Thrust vectoring
 

brian!!! knock it off!!


wrote:

For years, on occasion, I've have been involved with teaching someone
boat handling, using single and/or twin screw inboards.....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?

otn

Probably not, judging by one or two of the responses here.
Perhaps it would be easier for you to demonstrate the effect of
sucking a fluid past a board placed in the forward stream line.

Take a hand vacuum (I used a Bissett) and close to the suction nozzle,
place a sheet of paper parallel to the air flow into the nozzle.
Place the edge quite close to the nozzle's side.
You will see the paper move sidewards towards the airflow into the
vacuum if the flow speed is unequal on each side of the paper.
Bernouilli of course. The effect is quite small, but readily visible.

Faster flow leads to lower pressure, of course.
Extending the demo to the rudder placed behind the propellor spinning
in reverse to show the small side force on the rudder should then be
easier to communicate, I'd think.

Brian Whatcott

Thrust vectoring
 

over the nee, brian was sucking smoke.

Thanks. G Wasn't really interested in how you reacted to others
responses, only in how YOU reacted.
As for the rest, you seem to have also grabbed onto the "reverse"
aspect, which is NOT what I was discussing at all. I repeat, "When
kicking an engine ahead" Thrust vectoring does not apply when going
astern, unless G you have Flanking Rudders.... which is another story
entirely.

otn

Brian Whatcott wrote:

Probably not, judging by one or two of the responses here.
Perhaps it would be easier for you to demonstrate the effect of
sucking a fluid past a board placed in the forward stream line.

Take a hand vacuum (I used a Bissett) and close to the suction nozzle,
place a sheet of paper parallel to the air flow into the nozzle.
Place the edge quite close to the nozzle's side.
You will see the paper move sidewards towards the airflow into the
vacuum if the flow speed is unequal on each side of the paper.
Bernouilli of course. The effect is quite small, but readily visible.

Faster flow leads to lower pressure, of course.
Extending the demo to the rudder placed behind the propellor spinning
in reverse to show the small side force on the rudder should then be
easier to communicate, I'd think.

Brian Whatcott

Thrust vectoring
 

manana

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?


On 25 Mar 2004 21:57:40 GMT, (JAXAshby) wrote:

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.

Thrust vectoring
 

Gould 0738 wrote in message
...

If the wind has you pinned against the dock, you won't
realize enough kick to
get the stern free.

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.

As Jere said earlier, 'Kick Ass'.

JimB

Thrust vectoring
 

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

Thrust vectoring
 

JAXAshby wrote in message
...

water *pushed* over a rudder can cause a rudder to turn a
boat, while water "pulled" over a rudder can not.

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

Thrust vectoring
 

JAXAshby wrote in message
...

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

Thrust vectoring
 

JAXAshby wrote in message
...
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

Thrust vectoring
 

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?

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)

Thrust vectoring
 

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"?

You've got my confusulated if you are......

Please elaborate.

Thanks