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.

You sort of have to think in reverse. When locked, the blades are
like an airfoil at an angle of attack of nearly 90 degrees, not very
efficient. When turning, the blades are at a low angle of attack and
more efficient. A parachute is less efficient per unit area than a
wing. When the prop freewheels, it's rotational speed makes the water
flow over it faster than when it is just dragged through the water at
boat speed. Whatever force is produced has to be reflected in drag on
the boat as a whole.

This is a very crude pointer towards understanding a complex set of
relationships so, others, please don't bother jumping all over me to
point out how it isn't a rigorous and complete explanation.

There are a narrow range of conditions when a freewheeling prop may
have less drag than a fixed one but they are not commonly encountered
in actual vessels.

--

Roger Long

"Mike G" wrote in message
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?


It's a bad analogy because a prop and a helicopter blade operate on
different principles. A prop is simply a screw whereas a helicopter blade is
an airfoil. When it rotates it generates lift in the same way that an
airplane's wing does. THis is not the case with a prop.


--
Peter Aitken

"Mike G" skrev i en meddelelse
ews.com...
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.

That's exactly, what we have tried, but the conditions were
not ideal, so we did not - unfortunately - come up with some
'conclusive empiric evidence' ...

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.

That's also what my intuition tells me, but listening to
other people in this group, I get the impression, that my
intuition is not valid in this case ...

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 do not understand - a new variable? My question is simple:
.... If all other things are equal, should I stop my
propeller or should I let i turn freely?

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

My problem is, that I can not see, that your above argument
is wrong ... but some other readers in this group tell me,
it is not correct ... Unfortunately, I do not
know/understand the laws of physics governing this situation
....

--
Flemming Torp

"Dave" skrev i en meddelelse
...
On Mon, 06 Jun 2005 19:31:31 GMT, "Roger Long"
said:

When
the prop is freewheeling, it is producing energy

A fairly basic principle: energy can be neither created
nor destroyed.

Isn't it right, that when the boat is sailing (just for sail
that is) you need some energy to keep the propeller from
turning - i.e. to keep it fixed - and that gives a certain
drag on the boat ....

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?

--
Flemming Torp

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

"Dave" skrev i en meddelelse
...
On Mon, 06 Jun 2005 19:31:31 GMT, "Roger Long"
said:

When
the prop is freewheeling, it is producing energy

A fairly basic principle: energy can be neither created
nor destroyed.

Isn't it right, that when the boat is sailing (just for sail
that is) you need some energy to keep the propeller from
turning - i.e. to keep it fixed - and that gives a certain
drag on the boat ....

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?


You have lousy intuition, Flemming.

CN

"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

Your intuition is wrong. You never loosen the grip, you just change
to a different kind of grip. Ever notice how hot brakes get? That's
because they are turning all that energy into heat. When you let the
prop go, it is still restrained by the friction of the bearings and
oil in the transmission. They get warmer because they are turning the
energy being produced into heat. That gets reflected in drag on the
boat.

When the prop is locked, the shaft break, clutch, or whatever is
holding it doesn't get hotter. What gets hotter is the water flowing
past the blades. That energy production gets reflected in drag on the
boat as well but there is less of it at normal sailing speeds and
freewheeling shaft rpm.

--

Roger Long



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


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?

--
Flemming Torp

OK, here is the answer you are looking for.

Unless you have a highly unusual powertrain set up and strangely
pitched prop, determine the position in which the most blade area is
shadowed by keel and hull. Mark the shaft inside. Stop the shaft in
that position. Sail the boat. It's very unlikely you'll go faster
doing anything else.
--

Roger Long

"Roger Long" wrote in
:

Gee, isn't this kind of "niggling and perfectionist", to quote a well
respected rec.boats contributor?


Oh, you'd have a fit if you looked into that bilge. There's dishwater,
some oil, some rust, a few tools too deep to reach even with the magnet,
how awful! It's a boat....not house beautiful. Everything from the sinks,
shower, everything but the head, dumps in her bilge to be pumped overboard
WITHOUT those nasty thru-hull fittings rotting away...(c;

Look at the next picture. This is the NEW engine Cap bought from a guy in
NC I met on this newsgroup. It's a pullout. See? We didn't even paint
it! Looks awful...runs fantastic. The original looked worse. It had over
8000 hours on it. This "new" one is only up to 900 hours, now. Cap'n
Geoffrey couldn't even destroy it trying to start it full of seawater when
that stupid water injection line to the dripless packing he had a shipyard
install BACKED UP seawater into the exhaust ports. Try sailing up Ponce
Inlet S of Daytona Beach in the rush of an outgoing tide past the
lighthouse and seawall so the tow operator can drag you all the way to
Daytona Marina. Scary stuff that day. Cutter Doc did a great job pickling
the engine. It looked like we pumped the Exxon Valdez oil slick out of the
crankcase...(c;

These pictures are old. I need to carry my camera down, now that she's
been fitted out, and take some new pictures in her. I'm usually too busy
when Geoffrey is around working on his to-do list...(c; He finally gave up
trying to pay me. He asked what I wanted for all my work. I told him,
"That's easy. Just take me with you." It's worked out great for both of
us....(c;