"*JimH*" wrote in :

Now tackle this one...how many sailors does it take to screw in a light
bulb?



Eight. One to hold the bulb, seven to turn the boat.

"*JimH*" wrote in :

2. He subsequently posted my real name (I had posted under an alias for
obvious reasons), address and wife's name to rec.boats.



I posted a map straight to his house in MD when he pulled that **** with
me.

"Larry W4CSC" wrote in message
...
"*JimH*" wrote in :

Now tackle this one...how many sailors does it take to screw in a light
bulb?



Eight. One to hold the bulb, seven to turn the boat.


Bzzzzt....wrong.


One to turn the bulb and .................(can we make fun of ourselves at
this point?)............

*JimH* wrote:
Why 65 replies on a simple question. Amazing. ;-)


Hear is number 66

Every body is getting drag mixed up with lift. The reason you should
not allow the propeller to spin freely is not because of drag it is
because of all the lift it generates opposite the direction of travel
of the boat.

Drag is always in the direction of the relative fluid flow that causes
it. The relative fluid flow that causes the drag on the spinning prop
is not at all the same (direction and speed) as the relative fluid flow
that is influencing the rest of the boat hull but every one is
determining drag as though it was the same. The more the prop rotates
the more the relative fluid flow influencing it becomes circular, and
the more the direction of the lift it generates becomes parallel to the
prop shaft. As the prop starts to spin the angle of attack becomes less
generating more lift and less drag because the prop is operating well
below the stall angle.

The best aeronautic analogy of this would be an autogyro. The rotor is
tilted slightly rearward to facilitate its rotation from on coming
relative airflow. The rotating blade not only generates enough lift to
support the weight of the aircraft but unfortunately some of the lift
has a rearward component. Igor Benson wrote one of the disadvantages of
the auto gyro is all the drag it generates that opposes thrust. He got
drag mixed up with lift also. Lift and drag are very very similar.
Aircraft have flown non stop around the world in spite of drag while
others have done it as a result of drag.

Also be aware that the water is not flowing around the prop the prop is
moving while in a fluid (rotation) as it moves linear thru a fluid. If
the fluid were moving around the prop drag would cause its motion not
oppose it. Thanks to aeronautic texts we all know that to move thru a
fluid you have to overcome drag. This represents a fraction of what
drag is all about for it is also true that to remain still while in
moving air you must also over come drag. I have about reached the
limits of my third grade education that took me four years to get.

"*JimH*" wrote in :

(can we make fun of ourselves at
this point?)............


Yes, as long as someone gets aggravated and it starts a flamewar that goes
on for months like kindergarteners in a sandbox fighting over a truck.

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
remove the "dot" from after sail in email address to reply

67

Way back whenI was in school I remember something along the lines that
"a stopped propeller generates drag equal to the area of the stopped
blades,
but a spinning propeller generates drag equal to the AREA OF THE DIAMETER of
the propeller arc."

This would be a vote for stopped propeller equals less drag.

E. Currier

Lift is just drag directed upwards. The difference between lift and
drag is really just a thought convenience and not a physical
destination. The lift in an airplane wing has a slightly backwards
component which is why adding weight slows the airplane.

The term "going ballistic" refers to pushing over the nose of a fast
plane so that the plane goes into a zero G state. Since the plane no
longer weighs anything, the wings no longer have to create lift
(therefore it is "ballistic" like a bullet), drag is dramatically
reduced, and speed increases. This requires a downward curved flight
path so can't be maintained for long.

--

Roger Long

Helicopter - I believe that you will find that helicopters are
sometimes able to self-rescue because the pitch of the blades can be
changed. As the helicopter falls through the air, the blades are
pitched to spin the blades rapidly in one direction. Just before the
helicopter crashes, the pilot adjusts the "collective" to switch the
blade pitch. The momentum of the spinning blades keeps them going for
a short time. With the blades now pitched to create lift, the
momentum-created lift can be enough to cushion the landing. If the
pilot adjusts the collective too soon, the helicopter will come almost
to a hover and then begin to fall rapidly to the ground.

Larry's physics strike me as correct -- up to a point. If just enough
power is applied to the shaft to turn the prop to match the water flow
past it, the drag ought to be zero. If more power is applied, the
"drag" would be negative. If less power is applied, the drag will
increase right up until the prop is almost stationary. When the prop
become completely stationary, however, a new condition is created.
The blades stall. Going back to an aircraft analogy, as the wings of
a fixed-wing aircraft are flown to take a bigger and bigger bite of
the air, the drag will significantly increase. But, once you go one
step too far and the wing stalls, the plane will drop like a stone.

While Larry's idea of the increasing drag as the prop slows its
rotation makes perfectly good sense, it misses the transition to less
drag when the blades stop moving and stall.

OK, so maybe it is BS. :-D But it is pretty good sounding BS.
And its my story and I'm sticking to it.

BTW, not only is Denmark a great country, Danes are very fine people.

Lee Huddleston
s/v Truelove

(Lee Huddleston) wrote in
:

OK, so maybe it is BS. :-D But it is pretty good sounding BS.
And its my story and I'm sticking to it.


Hmm....I'll buy into that analogy, too. Thanks.

We still need someone to drag 'em behind the dingy to test these theories
out in a scientific manner....(c;

--
Larry

You know you've had a rough night when you wake up and your outlined in
chalk.