Shaft Brake

[Carl Thunberg]

I stand corrected. It is Newton's first law.

Think of the force of the water on the blades as the driving force. (This is really a simplification, because in reality it's a non-uniform pressure, that we convert to a resultant force by integrating over the area.) The resisting force is the grip on the shaft. The object became at rest by your application of an ever-increasing resisting force so the driving and resisting forces became in static equilibrium. That propellor is still subject to exactly the same driving forces as before. No more, no less. You just overcame them by the application of an external resisting force. As soon as you release the resisting force, the driving force causes the prop to move again. None of this brings us any closer to the question of whether a free-wheeling or a locked prop offers less fluid drag.

This really is a fluid dynamics problem in the end. I think vortex shedding may play a role here, but I don't honestly know. At the slow speeds we're talking about, the difference is probably infinitessimally small. Personally, I won't lose a minute of sleep over this. The America's Cup guys can lose sleep over this stuff.

[darmoose]

I am sorry, but everyone is way overcomplicating this with scientific formulas and such.

In your explanation, can you tell me where the extra force comes from that requires me to grip the shaft tighter in order to hold it at lower RPMs or at a stopped position.

Nothing has changed.... the boat is sailing at the same speed, the water is pushing against the propeller at the same velocity...oh, wait, the propeller is rotating slower. But how could that cause me to have to grip the shaft tighter to maintain the slower rotation?


darrell

[Neil Gordon]

Thought you might be interested in this one:

"An aircraft with the propeller stopped will glide
significantly farther than one in which the propeller is turning."

I found it here, although it didn't say why:

http://answers.google.com/answers/threadview?id=503506

[darmoose]

Neil,

Thats nice. I know you have been following this thread. Can you answer the very simple questions I asked Carl.

In fact, can anyone out there tell me where the force comes from that requires me to grip the shaft tighter in order to hold it at the lower RPM, or in a stopped position??

Darrell

[Oswego John]

Darrell,

Super O J here. You were already told where it comes from. Either you only scan what others have replied with, or you don't retain or apply what others have offered for a possible solution.

Super O J

Eureka. Beware Greeks bearing vacuum cleaners. You can be easily sucked in. (Archie M.)

[darmoose]

O.J.

I am not the one speaking in the abstract here. I am not trying to fool anyone here either. I do read all comments about what i have put forth.

I do not remember anyone explaining where the additional force that i am speaking of is coming from. If you know, or know who explained it please share it .

I set up a very straightforward and simple set of facts. I make the appropriate observations. And I ask the very focused and pointed questions that are at the heart of this controversy.

All i get back is complicated formulas, or references to laws that dont necessarily apply, or whatever overcomplications someone can dream up.(rocks falling off hills or wing tips of airplanes)

I dont wish to offend anyone, but obviously, that wish is not reciprecated.

It's obvious what i believe about this subject. And i dont wish to go on forever about this, but i would appreciate someone on this board addressing my questions directly, and proving me wrong.

I wonder why John has not said anything on this thread. I dont think that John would want to let stand an incorrect solution to this controversy that he initiated. And, i dont think John can answer the questions i asked Carl.(let me make it clear, i wasnt trying to embarrass or trick Carl in anyway, i hope he understands that) i merely thought he had the background, like i thought of you, to see how simple this problem was, and how pure my solution was. I was mistaken.

Have you never seen something so clearly, believed something so resolutely. I know, this is silly and unimportant, but it is also something that all of us have spent over two years discussing.

darrell

[bottomscraper]

Nothing has changed.... the boat is sailing at the same speed, the water is pushing against the propeller at the same velocity...oh, wait, the propeller is rotating slower. But how could that cause me to have to grip the shaft tighter to maintain the slower rotation?
darrell

Darrell,

If all this were known to be true I think you would be correct. The problem is the assumptions. You are assuming the boat speed stays the same and the force pushing the boat is the same. I don't think this is necessarily a valid assumption.

If we had a test tank and some way to pull the prop both locked and free at a constant velocity while measuring the force we would have an answer. This would be a case with only one variable.
(Truth be told i would only be the answer for that particular prop but maybe with a few different props we could get some warm fuzzies about our answer.)

My gut feeling is that you are correct but it is only a gut feeling. Lacking any other real data I will continue to let my prop spin free. As an additional argument consider a wood screw. Try pulling it out without rotating vs unscrewing it. Which is easier? Again this seems like a good argument but it's surely not conclusive.

Rich

[SPIBob]

Just woke up from my nap. Can’t believe this thread is still running. Since simple Newtonian principles haven’t persuaded everyone, let’s try an Einsteinian approach, using what he called a thought or mind experiment. If it was good enough for Einstein to illustrate relativity theory, it should be good enough to illustrate what’s happening with an Archimedean screw moving through the water.

To start with, we need a screw propeller that can measure the water pressure on the leading (forward facing) and trailing (aft facing) sides of the screw blades. No problem for a thought experiment.

This will be a fixed pitch prop. The pitch tells us how far the prop would move through the water with one complete revolution if there was no drag and no forward propulsion involved. If the pitch is, say, ten, and the prop moves through ten inches of water during one revolution, we would see zero pressure on both leading and trailing sides of the screw’s blades. Let’s call that its pitch rpm.

Point of sail, wind, and current will remain the same throughout. Speed will always be through the water.

We start out motor sailing. With the sails up, the transmission in forward and the engine rpms at max, the boat is making, say, six knots. We would register the maximum pressure that we are going to see on the aft sides of the screw’s blades. The prop is turning many, many revolutions while moving through ten inches of water and giving us maximum forward propulsion.

Now we begin to gradually throttle back on the engine. Screw rpms decrease, the boat’s speed decreases, and we see decreasing pressure on the aft sides of the screw’s blades.

Eventually the rpms will slow to the point where the screw moves through ten inches of water during one revolution. There will be no pressure on either side of the blades. The prop is creating neither drag nor propulsion. It has reached its pitch rpm. The boat is being propelled by the sails only. Let’s say we are now doing three knots.

If we now decrease engine rpms further to the point where the screw prop begins to move slightly more than ten inches through the water during one revolution, it has fallen below its pitch rpm, and we will see our first increase in pressure on the leading sides of the blades. Drag has begun. Our speed will drop below three knots.

If we now kick the tranny into neutral, the prop will continue to spin---freewheeling--- but now, without the assistance of the engine, at an even lower rpm. There will be a further increase in pressure on the leading sides of the blades, hence more drag and slower boat speed.

If we now begin to use force to slow the prop below its freewheeling rpm, say by increasing the friction of a prop shaft brake, pressure will further increase on the leading blade sides because the prop is moving further and further through the water during one revolution. It is spinning further and further below its pitch rpm. Again, more drag, slower speed.

With enough additional force on the shaft brake, we can stop the prop. More leading side pressure, more drag, slower speed.

Now we shift into reverse. The prop begins to rotate in the opposite direction. Even more pressure on the leading sides, even more drag, even slower speed.

At max engine rpms in reverse we will see the highest pressure on the leading sides of the blade and maybe even enough drag to cancel the propulsive force of the sails and stop the boat.

That’s it. The effect of the screw prop on the motion of the boat through the water will vary, in a continuum, from maximum propulsion to neutral to maximum drag depending on the direction and rpms of the screw prop. In this continuum, drag begins with the screw prop still in forward rotation just below its pitch rpm, and drag increases as the prop slows further and eventually stops and starts spinning in reverse.

Whew! This thought experiment stuff is tiring. Think I’ll take another nap.

Bob

[Carl Thunberg]

Darrell,
I may not be understanding your question quite right here, because my answer is pretty simplistic. You asked why you need to apply increasing pressure to bring the shaft to a complete stop. The answer is because you need to apply a sufficient amount of resisting force to overcome all of the driving force. For a single boat, moving through the water without heeling at a uniform speed, the driving force is a constant. If you apply only partial resistance, you won't bring the shaft to a stop. You'll only slow it down.

It's not that different from your car. Just for fun, let's say your car is being pushed by a uniform force of five offensive linemen from (you guessed it) the New England Patriots. You're trying to resist them by braking. If you only apply a slight amount of pressure on the brake pedal, you won't stop your car. You have to apply enough pressure on your brake pedal to overcome those five offensive linemen. I hope I haven't trivialized your question by this answer, but it's the best I can muster at 10:30. Time for bed.

[Al Levesque]

Bottomscraper hit on a great answer with the wood screw analogy. While wood is not a fluid it serves as an explanation. If the wood screw is pulled hard enough, without turning, it will eventually strip out the threads and then watch how easily it pulls out the rest of the way. That is a great analogy for a stalled prop.
The stalled prop no longer has threads to keep it from pulling through the water.

[M. R. Bober]

The extra force comes from your forearm acting through your wrist and applied to your fingers via the hand. Happy to clear this up for you.

Mitchell Bober
Sunny Lancaster (where pressure is directly proportional to depth) VA

[darmoose]

Well, I had a very good nights sleep and woke up this morning to find that there is some support out there, and some encouragement from a few others that are starting to think for themselves. I appreciate the beautiful "continuum" Einsteinian explanation shared by SPIBob, like his earlier post, he is right on, eloquent, and thorough. I hope he has another great nap before he next joins us.

Rich Abato,

you wrote in your last post "If all this were known to be true I think you would be correct"

Well, all of these things are true, because they are the perameters that i set up for this thought experiment. This is a hypothetical question to be answered within the confines of the perameters that I set up. For clarity's sake, the boat does slow very slightly because the increased drag from the propeller turning slower causes it to. However, i t does not slow enough (and never will) to change our overall results (not even close). And finally, just to be clear, this result will hold up for any fixed propeller at any pitch that
will drive a boat.

Free thinking problem solvers quite often do not need an apparatus like a test tank to solve a problem through deductive reasoning and logic and an understanding of the physical world and how it works. Just look at what John Vigor tried to prove with only thought. He, unfortunately, is wrong, not necessarily about helicopters, but certainly about how they apply to this question.


Carl T.

I appreciate you staying with me on this, and your first sentence of your last post is correct. I think you did not understand the question. I did not ask why I needed to apply additional force to the brake to bring the shaft to a stop, but rather, I asked why, in order to slow the shaft or bring it to a stop?

So, you see that even to slow the shaft slightly requires more force on the shaft brake, because slowing the shaft will slow the propeller, increasing drag because the wind is still blowing on the sails and trying to maintain speed.

Folks should consider the possibility that I, as well as SPIBob are correct here. This stuff is based in physics, we ain't makin it up. If you read SPIbobs two posts and understand them, there is no room to get around the conclusions being drawn. (i wish i were as eloquent as he)

Everyone have a great day

Darrell

[Derek Matheson]

Dear Mr SPIBob:

Your thought experiment is a good read, but you have an incorrect assumption, to quote: "The effect of the screw prop on the motion of the boat through the water will vary, in a continuum, from maximum propulsion to neutral to maximum drag depending on the direction and rpms of the screw prop".

This statement is incorrect. The prop blades when spinning, whether powered or not, are airfoils that are moving through the fluid in accordance with their known characteristics at some small angle of attack, usually a couple of degrees, or mostly parallel to the fluid flow. When not spinning, they are "stalled" and the direction of fluid flow is mostly perpendicular to the blade. This condition is not shown on airfoil lift/drag charts. It is a "singularity" that upsets your idea of a "continuum".

Relating this back to the original original question . . . a stalled airfoil will produce far less lift than an airfoil with flow over its surface. So, a locked propeller will produce less drag than a spinning propeller.

Darrell:
Back to your original question. Why do you have to exert more force (torque) on the propeller shaft to bring it to a complete stop? I am with Carl T on his answers, but would like to add to them. Yes, you would have to exert more torque on the shaft at different rpm's. At full freewheeling, you would exert no force (braking torque). As you progressively exert more braking torque, the propeller would slow down until the propeller torque equalled the braking torque. And . . . this is important . . . the power extracted from the propeller equals the power dissipated as heat in the brake. However, when you exert enough braking torque to stop the propeller shaft from turning, you once again encounter a singularity, as the brake will extract zero power from the stopped shaft. It doesn't matter how much more force you then apply to the shaft, as the shaft cannot be any more "stopped".
So, when dealing with propeller blades, thenk of them as airfoils, and when dealing with things that are moving, think about power, not force.
Chew on this one for a while: The torque you would feel on the shaft is related by the blade angle (propeller pitch) to the drag on the propeller. Now, go back to my previous post, follow the link I gave and study the chart. It shows how propeller pitch directly affects a decision to lock or freewheel.

As for tank testing, I am all for it and suggest a very large very available tank. We have one here called the Chesapeake Bay, and if you can gather the funding, we can design an experiment to validate your ideas.

Unfortunately, I am have now run out of coffee, and have a line of people out side my office . . .

Good Luck to All,

Derek M.

[darmoose]

Dear Mr SPIBob:

Your thought experiment is a good read, but you have an incorrect assumption, to quote: "The effect of the screw prop on the motion of the boat through the water will vary, in a continuum, from maximum propulsion to neutral to maximum drag depending on the direction and rpms of the screw prop".

This statement is incorrect. The prop blades when spinning, whether powered or not, are airfoils that are moving through the fluid in accordance with their known characteristics at some small angle of attack, usually a couple of degrees, or mostly parallel to the fluid flow. When not spinning, they are "stalled" and the direction of fluid flow is mostly perpendicular to the blade. This condition is not shown on airfoil lift/drag charts. It is a "singularity" that upsets your idea of a "continuum".


MY Fellow CDer Derek, I fear you have really stepped in it now. Let me say before I start that i do appreciate your effort, even though somewhat misguided. It is much better to be discussing something like what you have said than most other vague , abstract comments like most naysayers proffer.

Now, if i understand the above, you basically dont believe in the "continuum" idea, so, lets explore that a little.

You apparently are ok with SPIBobs explanation from the starting point where the propeller is operating at maximum thrust all the way down (lowering the RPMs) to the point where he reaches the "freewheeling" state where there is no pressure on the fore or aft edges of the propeller.

You are apparently ok with the explanation from the "freewheeling" state down through the state where the RPMs are being lowered by the engine or the shaft brake, during which time the drag is increasing until we get to the point of actually stopping the the propeller.

Now, with the propeller stopped, you believe that drag suddenly decreases so long as the propeller remains stopped. I assume you have no problem with SPIBobs explanation of what happens once the transmission is put into reverse gear, and so then believe that somehow drag begins to increase once again when the propeller starts to spin in reverse.

SO, heres my question to test your idea.

If i am sailing along at 3,4,5 kts with my propeller stopped by the shaft brake, obviously i have pressure on the brake to hold the shaft still. As i begin to release the pressure on the brake very slowly, i will reach a point where i am barely holding the shaft still, and when i release just a very little more pressure the shaft will begin to turn as the drag begins to overcome the pressure i am applying to the brake.

If your idea is correct, how could what i have written above be true? You have claimed that the stopped propeller is creating less drag than the slowly rotating propeller. If that were true, i could not start the propeller turning once again (from a stopped position) by releasing pressure on the brake, as the turning propeller is supposed to have more drag than the stopped propeller according to you.

It just occurred to me while i was completing this query, that a much simpler question for you is, if i want to go from the slowly rotating propeller to the stopped propeller while i am sailing along, why would i need to increase the pressure on my shaft brake, if the stopped position causes less drag?

If you can answer either of these questions satisfactorily, you can convert me.

Darrell

[Carter Brey]

Dave Gerr is the author of "The Propeller Handbook," the definitive reference on the subject. On page 108 he says, and I quote:

Locked Propeller or Free to Rotate for minimum Drag?

This brings us to the old argument as to whether a propeller produces the least drag when it is free to rotate or locked. The answer is both, depending on the configuration of the hull, keel and propeller. (Note, though, that some gearboxes are not lubricated unless the engine is running; if so, their bearings will be destroyed if the shaft is allowed to rotate.) If the propeller is neither folding nor feathering, and is exposed to the water flow-- as with a propeller on a strut well aft of a fin keel-- it will generate the least drag when it is free to rotate. If, on the other hand, a fixed two-blader can be well-hidden behind the keel, it will produce less drag when locked vertically. Fully-feathering propellers should be locked vertically, if possible, while folding propellers need not be locked since they show so little area and have no tendency to rotate when folded.

Cheers,
Carter Brey
Sabre 28 Mk II #532 "Delphine"
City Island, NY
...now with folding prop, but formerly freewheelin'