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