Steven Shelikoff wrote:


But it is changing. That's the variable we're changing, the angle of
the shaft. I.e., as the shaft angle gets closer and closer to coming
straight out of the boat with no downward angle, the prop walk gets less
and less. Or stated another way, the greater the angle of the shaft the
greater the prop walk. And that's due to the torque of the shaft, which
is applied more and more as a force to rotate the boat as the downward
angle of the shaft is increased. When the shaft is straight back with
no downward angle, the torque force from the shaft causes the boat to
list but doesn't rotate it in the water.



The rotational force from the prop remains the same. But what it does
to the boat depends on the angle of the shaft relative to the boat. If
the shaft is vertical, it will try and rotate the boat in the horizontal
plane, i.e., prop walk. If the shaft is horizontal, it will try and
rotate the boat in the vertical plane, i.e., list. Anywhere in between
vertical and horizontal and some of the force will try and rotate the
boat while some will cause list with the list increasing and the prop
walk decreasing the closer to horizontal you get.

Steve

Using your theory, Steve, the stern would walk the same way whether the
prop was a left hand or right hand wheel... and this is not the case.

I have always been under the impression, on straight inboards, that the
lower half of the propellor, the arc furthest from the boat/hull
surface, is the one that does the most work/thrust (eg. surface piercing
drives, etc.). The prop has better "bite" in the less turbulent water
away from the hull. This has been my experience too...

Correct Craft & Century inboards: RH prop, walks to port in reverse,
favors left turns in forward... (the opposite of your explanation if
taken to the extreme of a 90 degree propshaft angle).

Mastercraft inboard: LH prop, walks to starboard in reverse, favors
right turns in forward. (again, the opposite of your explanation if
taken to the extreme of a 90 degree propshaft angle).

The rotational force of the shaft/prop is transferred primarily to the
attitude of the boat on a single screw craft.

Rob

On Sat, 05 Jun 2004 11:06:08 -0400, trainfan1
wrote:

Steven Shelikoff wrote:


But it is changing. That's the variable we're changing, the angle of
the shaft. I.e., as the shaft angle gets closer and closer to coming
straight out of the boat with no downward angle, the prop walk gets less
and less. Or stated another way, the greater the angle of the shaft the
greater the prop walk. And that's due to the torque of the shaft, which
is applied more and more as a force to rotate the boat as the downward
angle of the shaft is increased. When the shaft is straight back with
no downward angle, the torque force from the shaft causes the boat to
list but doesn't rotate it in the water.



The rotational force from the prop remains the same. But what it does
to the boat depends on the angle of the shaft relative to the boat. If
the shaft is vertical, it will try and rotate the boat in the horizontal
plane, i.e., prop walk. If the shaft is horizontal, it will try and
rotate the boat in the vertical plane, i.e., list. Anywhere in between
vertical and horizontal and some of the force will try and rotate the
boat while some will cause list with the list increasing and the prop
walk decreasing the closer to horizontal you get.

Steve

Using your theory, Steve, the stern would walk the same way whether the
prop was a left hand or right hand wheel... and this is not the case.

Actually, no it wouldn't. The torque is in a different direction
depending on whether the prop is right or left handed so the boat would
turn in a different direction.

I have always been under the impression, on straight inboards, that the
lower half of the propellor, the arc furthest from the boat/hull
surface, is the one that does the most work/thrust (eg. surface piercing
drives, etc.). The prop has better "bite" in the less turbulent water
away from the hull. This has been my experience too...

This is true. That's one of the causes of prop walk, not the only one
though. That's why a boat with the prop further from the hull will have
less prop walk. But that's not what the question was. The question is
why a small change in the shaft angle (say only 10 or 20 degrees) will
have a dramatic effect on the amount of prop walk. To have that large
an effect with such a small change you have to have a large total force
and the one you mentioned above creates a very small force. But when
the shaft is completely horizontal, it's the main propellor related
force causing prop walk. Tilt the shaft down a little and it becomes
overwhelmed by the torque force.

Correct Craft & Century inboards: RH prop, walks to port in reverse,
favors left turns in forward... (the opposite of your explanation if
taken to the extreme of a 90 degree propshaft angle).

But the prop shaft is not 90 degrees. If it were, the boat would turn
opposite the direction of the prop since it's the only prop related
force being applied. Also, not all boats with a RH prop will walk the
same way. It depends on the specific configuration of the boat
including shaft angle, type of prop, how far the prop is from the hull,
etc. My boat is a single screw inboard with a right hand prop and it
backs to starboard with a pretty severe case of prop walk.

Mastercraft inboard: LH prop, walks to starboard in reverse, favors
right turns in forward. (again, the opposite of your explanation if
taken to the extreme of a 90 degree propshaft angle).

Same comments as above.

The rotational force of the shaft/prop is transferred primarily to the
attitude of the boat on a single screw craft.

Exactly what I said. i.e., the torque from the prop will primarily
cause the boat to list when the shaft is at or near horizontal. As you
increase the downward angle (or upward angle for that matter although I
don't know of many boats with an upward pointing shaft) the rotation
force from the shaft/prop will more and more cause the boat to rotate
and less and less cause the attitude (list) to change.

You can realize just how much rotational force from the shaft/prop there
is since it's big enough to cause a noticable list. Apply the same size
force from the paddle wheel affect in the direction to cause list and
you probably won't notice a thing.

I'll agree that on most boats, the rotational force from the torque of
the prop/shaft is not the main contributor to prop walk and is a much
greater contributor to list. But what we're talking about here is what
forces that contribute to prop walk change as the shaft angle changes.
And the torque effect certainly does.

Steve

On Sat, 05 Jun 2004 17:12:28 GMT, (Steven Shelikoff)
wrote:
My boat is a single screw inboard with a right hand prop and it

Strike that. Just after I wrote it I realized it was wrong but hit the
send button before I deleted it. My boat has a LH prop with a starbord
walk. But the shaft angle is not very severe so the torque effect is a
small contributor to rotation and large to list. Change the shaft angle
and that could change.

Steve

Another P.S.

The change in direction of applied force from the torque of the
prop/shaft is only one part of the theory. The rest is the change in
the amout of paddle wheel effect as the angle changes as well as a
change in the amout of available thrust in the proper direction to
counteract the prop walk as the shaft angle changes.

I'm always willing to adjust the theory.

Steve

steve, bone up on "P-torque" or "P-thrust" or more accurately "asymetrical
thrust". It is a common problem for aircraft with "conventional" landing gear
(meaning two wheels up front and one on the tail), and has been fully
understood since the early days of WWII.

Another P.S.

The change in direction of applied force from the torque of the
prop/shaft is only one part of the theory. The rest is the change in
the amout of paddle wheel effect as the angle changes as well as a
change in the amout of available thrust in the proper direction to
counteract the prop walk as the shaft angle changes.

I'm always willing to adjust the theory.

Steve

gene, you are one horse**** pilot, if you are any pilot at all, for not being
able to tell the difference between conventional gear and trikes.

Let me inform you a tad. when the aircraft is rolling with the tail down, the
angle of attack one side of the prop circle is greater than on the other side,
because of the forward movement of the craft. The side of the prop with great
a of a developes more thrust and pulls the aircraft forward faster than the
other side of the prop. than means the aircraft pulls to one side. This is
often (erroneously) referred to a P-Torque, because it feels like the engine is
torqueing the aircraft to the side. In fact, it is P-Thrust, or asymetrical
thrust that is causing the aircraft to veer.

be carefull, gene. Remember what the FAA taught you, "Safety is no accident".

You need to do some boning up on your pilot skills, gene, or maybe hang up your
googles.

(JAXAshby) wrote:

steve, bone up on "P-torque" or "P-thrust" or more accurately "asymetrical
thrust". It is a common problem for aircraft with "conventional" landing
gear
(meaning two wheels up front and one on the tail), and has been fully
understood since the early days of WWII.


So..... what conundrum of JaxWorld prevents this from affecting
tricycle gear aircraft, as well?? (Sure takes a lot of right rudder
to keep every aircraft, conventional or tricycle, I have flown on the
center line.) You *do* realize, of course, that most of the forces
felt in this regard are from propeller swirl impinging upon the
vertical stabilizer?

And, that said, given the arm of a 16" boat screw vs a 8 foot or
better aircraft propeller, it makes the effect of P-factor minimal, at
best, in a nautical scenario (unless, of course, you would like to
include things like WWII battleship propellers, just to prove the
laboratory experience in deference to real life).
--
23' Grady White, out of Southport, NC.

http://myworkshop.idleplay.net/cavern/ Homepage
http://www.southharbourvillageinn.com/directions.asp Where Southport,NC is
located.
http://southharbourvillageinn.linksysnet.com Real Time
Pictures at My Marina
http://www.thebayguide.com/rec.boats Rec.boats at
Lee Yeaton's Bayguide

"JAXAshby" wrote in message
...
Snip personal attack

Let me inform you a tad. when the aircraft is rolling with the tail down,
the
angle of attack one side of the prop circle is greater than on the other
side,
because of the forward movement of the craft. The side of the prop with
great
a of a developes more thrust and pulls the aircraft forward faster than
the
other side of the prop. than means the aircraft pulls to one side. This
is
often (erroneously) referred to a P-Torque, because it feels like the
engine is
torqueing the aircraft to the side. In fact, it is P-Thrust, or
asymetrical
thrust that is causing the aircraft to veer.

be carefull, gene. Remember what the FAA taught you, "Safety is no
accident".

Snip more personal attack

(JAXAshby) wrote:

Lordy, a lucid and correct answer from Jax.

Who would have thunk it!

Mark Browne

Used to be on a ship that would trim by the head ( 3' +) when fully loaded.
Put the engine half astern - back like a sumbitch to port (fixed pitch RH).
Even keel - half astern - back like a sumbitch to port. Trimmed by the stern -
half astern - back like a sumbitch to port.
Power used, is another one of those factors (otn missed that one) which will
increase propwalk.
It's not always easy to tell which factor may or may not be increasing or
decreasing or even IF there is any difference because of it.
For my money, the basic issue (propwalk) is caused by prop rotation and all
other factors may assist or decrease it, but the basic "phenom" occurs when you
rotate the prop.

Shen

On 05 Jun 2004 22:44:53 GMT, (Shen44) wrote:

the basic issue (propwalk) is caused by prop rotation and all
other factors may assist or decrease it, but the basic "phenom" occurs when you
rotate the prop.

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

Of course, but what is the cause of the asymetric thrust? THAT is the
question. Everyone agrees that a prop with a horizontal shaft still
exhibits prop walk, implying that the bottom the prop is more
efficient at providing thrust than the top. Lots of theories have
been provided but none that seem totally convincing since prop walk
still exists to one degree or another on deep props, that have plenty
of hull clearance.

I'm sorry I missed the original post. I'm responding to the thread, not this
post in particular.

The "p-factor," of the upcoming and downgoing propeller blades having
different angles of attack, is the least signifcant reason for asymmetric
propeller thrust, although the most often touted. I came across a little
article in Flying years ago which did the math, and the effect is very
small. There are something like five reasons that a tail-dragger will yaw
one way, the p-factor being only one.

In my previous boat, Stella B, (www.ctlow.ca/StellaB/StellaB.html), I had
about the same asymmetric thrust whether the outdrive was trimmed in or out.
So much for prop angle.

The main effect is from the spiral prop wash. In reverse, where asymmetric
thrust is virtually always more pronounced, the top half of the prop wash
vortex strikes the hull, and pushes it sideways. The effect will vary
depending on the underwater hull shape, the angle of the prop shaft, design
of prop ...

So, a right hand propeller in reverse turns counter-clockwise. The top half
of the spiralling prop wash is moving to port, and pushes on the hull,
yawing the stern to port. The bottom half othe spiral is mostly in clear
water, pushing on nothing.

I hate to say "end of discussion," because that sounds arrogant, and I still
have things to learn, but I am very sure that for most practical purposes,
that is it.

Charles

====

Charles T. Low
- remove "UN"
www.boatdocking.com/BDPhoto.html - Photo Contest
www.boatdocking.com
www.ctlow.ca/Trojan26 - my boat

====

"trainfan1" wrote in message
...

Using your theory, Steve, the stern would walk the same way whether the
prop was a left hand or right hand wheel... and this is not the case.

I have always been under the impression, on straight inboards, that the
lower half of the propellor, the arc furthest from the boat/hull
surface, is the one that does the most work/thrust (eg. surface piercing
drives, etc.). The prop has better "bite" in the less turbulent water
away from the hull. This has been my experience too...

Correct Craft & Century inboards: RH prop, walks to port in reverse,
favors left turns in forward... (the opposite of your explanation if
taken to the extreme of a 90 degree propshaft angle).

Mastercraft inboard: LH prop, walks to starboard in reverse, favors
right turns in forward. (again, the opposite of your explanation if
taken to the extreme of a 90 degree propshaft angle).

The rotational force of the shaft/prop is transferred primarily to the
attitude of the boat on a single screw craft.

Rob