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.

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).

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

In article ,
says...

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.

Well, next time you haul you mark the shaft inside the boat to indicate
when the prop is up and down behid the keel or skeg or whatever. After
twenty years, I might yet get around to doing that.

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.

Ours doesn't freewheel in reverse. And the cost of a feathering prop really
put me off.

Gogarty wrote:

In article ,
says...

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.

Well, next time you haul you mark the shaft inside the boat to indicate
when the prop is up and down behid the keel or skeg or whatever. After
twenty years, I might yet get around to doing that.

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.

Ours doesn't freewheel in reverse.

So you don't really need it then.

And the cost of a feathering prop really
put me off.

The feathering prop also gave us increased speed. That alone made it
worth the money. A folding prop and a feathering prop are different
and we have the less expensive one. Plus, we found when we took the
prop off that we had a scored shaft which had to be replaced.


grandma Rosalie

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

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.

"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

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" skrev i en meddelelse
...

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





Now we are getting close to 'basics' Roger ... but, but ...
as I wrote in the introduction, it has so far been very
difficult to get hard evidence from the log when trying to
let the propeller run and have it locked, as the speed of
the boat is a function of so many things, and I'm convinced
that there is not a big difference - so may be my question
is of a more theoretical type, as reliable data are hard to
get in the real world ... In a bassin, it might be easier
.... I have seen som reports, where different kinds of
propellers - folding with two blades, folding with three
blades, fixed with three baldes etc. were compared ... and
the result indicated differences in 'thrust' and in speed
up to between ½ - 1 in worst case ... But I have not seen
any reports on the comparison between a locked and a free
wheeling propeller ... but I have certainly got a lot of
input ... also from the aviation world, that I know nothing
about ... thank you.

But your final proposal is very logical, operational and
easy to implement ... when the water gets warmer, we might
do what you have recommended ... or do as Larry - use the
energy coming from the rotating propeller - og invest in a
folding propeller ... time will show, and thank you so much
for your keen interest in learning me some physics ...

Flemming Torp wrote:
"Roger Long" skrev i en meddelelse
...

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.

Now we are getting close to 'basics' Roger ... but, but ...
as I wrote in the introduction, it has so far been very
difficult to get hard evidence from the log when trying to
let the propeller run and have it locked, as the speed of
the boat is a function of so many things, and I'm convinced
that there is not a big difference - so may be my question
is of a more theoretical type, as reliable data are hard to
get in the real world ... In a bassin, it might be easier
... I have seen som reports, where different kinds of
propellers - folding with two blades, folding with three
blades, fixed with three baldes etc. were compared ... and
the result indicated differences in 'thrust' and in speed
up to between ½ - 1 in worst case ... But I have not seen
any reports on the comparison between a locked and a free
wheeling propeller ... but I have certainly got a lot of
input ... also from the aviation world, that I know nothing
about ... thank you.

But your final proposal is very logical, operational and
easy to implement ... when the water gets warmer, we might
do what you have recommended ... or do as Larry - use the
energy coming from the rotating propeller - og invest in a
folding propeller ... time will show, and thank you so much
for your keen interest in learning me some physics ...



Roger's advice is correct, but it does sidestep your original
question. Locking a prop in the "shadow of the hull" is generally a
winner. Even with a 3-blade prop you should have one blade lined up
with the hull. With a two blade prop the gain can easily be enough to
win a race, or come in before dark on long passage.

I've heard of two studies that addressed this. One, a publicized MIT
study showed that free-wheeling had less drag. However, this did not
directly model yacht props so it really doesn't apply. (In fact,
their prop wasn't actually free, it was powered at a speed that
minimized turbulence.) Other study, which I've heard of third hand,
"proved" that locked was less drag for yacht props. Dave Gerr, in his
"propeller Handbook," says rotating is less drag (unless you can lock
it behind the keel) but he doesn't give a reason or cite a reference.

Consider two cases: first take a hypothetical "flat prop" with zero
pitch. Obvious, it won't spin and it will have a lot of turbulence,
indicating a lot of drag. Now give it a bit a pitch. It will start
to spin, but the small pitch will mean that it has to spin very fast
fast to match the boat speed. Since it can't, there will still be a
huge amount of turbulence. Will it be less than the flat blade? Hard
to say, because there are a variety of factors - the shape of the
turbulence is different and the rotation means that a larger volume of
water is disturbed. This approximates the yacht prop where the free
rotation is very unlikely to match the boat speed and the turbulence
will be high and quite complex.

Now consider a feathering prop, where the blades are lined up with the
flow and cause no turbulence. Give them a some pitch - as long as the
flow is smooth there will be little drag. Increase the pitch
(actually a smaller number since feathering the pitch is infinite) to
the fastest spin with low turbulence. The drag will still be low at
this point. Now lock it - the turbulence (and the drag) will jump
up. Here's a case where the free rotating prop will have less drag
than the locked one, but it probably does not resemble a typical yacht
prop. It does come close to large ship props, so its not surprising
that there are some studies and anecdotal evidence from other forms of
props, that say free wheeling is less drag.

My point is (as I mentioned in my first post) that its impossible to
determine intuitively the drag for the case of a yacht prop, where the
turbulence is high whether locked or not, and analogies from other
type of props simply don't apply. The issue of the heat buildup in
the transmission is a "red herring." It is a significant side issue,
since yacht transmissions do have a lot of friction, and thus heat
problems, when free wheeling, but the dominant factor is the
turbulence. This approach tries to measure the energy extracted from
the water by the work done by the shaft, but this is only valid if the
turbulence is low and the prop is working in a relatively efficient
mode. When the turbulence is high, most of the energy goes into
heating the water, and this is rather difficult to measure.