Stuffing box... Sailing.. prop left to spin

[model 10]

[QUOTE=a64pilot;2628788]You can’t kill the C-85 that is used in a 140 by mixture,


Sorry thought you had the C-90.

[a64pilot]

[QUOTE=Ecos;2628817]

Originally Posted by a64pilot

You can’t kill the C-85 that is used in a 140 by mixture,





Sorry thought you had the C-90.

I wish, the C-90 is I think pretty rare in a 140 and I think use the same Stromberg carb? It is a better seaplane engine as it produces more torque.
There has been an ongoing debate about the mixture control on a Stromberg carburetor forever, almost all of them were wired so there were inop and the mixture control plate removed. Very few are operational and parts of course are almost non existent
http://www.bowersflybaby.com/tech/st...re_secrets.pdf

[Wotname]

With apologies to the OP for continuing the thread drift...

For anyone wanting to read more about prop drag (locked or freewheeling) please read this thread http://www.cruisersforum.com/forums/...ar-148717.html.

And really, I thought regular CF readers knew the facts by now but I note some are still promoting fiction.

And oh, by the way, comparing airplane props to boat props is like comparing tomatoes to strawberries - both are red and both are fruit but that is about all...

Originally Posted by jmschmidt

As it's been said many times, a free spinning solid prop creates parasite drag equal to the surface area of a solid disk the diameter of the prop. A prop on a locked shaft creates parasite drag equal only to the combined surface area of the blades themselves. A two-blade prop, not much drag if the shaft is locked. A four or five blade prop with a locked shaft, a lot more drag but still not as much as if it were left free spinning. A lot of people don't believe that but it's true. Check with an engineer. A full feathering prop, doesn't matter because it shouldn't spin much, if at all, when feathered. If you transmission manufacture states not to lock the shaft by leaving it in gear when sailing, it's best to install a separate shaft lock and leave the transmission in neutral. Using your shaft to run an electric generator produces tons of drag. The faster you sail, the more electricity it produces same the greater the drag. Where do you think the electrical energy comes from? Same with a wind driven generator on a boat, sail or power, produces tons of drag too. When sailing it's best to lock the prop on a wind generator too. Unless you really need the electricity, run your wind generator when anchored. There is no free lunch in the world of physics.

The reason most people don't believe it is because what you postulate is simply not true.
So ask engineer or better still factually dispute this analysis (from post #100 on page 7 of the above linked thread):

For a rational and bullet proof theoretical explanation, please read on. By the way, you won't need to know any fancy maths or understand complicated formulas, just a basic understanding of first principles of physics.

First, let's get some basic physics dealt with.

Energy can't be created or destroyed, just changed from one form to another.
All the forces on a sailboat can be resolved into 6 vectors (2 for each axis).
When the vectors are equal and opposite, velocity remains constant, when they are not, acceleration (positive or negative) occurs.

OK, next look at our situation; we only have two conditions to consider

1. The prop is allowed to rotate as freely as it can be given the confines of the gearbox being placed into neutral - we will call this freewheeling for the purposes of this discussion.
2 The prop is held stationary by either a shaft lock of by placing the gearbox into gear (ahead or astern) and thus having the engine compression preventing it from rotating - we will call this locked.
These are the only two conditions available to us. Note, we are not considering feathering or multi pitch props.

Now comes the explanation…

We only need to consider two of the six vectors: thrust and drag. Any forces that cause the the boat to move forward can be resolved into one vector - thrust. Any forces that hinder this forward motion can be summed together and becomes drag. When the thrust and drag are equal, the velocity of the boat remains constant. However, if some force acts on the boat to increase the thrust vector, the boat will accelerate until the drag increases and balances out the new thrust vector. It will then remain at this new velocity until something else changes. Of course, the reverse is also true.

So in our simple explanation, we have a sailing boat complete with a fixed bladed prop and and prop shaft. However, instead of a gearbox and engine, just consider the inside end of the shaft is fitted with a simple crank than can be turned by hand. I assure you that this arrangement acts in a similar manner as a gearbox and engine as far as the forces in question is considered.

Provided nothing prevents the crank from turning, the prop, shaft and crank will start to rotate as soon as we get some boat speed happening. For our purposes, we will say this occurs around 3 kts STW.

So assume we are sailing along at 5 kts with steady breeze, the prop will be rotating - for our example, let's say it is doing 30 RPM in a clockwise direction.
It is freewheeling in essence. If our velocity is constant (5 kts), then the thrust and drag vectors must be equal and opposite.

Now put you hand on the crank and by pure muscle power, speed up the rotation to say 60 RPM in the same clockwise direction. Clearly this is going to take some effort. So what happens to this extra energy that has been introduced into our steady system. Just as clearly, the prop is now going to be providing some additional force on the boat - in fact, it will increase the thrust. So now the thrust is greater than the drag and the boat will accelerate to a new and increased velocity. It will continue to accelerate until the drag caused by the extra speed balances out the the new thrust vector. At this point, the velocity will remain constant again but will now be be say 6 kts. While you keep turning the crank with the same vigour, the boat will now be doing 6 kts.

When you think about it, this what occurs when you start the engine while motor sailing.

OK you get tired and let the crank go, the boat will slow down back to 5 kts and the prop and crank will be freewheeling again.

For any one still reading, this is all quite intuitive and surely non-controversial.

Now lets say, you put your hand back on the crank and apply some effort in the opposite direction and slow the crank down to say 15 RPM. Again, this will take some effort but in the opposite direction as before. So what happens to this energy that has now been placed into the system. It is not hard to understand that it must be adding to the drag. Remember that the effort to speed up the crank added to the thrust, the effort to slow the crank is opposite and therefore must be added to the drag vector. Really, this is intuitive once you think about it. So if the drag vector is now increased compared to the thrust vector, the boat must slow down until the vectors equalise. This occurs as the slower motion decreases the other drag effects until the total drag vector equals that of the existing thrust. For this argument, let's say the new velocity is 4 kts. Once again, if you let go the handle, the energy you were applying to slow down the prop will be removed from the system and the prop RPM will increase back to 30 RPM as will the boat speed increase back to 5 kts.

Almost done

This demonstrates that forcing the prop to slow down from it's natural freewheeling speed cause the boat to slow down. It doesn't take much imagination to see that applying even more force to slow it down to a standstill MUST decrease the boat speed even further. Likewise, releasing it from a locked position and allowing it rotate freely, must increase the boat speed.

Remember, it is all about the total energy in the system. Adding energy in one direction will cause an increase in the thrust vector and adding the same amount of energy in a opposite diction will increase the drag vector.

Thus when compared to a freewheeling prop, starting the engine (adding energy) will make the boat go faster while adding energy in the other direction direction (locking the prop) will slow the boat.

And oh, I almost forgot, drag is always proportional to speed; it isn't a fixed quantity for any particular boat.

[joecat22]

The above dissertation was previously summarized in post # 21:
The energy that creates the rotation of the shaft is taken from the energy acting on the prop blades and thus reducing the drag energy.

[a64pilot]

Stopping or locking a prop, there is zero additional energy put into the system.

I bring up airplane props, cause a significant number of us are pilots, and almost certainly during training we were demonstrated how much difference there is in glide ratio between a windmilling and a stopped prop, so it’s natural for us to think a stopped Boat prop is less drag.
I was certain that it was at first, but I was wrong.

MIT studies and of course Maine Sail’s testing and others all proved me wrong.

[nigel1]

I can you some real world experience on freewheeling a prop and boat speed.
I skipper large tugs, typically 16,000 to 24,000 HP, and all twin screw with four engines. Either one or two engines can be coupled to each prop shaft depending on the bollard pull required.
So we strife to find the best balance between speed and fuel consumption and what we found was this.
If we have only one engine running, and driving one prop shaft, with the second shaft stopped on the shaft brake, the speed through water is roughly 7 kts. Now if we de-clutch the non running prop shaft from the gear box, and release the shaft brake, and set the prop pitch to maximum, that prop will now start to free wheel, and within a few minutes, boat speed increased to 9.5 kts.
So, I'm with the school of thought that a free wheeling prop reduces drag and increases speed

[a64pilot]

Nigel,
I know your correct, but to be a true equal test, you need the prop pitched the same when it’s locked and turning. Of course you weren’t testing fixed vs turning, you were seeking lowest fuel consumption and best speed.
I have no idea if that is possible.

The MIT prop testing was way over my head, I didn’t try to follow the math and can’t even understand the charts, but among other things clearly showed windmilling to be less drag for the several fixed pitch props they tested, and they weren’t weird props either they were Campbell Sailor and Michigan Wheel if I remember.

[joecat22]

Originally Posted by joecat22

The above dissertation was previously summarized in post # 21:
The energy that creates the rotation of the shaft is taken from the energy acting on the prop blades and thus reducing the drag energy.

Energy In (moving water acting on prop blades) = Energy Out (shaft rotation + drag)

Take away the shaft rotation and drag has to increase in order to keep the equation balanced. This is called the Law of Conservation of Energy.

[a64pilot]

Originally Posted by joecat22

Energy In (moving water acting on prop blades) = Energy Out (shaft rotation + drag)

Take away the shaft rotation and drag has to increase in order to keep the equation balanced. This is called the Law of Conservation of Energy.

That sounds logical, but using the same reasoning, explain how a stopped airplane prop has much less drag than a windmilling one?

A spinning aircraft prop, or helicopter rotor system has almost the same flat plate drag as a solid disk, stopped they only have a very thin profile that is presented to the airstream, and therefore low drag.

Why is a Campbell Sailor prop built like it is?

[joecat22]

I believe the difference is that in an airplane you are traveling at much greater speeds through a compressible fluid and the prop will be spinning much faster. Thus the physics become much more complicated. I have a friend whose brother is an aerospace engineer, I'll try to get a better answer.

[joecat22]

So upon checking with the aerospace engineer and pilot, he says that the free spinning prop on an aircraft stays connected to the drive train. Thus the "free spinning" prop is turning over the engine. This of course is not even close to comparable to a sailboat with the transmission in neutral. Of course there will be tons of drag under this condition.

[a64pilot]

Sometimes they are, but not always.
Read Hopcars post

On edit, I suspect though that your correct, the energy to spin that prop whether coupled to the engine or not comes from somewhere of course, where it comes from in an airplane is the potential energy of altitude.
This is why initially I was sure that a spinning boat prop was more drag that one locked, but there is too much testing that says otherwise, I was wrong.
I was even a few years ago arguing that here on this forum.

[a64pilot]

Originally Posted by HopCar

Not all airplanes can feather their props and they also benefit from stopping the prop when gliding.



Many years ago I did tests with an ultralight aircraft equipped with a fixed pitch prop. The engine was connected to the prop with a centrifugal clutch. When the engine was off, the prop was free to spin. I fitted a brake on the clutch so I could stop the prop from spinning. I also had a very sensitive variometer (vertical speed) and air speed gauge.



I would fly to altitude, shutoff the engine and note the air speed and sink rate.

Then I’d stop the prop, the air speed immediately increased significantly. When I slowed the air speed to match that of the spinning prop, the sink rate would significantly decrease. As I recall it made a 200 to 300 feet per minute difference.



At the time, someone smarter than me, calculated that the drag of the spinning prop was close to the drag of a plywood disc of the same diameter.

Here it is

[a64pilot]

Pitch on an airplane prop also makes a HUGE difference.
A normal turbine engine is essentially decoupled from the prop at idle when the aircraft is in flight. On the certification test flights, I would rig the crop duster we built to a very minimum pitch, so that when you pulled the aircraft to idle it would require a rate of descent of about 3000 foot per min to maintain approach speed.
Most Ag pilots liked it as when you brought it to idle on short final You had no real glide and there was a lot of braking action.
See Ag pilots don’t make money on the ground and want to shorten that as much as possible, they even land downwind to the loader so a simple turn around and takeoff, no taxiing, cause that is wasted time.
So with the min pitch stops you could rig the aircraft to really float on landing or fall out of the sky.
Depending on pitch a turning prop not coupled to anything can give you an enormous amount of drag, that is why they must be able to be feathered on a turbine, cause unfeathered, usually flight can not be maintained if you lose one in a multi engine airplane.
Most if not all US turbine engine aircraft auto feather upon loss of oil pressure, so if the engine quits, it automatically feathers.

I assume pitch on a boat prop also makes a huge difference, lots of pitch and freewheeling may not be much drag at all, cause among other things, lots of pitch would mean a slow rotation.

[model 10]

[QUOTE=a64pilot;
A normal turbine engine is essentially decoupled from the prop at idle .
Most if not all US turbine engine aircraft auto feather upon loss of oil pressure, so if the engine quits, it automatically feathers.

And I think most have torque sensing too?
You have a free turbine bias or something?