"Larry W4CSC" skrev i en meddelelse
...
"Flemming Torp" fletopkanelbolle2rp.danmark wrote in
:

My intuition tells me that when you ' loosen the grip' on
the propeller shaft and let the propeller turn freely, it
reduces the drag on the boat. Right or wrong?



It'd be real easy to test. Get one of those plastic props
for a small
outboard, put it on a shaft. Pull it loosely behind the
boat,
FREEWHEELING, with no load (which produces no work) using
a simple fish
weighing scale to measure its drag. Then, lock it to the
shaft so it can't
turn and drag it at the same speed, yet again.

Wanna bet the locked prop draggin' through the water
creates lots more drag
than the one locked that can't turn?

Take the dingy out for a spin. Run it wide open then drop
the engine into
neutral, freewheeling the prop, and see how long it takes
to stop dead.
Crank it back up and run it wide open again. This time,
leave it in gear
and just pull the safety lanyard making it lock the prop
dead still.
Retime how long it takes it to stop. More the drag, the
quicker the stop.
You'll find it stops MUCH quicker pulling that STALLED
prop through the
water than one FREEWHEELING.

As Lionheart's freewheeling shaft alternator is TURNED ON
to produce power
(producing torque on the alternator shaft), it SLOWS the
boat as the
alternator SLOWS THE PROP. The heavier it pulls (more
torque produced),
the more it slows the boat and prop. Freewheeling props
are producing a
minimal amount of torque...and work with no or little
load...as load
increases, to the point of being stalled at maximum
torque, the drag-
converted-into-torque INCREASES, not decreases!

If you were to spin the prop faster than its drag is
capable of, you will
pass through a point of ZERO DRAG just at the point where
increasing the
speed of the prop PRODUCES THRUST. So, as prop speed
DECREASES, drag
INCREASES! As prop speed INCREASES, it passes through
zero drag at the
point where it starts to produce thrust by turning it even
faster!

I'm not a physicist, but I play one on Usenet....(c;
Your assignment for tomorrow's class is pages 287 through
312. Do the
workbook exercies on workbook pages 42-43 for turn in.
The test will be on
Friday at 2PM.

It might also be a good time to point out that the
HELICOPTER rotor CHANGES
DIRECTION as the chopper crashes. It is FORCED in the
OPPOSITE direction
from freewheeling to provide DOWNDRAFT to hold the craft
aloft by the
engine. As it decends freewheeling, unless you can
reverse the pitch of
the rotor, the RISING air through it will make it spin in
the opposite
direction.....making it not relevant to any of our
discussion here as props
are fixed pitched and already going in the direction of
flow caused by
thrust.

Geez....


Thank you for proposing an interesting experiment ... I will
try to set up this experiment during the summer holidays ...

In the meantime, I must admit, I'm getting pretty much
confused ... I have just appreciated and accepted Roger
Longs 'lecture' on drag and rotation speed etc., where the
conclusion was: The locked propeller gives the maxium speed
compared to the freewheeling propeller ... And now, you
testify just to the opposite ...

My intuition was exactly as you write - I quote: Wanna bet
the locked prop draggin' through the water creates lots more
drag than the one locked that can't turn? Unquote. I think I
understand what you mean, but being a Dane, and having read
it quite a few times makes me a little uncomfortable - is
the sentence correctly phrased?

The beauty of your proposed experiment is, that it is very
operational with a little plastic propellerthing and the
weight from my wifes kitchen, a string and a piece of wood.
And you can repeat the experiment several time under
different conditions ... I will have to find out have to
keep the propeller from turning and at the same time measure
the drag on the weight, without affecting the weight
measurement ... I don't have a motor on the dinghy ...

I had serious problems understanding the helicopter analogy
.... and the way you phrase the case is the way, my intuition
saw the situation - I quote: It might also be a good time
to point out that the HELICOPTER rotor CHANGES DIRECTION as
the chopper crashes. It is FORCED in the OPPOSITE direction
from freewheeling to provide DOWNDRAFT to hold the craft
aloft by the engine. unquote. Therefore, I do not see, that
this analogy explains anything related to my question.

May be an old quotation is in place now: I'm still confused,
but - hopefully - at a higher level ??? As to the
assignment, I'm afraid I forgot my textbook in the office,
and I will be working from other places the rest of the week
.... sorry!

I'm 'afraid' this debate is not over yet, but you have given
me inspiration to make some funny experiments, that might
give me and my brother in law a clear and convincing answer,
so we can settle our little dispute ... thankyou!

--
Flemming Torp