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

Anybody in this group that can convince one of us, that
he
is not right? ...



Let the boat drive the prop when sailing. That shaft
produces 15-25A of DC
power on Lionheart from her shaft-driven alternator. No
sense wasting good
power draggin' a stalled prop behind you.

Larry, I certainly enjoy most of your constructive and
'witty' comments in this newsgroup, and I learn a lot and
from them. But this comment does not adress my question: ...
When will my boat run fastest: 'Free' running propeller or
fixed/locked propeller? ... Simple question, complicated
answers, I understand ...

I know this is a very partial question ... but that is to
keep the issue as simple as possible ... *just speed* ... no
economy or energy considerations, no maintenance og mechanic
considerations, no ecology or noise consideration - I'm just
interested in understand the relation between the status of
the propeller and the speed og the boat - all other things
equal ...

--
Flemming Torp

"Mike G" skrev i en meddelelse
ews.com...
In article
,
says...
On Mon, 06 Jun 2005 08:01:55 -0400, Gogarty
said:

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.

Bad analogy. In a helicopter when the rotors are
freewheeling after the
engine dies they're still rotating in a direction that
generates lift.



I just have to ask. In what way is it a bad analogy?

SNIP

Personally I figure, locked or freewheeling, the
difference in the
amount of drag caused by a kicker's prop is going to be so
small as to
be a total non issue unless you are into times in parts of
a second in
which case you pull the damn thing out of the water
anyway.


--
Mike G.
Heirloom Woods

www.heirloom-woods.net

I agree, that the practical difference might be dismissal
.... but in a race every second counts per NM ... and it is
not easy to get a fixed propeller out of the water ;-) - and
I'm pretty curious of knowing the correct answer to my
question - preferably backed by solid science as I'm not a
scientist ...

When cruising, I would always lock the propeller - or follow
Larry's advice - use the energy generated ... that is
something I will consider ti implement in the future ...

--
Flemming Torp

In article , "Flemming
Torp" fletopkanelbolle2rp.danmark says...
Being a sailor with no experience with aviation, I do not
understand the helicopter analogy.
In my opinion, a helicopter driven by the engine corresponds
in my understanding to a sailboat, where the propeller is
set to drive the boat *backwards* ..... When the propeller
on the boat 'gets the chance' to run without the force of
the motor, that is just sailing forward by sail, I expect
the propeller to turn in the opposite direction ... that is
'forward' ... If that happens in a helicopter, I would
expect the helicopter to hit the ground very fast ... don't
you? I do not assume, that there is a lot of inertia in a
propeller or shaft en a sailboat - may be that is the case
in a helicopter, and that could explain the smooth landing
without the assistance from the motor - or I'm I completely
wrong here? ...



The boat prop continues to turn in the same direction as it did when it
was the method of propulsion since there is no change in direction of
the water flow. In other words nothing forcing it to counter rotate.

In the case of the helicopter technically you are correct and a reversal
of direction would change the rotation of the blades. However the rotors
are connected to the engine via gears and can only rotate in the "lift"
direction. When the helicopter starts to fall due to loss of power the
rotors, slowed buy the drag of gears and engine, are forced to spin in
the lift direction causing "drag" that slows the copter to a rate where
impact with the ground is, at least, survivable.



--
Mike G.
Heirloom Woods

www.heirloom-woods.net

"Dave" skrev i en meddelelse
...
On Mon, 6 Jun 2005 13:06:28 -0400, Mike G
said:

I just have to ask. In what way is it a bad analogy?

Possibly you are thinking the freewheeling prop would
somehow generate
some forward motion. Not likely or you would have a
perpetual motion
machine.

Those with an understanding of basic physics will
understand my point. It's
clear that a detailed explanation to you would be
pointless, as you would
have figured it out by now if you had the basic
understanding.

Please give it a try anyway ... please ... ;-) ... I'm
curious, but not a physicist ...

--
Flemming Torp

"Mike G" skrev i en meddelelse
ews.com...
In article ,
"Flemming
Torp" fletopkanelbolle2rp.danmark says...
Being a sailor with no experience with aviation, I do not
understand the helicopter analogy.
In my opinion, a helicopter driven by the engine
corresponds
in my understanding to a sailboat, where the propeller is
set to drive the boat *backwards* ..... When the
propeller
on the boat 'gets the chance' to run without the force of
the motor, that is just sailing forward by sail, I expect
the propeller to turn in the opposite direction ... that
is
'forward' ... If that happens in a helicopter, I would
expect the helicopter to hit the ground very fast ...
don't
you? I do not assume, that there is a lot of inertia in a
propeller or shaft en a sailboat - may be that is the
case
in a helicopter, and that could explain the smooth
landing
without the assistance from the motor - or I'm I
completely
wrong here? ...



The boat prop continues to turn in the same direction as
it did when it
was the method of propulsion since there is no change in
direction of
the water flow. In other words nothing forcing it to
counter rotate.

In the case of the helicopter technically you are correct
and a reversal
of direction would change the rotation of the blades.
However the rotors
are connected to the engine via gears and can only rotate
in the "lift"
direction. When the helicopter starts to fall due to loss
of power the
rotors, slowed buy the drag of gears and engine, are
forced to spin in
the lift direction causing "drag" that slows the copter to
a rate where
impact with the ground is, at least, survivable.



--
Mike G.
Heirloom Woods

www.heirloom-woods.net

I'm sorry - Now, I don't understand the argument or the
analogy from the helicopter to the boat of my brother in law
with three fixed blades ... Should he lock it or let it
'freewheel'?

--
Flemming Torp

In article , "Flemming
Torp" fletopkanelbolle2rp.danmark says...
I agree, that the practical difference might be dismissal
... but in a race every second counts per NM ... and it is
not easy to get a fixed propeller out of the water ;-) - and
I'm pretty curious of knowing the correct answer to my
question - preferably backed by solid science as I'm not a
scientist ...

When cruising, I would always lock the propeller - or follow
Larry's advice - use the energy generated ... that is
something I will consider ti implement in the future ...

--
Flemming Torp





Well, if you are racing and can't get the prop out of the water I can
see where it may be of concern but if you're looking for a solid science
answer I'd have to venture to say that there are so many variables
involved only a practical application with the boat involved would work.

Drag is drag and if you can set a straight course with a constant speed,
difficult under sail even in ideal conditions but still do able, just
locking and unlocking the prop and seeing what it does to your speed
should provide the answer.

However, I'd have to opine that there is a difference between a free
wheeling prop and one that is not only spinning but driving an
generator/alternator. There is no free ride. A prop working a generator
requires more energy then a free wheeling one and the only place that
energy can come from is the forward speed of the boat.

Again, the difference may be negligible but if you are concerned with
thousands of a second, as you would be if racing, you have now
introduced one more variable that has to be considered.

I'm afraid you can't really get a "just speed" answer and expect an
answer backed by science without considering conservation of energy,
every action has an......... and so on.

I can't prove it but my money would be on a truly free spinning prop. A
fixed prop wants to spin and is going to use energy trying to do so.
It's going to fight forward motion harder then a free spinning prop. A
prop with a generator on it will spin but not without the loss of energy
required to also turn the generator, a free spinning prop requires only
the energy required to spin it. It'll, pardon the pun, go with the flow.
Some energy penalty but less then the first two scenario's

Again, the only place the energy required can come from is the forward
motion of the boat.

--
Mike G.
Heirloom Woods

www.heirloom-woods.net

Mike G wrote:
In article ,
says...

On Mon, 06 Jun 2005 08:01:55 -0400, Gogarty said:


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.

Bad analogy. In a helicopter when the rotors are freewheeling after the
engine dies they're still rotating in a direction that generates lift.




I just have to ask. In what way is it a bad analogy?

Perhaps you should consider an autogyro:
http://www.centennialofflight.gov/es...togyro/DI8.htm



....


Personally I figure, locked or freewheeling, the difference in the
amount of drag caused by a kicker's prop is going to be so small as to
be a total non issue unless you are into times in parts of a second in
which case you pull the damn thing out of the water anyway.

Well, each to his own. It is often claimed that the difference can be
up to a half of a knot. On a hundred mile passage that can be about an
hour. Given a choice, I'd rather come in before dark. My previous
boat had a two blade which I always locked aligned with the hull; my
current boat has folding props.

I'll try.

The answer will be found in conservation of energy. The energy books
have to balance in every system. The amount if drag of the water
flowing over the prop will exactly equal the energy being produced.
With the prop stopped, it is simply pull aft on the shaft. The prop
isn't very efficient that way. Try propelling a boat by putting the
prop out ahead on a long line and then pulling it in quickly. When
the prop is freewheeling, it is producing energy that is absorbed by
heating up the bearings and lube oil in the transmission, (or in
Larry's case by charging batteries).

It's slightly counterintuitive but, whatever energy is produced has to
have an exact mirror image in drag on the boat. Since the prop is
making energy more efficiently while turning, there has to be more
drag.

--

Roger Long

"Flemming Torp" fletopkanelbolle2rp.danmark wrote in message . ..

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

Anybody in this group that can convince one of us, that
he
is not right? ...



Let the boat drive the prop when sailing. That shaft
produces 15-25A of DC
power on Lionheart from her shaft-driven alternator. No
sense wasting good
power draggin' a stalled prop behind you.

Larry, I certainly enjoy most of your constructive and
'witty' comments in this newsgroup, and I learn a lot and
from them. But this comment does not adress my question: ...
When will my boat run fastest: 'Free' running propeller or
fixed/locked propeller? ... Simple question, complicated
answers, I understand ...

I know this is a very partial question ... but that is to
keep the issue as simple as possible ... *just speed* ... no
economy or energy considerations, no maintenance og mechanic
considerations, no ecology or noise consideration - I'm just
interested in understand the relation between the status of
the propeller and the speed og the boat - all other things
equal ...

--
Flemming Torp

Lock the shaft for speed. Don't allow the prop to freewheel.

It's that simple.

CN

"Roger Long" skrev i en meddelelse
...
I'll try.

The answer will be found in conservation of energy. The
energy books have to balance in every system. The amount
if drag of the water flowing over the prop will exactly
equal the energy being produced. With the prop stopped, it
is simply pull aft on the shaft. The prop isn't very
efficient that way. Try propelling a boat by putting the
prop out ahead on a long line and then pulling it in
quickly. When the prop is freewheeling, it is producing
energy that is absorbed by heating up the bearings and
lube oil in the transmission, (or in Larry's case by
charging batteries).

It's slightly counterintuitive but, whatever energy is
produced has to have an exact mirror image in drag on the
boat. Since the prop is making energy more efficiently
while turning, there has to be more drag.

--

Roger Long



Thank you for trying, Roger. Sorry, but I still have some
doubts. In my opinion it requires quite a lot of energy just
to keep the propeller fixed when sailing. It is not
possible - for instance to hold the propeller shaft by
hand - so the shaft does not turn - as Rosalie confirmed ...
So there must be quite a constant pressure on the gear box,
when just sailing and the prop locked by the gearbox. In my
opinion, it could be compared to a kind of turbine, where
you keep the blades fixed from turning ...

My intuition tells me, however, that if you 'let the
propeller go', the total resistance/drag will be less and if
you start braking the turning propeller, the resistance/drag
gets bigger, and that resistance can be converted to
electricity - as explained by Larry -isn't that right? But I
must admit, that I don't know - or may be worse: Don't
understand - the Law of physics behind the right answer ...

--
Flemming Torp

PS - I have followed your interesting 'report' of the last
few months of projects on your boat ... good and inspiring
reading - thank you!