chapmans described the effects of a downward slanting propshaft. It seemed
some people were under the impression that that was the only issue. End plate
effect is another and different issue issue.


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?

assumption was that no current existed outside that which the prop caused.

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.

yes.

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!

I did see the comment and thought I was expanding it. sorry if my words did
not convey that.


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.

you have mostly described "end plate" effect, though end plate effect has more
implications.


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.")

it is hard to say it has even a minor part. the density of water is for all
practical purposes the same at any depth.


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 agree. end plate effect is also an issue.


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!

I brought it up because I have found that most boaters think a blast of the
engine in reverse will affect the rudder. I have also seen boaters who were
told by marinas to spend major bux to move a prop back closer to the rudder to
"help fix" the lack of rudder response backing up. Ya gotta have a boat moving
backwards through the water to have the rudder effective.


Dave

Subject: Which way does a boat turn?
From: (JAXAshby)
Date: 03/29/2004 15:02 Pacific Standard Time
Message-id:

Simple question.

Q: Which way does the stern of a boat t-boned to a dock turn when, with the
rudder turned to port and the tranny in forward, you give a 2 second burst of
the engine?

A: I think we all agree the stern will move starboard.



Q: same question but rudder turned to starboard?

A: the stern will move to port.



Q: same question but rudder centered?

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


Thats ok Jax .... we all understand you're a "newbie". Anyone with any
experience would have stated prop rotation, as it's important to the answers.
Just tell them you were using a left hand prop for the first set of questions
and a right hand for the second set of reverse questions.
BTW, you should also add that this exercise is without wind or current.
With wind or current, especially in the astern mode, depending on the boat,
things may not work according to Hoyle.

Shen

PS. It's up to a $1,000/hr .... I can see you'll need special handling, and I
'll have to waste 50 min out of every hour, just trying to keep you
concentrating.