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