BoatBanter.com - View Single Post - Angle of prop shaft

On 07 Jun 2004 03:25:31 GMT, (Shen44) wrote:

Subject: Angle of prop shaft - theoretical question.
From: Wayne.B
Date: 06/05/2004 21:28 Pacific Standard Time
Message-id:

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.

First off, ignore any parallel discussions of aircraft propellors. An aircraft
propellor work in one medium ... air. A boat propellor impacts two mediums ...
water and air (if you need confirmation of this, look astern of a boat
underway, under power ... you will see a wash/water, being lifted above the
surface in a rooster tail or lessor version thereof.
Now, go to a boatyard and stand in front of a propellor on some boat out of the
water.
Look at the angle/pitch of the blade, then start slowly rotating the blade
(assuming RH and you are standing in front of it) and visualize you are pushing
against water. As you rotate, (starting in the vertical up position) you will
note that the blade angle/pitch is pushing back and down against the water.
Water is not compressible. By pushing back and down, you are pushing against a
"solid" mass of water and are most efficient.
Now, as the propellor rotates it also pushes to the side against that "solid
mass of water, but as it starts rotating up, the solid mass becomes a relative
small column of water with air above it (air - highly compressible).
The propellor blade is now pushing back, but also up, and lifting that water up
and away - i.e., no longer a solid wall of water, into the air, and it becomes
less efficient.
The same applies as the blade re aproaches the vertical.
The forces of greatest efficiency are on the downward and sideward rotation of
the blade (pushing the stern to stbd when going ahead -left turn).
The same applies to backing, but because the prop is now pulling the boat, the
effect is far more noticeable.
Someone is bound to discuss the overhang of the hull, over the prop, should
negate this. Not so. That overhang is relatively short and variable, so that
although it may affect the degree, it will not stop the results.
BG And this is my theory and I'magonna stick to it......for now.

That theory sounds compelling at explaining a difference in the
efficiency of the blade during it's downward vs. upward travel (if
there's no overhang.

This part of your explanation would create an
upward force. It would also create a listing torque force since the
upward force of your theory above is applied off the center of the prop.
But since the column of water is the same in both the sideways
directions, where does the sideways force come from to create prop walk?

Steve