Take a spring from bow to shore. Put lots of fenders up
front. Apply full helm towards the shore. Slowly add power.
The stern will walk away from the shore until you can back
off from the quay in comfort.

Of course, but what does that have to do with an observation that with the helm
amidships you won't achieve enough side thrust from the prop to kick the stern
away from the dock?

In a *moderate* wind, the spring line is usually unneccessary. Removing from
the question originally posed to the board any variable that says the rudder
cannot be used allows the rudder to be turned toward the dock, and a brief
application of forward will indeed "kick out" the stern. (Just have to make
sure that you don't whack the stem in the process)

Hello otn,

I wasn't quite sure what you were driving at until I read your last
sentence and then it all became crystal clear. So the answer is "yes"
I would understand what you were saying and "yes" it would help me
understand the concept. But even though the term "thrust" is easy for
most people to understand, the concept of a "vector" (magnitude and
direction) may not be. But only you know who your target audience is,
and whether they would understand what a vector is. Hope this helps,

Bob Whitaker
"Free Spirit"


otnmbrd wrote in message hlink.net...
For years, on occasion, I've have been involved with teaching someone
boat handling, using single and/or twin screw inboards.
Naturally, (especially on twin screw+) the issue of rudder use arises (
from here we will consider this a twin screw+ discussion).
So that everyone knows, I am a STRONG proponent of rudder use, but I
understand many of the arguments against (G just don't agree with all
of them or feel the argument doesn't really address the issue).
In all of my discussions, I have had a problem with the term "steering",
as in "the boats moving too slow to steer with rudders", or "rudders are
not effective at these speeds".
At any rate, I knew I was never able to explain my point clearly and
concisely.
Recently, for some unknown reason, I remembered a TV show on jet
fighters which discussed (I believe) thrust vectoring, and it dawned on
me that this may be just the term to describe what I am trying to get
across to those I am teaching.
With this in mind, for those with twin screw boats, if I told you that
rudders were important tools of boat handling, but not to be considered
for steering, rather for "thrust vectoring", when maneuvering around a
dock, etc., when kicking an engine ahead, both positive and negative
...... would you understand what I was saying?

otn

On 25 Mar 2004 01:32:19 -0800, (Bob Whitaker)
wrote:
I wasn't quite sure what you were driving at until I read your last
sentence and then it all became crystal clear. So the answer is "yes"
I would understand what you were saying and "yes" it would help me
understand the concept. But even though the term "thrust" is easy for
most people to understand, the concept of a "vector" (magnitude and
direction) may not be. But only you know who your target audience is,
and whether they would understand what a vector is.
===============================

From a teaching standpoint, I think it might be more clear, and to a
wider audience, if you referred to the concept as "directed thrust",
i.e., using the rudder to direct the prop thrust to port or starboard.
It means the same thing but to me it's more intuitive.

I think the most difficult concept to visualize is "prop walk". It's
not instantly clear what is generating the side force, or in which
direction. Prop walk is important to understand because it seems to
cause much of the consternation when backing a single screw. I like
to think of it as a jet of water created between the prop tips and the
hull, but I'm not sure if that's an accurate visualization or not. If
true, it would imply that deeply mounted props, away from the hull,
should generate less "walk" but I'm not sure if that is valid.

It seems to be a consensus among experienced twin screw captains that
leaving the rudders amidship is good practice for most maneuvers.
There are exceptions of course but having the rudders amidship leads
to more predictable response in my experience.

Comments?

I like
to think of it as a jet of water created between the prop tips and the
hull, but I'm not sure if that's an accurate visualization or not.

it is caused by asymetrical thrust of an angled prop shaft. When backing up the
blade coming up to the hull has a much greater "angle of attack" than the blade
going down from the hull. Thus more thrust on one side than the other.

(there is also asym thrust when going forward, but the rudder can compensate
for this as long as the prop is *pushing* water over the rudder, or the boat is
moving forward)

It is also caused to some extent by the contrainment of the prop wash against
the hull on the up side blade, compared to no constrainment on the down side
blade.

the prop will will "walk" in the direction of movement of the bottom blade.

(speaking of prop walk)
I like
to think of it as a jet of water created between the prop tips and the
hull, but I'm not sure if that's an accurate visualization or not.

JAXAshby wrote:
it is caused by asymetrical thrust of an angled prop shaft.

Oh, really? If that is true, then a saildrive or a design with a
perfectly horizontal prop shaft would not have any prop walk.



It is also caused to some extent by the contrainment of the prop wash against
the hull on the up side blade, compared to no constrainment on the down side
blade.

If that were true, then surface drives would not exhibit any prop walk
either.


the prop will will "walk" in the direction of movement of the bottom blade.

Unlike the previous parts of your post, which is just plain wrong, this
is backwards. Prop walk will push the stern in the opposite direction to
that of the bottom arc of the propellor.

Fresh Breezes- Doug King

On 25 Mar 2004 15:17:47 GMT, (JAXAshby) wrote:
it is caused by asymetrical thrust of an angled prop shaft. When backing up the
blade coming up to the hull has a much greater "angle of attack" than the blade
going down from the hull. Thus more thrust on one side than the other.

(there is also asym thrust when going forward, but the rudder can compensate
for this as long as the prop is *pushing* water over the rudder, or the boat is
moving forward)

===========================

OK, the implication of your assertion is that a prop shaft parallel to
the water line, and with little or no hull/aperature entrapment, will
exhibit little or no prop walk. Many full keel lobster boat and
trawler types come close to meeting that criteria.

Does anyone know if that holds up to real world experience?

OK, the implication of your assertion is that a prop shaft parallel to
the water line, and with little or no hull/aperature entrapment, will
exhibit little or no prop walk. Many full keel lobster boat and
trawler types come close to meeting that criteria.

Does anyone know if that holds up to real world experience?


A prop is shaped to move water most efficiently when in forward gear. This is
one reason that it takes more rpm to achieve a given speed in reverse. Some
props are worse than others......(folding props under some sailboats are a
total joke in reverse and you need to put the brakes on pretty early- even at
close to zero knots).

Twin screw boaters have experienced the phenomenon where the reverse wheel
requires a little more throttle than the forward to achieve an almost perfect
pivot.

When a single screw is in reverse, the stern will tend to follow the direction
of the top blade of the prop rotation, not the bottom. Example, a rh propeller
turns clockwise. Viewed from astern, the top blade goes to starboard and the
bottom blade to port when the vessel is in forward gear.

When in reverse, the top blade is moving to port and the bottom blade to
starboard. RH prop vessels routinely back to port.

Prop walk is always present, whether in forward gear or reverse. Yes, the
amount of angle on the prop shaft will increase the amount of prop walk. Prop
walk is not so much a problem in forward because the keel and rudder apply
greater directional influence than the side thrust of the prop.

After a while, we single screw boaters (yes, my wife knew all about that
deficiency when she married me)...learn to use a balance between prop walk and
rudder to steer in reverse. For example, my boat tends to back to starboard. To
back to port, we need a full left rudder and even then we won't start moving to
port until we pick up a bit of speed and the pressure of the water flowing over
the rudder is greater than the pressure produced by the prop walk. To move more
quickly to port, (once we have sternway established), we can take the engine
out of gear, momentarily, so the rudder isn't fighting the prop.

JAXAshby wrote in message
...

it is caused by asymetrical thrust of an angled prop
shaft. When backing up the
blade coming up to the hull has a much greater "angle of
attack" than the blade
going down from the hull. Thus more thrust on one side
than the other.

Interesting theory. Can't think where you got that from.
Better check the trig though. 1 ft/sec astern, typical prop
tip speed about 50ft/sec, lets say one 1 degree.
Differential effect of 20deg shaft angle, 1-cos20 = 0.06deg.
Lets say 1/20 degree. Compared to a typical prop pitch of
20deg or so that means that 1/400 of your thrust (800 lb?
reduced to 2lb) is being exercised over a moment arm of 16
inches to turn your vessel.

OK, that's coarse maths from the back of an envelope, with a
margin of error of maybe an order. But I still don't think
that even 30 ft/lb is going to turn your vessel. That's
what
I use to tighten my nuts. And it reduces to zero when you
have zero stern way.

So your theory can only true when the boat is actually
travelling in reverse (your definition of backing up?).
It is utterly trivial compared to the paddle wheel effect.
You can test this statement by selecting reverse while
moving slowly forward. The vessel won't kick first
one way, then the other. It'll go the paddle wheel way.

It is also caused to some extent by the contrainment of
the prop wash against
the hull on the up side blade, compared to no
constrainment on the down side
blade.

Don't understand that.

All forces are the result of changes in momentum. The wash
spirals away from the prop. Read on.

On the upper side, the lateral speed of the spiral is slowed
by friction against the ship's hull. The lower side much
less so. So the lower lateral momentum added is greater
than the upper.

The result is a force as if paddled by the lower blades.

You could also think of it as the frictional force exerted
on the hull by slowing the lateral speed of the upper part
of the spiral.

Whichever, it's the opposite direction to your theory,
which, in turn, doesn't tie in with my experience.

JimB

jim, asym thrust has been a known factor since before WW2.

it is caused by asymetrical thrust of an angled prop
shaft. When backing up the
blade coming up to the hull has a much greater "angle of
attack" than the blade
going down from the hull. Thus more thrust on one side
than the other.

Interesting theory. Can't think where you got that from.
Better check the trig though. 1 ft/sec astern, typical prop
tip speed about 50ft/sec, lets say one 1 degree.
Differential effect of 20deg shaft angle, 1-cos20 = 0.06deg.
Lets say 1/20 degree. Compared to a typical prop pitch of
20deg or so that means that 1/400 of your thrust (800 lb?
reduced to 2lb) is being exercised over a moment arm of 16
inches to turn your vessel.

OK, that's coarse maths from the back of an envelope, with a
margin of error of maybe an order. But I still don't think
that even 30 ft/lb is going to turn your vessel. That's
what
I use to tighten my nuts. And it reduces to zero when you
have zero stern way.

So your theory can only true when the boat is actually
travelling in reverse (your definition of backing up?).
It is utterly trivial compared to the paddle wheel effect.
You can test this statement by selecting reverse while
moving slowly forward. The vessel won't kick first
one way, then the other. It'll go the paddle wheel way.

It is also caused to some extent by the contrainment of
the prop wash against
the hull on the up side blade, compared to no
constrainment on the down side
blade.

Don't understand that.

All forces are the result of changes in momentum. The wash
spirals away from the prop. Read on.

On the upper side, the lateral speed of the spiral is slowed
by friction against the ship's hull. The lower side much
less so. So the lower lateral momentum added is greater
than the upper.

The result is a force as if paddled by the lower blades.

You could also think of it as the frictional force exerted
on the hull by slowing the lateral speed of the upper part
of the spiral.

Whichever, it's the opposite direction to your theory,
which, in turn, doesn't tie in with my experience.

JimB

Wayne.B wrote:


From a teaching standpoint, I think it might be more clear, and to a
wider audience, if you referred to the concept as "directed thrust",
i.e., using the rudder to direct the prop thrust to port or starboard.
It means the same thing but to me it's more intuitive.

G Thanks .... as I say, "thrust vectoring" may need work.

I think the most difficult concept to visualize is "prop walk". It's
not instantly clear what is generating the side force, or in which
direction. Prop walk is important to understand because it seems to
cause much of the consternation when backing a single screw. I like
to think of it as a jet of water created between the prop tips and the
hull, but I'm not sure if that's an accurate visualization or not. If
true, it would imply that deeply mounted props, away from the hull,
should generate less "walk" but I'm not sure if that is valid.

I consider prop walk to be due to the angle/pitch of the blade, pulling
the prop to one side, on the downward stroke and pushing to the same
side on the upward stroke (rather simplistic), which really has nothing
to due with shaft angle (look at many trawler types and ships - little
shaft angle but significant walk, plus the props are generally away from
the hull).... again, this is MY way of explaining it.

It seems to be a consensus among experienced twin screw captains that
leaving the rudders amidship is good practice for most maneuvers.
There are exceptions of course but having the rudders amidship leads
to more predictable response in my experience.

Comments?

G This last paragraph is probably my main reason for trying to find a
"simplistic" yet effective way of discussing this.
First off, how you handle a particular twin screw boat will depend on a
number of factors:
1. the boat - length, breadth, hull shape, windage
2. the props - inboard turning or outboard turning
3. the conditions - wind and current
4. the operator - what works for one person, does not always work for
another.
5. the rudders - large, small, old fashion, semi balanced, balanced.

I consider myself to be a fairly experienced twin screw boat handler,
but contrary to the above, I normally will start off using rudders and
will continue to do so, until and unless I find them of no value.
Now, this works for me - it's not written in stone, but my main feeling
is that I want the rudders to be a familiar option, if a REAL need
should arise, i.e., I don't have to give their use a second thought.

Again, thanks for your comments.

otn