Shaft Brake

[Oswego John]

Be careful, be veeery careful.

I now realize how the salesman selling Eureka vacuum cleaners is involved with this thread.

There is an aura of a well concealed, insidious VORTEX, eagerly awaiting to entrap all unsuspecting smarty-pants boat owners into its suction.

Beware. Even the very elect shall be hoodwinked

Abandon hope, all ye who enter the portals of this thread.

Fair warning,
O J

[darmoose]

To All Contributors and Readers

I know this discussion has gone on far too long, and everyone has grown tired of it. I wish dearly that i had stumbled across the "shaft brake" analysis long ago, and not spent soo much time discussing helicopters and airplanes and the like. Nevertheless, having stumbled onto the "shaft brake" (also refered to as a friction brake), and seeing how it applies directly to this problem, and giving it alot of thought, i believe we are on the verge of reaching a consensus.

I believe we, as a board, are also on the verge of solving a problem that has perplexed sailors and boat designers for centuries, and i hate like hell to give up now.

I want to address some of the comments being made about all the other factors that come into play like heeling, and torque, and axial vs. rotational forces, coefficiants of friction, boat speed, and anything else that you think has an affect on propeller drag.

Most or even all of these factors may be real. Each of them has either a positive or a negative impact on propeller drag. They all combine to create the resultant drag or resistance offered by the propeller to the water that the boat is moving through.

All of these factors have no other place or way to manifest themselves other than to either add to or take away from the speed of the props rotation as it freewheels through the water.

The speed of the boat being driven by a steady wind (or being towed) provides the energy, the prop provides the resistance, the resistance causes the prop to spin which causes the shaft to spin.

Our task is to measure if not the drag or resistance itself, then something "directly proportionally related". Something that rises and falls exactly in the same way that the drag created by the propeller rises and falls as we test for drag at freewheeling RPMs all the way down to the stopped or locked propeller condition.

The propeller, the shaft, and the brake are connected like the gears in a watch. When one moves they all move.

We do not need to measure drag directly, and it matters not about axial vs. rotational forces. The propellers rotation embodies and sums up everything that is impacting it.

We could easily put an instrument on the shaft brake that would tell us the force we are applying every time we squeeze or release the shaft, but that is not necessary to obtain the results we seek.

The boat is being driven along at a steady speed, the propeller is being pulled through the water causing it to spin, the shaft is spinning equally, and we are operating our shaft brake to test the resistance at any number of RPMs. Everyone knows that in order to reduce the RPms we will need to squeeze and to increase the RPms we will need to release.

Indeed, if any of the factors (secondary forces) that have been identified were big enough to change this, then somewhere along the RPM scale, between freewheeling and the stopped position, you would need to release to lower RPMs or squeeze to increase RPMs. Does anyone really think that place exists?

As to measuring the drag of a stopped prop, Derek makes the point that usually it takes more force to slow a shaft than to keep it stopped due to static vs. dynamic coefficients of friction. I think he is implying that if we start the tests with a locked or stopped prop the results will be different. Remembering that we are in a boat moving steadily along, i must beg to differ.

Under no circumstance that i can imagine, if you started with a stopped propeller, would you increase pressure (squeeze) to start the propeller spinning.

Final note to our friend (the boatless tadpole and overly self critical) Sea Hunt. We all respect Mr. Vigor, but i bet he would be the first to admit to being human, and not infallable. And after all, lets not forget for a moment that he started this mess.

Is there not one poor old CDer out there that understands and agrees with me? I could sure do with even a sliver of support. If not, then somebody please drive a stake through my heart, and put me out of my misery. Please dont give me any more strange forces or airplane wing tips, just show me why, if a stopped prop creates less drag, i cant squeeze the brake when the prop is stopped and being held by the brake, to start the shaft turning.

Darrell

[Stan W.]

darmoose,

I think Derek is correct that your theoretical shaft brake only measures torque at the shaft. The question is whether shaft torque is a proxy for hydrodynamic drag. It seems to me they are two different things. If wind power is driving a freewheeling prop through the water, some of that thrust will be lost to torque and some will be lost to drag. If wind power is driving a locked prop through the water, some of the thrust still is lost to torque and some still is lost to drag. It seems logical that less thrust will be lost to torque if the prop freewheels, but what does that tell us about drag? Isn't it still possible that the reduction in energy lost to torque could be offset by an increase in energy lost to drag?

[Neil Gordon]

I'm not an engineer, either, but I am a sailor of sorts. So I started thinking of the prop vs. sail analogy. Try this:

(i) Why is a beam reach the fastest point of sail? Right... because as the boat is pulled forward, the force of the wind on the sail stays the same. (Sure, the forward motion moves the apparent wind forward, but disregard that vector for now.) By analogy to the spinning prop, shouldn't the path of least resistance (i.e., moviing the boat) reduce the force?

(ii) Let go of the tiller and the boat heads up into the wind and stops. Why? because the path of least resistance wants to turn the sail to the easiest airflow. Letting go the sheet does the same thing to the sail, without turning the boat. But hold the boat on the beam reach and you go fast.

(iii) The prop blade wants to do the same thing, which is lift/turn in response to the force. But... since it's attached to the shaft, the blade can't feather itself, it can just spin, in a futile attempt to improve its position relative to the flow. But no matter which way it's turned, the force of the water is the same.

(iv) My conclusion is that if we only had the flow over the blades to consider, locked and unlocked would be the same. I'll suppose that the difference is caused by differences in water flow as it's disrupted by either the fixed or spinning blades, as the case may be. So it's not the energy absorbed by the prop or the spinning or locking, it's the position of the blades, fixed or moving, that makes for the most efficient way for the water in front to get to be the water in back.

Comments from the engineers?

[darmoose]

Let me just throw this in here.

There are any number of examples in engineering and science where we measure a "proxy"(i like that word, thank you Stan) to determine what we really want to know.

In our own marine industry, several things that we are all very familiar with come to mind.

Radar does not measure distance or speed, it measures time to send and recieve a signal, then it calculates speed as well as distance.

Depth finders measure speed of signal to get depth. GPS measures speed of signal to get distance to get position (i think).

In my shaft brake theory, while i believe it is actually very accurate, accuracy is not very important to reach a conclusion. All the components are directly related, and all we need is an indication of the direction forces are moving, up or down, to draw our conclusions about resistance.

I am sure we could all come up with lots of examples where measuring "proxies" are used discover some related information.

[Oswego John]

Beware the all-powerful vacuum. It'll suck you into its grasp.

Even the strongest can't resist. If not sooner, then later.

But it will get you.

[Al Levesque]

I think the board is very close to an understanding. The axial forces are the only ones that affect the boat's axial motion. An understanding of the relationship of prop rotation to axial force or drag is important. However, there is one other important factor and that is the relationship of the prop to the water that it operates in. If the prop is stalled, all bets are off. A locked prop is stalled and causes drag in an entirely different way. The stalled prop and relatively low axial drag has been observed frequently in aircraft. The same principles apply to water as to air.

[darmoose]

O.J.

Thats funny O.J. I am not sure what your aim is here, but it is funny.

Neil,

A couple of your points are well taken, a boat, like a propeller will take the path of least resistance through the water if left to its own devices.

In your (i) a boat on a beam reach is not being left to its own devices, and is not therefore taking the path of least resistance. You are holding the sail against the wind with your sheet and you are holding the rudder, and so you are directing a path of greater resistance in order to sail.

In your (ii) the boat is taking the path of least resistance when rounding up, however, it is not reducing the force (being the wind which stays the same) but it is reducing the resistance to the wind by changing course. The spinning propeller does the same thing. It does not reduce the force of the water being driven against it by the movement of the boat, but it does reduce the drag (resistance to the water) by spinning (changing course). You come close to recognizing this in your (iii) and (iv)

I once said something along the lines of this, "a sailboat sailing along at any given speed with the propeller unrestricted and freewheeling has the propeller spinning at a RPM that is in sinc with the speed of the boat. And if you slow the propellers RPMs artificially, you will slow the boat."

I think it was Mitch Bober (Mitch, forgive me if it wasnt you) who took issue with me, and said that anytime he turned on his engine and motor sailed with the engine at any RPM (even very slow) he would pick up speed. That is probably true, but I suspect that he was probably sailing with a locked propeller before he turned on the engine, so that any RPM would have been an improvement.

I think if one is sailing along, say at 5kts, and the propeller is freewheeling (it is spinning pretty good at that boat speed), that if you turn on your engine, you wont gain any boat speed until your propeller exceeds the previous freewheeling RPMs.

This brings me to another thought. It makes no difference what causes your propeller to spin, but if on that sailboat above sailing along at say 5kts, your propeller can be made to spin faster than the "in sinc RPM", you will get additional thrust and pick up speed. This is so obvious, so why do we have such a hard time understanding that if you slow your propellers RPMs artificially below the "in sinc RPM" for your current boat speed, that you will slow the boat, the same as if you lock the propeller.

Chew on that a little bit

Darrell

[Derek Matheson]

Al Levesque stated most eloquently: "A rotating prop has different dynamics than one that is stalled." Thank you for that key to this puzzle, Al.

Aha! So a rotating propeller is like an airplane wing that is not stalled, and therefore producing lift, keeping the airplane up in the air, a weighty job. In this sense, a rotating marine propeller that is not stalled will produce thrust when powered, and lots of it. And when driven by the flow of water past it, it will produce negative thrust. We call this drag.

So a stalled wing will not hold an airplane up in the sky very well, and a stalled marine propeller will not produce negative thrust, or drag, very well.

Quid est demonstrandum somewhat, and I remain . . . locked.

[Matt Cawthorne]

While on my way to maine last summer I was trying out my new generator. I did a number of trials switching between freewheeling, a little drag on the prop via my shaft driven generator and locked. I never could see any change in speed. I tried it at 4 and 5 knots. There may be an impact, but on my boat with my prop and a 0.1 knot readability on the knotmeter one could remain content to do what they feel best and not be proven wrong.

Matt

From sunny Arizona where the ocean is just too far away.

[darmoose]

I have never felt so impotent in my entire life. Thinking about taking a whole bottle of Viagra (or something), but my wife explained to me that it is not for this type of impotency. Seems that nothing i say makes any difference to anybody at all. I dont even know if its read. I only know that it's not gettin through.

Maybe i'll try asking a question?

In our sailboat, which is sailing along at say 5kts, under a steady wind, my propeller shaft in spinning along unfettered.

I apply some pressure to the shaft brake (or i could have Super O.J. grab the shaft with his hand and squeeze a little), and the shaft slows down a little. (i make the assumption that the propeller slows down too, cause last time i looked it was connected to the shaft, hope it still is?)

We do this several times, every time we wish to slow the shaft we squeeze, and every time we want to speed up the shaft we loosen our grip.

Being the analytical nutcase that i am, always trying to learn something. What am i measuring, or what am i learning when i am doing this?

Obviously i can use some help.

Darrell

[Al Levesque]

When you grab the shaft and try to prevent it from turning you are sensing and opposing torque. Torque is "around" the shaft. It does nothing to the axial motion of the boat per se. Only when a prop is operating under the right conditions does it convert torque to thrust, or drag, efficiently. A stalled prop is not efficient, it is not operating under the right conditions. For that reason, it does not do a good job of creating drag. Be comforted though, the difference is not that great and it varies at different speeds. That's what makes it so hard to judge without precise measurements under controlled conditions.

I bet John knew all that but he gave us a great subject to think about through this miserable weather.

[Carl Thunberg]

I was really really hoping never to weigh in on this particular topic because reasoning alone will never solve this problem. Whoever mentioned tank testing was right. I'd be very curious to know what the Volvo Open Ocean racers do, but I have no contacts at that level. Chances are, they have a folding prop anyway.

The shaft analogy doesn't work because of Newton's Second Law. A body in motion stays in motion. A body at rest stays at rest unless acted upon by another force. The propellor must spin when dragged through the water because it is subjected to an outside force, namely the velocity of the water impacting the blades (of course, in reality the water is standing still, and the boat is moving). The same thing happens when you blow on a pinwheel. So the fact that you have to apply pressure to stop the shaft only verifies Newton's Second Law. It really doesn't speak to the issue of drag, which is fundamentally a fluid dynamics problem. Honest, we are listening Darrell, but the shaft analogy just doesn't work.

I wish I could give you a simple equation for computing the magnitude of the force applied by the water on the blades, but it gets into impulse-momentum concepts and the equations get complex very quickly.

[Neil Gordon]

Thinking about taking a whole bottle of Viagra (or something), ...

Just be sure to remember the 4 hour rule!

[darmoose]

[quote=

The shaft analogy doesn't work because of Newton's Second Law. A body in motion stays in motion. A body at rest stays at rest unless acted upon by another force. The propellor must spin when dragged through the water because it is subjected to an outside force, namely the velocity of the water impacting the blades (of course, in reality the water is standing still, and the boat is moving). So the fact that you have to apply pressure to stop the shaft only verifies Newton's Second Law. It really doesn't speak to the issue of drag, which is fundamentally a fluid dynamics problem. .[/quote]

Carl,

I want to talk to you for a moment. I was hoping that you would post sooner or later, because i think in the past you indicated that my arguments seemed to make some sense. Believe me , I ain't crazy, so, please stay with me just for a moment on this.

First, i think you are refering to Newtons first law above (i looked it up)

Second, I agree with everything you say above, except the last sentence

I believe the fact that i have to apply pressure to either slow or stop the shaft means exactly this: (please read this slowly, think about each statement below, and understand the meaning)

1. the boat is continuing under sail at about 5kts

2. the water continues to push against the propeller, just like you described above, at the same velocity

3. the propeller is either spinning slower or stopped because of the pressure i have applied

So, here's the question..... Why is it taking this additional pressure to hold the propeller at either the slower RPM, or stopped?

I think there is only one answer. Its because the propeller, at this slower RPM (or in a stopped condition) is providing more resistance (drag) to the water than it did before we applied any pressure, and so the propeller is trying even harder to turn the shaft.

I have got goose bumps because i think it has never been clearer.

Darrell