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 ...

In article , "Flemming Torp" fletopkanelbolle2rp.danmark wrote:


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?


I guess I'm too long winded.

It is my guess that a freely turning propellor would produce
less drag than a locked propellor. This is assuming that
there is no load on thepropellor other than the friction of
the bearings that support the propellor shaft.

here's more various discussion from nother forum.

it's more geared to mounting alternators, but gives insight to prop
drag.

http://www.ybw.com/forums/showflat.p...e=18& fpart=1

This has been going on for a long time this argument. And I don't mean just
in this forum for the last few days.

In the late fifties (of last century), my father was building himself a
steel ocean racing yacht. The prop drag controversy was alive and well then

There was a handicap allowance at the time for prop based on diameter, and
some silly ineffectual props that looked likesomething off a model airplane
were being tried. Sometimes two in tandem, and they were of course locked in
line with the deadwood. Safety regulations required that the boat be able to
demonstrate performance under power. In still air and calm water, of speed
in knots equal to the square root of the wateline length in feet. I remember
a 58ft 10 Meter Class yacht I was crewing on at the time being unable to
make headway into a 25 Knot breeze. She had about 50 SHP, but a small prop
that was able to transmit only a small fraction of the engine power into
thrust.

Dad didn't want any of this nonsense, if his boat needed an engine for
safety reasons, then it was going to be able to use the power of the engine
and go to windward if necessary despite the weather. But he wanted it to be
competitive.

He got a piece of steel tube about 3 inches in diameter and 18 inches long
and machined inside each end to take the outer ring of a tapered roller
bearing. The inners of the bearings were installed on the shaft with
shoulders so that the bearings were opposed and transmitted the forward and
reverse thrust to the outer rings. Outside of the bearings at each end of
the tube was a normal oil seal, installed the right way round so as to keep
oil inside the tube. The oil inside the tube was pressurised by a header
tank mounted about 3 ft above the waterline.

The tube was mounted at the trailing edge of the keel aperture, with the
prop sized to the 40 HP diesel engine.

Inside the boat between the shaft and the engine gearbox there was a dog
clutch, so that the prop and shaft could be completely disconnected from the
gearbox.

The unit had so little friction that on the slip on a windy day the prop
would revolve in the breeze.

Driven off the shaft by a small chain was an aircraft tachometer generator,
and in the cockpit the tachometer, which was calibrated to read knots. It
was about as accurate as anything I've ever used. I once ran a DR plot based
on it that was 10 nm out at the end of a 600 nm Sydney Hobart race. 1.5%?

Dad ran into the third or fourth owner of the boat in 1980, and when told
that the stern tube assemble had just required repair took delight in being
able say what a pity it was, as the guarantee had just run out.

Going back to the argument about locking or freewheeling: this subject
occupied numerous off watch race hours without as I remember any consensus.
Had we had a GPS or paddle wheel log then it would have been easy to set the
boat up with the prop locked, and then unlock it to see if the speed
increased. Though I doubt we would have done this during a race, and in
light winds (when the effect will be greatest) we never wanted to sail if it
wasn't a race. We had this beaught engine with a big prop.

Why 65 replies on a simple question. Amazing. ;-)

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

In "*JimH*" writes:

Why 65 replies on a simple question. Amazing. ;-)

Because it is not as simple as you and the majority of posters seem to
think. By the way this discussion is repeated about once in three years
and same unfounded arguments and simplifications are represented over
and over again.

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

There are not enough wise men to enlighten the ones that prefers to stay
ignorant. Looks like no lightbulbs are needed.

- Lauri Tarkkonen

"*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.