Thrust vectoring
 

I like
to think of it as a jet of water created between the prop tips and the
hull, but I'm not sure if that's an accurate visualization or not.

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.

(there is also asym thrust when going forward, but the rudder can compensate
for this as long as the prop is *pushing* water over the rudder, or the boat is
moving forward)

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.

the prop will will "walk" in the direction of movement of the bottom blade.

Thrust vectoring
 

My first few lessons in small powerboat handling as a young lad included "thrust
vectoring" examples, although those terms were never used. I remember vividly
bringing the launch (about 18 feet with an Atomic 4 and a "stick" tiller) bow to
the dock and wagging the butt back and forth with the tiller. The exercise was
repeated in reverse hanging off a bow line, proving (contrary to jaxie's claim)
the affect works in reverse, although with greatly diminished results. These
exercises were an eye-opener for me, since as a sailor, I had never thought in
terms of water flow created by a prop.

A few years later I had another lesson (I should have known better by then) when
I tried to power off of a dock at Castine, ME with a very strong current
flowing. An old hand showed me the "power against a spring line" trick. I was
amazed that so much power could be brought to bear.

These techniques are best taught "hands on," in a small boat, if possible, so
that one can feel the power the prop can generate. Whether the words "thrust
vectoring" are used depends on the audience.





"otnmbrd" wrote in message
hlink.net...
Interesting. I can't think of too many boats that "pull" water over a
rudder for steering, except in reverse, and this is well known to be,
generally, of little effect.
Of course, considering the source of that statement, we realize it has
no value, other than to say he didn't understand what I was saying, nor
does he have any knowledge of the subject.
G I'll call that a "negative", with reservations.....
Come-on group, I'm looking for some pos/neg responses from people with
some knowledge and experience, not jaxasses .....

Thrust vectoring
 

The exercise was
repeated in reverse hanging off a bow line, proving (contrary to jaxie's
claim)
the affect works in reverse,

geezus kriste, jeffies, you claim to have a degree in physics and have NEVER --
to this moment -- heard of classic "under water lawn sprinkler" paradox that
ALL physics students learn.

Get your wife to explain it to you. It is impossible to steer by *pulling*
water over a rudder with a prop. Can't be done, except when under the
influence of hard drugs.

Thrust vectoring
 

here you go, jeffies and wife. the underwater lawn sprinkler as shown by
Feynman (remember him?) on the first hit.

jeffies, people who claim to have a degree in physics should be able to
remember this stuff.

http://groups.google.com/groups?q=%2...an&hl=en&lr=&i
e=UTF-8&selm=4l58m4%24359%40newsbf02.news.aol.com&rnum =1

Thrust vectoring
 

(speaking of prop walk)
I like
to think of it as a jet of water created between the prop tips and the
hull, but I'm not sure if that's an accurate visualization or not.

JAXAshby wrote:
it is caused by asymetrical thrust of an angled prop shaft.

Oh, really? If that is true, then a saildrive or a design with a
perfectly horizontal prop shaft would not have any prop walk.



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.

If that were true, then surface drives would not exhibit any prop walk
either.


the prop will will "walk" in the direction of movement of the bottom blade.

Unlike the previous parts of your post, which is just plain wrong, this
is backwards. Prop walk will push the stern in the opposite direction to
that of the bottom arc of the propellor.

Fresh Breezes- Doug King

Thrust vectoring
 

On 25 Mar 2004 15:17:47 GMT, (JAXAshby) wrote:
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.

(there is also asym thrust when going forward, but the rudder can compensate
for this as long as the prop is *pushing* water over the rudder, or the boat is
moving forward)

===========================

OK, the implication of your assertion is that a prop shaft parallel to
the water line, and with little or no hull/aperature entrapment, will
exhibit little or no prop walk. Many full keel lobster boat and
trawler types come close to meeting that criteria.

Does anyone know if that holds up to real world experience?

Thrust vectoring
 

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

Thrust vectoring
 

OK, jaxie, please explain Feynman's Sprinkler Paradox and give us your answer.
And why do you think this is relevant?

Are you claiming that an object placed on the "suction side" of a propeller will
not feel an affect, that is it cannot be deflected to one side if it
asymmetrical?


"JAXAshby" wrote in message
...
The exercise was
repeated in reverse hanging off a bow line, proving (contrary to jaxie's
claim)
the affect works in reverse,

geezus kriste, jeffies, you claim to have a degree in physics and have
NEVER --
to this moment -- heard of classic "under water lawn sprinkler" paradox that
ALL physics students learn.

Get your wife to explain it to you. It is impossible to steer by *pulling*
water over a rudder with a prop. Can't be done, except when under the
influence of hard drugs.

Thrust vectoring
 

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

Thrust vectoring
 

Sorry Jax, sometimes you get it right, but this time your habit of
stating absolutes where not appropriate is showing again. The
underwater lawn sprinkler is an imperfect model for the mechanics of a
boat's prop and rudder. Now, a prop in reverse creates very little
water movement past the rudder compared to one in forward, but very
little is not the same as none. As long as there is water being moving
past and deflected by the rudder there will be some lateral force
generated. Not much in this case, but some. This can be readily
demonstrated as another poster pointed out.

JAXAshby wrote:
The exercise was
repeated in reverse hanging off a bow line, proving (contrary to jaxie's
claim)
the affect works in reverse,


geezus kriste, jeffies, you claim to have a degree in physics and have NEVER --
to this moment -- heard of classic "under water lawn sprinkler" paradox that
ALL physics students learn.

Get your wife to explain it to you. It is impossible to steer by *pulling*
water over a rudder with a prop. Can't be done, except when under the
influence of hard drugs.



--
Dan Best - (707) 431-1662, Healdsburg, CA 95448
B-2/75 1977-1979
Tayana 37 #192, "Tricia Jean"
http://rangerbest.home.comcast.net/TriciaJean.JPG