"Flemming Torp" fletopkanelbolle2rp.danmark wrote in
:

My intuition was exactly as you write - I quote: Wanna bet
the locked prop draggin' through the water creates lots more
drag than the one locked that can't turn? Unquote. I think I
understand what you mean, but being a Dane, and having read
it quite a few times makes me a little uncomfortable - is
the sentence correctly phrased?


I owe you an apology. I would bet the freewheeling prop drag is LESS than
the locked prop drag. I'm sorry I made a joke of my English. I should
have read your header before putting my foot in my mouth with the joke.

Beautiful country, Danmark. We can only pray leaking, abandoned Russian
nuclear facilities don't pollute Scandanavia. I read bellona.no websites
about it. How awful.

"Flemming Torp" fletopkanelbolle2rp.danmark wrote in
:

I'm 'afraid' this debate is not over yet, but you have given
me inspiration to make some funny experiments, that might
give me and my brother in law a clear and convincing answer,
so we can settle our little dispute ... thankyou!



The thread is wonderful. So many ideas and theories. Thank you for
starting it.

"Roger Long" wrote in
:

Under some circumstances, a freewheeling prop may have less drag than
a fixed one. Props on normal transmissions are not freewheeling
however. There is enough drag in the shaftline, bearings, and
transmission to upset things. Outboards, with their clutches right in
the lower unit are closer to freewheeling.



But the point is, as I load the shaft alternator, and therefore the
freewheeling prop, the boat slows more and more as the load increases,
terminating in worst case drag as the prop stalls and stops.

As the load decreases, and the prop turns faster, speed increases. If you
take this beyond the natural drag of the transmission and cutlass bearing
limiting prop speed, by adding slight power to it, there comes a point at
which the prop crosses zero drag trying to pull the shaft out and starts
pushing harder and harder on the shaft as speed increases, where drag goes
through zero and turns into THRUST.

I've never seen it plotted. It might not be a linear function but appears
not to have any weird peaks or valleys in the curve....
|\
D | \
R | \ It's not linear, I'm sure.
A | \ Lots more physics going on.
G | \ Somewhere it stalls.
| \
|------\--------------"freewheeling drag"
0 PROP SPEED - (prop turns = boat speed = no drag here)
| \
T | \
H | \
R | \
U | \
S | \ Down here, somewhere it cavitates.
T | \

God I hate text graphing....(c;

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.

In "Flemming Torp" fletopkanelbolle2rp.danmark writes:


"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 do not create energy by letting the propellor run. In one answer
earlier it was pointed out that a stalled propellor is very different
than a rotating propellor. If there were no friction in your propeller
shaft when it is running freely the energy needed to spin the propeller
could be less than the energy needed for the water to around the
propeller blades. If you have a two blade propeller and it is in
vertical position behind your keel, the water flow behind the keel is
already somehow turbulent and the drag is fairly small. Then in most
cases it will cause less drag if it is not rotating.

There are so many facotrs involved; the sahpe of the blades, the pitcc,
the aspect ratio the shape of the rear of your boat etc. so it is
impossible for someone even very knowledgeable person to give you an
answer just by reading your description of the situation.

There are (fairly reliable) measurements of several sailors sailing and
then stopping and letting the propellor to rotate about the changes of
the speed. The problem is that in some cases it is faster to let spin
and other cases (boats) it is faster to not let to spin. In a case of a
three bladed propellor in a 40 feet Colin Archer the difference has bee
almost one knot.

So I suggest you keep on recording speed differences, there must be some
situation where you wave and weather conditions keep for more or less
constant for some 20 minutes, so you can stop and let your propeller to
roatate by putting the gear on and putting it on neutral.

You keep saying that you do not know physics. There are plenty of good
(and interesting and well written books) it would be easer to choose
from the answers of this group the relevant ones if you were not
ignorant. I can promise you that the speed of your boat is not affected
by the "votes" given in this discussion.

- Lauri Tarkkonen

In Larry W4CSC writes:

"Flemming Torp" fletopkanelbolle2rp.danmark wrote in
:

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?



It'd be real easy to test. Get one of those plastic props for a small
outboard, put it on a shaft. Pull it loosely behind the boat,
FREEWHEELING, with no load (which produces no work) using a simple fish
weighing scale to measure its drag. Then, lock it to the shaft so it can't
turn and drag it at the same speed, yet again.

Wanna bet the locked prop draggin' through the water creates lots more drag
than the one locked that can't turn?

Your example has one flaw. The sailboat propeller is not loose, it is on
a shaft with some bearings and the transmission wheels and oil etc. Do
you know that some boat trhansmissions get very hot if you let the
propeller to free wheel? Some energy is needed to heat it up.

- Lauri Tarkkonen

Take the dingy out for a spin. Run it wide open then drop the engine into
neutral, freewheeling the prop, and see how long it takes to stop dead.
Crank it back up and run it wide open again. This time, leave it in gear
and just pull the safety lanyard making it lock the prop dead still.
Retime how long it takes it to stop. More the drag, the quicker the stop.
You'll find it stops MUCH quicker pulling that STALLED prop through the
water than one FREEWHEELING.

As Lionheart's freewheeling shaft alternator is TURNED ON to produce power
(producing torque on the alternator shaft), it SLOWS the boat as the
alternator SLOWS THE PROP. The heavier it pulls (more torque produced),
the more it slows the boat and prop. Freewheeling props are producing a
minimal amount of torque...and work with no or little load...as load
increases, to the point of being stalled at maximum torque, the drag-
converted-into-torque INCREASES, not decreases!

If you were to spin the prop faster than its drag is capable of, you will
pass through a point of ZERO DRAG just at the point where increasing the
speed of the prop PRODUCES THRUST. So, as prop speed DECREASES, drag
INCREASES! As prop speed INCREASES, it passes through zero drag at the
point where it starts to produce thrust by turning it even faster!

I'm not a physicist, but I play one on Usenet....(c;
Your assignment for tomorrow's class is pages 287 through 312. Do the
workbook exercies on workbook pages 42-43 for turn in. The test will be on
Friday at 2PM.

It might also be a good time to point out that the HELICOPTER rotor CHANGES
DIRECTION as the chopper crashes. It is FORCED in the OPPOSITE direction
from freewheeling to provide DOWNDRAFT to hold the craft aloft by the
engine. As it decends freewheeling, unless you can reverse the pitch of
the rotor, the RISING air through it will make it spin in the opposite
direction.....making it not relevant to any of our discussion here as props
are fixed pitched and already going in the direction of flow caused by
thrust.

Geez....

In JR Gilbreath writes:

If there truly was less drag on a locked prop than a freewheeling one
you would not have to put it in gear to lock it, a prop out of gear
would not even freewheel it there was more drag.

The rotating propeller after it gets some speed is a very different
animal that the locked propeller.

- Lauri Tarkkonen

In "Roger Long" writes:

The dinghy outboard thought experiment is not valid because the whole
system is coming to a stop and the prop is slowing as the boat is
slowing. Do it while towing the dinghy behind the big boat at a steady
speed with a scale on the tow rope and you may get a different result.

Under some circumstances, a freewheeling prop may have less drag than
a fixed one. Props on normal transmissions are not freewheeling
however. There is enough drag in the shaftline, bearings, and
transmission to upset things. Outboards, with their clutches right in
the lower unit are closer to freewheeling.

--

Roger Long

I will second Roger on this. The dinghy experiment would be valid on a
very naiive level of thinking and the experiment with the fishing lure
is invalid because the boat propellors is NOT freewheeling. There is
quite a bit of friction involved. In some cases the gearbox get too hot
to hold a hand on it because the transmission fluid is not pumped around
and while lubricating it is also transmitting the heat away.

- Lauri Tarkkonen

"Roger Long" wrote in message
...
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



Your explanation is pure nonsense. When the prop is fixed the energy created
is dissipated as heat in the water. When it is free to rotate some is
dissipated in the grease and bearings and some as heat in the water.


--
Peter Aitken

Under some circumstances, a freewheeling prop may have less drag than
a fixed one. Props on normal transmissions are not freewheeling
however. There is enough drag in the shaftline, bearings, and
transmission to upset things. Outboards, with their clutches right in
the lower unit are closer to freewheeling.

Roger Long


I will second Roger on this. The dinghy experiment would be valid on a
very naiive level of thinking and the experiment with the fishing lure
is invalid because the boat propellors is NOT freewheeling. There is
quite a bit of friction involved. In some cases the gearbox get too hot
to hold a hand on it because the transmission fluid is not pumped around
and while lubricating it is also transmitting the heat away.

- Lauri Tarkkonen


Actually, the dingy experiment is on the right track. It seems to me
that the energy dissipated in the prop/shaft/transmission/oil train is
irrelevant. That is energy that *would* had been dissipated in
redirecting water flow. Look at it simply; you have *one* energy source,
and that's the water 'stream' past the prop (not quite accurate of
course, since the overall drag will affect the 'stream' velocity, but
it's an easier way to envision it). The amount of energy extracted from
that source will be the result of 1) frictional losses, 2) inertial
forces, i.e. energy required to redirect the mass of water striking the
prop surfaces (not straightforward to calculate), and 3) impact forces
(momemtum, if we're thinking in terms of a 'stream'). Now, the only
issue is whether the drag at the prop is more or less when fixed. So
*if* the freewheeling prop has less drag than the fixed prop, then so
will the turning prop attached to the prop/shaft/transmission/oil train.
To the extent that energy is dissipated by the drive train, the prop
will be rotating slower, and the system will behave somewhere in between
fixed and freewheeling.

Now, whether the fixed or turning has less drag, under any particular
set of parameters, I don't know. I suspect it's a wash. Clearly a
totally freewheeling prop would have less, as the impact forces are
reduced, as is the angular deflection of the water stream, leaving
mostly frictional losses. I suspect though, as appears the case on my
Catalina 30, that the prop when turning significantly slower than it's
pitch rate (i.e. dragging the shaft and trans with it) creates nearly
the same drag as the fixed prop.

Keith Hughes