Give the dipsquat a break. He's still trying to "google" the
difference between inboard and outboard turning props on a twin screw
inboard ..... with no luck.

Shen44 wrote:
Subject: Which way does a boat turn?
From: (JAXAshby)
Date: 03/29/2004 15:02 Pacific Standard Time
Message-id:

Simple question.

Q: Which way does the stern of a boat t-boned to a dock turn when, with the
rudder turned to port and the tranny in forward, you give a 2 second burst of
the engine?

A: I think we all agree the stern will move starboard.



Q: same question but rudder turned to starboard?

A: the stern will move to port.



Q: same question but rudder centered?

A: the stern will move to port due to prop walk



Thats ok Jax .... we all understand you're a "newbie". Anyone with any
experience would have stated prop rotation, as it's important to the answers.
Just tell them you were using a left hand prop for the first set of questions
and a right hand for the second set of reverse questions.
BTW, you should also add that this exercise is without wind or current.
With wind or current, especially in the astern mode, depending on the boat,
things may not work according to Hoyle.

Shen

PS. It's up to a $1,000/hr .... I can see you'll need special handling, and I
'll have to waste 50 min out of every hour, just trying to keep you
concentrating.

sherr wrote the following useful material:

You STILL trying to figure that one out, Jax?

SV

You STILL trying to figure that one out, Jax?

SV


which one is that?

the rule is :a RH prop paddles to port in reverse. you figer the rest
rick

On 29 Mar 2004 23:02:24 GMT, JAXAshby wrote:

Simple question.

Q: Which way does the stern of a boat t-boned to a dock turn when, with
the
rudder turned to port and the tranny in forward, you give a 2 second
burst of
the engine?

A: I think we all agree the stern will move starboard.



Q: same question but rudder turned to starboard?

A: the stern will move to port.



Q: same question but rudder centered?

A: the stern will move to port due to prop walk



Q: same question, rudder centered, but tranny in reverse (bow of boat
tied to
dock)?

A: the stern will move to port due to prop walk



Q: same question (tranny in reverse) but rudder to port?

A: the stern will move to port due to prop walk



Q: same question (tranny in reverse) but rudder to starboard?

A: the stern will move to port due to prop walk



Anybody but sherr doubt that? (forget the "good professor". he uses the
word
"prop" when he means "rudder" and he wants to argue that friction in the
rudder
bearing makes the difference)



--
Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/

"JAXAshby" wrote in message
...


Q: same question, rudder centered, but tranny in reverse (bow of boat tied
to
dock)?

A: the stern will move to port due to prop walk


I would have said to starboard, reason being that I would expect it to be
the opposite of the prop walk direction when in forward gear.

I have heard several different theories for "prop walk".

"Asymmetrical thrust" is certainly one, and obviously the one being debated
the most right now.

Cavitation is another. Consider that both surfaces of the prop are being
used. The "push" side of the prop can generate unlimited PSI, constrained
only by what can be delivered down the shaft and the strength of the prop
itself. The "Pull" side of the prop can only develop the pressure of the
ambient.

Consider a prop that is spinning just under the surface of the water. As
the tip of the blade passes over the top, the total pressure that can be
developed on the "pull" side is 14.7 PSI. (at the tip). As the tip of the
blade passes down the bottom side it is in deeper water, and hence will have
more ambient pressure. If the prop is 12 inches in diameter then it would
have a foots worth of water more pressure, or approximately another 1/2 PSI
or so. This difference in pressure top to bottom gives the bottom of the
prop just a little more bite, and the prop wants to walk.

There can be other reasons why a given boat kicks one way or the other.
Some inboards I have seen have the rudder mounted just off center, which
allows removal of the prop shaft without having to remove the rudder (which
it would hit if the rudder was on center). I would expect an off center
rudder to have some effect on prop walk.

What is the dominant effect? I have no clue. I don't care. I do know that
my single engine inboard is impossible to steer reliably in reverse. It
usually (but not always) kicks to port in reverse. Which is why when you see
me going down a tight fairway in the marina I will be hugging the left side
as I can turn right much better than I can turn left.

Rod McInnis

On Tue, 30 Mar 2004 18:09:21 -0800, "Rod McInnis"
wrote:


"JAXAshby" wrote in message
...


Q: same question, rudder centered, but tranny in reverse (bow of boat tied
to
dock)?

A: the stern will move to port due to prop walk


I would have said to starboard, reason being that I would expect it to be
the opposite of the prop walk direction when in forward gear.

I have heard several different theories for "prop walk".

"Asymmetrical thrust" is certainly one, and obviously the one being debated
the most right now.

Cavitation is another. Consider that both surfaces of the prop are being
used. The "push" side of the prop can generate unlimited PSI, constrained
only by what can be delivered down the shaft and the strength of the prop
itself. The "Pull" side of the prop can only develop the pressure of the
ambient.

Consider a prop that is spinning just under the surface of the water. As
the tip of the blade passes over the top, the total pressure that can be
developed on the "pull" side is 14.7 PSI. (at the tip). As the tip of the
blade passes down the bottom side it is in deeper water, and hence will have
more ambient pressure. If the prop is 12 inches in diameter then it would
have a foots worth of water more pressure, or approximately another 1/2 PSI
or so. This difference in pressure top to bottom gives the bottom of the
prop just a little more bite, and the prop wants to walk.

There can be other reasons why a given boat kicks one way or the other.
Some inboards I have seen have the rudder mounted just off center, which
allows removal of the prop shaft without having to remove the rudder (which
it would hit if the rudder was on center). I would expect an off center
rudder to have some effect on prop walk.

What is the dominant effect? I have no clue. I don't care. I do know that
my single engine inboard is impossible to steer reliably in reverse. It
usually (but not always) kicks to port in reverse. Which is why when you see
me going down a tight fairway in the marina I will be hugging the left side
as I can turn right much better than I can turn left.


Our boat exhibits no prop walk that I can detect. It also can't steer
from propwash in forward. It must be moving for any steering to occur.
The prop (18-inch Martec) is 12 feet forward of the spade rudder and
fairly close to the keel, so it has little lever arm to turn the boat.

A friend has a boat (Bristol 35.5) that walks horribly. It has a
Maxprop that is much further aft than ours in a boat about the same
length. I suspect the Maxprop is set for excessive pitch, because it
doesn't produce much thrust, but walks like crazy.



Rodney Myrvaagnes NYC J36 Gjo/a

"WooWooism lives" Anon grafitto on the base of the Cuttyhunk breakwater light

Our boat exhibits no prop walk that I can detect. It also can't steer
from propwash in forward. It must be moving for any steering to occur.
The prop (18-inch Martec) is 12 feet forward of the spade rudder and
fairly close to the keel, so it has little lever arm to turn the boat.

yeah, 12 feet doesn't leave much "push" still in the prop stream, and too a
prop at about the pivot point can't do much twisting of the boat.

"Rodney Myrvaagnes" wrote in message
...

Our boat exhibits no prop walk that I can detect. It also can't steer
from propwash in forward. It must be moving for any steering to occur.
The prop (18-inch Martec) is 12 feet forward of the spade rudder and
fairly close to the keel, so it has little lever arm to turn the boat.

I assume that this is a sail boat, which would put this into a "large
rudder" catagory. Sailboats, by their very nature, need to be able to steer
at slow speeds without any thrust from the prop. Thus, they install a rudder
that is large enough to provide adequate steering at dead slow speeds.

A typical power boat, on the other hand, uses a very small rudder that is
located as close as possible to the prop. The rudder is rarely taller than
the prop is. Such a rudder is very effective when it can deflect the stream
of water that is being pushed by the prop, but has minimal effect when the
prop is not turning.


Rod

On Wed, 31 Mar 2004 11:56:00 -0800, "Rod McInnis"
wrote:


"Rodney Myrvaagnes" wrote in message
.. .

Our boat exhibits no prop walk that I can detect. It also can't steer
from propwash in forward. It must be moving for any steering to occur.
The prop (18-inch Martec) is 12 feet forward of the spade rudder and
fairly close to the keel, so it has little lever arm to turn the boat.

I assume that this is a sail boat, which would put this into a "large
rudder" catagory. Sailboats, by their very nature, need to be able to steer
at slow speeds without any thrust from the prop. Thus, they install a rudder
that is large enough to provide adequate steering at dead slow speeds.

A typical power boat, on the other hand, uses a very small rudder that is
located as close as possible to the prop. The rudder is rarely taller than
the prop is. Such a rudder is very effective when it can deflect the stream
of water that is being pushed by the prop, but has minimal effect when the
prop is not turning.

Yes, ours is a sailboat. We rented a canal boat once that had a
semibalanced barn door rudder clost to the prop. It could turn
practically in its own length. The appearance of the propwash suggests
the rudder completely covered the prop circle, shooting out the side
when hard over.



In reverse it did walk, but very predictably. As icing on the cake, it
also had a bow thruster.
Rod



Rodney Myrvaagnes NYC J36 Gjo/a


"Curse thee, thou quadrant. No longer will I guide my earthly way by thee." Capt. Ahab