Wayne.B wrote in message
...

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?

True. You only need rudder if you're trying to achieve a
lateral shift in position.

This can be nicely illustrated in zero wind and current if
you position close to a buoy, apply full rudder, then
balance one engine astern and the other forward to give zero
yaw rate and boat speed, and the boat will slowly (deep
keel) or quickly (shallow keel) move laterally away from the
direction you've applied rudder.

The 'Kick Ass' effect, with rotation cancelled by
differential power!

JimB

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.

Much depends on the size of the rudders.
Twin screw boats tend to have much smaller rudders than single screw for a
variety of reasons. Some of these reasons have to do with rudder support when
the rudder is not directly aft of the keel, others involve the reduction of
drag to achieve greater speed, and still others calculate the combined area of
both rudders.

In the final analysis, on most twin screw vessels the rudder has a very
marginal ability to change the direction of the boat through the water compared
to the application of unequal thrust from the engines.

I'll be out on a speedy twin screw boat later this morning to collect some data
and
get some photos. I fully expect that at crusing speed or better and with equal
thrust from the engines the turning circle of this 42 footer will be close to
1/8 mile in diameter. And that won't be particularly unusual. Obviously not
much rudder in play.

When close quarter maneuevering we consider the wind and current and compensate
for any significant forces.
Seems to make sense that one should respond with the most efficient and
significant force available....whether that's
unequal thrust, a big rudder, or an oar.

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"

Go to http://www.videos.sailingcourse.com/pivot_turn.htm

On a related page,
http://www.videos.sailingcourse.com/...pring_line.htm

"A forward spring line is placed from the stern of the sailboat to the dock. In
reverse gear, the sailboat is slowly backed. The spring line prevents the
backward motion of the boat and swings the bow away from the dock. One may also
use the rudder to vary the direction of boat thrust, to aid in holding the boat
to or in swinging the boat away from the pier. "



"Rich Hampel" wrote in message
...
Go to http://www.videos.sailingcourse.com/pivot_turn.htm

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

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

otnmbrd wrote...
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".

Yep, much more appropriate; but that leaves me with no opinion. Hope
you derive something useful here. Have you asked your students if they
have any analogies or concepts to offer when you see the lightbulb of
understanding go off in their heads that first time?

I'm always learning.
Actually, most people I get into these discussions with (I'm not an
instructor in any sense of a formal course, though I've taught more than
one "newbie") are people with experience that have a problem or need to
figure out a reason for what they are seeing/experiencing. Frequently
the discussion revolves around "inboard" turning and "outboard" turning
props (no one here picked up on that difference, which I find
interesting, yet many were concerned with using rudders when going
astern, which I was not discussing).
I'm still hoping to see some other responses to my responses, since how
others see things, can be as educational as how I see things BG.

otn

Frank Maier wrote:
otnmbrd wrote...

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".


Yep, much more appropriate; but that leaves me with no opinion. Hope
you derive something useful here. Have you asked your students if they
have any analogies or concepts to offer when you see the lightbulb of
understanding go off in their heads that first time?

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?


No, I would think that most people would be more confused.

To begin with, I would start by pointing out that there is a big difference
between how an inboard reacts and how I/Os or outboards react. There is
also a big difference between what I call "large rudder" and "small rudder"
boats.

A typical sail boat has a "large rudder" which becomes effective, in either
forward or reverse, as soon as the boat is making way.

A typical power boat has a "small rudder" which either needs a significant
amount of boat speed or to have the prop pushing water past it. These
rudders are generally ineffective in reverse.

The basic concept of twin engine inboards is that you consider the rudder to
only be a factor when the engine is in forward. Ignore it for the engine in
reverse.


Rod