Steven Shelikoff wrote:
But the force generated is actually pretty high.
Not really. If it was, it would overcome the ahead or astern component.
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.
Because of propwalk and the fact that your prop is in the stern and
everything forward of that follows your prop, much like the "tail of a dog".
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.
If I remember correctly you have a LH prop which means this is totally
understandable and expected.
No, that wash is coming from the upward and to the right (for a right
handed prop).
No, only some of it. G it's amazing what you can see and watch over a
period of years. Because of the proximity to the surface, the blade,
starting at 000*, pushes water right, then down, BUT, again, because of
the proximity to the surface (no real column of water above that
directly in line with the pitch of the blade) it pushes water up into
the air, thus not having full efficiency. Once again, my feeling is that
it doesn't reach full efficiency until @ 045*, and moving the prop
further underwater doesn't totally negate this fact/effect, just reduce it.
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,
Maybe, depends on the pitch, but for our discussion, ok. (as long as you
realize the direction is also UP toward the surface .....
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.
Understood, but I feel the column of water above this is not "solid" ...
when you push on something, if it can't go straight, it goes to the
side, in this case up and to the surface .... path of least
resistance..... less efficiency.
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.
The main left component is from 090* to 180* during maximum prop
efficiency. From 180* - 270* the "left" component starts at maximum and
constantly decreases, as the upward component increases (nothing happens
individually, everything happens concurrently).
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.
NO
It's decreasing as
you go from 180 to 270 and then increasing again as you go from 270 to
360.
No. The "left" component is decreasing as you go from 180-270, but the
right component (from 270-000) is staying relatively low (compared to
the left component from 090 t0 180) because the direction is up toward
the surface at the same time as it is to the right, rather than
(090-180) down toward "solid" water and to the left.
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.
Rather than going into detail .... NO. You cannot compare a force from
180-270, to a force from 270-360. you MUST compare a force from 180-270,
to 000-090, and a force from 270-360, to 090-180.
The "length" of the water column from 090-180, is far greater than the
"length" of the water column from 270-360. The body (propellor pitch) is
constantly changing direction of "push".
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.
True, but that sideways force does NOT equal the force at 100-170,
because the column of water above it is less than the column of water
below 100-170.
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.
NO,NO,NO You cannot compare 000-090, to 090-180 or 180-270, to 270-360.
You Must compare opposites !! 000-090 and 180-270 or 090-180 and 270-360.
Of course, all this assumes no hull overhang.
It may be a factor, but not a cause EG as explained from my point
of view earlier.
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.
Nope, I'm narrowing down the important quadrants of push, germane to the
discussion.
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.
No. The column of water is important to the "net" right and left forces.
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).
No. This would only be true if the column of water was contained within
a pipe. It's not, so the water above the column can escape above the
surface, reducing efficiency.
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.
No, if that were the case, then your boat wouldn't experience propwalk.
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
LOL Well, it seems no one is really complaining, so have at it.
otn