OK, I admit to some shooting from the hip on the freewheeling prop
business. Let's see if I can make a substantial contribution to the
subject.

First of all, you have to recognize that a propeller, airplane or
boat, is just a wing going around in a circle. The relationships are a
little harder to understand because the mental model you try to make
in your head has to spin around. If you break it down into short
strips at fixed distances from the shaft centerline however, it is
fairly easy to understand.

If you would first like a non-technical explanation of wings, I can
recommend some articles published in Avweb, the major aviation
E-zine, by a frequent contributor to this and aviation newsgroups.
These articles created a firestorm of controversy among uninformed
pilots but, except for some minor errors, have never been seriously
questioned by an aerodynamics professional. One even uses them in his
college introduction to aerodynamics. Many of the concepts apply to
sails so may be of interest to this group. For example, you can find
out why sails do not stall dramatically as do airplane wings.

You can find these articles he

http://home.maine.rr.com/rlma/Articles.htm

One of the most important things to know about a wing, foil, or
propeller blade, is the Angle of Attack or angle of the flow, (AOA).
Here is a graph of an airplane wing showing the Coefficient of Lift
(CL) at various angles. This is a wing from the article but could just
as well be a prop blade, rudder, or hydrofoil.

http://www.avweb.com/newspics/stalldrb_figsb5.jpg


The CL x the speed squared tells you the amount of lift that will be
generated per unit of area. Note that it is at a maximum at an angle
of about 15 degrees. It is zero when the foil is moving edgewise
through the fluid, as you would expect. It then rises to maximum as
the angle increases and then starts to drop off with further increase
in AOA. The region to the right of the peak is the stalled region. It's
worth reading the article on stalls because you will learn that an
airplane wing stalling does not let the plane drop because lift
suddenly decreases. In fact, an airplane mushing into the ground with
a stalled wing is generating just as much lift as one flying level
but, I digress.

A wing is a foil being dragged along by the airplane's engine (or
gravity in the case of a glider). The un-powered prop we have all been
waving our arms and shouting about is being dragged along by the sails
via the hull. It is still just a wing.

The graph in the picture doesn't extend far enough to show the
relationship of a locked sailboat prop but you can mentally extend the
graph over to the right. The CL will be very low and the angle of
attack very high, up in the 70 - 80 degree range. This very
inefficient foil will still be generating "lift" which is backwards
and the drag slowing the boat.

Now, we mentally let the prop start to turn. This is the mentally
tricky part. The rotation of the prop causes the water to now seem to
be coming from a different direction. This added to speed of the boat
decreases the angle of attack. The faster the prop spins, the less the
angle of attack. If it spins fast enough, the AOA will reverse and
the prop will then be driving the boat. Only the engine can do this.

An aside, this is why prop blades are twisted, the tips are moving
through the water faster since they are farther from the shaft. The
boat speed is the same for the full blade. It has to be twisted so the
angle of attack will be the same along the length of the blade. This
is only perfect at one speed but props are maximally efficient only at
a specific design speed.

Anyway, look back at the CL graph. Decreasing the angle of attack from
the deeply stalled condition of the locked prop increases its
efficiency as a lifting device and the "lift" is towards the stern,
slowing the boat. At the same time, the speed of rotation is being
added to the speed of the boat, further increasing "lift" which is
actually drag in this case.

As we let the prop spin faster by decreasing bearing friction (or the
load on Larry's alternator) angle of attack continues to decrease and
drag increase. At some point however, the speed will get high enough
that the angle of attack will reach the peak of the CL curve. Further
increases in speed and further decreases in angle of attack will now
REDUCE the CL. Drag will start to drop off as maintained by several in
the other thread. If the prop can reach a high enough speed, it may
even drop below the drag it had when the shaft was locked. Since
lift/drag is being increased by higher speed (due to rotation) at the
same time it is being decreased by lower angle of attack, AOA has to
be pretty low to achieve this.

My contention was (although not clearly) that the typical auxiliary
sailboat powertrain is unlikely to obtaine a freewheeling state of
less drag than when locked. It can happen though. It may well happen
in Larry's dinghy outboard experiment.

We were all lining up on different sides yelling support for a simple
answer and there isn't one without understanding the relationships and
the specifics of RPM, pitch, and speed for each case.

--

Roger Long

Another way of looking at this issue is the analogy of turning a drill bit through a piece of wood
vs. pushing it through. If the speed of the freewheeling prop can match that of a 'screw' going
through the water, it seems like that would have the
minimum drag. If the speed is considerably different, it would have the effect of
pulling an almost flat object, the diameter of the prop, through the water, causing
appreciable drag. Seems like self-feathering props are designed to fix this problem,
and outboards taken out of the water while under sail, both try to minimize drag.

Sherwin D.

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%

"Roger Long" skrev i en meddelelse
...
OK, I admit to some shooting from the hip on the
freewheeling prop business. Let's see if I can make a
substantial contribution to the subject.

First of all, you have to recognize that a propeller,
airplane or boat, is just a wing going around in a circle.
The relationships are a little harder to understand
because the mental model you try to make in your head has
to spin around. If you break it down into short strips at
fixed distances from the shaft centerline however, it is
fairly easy to understand.

If you would first like a non-technical explanation of
wings, I can recommend some articles published in Avweb,
the major aviation E-zine, by a frequent contributor to
this and aviation newsgroups. These articles created a
firestorm of controversy among uninformed pilots but,
except for some minor errors, have never been seriously
questioned by an aerodynamics professional. One even uses
them in his college introduction to aerodynamics. Many of
the concepts apply to sails so may be of interest to this
group. For example, you can find out why sails do not
stall dramatically as do airplane wings.

You can find these articles he

http://home.maine.rr.com/rlma/Articles.htm

SNIP
--

Roger Long


Thank you for most interesting input Roger ... It seems to
be right, that "all complex problems has at least one simple
solution . . . . that does not work".

Unfortunately, I do not get a response, when I try to use
the above link ... Is it closed for Danes?

best regards

--
Flemming Torp

Unfortunately, I do not get a response, when I try to use the above
link ... Is it closed for Danes?

Yes, some of us are still upset about the Viking invasions

Seriously though, I have no idea why. I've had people read it all over
the world. Try later.

--

Roger Long

"Roger Long" skrev i en meddelelse
...
Unfortunately, I do not get a response, when I try to use
the above link ... Is it closed for Danes?

Yes, some of us are still upset about the Viking
invasions

As Carlsberg is saying in one of their adds: They took our
wiwes, but gave us the beer ... Sorry.

Seriously though, I have no idea why. I've had people read
it all over the world. Try later.

--

Roger Long



And you are sure it is as written in your message?
I've tried more than ten times, and I just get the message
that the page cannot be shown ...

--
Flemming Torp

I just clicked them both again directly from my post.

Try these direct links through Avweb.

http://www.avweb.com/news/airman/183261-1.html

http://www.avweb.com/news/airman/184307-1.html

You'll have to look in the second one for the CL diagram.

--

Roger Long



"Flemming Torp" fletopkanelbolle2rp.danmark wrote in message
. ..

"Roger Long" skrev i en meddelelse
...
Unfortunately, I do not get a response, when I try to use the
above link ... Is it closed for Danes?

Yes, some of us are still upset about the Viking invasions

As Carlsberg is saying in one of their adds: They took our wiwes,
but gave us the beer ... Sorry.

Seriously though, I have no idea why. I've had people read it all
over the world. Try later.

--

Roger Long



And you are sure it is as written in your message?
I've tried more than ten times, and I just get the message that the
page cannot be shown ...

--
Flemming Torp

"Roger Long" skrev i en meddelelse
...
I just clicked them both again directly from my post.

Try these direct links through Avweb.

http://www.avweb.com/news/airman/183261-1.html

http://www.avweb.com/news/airman/184307-1.html

You'll have to look in the second one for the CL diagram.

--

Roger Long


Thank you - both working! But still no answer on
http://home.maine.rr.com/rlma/Articles.htm ??

--
Flemming Torp
'kun en tåbe frygter ikke haven'