"me" skrev i en meddelelse
news:zn2pe.7783$nr3.5795@trnddc02...
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.

I'm not sure I get your conclusion ... ;-) Sorry, but do
you recommend me to let the propeller turn og should I stop
it from turning, if I'm only concerned with the speed of the
sailboat - when only using the sails?

--
Flemming Torp