"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

Comments interspersed:

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 believe he was talking about a RH, fixed pitch prop, no wind or
current.....stern should go to port in reverse, to stbd when ahead.

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

G Same here .... My opinion FWIW, prop walk is a function of the prop
and pitch. The angle of the shaft may increase it, but is not the basic
cause as will some hull shapes and prop locations (tunnels or nozzles),
increase or decrease it.....




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.

My first impression is that this would have minimal effect if any, G
would need some test, and although I've worked some boats with various
rudders types and locations, I can't say that I've ever noticed a
difference that I could attribute to the rudder.


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.

Going to assume that you have a RH fixed pitch prop. This is the norm,
however, wind and/or current can make a boat which normally backs to
port because of the prop, back to stbd from the "get go".
Steering your boat "reliably" in reverse, would involve many possible
solutions, including "your boat just won't steer reliably 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.

Interesting. Normally a single screw RH fixed pitch prop will steer more
easily to port than to stbd, unless you are talking a back and fill
turn, in which case you want to turn to stbd.

otn

My opinion FWIW, prop walk is a function of the prop
and pitch.

owners of AutoProps report limited prop walk as compared to their prior fixed
blade props. AutoProp blades continuously change pitch during a rotation so
that (more or less) total thrust on each side of the prop rev is the same.

Rod,

Actually, the asymmetric response to forward and reverse can be used
to do some neat tricks (which I use all the time). Try this, next
time you're out in your boat with some time to spend:

Motor out to a bouy (navigation, crab pot, whatever) in open water,
where there's no current. The bouy doesn't have to be very close --
it's only to gauge forward motion.

Stop the boat, put the rudder hard over to starboard, put the motor in
forward, and gun it. The first reaction will be that the stern kicks
to port. The next will be that the boat starts to move forward.

Immediately put the motor in reverse, LEAVING THE RUDDER HARD TO
STARBOARD. Apply moderate juice. The stern will continue to move to
port, and eventually the boat will come to a stop. Keep reversing,
and the stern will continue to move to port, due to the effects that
have been described already in this thread. Note where the boat is
with respect to the buoy, and continue backing until you've canceled
any forward motion and she's back where she started.

Now put the motor in forward and repeat the whole operation. With a
little practice, you can almost eliminate all the forward and aft
motions, and simply turn the boat inside her own length, by simply
pushing the stick forward and back, all the while holding the rudder
hard over. When you need to do this in close quarters (say, in a
marina with only a foot or two of room between your bow and the boat
ahead of you, and between your stern and the boat astern), you will
draw applause from onlookers.

Now, suppose some day you need to turn the boat in the opposite
direction, say, 90 degrees. Is there a similar way to turn her inside
her length, but in the opposite direction? No. So, simply turn her
the 270 degrees the way she wants to go (clockwise) until you've
accomplished the turn you need! Big ships do this all the time, and
it works great.

So no need ever again to hug the port side of your channel!

"Rod McInnis" wrote in message ...
"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

"Rob Overton" wrote in message
om...


Actually, the asymmetric response to forward and reverse can be used
to do some neat tricks (which I use all the time). Try this, next
time you're out in your boat with some time to spend:

Oh, I have have operated single engine inboards for 25 years. Believe me, I
am quite familiar with the tricks.




Now put the motor in forward and repeat the whole operation. With a
little practice, you can almost eliminate all the forward and aft
motions, and simply turn the boat inside her own length, by simply
pushing the stick forward and back, all the while holding the rudder
hard over.


The degree to which this works varies greatly with the boat and conditions.

15 years ago I owned a 1956 Stephens, a wooden hull boat with a single
inboard. This boat actually had a sizable keel, about an 18 x 4 inch timber
that hung down below the hull. The keel prevented excessive lateral motion
while allowing the boat to pivot.

I also owned a 1979 Pro Am tournament ski boat, inboard. This was a fairly
small boat with the engine mounted center. Its light weight and bulk of the
mass in the center would also make it want to pivot more about the center.
The 2000 Air Nautique that replaced it was similar. Also a center mounted
engine, but the boat was bigger and heavier, much more sluggish. It would
not pivot inside its own length, but it was close.

My current inboard is a 2003 Super Air Nautique. This is a V-drive inboard,
which puts the engine in the back. It is even a bigger and heavier boat.
Having the bulk of the weight in the very back makes the boat very sluggish
to turn. It doesn't pivot, it turns. It turns slightly better to the right
when going forward, and reverse tends to make the stern push to port so if
the channel is narrow I will make my turns that direction.

Add on top of this wind and current. If I was on a lake on a windless day I
could manage just about anything. I boat mostly on the California delta,
and have wind and tidal currents to deal with. I may have to contend with a
2 knot current pushing me one way while a 10 knot wind is pushing me a
different direction. Sometimes the current swirls around and totally messes
things up. The wind is always confused as it deflects off of covered docks
and the levee.


Rod

Being old and senile, I have to work things out with a mind
picture.

Re prop-walk...if I have a prop with flat blades aligned fore
and aft, I essentially have a "paddle wheel". If I turn the prop
clock-wise (as viewed from the rear of the boat), I'd
expect the stern to go to the right. Reversing the direction
would obviously make the stern go to the left.

If I re-pitch the prop so the flat blades are at right angles to the keel,
(cross-ways), I'd expect a bit of froth, but not much in the way of
stern reaction.

When I consider a *real* prop, the blades are aligned between
the two extremes above, and I'd expect the stern reaction to
also be between the two extremes.

Norm B