Thanks. Actually, I am talking about "wash over the rudder", but I'm
trying to get away from the term "Steering" and it's overall sense of
driving the boat, to concentrate on the effects/advantages of a "kick"
ahead when using or not using the rudder.

otn

Jere Lull wrote:


I don't think you're simply talking about the wash over the rudder. That
is clearly thrust vectoring, but should be easy enough to explain as
"kicking the ass over".

Thanks. Although it may not be technically correct, I think I could
easily explain the "vector" as the direction the thrust pushes too, due
to the rudder angle.
G needs work.

otn

Bob Whitaker wrote:
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"

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.

Get your wife to explain it to you. It is impossible to steer by *pulling*
water over a rudder with a prop. Can't be done, except when under the
influence of hard drugs.



It's possible to steer when sternway creates water pressure against the rudder.

Stick a flat board in the water and try to move it directly forward while
holding it at a fixed angle. :-)

Where can I buy one of those underwater lawn sprinklers that dispenses hard
drugs? :-)



NEVER --
to this moment -- heard of classic "under water lawn sprinkler" paradox that
ALL physics students learn.

Thanks. One of the things I tend to talk about is the potential
advantage of using rudders when twisting (swivel) as it tends to help
when you work into how you need to set your rudders to "walk".

otn

JimB wrote:

I like Jere's approach. It correctly describes both phenomena you
get using rudder with power; first, a rotation, secondly, a
lateral movement.

When teaching, I've found it useful to separate the two
phenomena.

First, how to point the boat. If you're static, that's using one
engine ahead and one astern to swivel around a point with rudder
neutral. If you're moving forward significantly (or aft) that's
using rudder or differential engine to steer.

Second, how to shift the boat (or part of it) bodily sideways.
That's using wind, and/or prop walk, and/or power with rudder
(the other engine being used to control the degree of fore/aft
movement), and/or bow thruster. The last three I certainly
understand as 'thrust vectoring' (I'm familiar with the term as
an ex VTOL pilot, and most numerate people would be OK with
'vectoring').

When teaching, though, I'd test my pupil's understanding of the
phrase before using it. After all, the student may be numerate -
as an accountant - rather than a physics major. And 'kicking ass'
does have a nice ring to it . . .

JimB, Yacht Rapaz, sadly, for sale to buy that nice new Greek
house:
http://www.homepage.ntlworld.com/jim...cification.htm

Thanks. Actually, I am talking about "wash over the rudder", but I'm
trying to get away from the term "Steering" and it's overall sense of
driving the boat, to concentrate on the effects/advantages of a "kick"
ahead when using or not using the rudder.

From a dead stop, with rudder amidships, you'll experience only a minor amount
of crabbing in the stern when selecting forward. IME. The forward efficiency of
the prop will begin moving the boat ahead so quickly that you can't count on
much useful kick in the stern.

If the wind has you pinned against the dock, you won't realize enough kick to
get the stern free. If the wind is blowing you off the dock, you won't need to
worry about it in any event.

otnmbrd wrote:
....snip...
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?

Just a thought... many people would understand the concept of "torque
steer" from their car-driving experience. Maybe that phrase would
work. Personally, I wouldn't find "thrust vectoring" an informative
phrase. Adn it seems to me that you're looking for a useful
description rather than a "scientific" explanation, right?

Frank

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

Thanks. Hands on IS the best teacher,G but we need to be able to
explain what just happened and why, when that poor newbie get's that
look of confused panic on his face, when what he just did, doesn't work.

otn

Jeff Morris wrote:
My first few lessons in small powerboat handling as a young lad included "thrust
vectoring" examples, although those terms were never used. I remember vividly
bringing the launch (about 18 feet with an Atomic 4 and a "stick" tiller) bow to
the dock and wagging the butt back and forth with the tiller. The exercise was
repeated in reverse hanging off a bow line, proving (contrary to jaxie's claim)
the affect works in reverse, although with greatly diminished results. These
exercises were an eye-opener for me, since as a sailor, I had never thought in
terms of water flow created by a prop.

A few years later I had another lesson (I should have known better by then) when
I tried to power off of a dock at Castine, ME with a very strong current
flowing. An old hand showed me the "power against a spring line" trick. I was
amazed that so much power could be brought to bear.

These techniques are best taught "hands on," in a small boat, if possible, so
that one can feel the power the prop can generate. Whether the words "thrust
vectoring" are used depends on the audience.

Thanks. In answer to your question, yes, but "torque steer" would not
work for me, as I would be apt to apply that to "prop walk".

otn

Frank Maier wrote:
otnmbrd wrote:
...snip...

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?


Just a thought... many people would understand the concept of "torque
steer" from their car-driving experience. Maybe that phrase would
work. Personally, I wouldn't find "thrust vectoring" an informative
phrase. Adn it seems to me that you're looking for a useful
description rather than a "scientific" explanation, right?

Frank