BoatBanter.com - Thrust vectoring

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Thrust vectoring

gould, you are right *IF* the boat is moving. However, not right if the boat
is not moving. That's a fact of physics. What's more I explained in easy
terms why. What's more I gave unimpeachable references to backup what I
explained. In addition, I gave the name of a famous physicist to google to
your hearts content, as this particular physicist actually made a movie of the
effect because so many people could not believe something they did not
understand intuitively.

the prop has zero effect on the rudder when in reverse. zip. nada. squat.
nothing.

it can't.

Let me help you out here. a rudder has NO effect on which way a boat is
going
in reserve because of the prop. None. Nada. Zip. Zero. Nothing.

That's a fact of physics, schlackoff. get used to it.

Your premise is wrong.

As the vessel goes in reverse, water presses against the face of the rudder
if
it is turned to port or starboard. The greater the speed, the more the water
force on the rudder. At sufficient speed, the force of the water against the
rudder overcomes the prop walk and the boat will follow the rudder, not the
prop.

When speed is reduced, the vessel will reach a point where the prop walk
force
is greater than the rudder force and the rudder will not be useful to
steering.

It could be argued that at sternway at all, there is some effect from the
rudder. It may not be enough to overcome the prop walk at the slowest speeds,
but in principle the prop walk is finitely reduced
with even the slightest pressure against an opposing rudder.

The water hitting the rudder is not being pulled over the rudder by the prop,
any more than the bow wake is created by water being pulled against the bow
when going forward. I think everybody agrees on that. But to say a rudder as
NO
effect on a boat going in reverse because of prop walk is not correct. The
rudder exerts increasing influence as speed increases.

This may not be true for boats with itsy bitsy rudders, btw. It might not be
possible to get enough water pressure on an 8 1/2 X 11" rudder to offset prop
walk. :-)

Thrust vectoring

JAXAshby wrote in message
...
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.

Correct. I was just pointing out some other low prop walk cases.

JimB

Thrust vectoring

Shen44 wrote in message
...
Subject: Thrust vectoring

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.

To be exact, a Brixham Trawler, a 75ft gaff rigged ketch,
designed with a powerful rig for sail trawling. The keel is
straight from stem to stern, about 1m draft at the stem, 3m draft
at the stern. The rudder is mounted 20degrees off vertical. The
aft hull is flat, wide beamed, and lifts above waterline at the
rudder post. The prop is mounted 1.5m deep, in a 60cm (2ft!) arc
cut into the keel just forward of the rudder hinge. The effect is
that the reverse wash of the prop runs down each side of a
vertically straight keel - which I assume kills the spiralling of
the prop wash on both sides equally. Whereas most modern vessels
have the upper part of their prop wash losing energy to the hull
bottom through friction, and no other interference.

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.

I think the key factor here is the cruciform blades supporting
the cylinder in front of the prop, which act as flow
straighteners. No spiral flow along the hull = no prop walk.

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.

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.

If the prop's deep, but close to a horizontal surface (the hull)
you'll get good prop walk. Because the hull, through friction,
kills the lateral velocity of the spiralling wash on top, but the
lower spiral rushes off, giving you asymmetric reverse thrust.
Paddle wheel effect if you like. If the prop is well clear of the
hull, just look at the shaft supports to design prop walk out. If
you've got a vertical blade supporting the shaft, stick a
balancing skeg below it. Keeps the ropes clear too.

JimB

Thrust vectoring

gould, you are right *IF* the boat is moving. However, not right if the boat
is not moving.

We agree.

I took issue with your statement:

Let me help you out here. a rudder has NO effect on which way a boat is
going
in reserve because of the prop.

because the phrase "which way a boat is going in reserve (sic)" implies the
boat is in motion. Rather obviously, if the boat is going somewhere, it must be
in motion.

Putting the boat in reverse will (after a split second) result in sternway
unless the engine is not running
or the vessel is lashed to the dock. When the sternway becomes sufficient, the
rudder exerts greater influence on the direction of boat than the prop walk
does.
In my experience. Unlike some here I don't have a string of degrees and
abbreviations following my name, (unless S.O.B. counts), so I can't argue the
applicable theories of physics or fluid dynamics....... but by the same token I
don't have to conduct an "experiment" to know how a single screw with a fair
size rudder behaves in reverse. :-)

Thrust vectoring

gould, you are right *IF* the boat is moving. However, not right if the
boat
is not moving.

We agree.

I took issue with your statement:

Let me help you out here. a rudder has NO effect on which way a boat is
going
in reserve ******* because of the prop*********.

sorry, the sentence structure may not have been clear. I was saying the prop
caused no change in boat direction because of the rudder. The prop and rudder
are net zero between them

yup, the prop can make the boat move and then the rudder can steer the boat.
but the prop wash in reverse does nothing to the rudder, as opposed to prop
wash in forward which does.

sorry about my sentence structure not being clear as to what I was trying to
say.

because the phrase "which way a boat is going in reserve (sic)" implies the
boat is in motion. Rather obviously, if the boat is going somewhere, it must
be
in motion.

Putting the boat in reverse will (after a split second) result in sternway
unless the engine is not running
or the vessel is lashed to the dock. When the sternway becomes sufficient,
the
rudder exerts greater influence on the direction of boat than the prop walk
does.
In my experience. Unlike some here I don't have a string of degrees and
abbreviations following my name, (unless S.O.B. counts), so I can't argue the
applicable theories of physics or fluid dynamics....... but by the same token
I
don't have to conduct an "experiment" to know how a single screw with a fair
size rudder behaves in reverse. :-)

Thrust vectoring

"motorsailor", eh? that's a hermaphordite that nobody on either side of the
bed wants to play with. a twig too small to be a stick, and too long to be a
clit.

What Irwin 37? If you think I own one, you're thinking of the wrong guy .....
sides, I'm thinking of a "motorsailer" so I can enjoy the pleasures of both
worlds.

Shen

on an Irwin 37? what watertight door?

Aww now, come on Shen. Why not ask the dummy to ride with you for a
week for some real world experience .... just be sure he signs the
release form ... dipsquat will probably fall on his ass, the first time
he goes through a watertight door.

Thrust vectoring

there you have if, folks. schlackoff declares himself a greater physicist than
Feynman. Wait until the Nobel Committee learns fo schlackoff. Fame and
fortune will soon be his.

go ahead, schlackoff. try to back up a boat using the prop and rudder. do
it
around expensive boats, and cops too if you can. Have a beer or two first,
'cuz it is so easy.

It is, if you know how to do it and can make use of the various factors
involved, and/or compensate between their effects.

schlackoff, yesterday you flip-flopped rather nicely. today, you are back
to
saying what you originally said. i.e. that you are a dummy.

It would appear that you don't have a clue WHAT I said, much less understand
the meaning.

Shen