Consider a yacht with a frictionless shaft.
Consider that we have taken a section thru one of the blades, and we will
represent this always level to the page (because I cannot figure out how to
draw it at an angle)
I will represent the prop section as a horizontal line, and leave you to
imagine a curved line above it..
Imagine a curved top surface above the line (forward
on the boat)
Leading edge ________________________________ trailing edge
STAGE 1. We are under power. The engine is driving the prop blade to the
left, and the result of the boat speed and propeller pitch gives a vector
flow onto the blade from below (aft on the boat) at an angle of attack of
say 8 degrees:
\ lift up (forward) at 8 deg drag
Leading edge ________________________________ trailing edge
Flow (this arrow should be tilted up at 8 deg.)
Our prop blade is now developing "lift" that is pushing the prop shaft
forward.
It is also developing drag from three causes:
Friction drag from the flow of water over each side of the
blade.
Form drag because the thick blade has to push the water out of
the way.
Induced drag as a result of developing lift.
STAGE 2. We just a moment ago put the transmission in neutral, and the prop
has slowed to the point where the result of the boat speed and propeller
pitch gives a vector flow onto the blade from the left at an angle of attack
of 0 degrees:
drag
Leading edge ________________________________ trailing edge
Flow (this arrow should be level at 0 deg.)
The prop blade is now developing no lift.
And is now developing drag from only two causes:
Friction drag from the flow of water over each side of the blade.
Form drag because the thick blade has to push the water out of
the way.
And these two drags combined will continue to make the prop slow until:
STAGE 3. We are under sail with a freewheeling prop. Water flow from ahead
is driving the prop blade to the left, because the result of the boat speed
and propeller pitch gives a vector flow onto the blade from above at an
angle of attack of say 8 degrees:
Flow (this arrow should be tilted down at 8 deg.)
Leading edge ________________________________ trailing edge
/ lift down (aft) at 8 deg drag
Our prop blade is now developing "lift" that is pushing the prop shaft aft.
It is also developing drag from three causes:
Friction drag from the flow of water over each side of the
blade.
Form drag because the thick blade has to push the water out of
the way.
Induced drag as a result of developing lift.
Note that the amount of lift produced by the prop will be just enough to
keep the prop turning. Any more and the prop will speed up and the vector
flow onto the blade will reduce, reducing the lift. Any less and the vector
will increase, increasing the lift.
Note that what I have called lift produced by the freewheeling prop is
actually drag when resolved to the boat.
Note that the section of the prop is upside down when freewheeling and the
lift drag characteristic will be quite a lot worse than when the flow is
from below in the imaginary drawings above.
STAGE 4. If you now introduce some shaft friction or load, the prop will
slow until the angle of attack of the prop produces enough lift to keep the
prop turning. If the shaft friction or load is great enough this may well
mean that the lift that the prop has to develop is greater than that if the
blade were locked and thus fully stalled.
NOTE. What I have tried to describe here is nothing like what happens on a
helicopter or autogyro where the angle of attack is always from the bottom
of the blade.
NOTE. Just a thought: A locked boat propeller with a disc area ratio of 60
% will have more effect surely than a locked airplane propeller with its
disc area ratio of 10%
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