bject: Thrust vectoring
From: (JAXAshby)
Date: 03/26/2004 13:53 Pacific Standard Time
Message-id:

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

HOLY EIGHT SMOKES !!!!!! Jax agrees with otn on something !!!!!
Hey OTN .... are you sure you meant to say that?
Hey Jax .... I bet you picked up on the difference between inboard turning and
outboard turning props, too .... you wanna esplain that?

Shen

JAXAshby wrote in message
...
jim, asym thrust has been a known factor since before WW2.

Since Pontius was a pilot, Jax. So you got the idea from
aviation.

What I was querying (though I didn't make that very clear) was
your application of the theory to boat prop walk, where the boat
velocities are trivial, the lateral dimensions of the prop are
trivial, and the inertia of the vessel is large. Your argument
was a bit like talking about the effect on boat pitch due to yaw
through the gyroscopic effect of a running engine. It's there.
But it's trivial.

I was also querying your other theories of the causes of prop
walk, which I certainly don't understand.

JimB

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

otnmbrd wrote in message
k.net...
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.

There's very little prop walk on traditional wood trawlers, when
the prop is set almost half way down a 3m deep keel in a cutaway.

Also, many Greek fishing vessels (enormously over-engined with
old bus engines 300hp driving a 30ft caique) to put long skegs in
front of the prop and big lateral plates extending sideways at
water level to stop air being drawn into these mammoth blades.
This also keeps nets and ropes clear, and seems to minimise prop
walk.

I don't know a Kort nozzle, but if it's a protective cylinder
around the prop with a skeg and cross shaped blades supporting it
in front, that's what I meant by 'shield'. Some old, single
engined tugs used to use these and I suspect the Greek approach
is a half way house.

Also on hydrofoils, where the prop is 2m deep with no hull
interference in sight. But that's a bit exotic, since most
hydrofoils are twin screw.

JimB

schlackoff, congratulations on your flip-flop. from instant one I have been
saying the boat MUST be moving for the rudder to be effective in reverse, while
you kept saying No the prop pulls enough water forward to make it work.

at least I am glad you saw the light. interesting how you changed position and
claimed it as your own all along.

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

schlackoff, English is beyond you.

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

HOLY EIGHT SMOKES !!!!!! Jax agrees with otn on something !!!!!
Hey OTN .... are you sure you meant to say that?
Hey Jax .... I bet you picked up on the difference between inboard turning
and
outboard turning props, too .... you wanna esplain that?

Shen

That implies no net lateral force.


that is correct.

There's very little prop walk on traditional wood trawlers, when
the prop is set almost half way down a 3m deep keel

That's because there is almost no "end-plate" effect from the hull.

Subject: Thrust vectoring
From: (JAXAshby)


ROFL (see my other post on steering)
You obviously don't know what I said, and I can guarantee you can't find a post
of mine which backs up your statement below (unless you take something out of
context)
Sorry Jax, as per usual, you are wrong .... I didn't flip-flop and never said
what you are crediting me with.

Shen


schlackoff, congratulations on your flip-flop. from instant one I have been
saying the boat MUST be moving for the rudder to be effective in reverse,
while
you kept saying No the prop pulls enough water forward to make it work.

at least I am glad you saw the light. interesting how you changed position
and
claimed it as your own all along.

Subject: Thrust vectoring
From: (JAXAshby)


So you can't explain the difference .... thought not.

Shen

schlackoff, English is beyond you.

Hey Jax .... I bet you picked up on the difference between inboard turning
and
outboard turning props, too .... you wanna esplain that?

Shen

Subject: Thrust vectoring
From: "JimB"
Date: 03/27/2004 04:21 Pacific Standard Time
Message-id: Sre9c.1164$aQ3.634@newsfe1-win


otnmbrd wrote in message
nk.net...
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.

There's very little prop walk on traditional wood trawlers, when
the prop is set almost half way down a 3m deep keel in a cutaway.

Please explain what you mean by "trawler" (fishing or yacht) and "cutaway" and
what the depth of the prop has to do with it.


Also, many Greek fishing vessels (enormously over-engined with
old bus engines 300hp driving a 30ft caique) to put long skegs in
front of the prop and big lateral plates extending sideways at
water level to stop air being drawn into these mammoth blades.
This also keeps nets and ropes clear, and seems to minimise prop
walk.

Interesting.


I don't know a Kort nozzle, but if it's a protective cylinder
around the prop with a skeg and cross shaped blades supporting it
in front, that's what I meant by 'shield'. Some old, single
engined tugs used to use these and I suspect the Greek approach
is a half way house.

I can see where this stuff together would cut down propwalk. A Kort nozzle can
be found on fishing boats, but your main application will be tugs. It's a
shaped cylinder, generally attached to the hull, which surrounds the prop,
entirely.
It's main function is to increase "bollard pull", but because it surrounds the
prop, the prop can not "pull" to one side (propwalk) since all thrust is
directed out the nozzle, either ahead or astern.

Also on hydrofoils, where the prop is 2m deep with no hull
interference in sight. But that's a bit exotic, since most
hydrofoils are twin screw.

JimB

Not sure why this reference to depth of a prop in relation to a hull. I may be
reading you wrong, but, for instance, take a single screw ship where the prop
is generally set fairly deep on the hull..... they tend to torque to a
fairtheewell.

Shen