On Wed, 09 Jun 2004 17:57:52 GMT, otnmbrd wrote:
I've always wondered if a submarine, 1,000 feet underwater, experiences
"propwalk" .... I'm betting it doesn't G.
At any rate, to save some unnecessary reading, by the numbers on some
points at issue:
1. We are not talking about some highly visible, easily measured,
variations in efficiency. These are very subtle, but sufficient to cause
the condition.
But the force generated is actually pretty high. For instance, due to
the full keel of my boat it's much easier to move it fore and aft vs.
spin it sideways. Yet when I throw the thing in reverse, all that
happens at first is prop walk. By keeping the rudder fully to port I
can, by alternating forward and reverse power every few seconds, spin my
boat completely around in very little more than it's LOA. I use that
"feature" to get into very tight spaces.
2. Due to it's relative proximity to the surface, I do not feel that a
blade is at maximum efficiency at this angle of rotation. My
sense/guess/feel, is that isn't doesn't reach maximum until about 045*.
Why? Sit on a tug, tied to the stern of a ship and watch "propwash" for
awhile. The wash is pushed to the side, breaking the surface during the
initial part of it's rotation. However ....
No, that wash is coming from the upward and to the right (for a right
handed prop). For example, say the prop is 2 feet deep. When the blade
is at 315 degrees, that's when it's pushing water out 45 degrees to the
right, which works out to be 2 feet to the right. By the time the blade
goes past 0 degrees, it's only pushing water sideways and then down.
The upward wash to the left of the boat is from the 180 to 270 part of
the rotation and the upward wash to the right is from the 270 to 0 part
of the rotation.
3. Ignore the quadrants 000-090, and 180-270 ( My sense - the prop
efficiency is in transition during these portions of rotation. In one
the efficiency is increasing and the other it is decreasing ... 0% net
Separating out all the other effects and only discussing the efficiency
of the effect we're talking about here, it's not increasing from 0 to
90. It's at maximum at 0 and stays there until 180. It's decreasing as
you go from 180 to 270 and then increasing again as you go from 270 to
360. For a prop that's not right at the surface the decreasing force to
the right as it goes from 180 to 270 balances out the increasing force
to the left as it goes from 270 to 360 because the balance point (i.e.,
where the force is at a minimum due to minimum efficiency) is right at
or very near 270 degrees. I.e., the size of the water column when it's
at 280 degrees is the same as when it's at 350, just in the other
direction. 290=340, etc.
But for a prop that's very near or at the surface, those forces don't
balance out. That's because as the prop gets closer to the surface the
balance point (where the force is at a minimum due to minimum efficiency
because of the smallest water column before you get to air) move further
around the rotation. In this case, the force at 280 /= the force at 350
and you have a net sideways force.
difference.)the blades are still pushing back, but there is no net
effect (arguably) which we can readily apply to "propwalk". Instead .....
4. Concentrate on the quadrants 090-180 and 270-000. From 090 to 180 the
blade is pushing back against a relatively solid column of water, down
against a relatively solid column of water and increasingly LEFT against
a relatively solid column of water. During this entire quadrant of
rotation, the blade is at maximum efficiency...... BUT, from 270-000 the
blade is pushing back relatively nearer the surface, up toward the
surface, and RIGHT towards and relatively close to the surface, where it
can and does break the surface or at least bulge the surface. So.....
And the quadrant from 0 to 90 exactly balances out the quadrant from 90
to 180. And the quadrant from 180 to 270 "almost" exactly balances out
the quadrant from 270 to 0. The smaller the ratio of prop size/prop
depth, the closer those quadrants (180-270 and 270-0) will balance out.
Of course, all this assumes no hull overhang.
5. My sense from this. The blade, in these two all important quadrants,
is more efficient between 090-180, than it is between 270-000. The
Most definitely. But you're ignoring the other two all important
quadrants.
differences if you add in depth of the hub of the prop, may diminish,
but for a boat floating on the water surface, the efficiency will never
be equal...... VBG ..... propwalk.
I disagree that there is no such thing as a "more solid" column of water.
That doesn't really matter since it's not important as long as you
realize that the "solidity" of the column of water (if there is such a
thing) is the same for the same angle to the right vs. to the left of
the prop.
I disagree that a propellor is at maximum efficiency at 000* (on a boat
floating on the surface of the water).
If it's not then there's something else at work then the effect we're
talking about here. Because at 0 degrees the blade is pushing against
an infinite column of water directly to the right (for a RH prop).
Well, maybe not infinite because of the curvature of the earth and
because there may be a shoreline before the curvature of the earth comes
into play.
I disagree that by moving the blade a few feet down in the water, you
will totally negate the effects or differences in blade efficiency.
Not totally. But it doesn't take going very far down before everything
*nearly* balances out left and right due to the effect we're talking
about.
I think we've probably hashed, thrashed, and rehashed this enough in the
NG, Steve. However, feel free to continue via E-mail if you wish.
Nah. I'd rather continue it here. Finally a civil boating related
thread in the midst of a sea of bickering and political crap.
Steve