Boat static in slip. Put in forward gear. move rudder port and
starboard, stern moves accordingly. Put in reverse gear, again same
result. 'splain dis to me,Luci?

If you have a large enough rudder and you can swing it fast enough, you will
see some movement of the stern when static in the slip. In fact, the movement
will be exactly the same whether forward or reverse is
selected....................as long as the engine isn't running. :-)

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.

Gould 0738 wrote in message
...
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?

Sorry! Missed the point that the helm was amidships, and
therefore agree your point. I have walked a stern out in reverse
with the bow tethered to the shore though.

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)

JimB

Single screw .... With full left rudder, kick ahead, bow will swing to
port .... due mainly to rudder and some prop walk (right hand prop).
With rudder hard left, kick astern, stern will swing to port .... all
prop walk, has nothing to due with rudder placement(rudder could be hard
left or hard right, initial affect will be the same).
The only way to shield a prop from prop walk, that I know of is a Kort
Nozzle or similar shield.

otn

JimB wrote:


Note that the effect is in the same direction as the burst
of ahead; ie, full left rudder yaws the vessel to the left
in either fwd or reverse (no 'suction' effect). And note
that I was talking about props sheltered from prop walk -
either through their depth, the use of shields or a lateral
offset.

JimB

Gould 0738 wrote in message
...
See my other post. If conditions are such that water is
ingested from one direction, and exits in another, there is
a change in lateral momentum. That's a lateral force,
usually behind the C of G. That's a yaw. That's turning a
boat.

I think you're making the assumption (without stating it)
that, engine in reverse, water will always be ingested from
dead astern; ie, the rudder is small and of such a distance
from the prop that it has no effect on the direction of
ingestion.

JimB

JimB: Are you attempting to make the case that a rudder
controls the direction
of a power driven vessel primarily by changing the side of the
prop on which
water is "ingested"?

No. I'm starting from the point that the rudder changes the
lateral momentum of water traveling past the stern, and this
creates a lateral force.

It's obvious that, engine ahead, the water comes from straight
ahead and can then be deflected laterally by the rudder to create
a lateral force.

With the engine in astern, I was saying that *if* the water
entering the prop is constrained to come from one side (big
rudder etc) then it's momentum is changed from having a lateral
component to having no lateral component, therefore a similar
force is developed.

However, you've made me reflect on that. Yes, there will be a
force on the rudder, since it is deflecting the water (and you
can feel this on the tiller). And I've witnessed (and had
explained to me) the effect on trawlers and tugs.
But I can't square this with the idea that, in astern with no
motion, the water ultimately has started with zero momentum (if
we're standing still) and is accelerated to have solely a fore
and aft element. That implies no net lateral force.

So something is missing here!

Perhaps there's a subtle change in prop walk in these types of
boats caused by the rudder angle in astern

Thanks for making me think . . .

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

nope. *pull* does NOT affect the rudder, only *push* does.

careful. you *must* be moving through the water for a
rudder to be effective
when trying to back up.

Not quite true. When 'trying to back up' (attempting
rearward motion) you will often use bursts of ahead over the
rudder to alter the pointing of a boat, whether you're
static or actually moving astern.

Very large diameter props, well sheltered from prop walk,
with a large rudder close to the prop (as on single prop
tugs and some fishing vessels), will, in astern, cause
rudder to alter the direction from which water is ingested
into
the prop. If the water is ingested from one direction,
and exits in another, that deflection causes a change in
lateral momentum - creating a force which in turn causes a
yaw. Of course, single screw tugs are not so common now, and
this thread was about twin screw vessels. But this used to
be a useful manoeuvring trick.

Note that the effect is in the same direction as the burst
of ahead; ie, full left rudder yaws the vessel to the left
in either fwd or reverse (no 'suction' effect). And note
that I was talking about props sheltered from prop walk -
either through their depth, the use of shields or a lateral
offset.

JimB

Single screw .... With full left rudder, kick ahead, bow will swing to
port .... due mainly to rudder and some prop walk (right hand prop).
With rudder hard left, kick astern, stern will swing to port .... all
prop walk, has nothing to due with rudder placement(rudder could be hard
left or hard right, initial affect will be the same).

true.


otn

JimB wrote:


Note that the effect is in the same direction as the burst
of ahead; ie, full left rudder yaws the vessel to the left
in either fwd or reverse (no 'suction' effect). And note
that I was talking about props sheltered from prop walk -
either through their depth, the use of shields or a lateral
offset.

JimB

ject: Thrust vectoring
From: "Rod McInnis"
Date: 03/25/2004 11:53 Pacific Standard Time
Message-id:



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.


I'd be interested to know why you think this, as I don't consider the rest of
what you wrote to be applicable to this statement.

Shen

Subject: Thrust vectoring
From: (JAXAshby)
Date: 03/25/2004 13:57 Pacific Standard Time
Message-id:

t can
also be applied to moving ahead,

no, it can not. water *pushed* over a rudder can cause a rudder to turn a
boat, while water "pulled" over a rudder can not.


Jax, quit worrying about push and pull.
Take a sailboat and start backing it with the engine.... get up the right
amount of speed and shut down the engine. You can now steer that boat because
of the forces of the water PASSING over that rudder, exert a steering force.
Hoist sails and trim them to start getting headway (you are not pushing water
over the rudder you are passing water over the rudder) and once you have
sufficient speed, this water passing over the rudder will exert a force to
steer the vessel.... hence it applies astern or ahead.
Too simple for you to understand? ...... oh well......

Shen