On Thu, 17 Jun 2004 17:56:12 GMT, otnmbrd wrote:

Finally got some paper, so's I could print out, sit back and read/view
the whole, as I find reading on the screen tends to "tunnel" my view.
As I said about 4-5 days ago, we've thoroughly thrashed and rehashed
this issue and are just chasing each other in circles.
Some final thoughts, and I'll bow out (G everyone sighing with relief).
I doubt you've ever watched a large ship prop being turned by steam as
they warm up the turbines, though you may have (from a distance) seen
some diesel ships start. I've watched both at very close range, for
years, and still do (Though I see few turbines anymore).
The effect is obvious, not only in what it shows, but in which blade
it's coming from .... you can't believe this .... ok ... on to the next
point.

I believe you can for some props, not for others. Also, if the prop is
very shallow, like with an unladen freighter, the effect is much more
obvious. But then the contribution to prop walk is much greater as well
if the prop is right at or just under the surface than when it's deeper,
like with a loaded freigher. So it follows that you should be able to
see it better when the prop is near the surface.

My feeling of the basic cause of propwalk, is the rotation of the prop.

Again, that's pretty obvious the cause, at least to me.

Overall, the efficiency of the prop between the 090-270 arc (where the
blade is pulling the boat to the right when going ahead) is greater than
it is between the 270-090 arc (when the blade is pulling the boat to the
left) .... repeat, overall.

I agree with that as well. But if you look at what the overall
difference and where it occurs, you'll see that the overall difference
is not that great. Not nearly enough to be the only cause of propwalk.
On some boats, it may be the largest. On others, there may be other
effects that contribute more. It all depends on the configuration.
I.e., hull overhang, prop depth and configuration, shaft angle, how fast
you're moving through the water, etc. One thing I think you're
forgetting is that sufficient hull overhang can [almost] totally negate
your theory and yet there still is prop walk with that hull
configuration.

Nothing which has been said here has altered that opinion/feeling.
This is the basic cause.

As you can see, I don't agree it's the "basic" cause. Then again,
neither do you since just above you said prop rotation is the "basic"
cause. It is a cause however.

Other factors enter in, to increase, decrease, and even negate this affect.
For instance, Charles Low mentioned the wash from the prop, going astern
(btw everything considered is a RH fixed pitch prop), would push against
the hull, causing the stern to move to port. My response being that this
was not the cause, but a factor. My feeling here is that while the wash
is pushing the hull left, the prop is pulling the hull right .... one is
canceling out the other, while the opposite blade is pulling the hull
left and this adds to propwalk. ( the additional amount will depend
greatly on hull form.)

There are also problems with the wash against the hull theory. The wash
had to come from the prop itself and both are attached to eachother.
Try pushing your left hand against your right hand and see if it moves
your body anywhere. I agree that there could be some effects from the
wash against the hull though. But again, not as much as you would think
if you envision it coming directly from the wash pushing against the
hull. Maybe if one side is moving faster than the other the bernoulli
effect will cause a pressure difference between the sides of the hull?
Anything's possible.

I've "backed" vessels in really deep water and not so deep water
....propwalk is there with little relative difference. However, when you
back in really shallow water, all bets are off. Frequently you will find
yourself going either way, or not turning at all, but my feel is, if it
"does" back to port, it will be at an increased rate.
Naturally, the biggest contributors or negators (is that a word?) are
wind and current ..... either/or can totally change the direction in
which your stern will back.

I don't count wind/current as causes of prop walk. Sure, they can
negate or add to the effects. But they would move your boat even with
the engine off so it can't be a cause of prop walk.

These are my experiences, feelings, views, based on 50 years on the
water, handling a multitude of vessels of all types and in all
conditions of water depth, bottom type, weather, currents, etc., .......
BG Yours may vary, be different, etc.,..... ok .....

Ok, I guess we're finished then. Big sigh of relief.

Steve