"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

comments intersperced

end-plate effect


But still an existing effect whatever you call it and different than
what was attributed in the mentioned Chapman's discussion?

yes, different. end-plate effect means the thrust is greater for the blade tip
coming "close" ("close" is a variable term) to an end plate, which constrains
the flow.

"asymetrical thrust" it is called, though Chapmans does not use the term


"A rose by any other name"?

it is a common term, though Chapmans didn't use it.

But if water flow is exactly parallel to the shaft (very
unlikely I agree) no SUCH effect would occur.

true.

doesn't have to horizontal, just has to different from the axis of the
prop.


Again, I agree but I was discussing the suitability of the use of
Chapman's words and he (was quoted in the proceeding discussion to have)
used the term "parallel to the water's surface and to the flow of water
past the blades" (which implies the flow of water is horizontal, because
the shaft could never be parallel to the water's surface and to the flow
of water at the same time if the flow of water he (Chapman) is referring
to here isn't parallel to the water's surface - which is assumed to be
horizontal in most simple cases!)

Chapmans described the effects of a downward slanting propshaft. It seemed
some people were under the impression that that was the only issue. End plate
effect is another and different issue issue.


caused by the (forward) motion of the boat through the
water.

doesn't have to be from boat movement, just has to be water movement.

So I trust you are not adding a new "fact" to the original discussion -
that the boat is tied to the dock but there is a current flowing past
the dock and the boat?

assumption was that no current existed outside that which the prop caused.

Why not add that there is a current parallel to
the dock pushing the stern to port or to starboard? I think the
original description would imply the boat and dock are in still water.

yes.

However if the "water movement" you refer to is just from the prop, I
think I covered that... (see below)


As the boat is jammed against the dock and not able to move through the
water there will be no horizontal flow of water due to forward motion.

no, the hull of the boat causes the water flow behind of the forward
pushing
prop to "line up" not in line with the prop shaft, thus asym thrust.


Again, see below. You left off part of my comment on this!

I did see the comment and thought I was expanding it. sorry if my words did
not convey that.


The only flow past the prop will therefore probably be a flow parallel
to the shaft

no, the hul gets in the way, at least if the hull is anywhere near the prop


You ignored a significant part of my statement: "(possibly modified
slightly by hull effects)" so in fact we are in agreement here.



the water leaving the ascending blade (on the
port side of the boat) may produce more push on the port side of the
hull than does the water swirling down from the descending blade

why is this? what has "swirling" to do with it?


The water leaves the prop in a sort of corkscrew fashion - that which
leaves the descending starboard blade will tend to corkscrew downward
and back away from the hull. That which leaves the ascending port blade
will corkscrew upward and back tending to cause a net push on the aft
sections of the hull.

you have mostly described "end plate" effect, though end plate effect has more
implications.


I have even heard the explanation that since the water is more dense at
the bottom of the rotation than at the top,

nah, the difference is virtually nothing. There is less than 1/2 psi
pressure
difference per foot of water depth and water compresses
soooooooooooooooooooooooooooooo little than even several thousand psi
wouldn't
make any difference.


And again aren't you just agreeing with me? ("Surely this density
difference is small so this contribution to the overall effect from this
must also be a minor part.")

it is hard to say it has even a minor part. the density of water is for all
practical purposes the same at any depth.


All I was trying to get across is that the effects causing prop walk are
multiple and in varying degrees and in the specific setup originally
described (tied bow to dock) attributing the action to one single effect
(the mentioned Chapman explanation) may not be fully justified.

I agree. end plate effect is also an issue.


I am sure we can both agree that to most boaters knowing the Physics
behind these effects is really unnecessary. Knowing what boat will do
in each situation and being able to use it to safely and effectivly
control the boat in a tight location is the thing!

I brought it up because I have found that most boaters think a blast of the
engine in reverse will affect the rudder. I have also seen boaters who were
told by marinas to spend major bux to move a prop back closer to the rudder to
"help fix" the lack of rudder response backing up. Ya gotta have a boat moving
backwards through the water to have the rudder effective.


Dave

"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

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

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

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

A paddle wheel has it's top portion out of the water, a prop doesn't.


"engsol" wrote in message
...
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

A paddle wheel has it's top portion out of the water, a prop doesn't.


"engsol" wrote in message
...
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

Lower that paddle wheel into the water, until it is just beneath the
water .... bet it will still pull to the right, because the blades on
the upward part of the rotation will be lifting the water and basically
throwing the "push" away (into the air) i.e., they'll be pushing, but
not at the efficiency of the blades on the downward turn.

otn

Scott Vernon wrote:
A paddle wheel has it's top portion out of the water, a prop doesn't.


"engsol" wrote in message
...

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

Lower that paddle wheel into the water, until it is just beneath the
water .... bet it will still pull to the right, because the blades on
the upward part of the rotation will be lifting the water and basically
throwing the "push" away (into the air) i.e., they'll be pushing, but
not at the efficiency of the blades on the downward turn.

otn

Scott Vernon wrote:
A paddle wheel has it's top portion out of the water, a prop doesn't.


"engsol" wrote in message
...

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