In article , Gogarty wrote:
In article ,
fletopkanelbolle2rp.danmark says...


My brother in law, and I have had a discussion of whether it
is best - from a pure speed point of view (no consideration
to the mechanics/oil/maintenance/gearbox etc. here ...)
whether you should let your "fixed three bladed propeller"
run/turn or keep it fixed (like put into gear) when sailing
just for the wind with your sails in a 34 feet cruiser
weighing roughly 5 T ... We have - unsuccesfully - tried to
find out using the log ... the results were not conclusive -
or one of us would not admit, that the other was right ... I
think we need a testimony from someone, that has a
'scientific based valid answer' ... or just knows for sure
...

The assumption is: There is no way to move, turn, 'collapse'
anything on the propeller - the 'blades' are fixed ... (hard
to explain in a language, that is not your own ... hope you
get my point).


1. Many transmissions require a running engine to keep them lubricated.
Such transmissions should be locvked in reverse to prevent freewheeling.

2. A freewheeling propeller creates more drag than a locked on.. Just
consider a helicopter. Engine out and rotors freewheeling, the aircraft
will go down safely. Rotors locked and it drops like a stone.


The analogys between aircraft and boat propellors do not
hold up when examined by people who understand the physics
involved. I have a very superficial understanding of the
matter, but I can see several problems with it.

An aircraft with a fixed pitch propellor will glide farther
with the propellor stopped because the propellor is bolted
directly to the engine crankshaft. If the prop is turning,
the engine is turning. If the engine is dead but still
being turned, the power to turn the engine is being
extracted from the air flowing through the propellor. The
power lost in turning the porpellor and engine shows up as
drag in the airstream which requires a steeper and shorter
glide to maintain a flyable airspeed.

If the engine can be separated from the propellor by placing
a transmission into neutral as I would expect the case to be
in a sailboat, I would guess that a free wheeling prop would
produce less drag than pulling the stalled propellor blades
through the water. It should take very little power to turn
a shaft riding in two or three bearings with no load on
them. The turbulence of the stopped propellor blades
dragging through the water at nearly right angles to their
streamline shape should put up a lot more resistance.

A helicopter rotor bears no resemblance to a boat propellor
because the rotor blades have variable pitch that can change
each blade individually. The pitch angle can be set so that
(in one exampe) the blade that is moving forward has a very
low pitch while the blade that is moving backward has a very
high pitch. This means that the individual blades are
constantly twisting and turning in their hub bearings as the
entire rotor assembly goes around. There is nothing similar
to that in any boat propellor that I have ever heard of.