wayne, please read what you wrote and you will find you are saying exactly what
I am saying that you are disagreeing with so strongly. the boat MUST be
**moving** for the rudder to have an effect in reverse.

please read what you wrote again.

When the sternway becomes sufficient, the
rudder exerts

Absolutely right.

give it a burst
of power in reverse ... get it
moving sternward at a knot or two,

let the boat coast backward on momentum while
maneuvering with the rudder.

Once the boat is moving the keel and
rudder take over

Once the boat is moving the keel and
rudder take over, forward or reverse.

in forward, you can use the prop blast to move the rudder. in reverse, you
can't.

but, yes, once the boat is moving in reverse the rudder can be used to turn the
boat.

Subject: Thrust vectoring
From: Brian Whatcott
Date: 03/28/2004 19:26 Pacific Standard Time
Message-id:

On 28 Mar 2004 19:11:58 GMT, (Shen44) wrote:


G Never hoid of this Feynman character, before this thread....
Shen

He won a Nobel prize.
He was an accomplished bongo player too, and had a taste for women.

But I think you can take it that his bongo-playing talent and Nobel
class research endeavours are pretty well independent from
boat-handling skills. No Nobellist has ever won a major boat race, as
far as I know.


G haven't heard of too many that are pilots, either.

Shen

Subject: Thrust vectoring
From: "JimB" j
Shen44 wrote in message

OK, think I see what you're saying.
First off, I used to run a 65 foot boat with a similar
configuration and
remember a beautiful old 45' ketch with the same thing ... both
had large
degrees of "propwalk".

Interesting. Some of the older boats had engines added later, of
course, and sensibly had prop shafts installed with an offset so
you could remove the shaft without taking the rudder off. That
changed the prop wallk picture a bit!

True There are many varying degrees of propwalk, for varying reasons (offsets,
kort nozzles, rope guards, to name a few). I normally board a boat and find out
type and rotation of prop to determine expected direction of walk, then as soon
as possible, if it's new to me I'll try to create a situatuion to test it out.
I've learned to expect a certain direction and amount of walk for some boats,
but always keep in mind that sometimes, especially on some boats they may walk
opposite to the expected (normally this will be a wind, current, or existing
turn situation, but not always.). .... at any rate, expect it, test it, and
then be ready for the unexpected.


Secondly, I think we may have different views of what is the
major cause of
prop walk (I attribute it to the pitch of the blades pulling to
a greater
degree on the "down stroke" side of the revolution, than the
"upstroke side").

OK. The only certainty about prop walk is that it must be caused
by the prop in reverse squirting more water to one side of the
boat than the other. We could discuss the mechanism for ages -
but there wouldn't be much practical use to that unless we were
designers!

Again true, although there is still prop walk when going fwd, it's just not
normally as noticeable.

BG Why would I want to design "propwalk" out? "Propwalk" is
my friend!

Especially pulling the stern in as you come alongside.... bit of
a sod when it's the wrong side though.

G Can also be used when docking the other side too, to stop the stern coming
alongside as you stop.
There is no doubt, that at times, prop walk is a pita, because it is contrary
to what you want and need to do, but in a high percentage of cases, it can be
made to work for you.

Shen

On 28 Mar 2004 17:29:30 GMT, (JAXAshby) wrote:

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.

Not another one of your "what I did on shore leave" Navy stories, JAX.
There are young people present.

R.

In article ,
Wayne.B wrote:

On 28 Mar 2004 15:14:03 GMT, (Gould 0738) wrote:
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.

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

Absolutely right. One of my favorite strategies for dealing with a
sailboat that has a nasty prop walk in reverse, is to give it a burst
of power in reverse where there is still some maneuvering room, get it
moving sternward at a knot or two, put the engine in neutral to kill
the prop walk, and then let the boat coast backward on momentum while
maneuvering with the rudder. Once the boat is moving the keel and
rudder take over, forward or reverse.


With our boat, that's the only way to do it. Until we're doing a knot or
two, we can at best keep the boat from pulling to port while the engine
is in gear. (a 16" prop *may* be a bit excessive for a 28' boat.)

Something that I've noticed, though I'm not sure if it's universal or
even really true: When I punch it, the prop walk seems to be minor. When
I want maximum prop walk, idle speed is the way to go. It could be just
that the duration is so much shorter, or that the higher thrust
straightens things out.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/

Something that I've noticed, though I'm not sure if it's universal or
even really true: When I punch it, the prop walk seems to be minor. When
I want maximum prop walk, idle speed is the way to go. It could be just
that the duration is so much shorter, or that the higher thrust
straightens things out.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/


Could it be due to a short duration of a "punch" in reverse, or due to the
factor that sternway (and therefore rudder effectiveness) increased enough at
the higher RPM ? :-)

Subject: Thrust vectoring
From: Jere Lull


I don't doubt that this is true for your boat (since you've observed it) but,
I'd bet it's not universal and would depend on the boat, set-up, and
conditions.
Why it occurs, would take someone familiar with boat props, and I would wonder
if it had anything to do with the fact that the prop might be working more
efficiently when starting at slower RPM's than at higher, especially if the
boat is DIW.
Another observation along the same lines .... I'm more apt to see the effects
of prop walk when going ahead, if boat speed is low.

Shen

Something that I've noticed, though I'm not sure if it's universal or
even really true: When I punch it, the prop walk seems to be minor. When
I want maximum prop walk, idle speed is the way to go. It could be just
that the duration is so much shorter, or that the higher thrust
straightens things out.

Not another one of your "what I did on shore leave" Navy stories, JAX.

can't be. I was never in the Navy.


R.

In article ,
(Gould 0738) wrote:

Something that I've noticed, though I'm not sure if it's universal or
even really true: When I punch it, the prop walk seems to be minor. When
I want maximum prop walk, idle speed is the way to go. It could be just
that the duration is so much shorter, or that the higher thrust
straightens things out.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/


Could it be due to a short duration of a "punch" in reverse, or due to the
factor that sternway (and therefore rudder effectiveness) increased enough at
the higher RPM ? :-)

I've been tending toward that explanation except that a few seconds of
near-idle reverse can slide the transom over a foot without moving very
far; punching it for a few seconds (2000 rpm out of 3600), we don't seem
to pivot at all. When I shift to neutral, we don't have authoritative
steerage way yet.

Our prop (16x10x3 AutoProp) is pretty close to the rudder... Could we be
getting enough flow across it to counteract the walk?

What got me thinking about it and taking closer notes was watching a new
36 or 38' Catalina on charter try to back into a slip at idle last
September. Actually rotated more than it backed. The rudder never got a
chance. I'd never seen a sailboat do that badly, though I've seen a few
powerboats humble some mighty fine helmsmen.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/