On Mon, 06 Jun 2005 12:42:52 GMT, Rosalie B. wrote:

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.


This is not what he asked. He said specifically
from a pure speed point of view (no consideration
to the mechanics/oil/maintenance/gearbox etc. here ...)

Plus not all transmissions can be locked in reverse to prevent
freewheeling. Ours cannot. Doesn't matter what gear you stop in, the
shaft will freewheel unless you stop it by putting a vice grip on the
shaft or something (IMHO a bad idea, but I know someone who does
this).


Not remotely applicable, autorotation (that rotating of the helicopter rotors
that you refer to in a uncontrolled descent) is caused because the pitch of the
rotor blades are deliberately and continuously adjusted to generate maximum
possible lift for that vertical airspeed. Additional differences - air is
compressible, water for all intents and purposes is not - this makes a
tremendous difference in the effect.

This is a comparing apples to oranges situation. The jury is out as to whether
it causes more drag propeller locked or rotating. Some highly respected naval
architects say one thing, others say the opposite. Until a scientific peer
reviewed study is released I will go with "I don't know and nobody else does
either" opinion.

JJ

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.

This is probably a good analogy, although not all aircraft principles
will translate to water and v.v. (as the Wright brothers found)

My answer would be the same as yours and also that if you have a two
blade prop, it should be locked in line with the keel if it is
possible to determine where that is etc.

In our case, the freewheeling prop made so much noise that one of the
first things we did was get a feathering prop. We do have evidence
that feathering the prop increases our sailing speed, in addition to
being quieter.

grandma Rosalie

James Johnson
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