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I am thinking of installing a shaft brake on Mystic Rose. I would like to be able to slow the shaft"s spinning, maybe even stop the shaft completely, while under sail. I dont want to use the transmission (by putting it in gear) because my Volvo manual (and the dealer) recommends against it on this particular model.
Can anyone explain how a shaft brake actually works? I am wondering about the force necessary to slow down the spinning shaft and even to bring it to a stop? Do you think it would take more or less force to actually stop the shaft than just to slow it down? (hhhmmm)
Sure would appreciate any wisdom that might be gleaned from this august board.
You managed to revitalize the helicopter rotor autorotation analogy. If you want to read EVERYONE's opinion about which causes the most drag just go back and search on 'heilcopter'. The thread was just about a year ago.
The only boat that I ever sailed on with a shaft brake was a Peterson 46. That had a disk on the shaft with a 'v' notch in it and a plunger that matched the 'v' shape. The plunger was spring loaded and it had either a solonoid or hydraulics that withdrew the plunger when the engine was running. The only trouble was that when the boat exceeded 14 knots (unpleasant as it had a 8 knot hull speed) the shaft had enough torque to kick the plunger out of the disk so that the prop would spin 1 rotation, at which time th plunger would re-engage with a rather loud thump that rung throughout the hull. The 14 knot condition only happened when we were skidding down the sides of some significant waves with more canvas than may have been prudent, but it unvariably made for no sleep for those off watch. Had I been the Captain I would have reefed a little sooner.
Although I was sorely tempted to reply with a double entendre about the possible pain associated with a broken shaft, my hands are clean. FWIW: I was towed a few years ago and the tow operator requested that I lock the shaft with a pair of visegrips.
Mitchell Bober
Sunny Lancaster (where helicopters do not fly faster with locked "assembly, rotary wing" except during descent.), VA
If all you do is slow it down, then you are going to be heating the shaft or the disk and wearing the brake pad material down. If you stop it, then there is no wear, just more drag. Right John?
If you have a chance to experiment try wrapping the shaft with leather or other tough material and clamping with visegrips. Pick a spot where the shaft is near the hull or use a block to rest the visegrips against. I wouldn't use the visegrips directly on the shaft for fear of scoring it. My gearbox and cutless bearing are stiff enough to prevent rotation at low speeds but I can easily turn the shaft by hand so I doubt there is much load, (unless you fly at 14 knots).
Al Levesque wrote:If you have a chance to experiment try wrapping the shaft with leather or other tough material and clamping with visegrips. Pick a spot where the shaft is near the hull or use a block to rest the visegrips against. I wouldn't use the visegrips directly on the shaft for fear of scoring it. My gearbox and cutless bearing are stiff enough to prevent rotation at low speeds but I can easily turn the shaft by hand so I doubt there is much load, (unless you fly at 14 knots).
AL,
Thats a very good idea. I think i might try to set something up to do just that.
So, if i do try this, going back to my original question, i would assume that as i am sailing along and the propeller shaft is freewheeling, i would need to apply ever increasing pressure to the experimental shaft brake to slow the shafts RPMs?
And then, if i actually wanted to stop the shaft, do you think i would need to increase the pressure some more, or would i need to ease back or release some pressure on the brake to bring the shaft to a halt?
First of all, I don't think I'd be messing with the shaft if it is in motion. Too many chances for it to catch you up in its idea of fun and hurt something. Put the tranny in gear first or leave it in gear as you kill the engine.
If you allow the strop to wrap around itself on the shaft and then clamp the free end(s) with a Vise Grip, then the pliers ought to jam themselves between the shaft and the hull as the shaft tightens the strop's grip.
I really don't think you want to mess with slowing the shaft unless you use something like a disk brake and like to remove brake dust from the bilge. The leather strop will go up in smoke pretty fast if it is merely slowing the shaft and not stopping it.
You might also keep in mind that there will be an abnormal force on the transmission's output shaft bearing if you slow or stop the shaft from turning. Seems to me that if any damage would occur as a result of this force, which is perpendicular to the shaft, that there would be more damage if the shaft is turning and producing heat at the bearing.
My thought was that the boat and shaft would be stopped when applying the visegrips. If changing from engine power, stop the engine, bring the boat into the wind, and then apply the visegrips. It might be good to be ready to head into the wind to remove the visegrips if the shaft turns anyway. It would be essential to remember to remove the visegrips before starting the engine! This should only be experimental to get an idea of the forces. Once under sail test the force by trying to rotate the shaft with the visegrips. I wouldn't expect much BUT I have never tried it.
Darrell,
Your recurring question about increasing and decreasing forces on the prop shaft reminds me of a recurring dream I've been having lately. In the dream Sir Isaac Newton and I are out for a ride in a helicopter. In response to my question about the physics of an Archimedean screw moving through the water, he instructs me thusly (my translation from the Latin):
My poor benighted man, a body in motion follows the path of least resistance, unless acted on by an outside force, in which case that body will now follow a path of increased resistance. An increase in energy now will be required to overcome this increased resistance if the body's original velocity is to be maintained.
My puzzled look convinces him that I'm slow to pick up on the obvious, even when it's pointed out to me. He continues:
A sailboat's prop, its Archimedean screw, left unfettered, will spin as it moves through the water. This is its path of least resistance. To slow or stop the spin requires the action of an outside force. The slowed or stopped prop now will be following a path of increased resistance. Ergo, an increase in energy proportional to the amount of energy required to slow or stop the prop will be required to maintain the prop's original velocity. Absent this increased energy, the prop's velocity through the water will decrease.
Ah ha! says I. Since the prop is attached to the boat (showing a little deductive prowess myself here), the boat's velocity will also decrease should anyone apply outside force to slow or stop the prop.
He seems mildly surprised that it finally got through to me, retracts an earlier remark about pearls before swine, and adds:
Yes, of course, unless additional energy is now applied to bring the boat back to its original velocity.
At this point in the dream, the copter's engine fails, we plunge toward the earth, and I always awake just before impact.
I had wanted to ask Zack about the physics of spinning helicopter blades, but the dream always ends to soon. I'm hoping that one of these dreams Jacob Bernoulli will come along for the ride.
A sailboat's prop, its Archimedean screw, left unfettered, will spin as it moves through the water. This is its path of least resistance.
Ah ha! says I. Since the prop is attached to the boat (showing a little deductive prowess myself here), the boat's velocity will also decrease should anyone apply outside force to slow or stop the prop.
He seems mildly surprised that it finally got through to me, retracts an earlier remark about pearls before swine, and adds:
Yes, of course, unless additional energy is now applied to bring the boat back to its original velocity.
Bob
Bob,
You have uncovered and lain avast my little ruse. Amazingly, after some four hundred views of this thread, only you seem to clearly understand that this little brain teaser was never about installing a shaft brake on my beloved Mystic Rose. To be fair, i think Matt and Mitch were on to me to.
Of course, this was always merely an offering of "proof positive", having only recently realized the obvious example which does indeed prove what i have always said, while so many others ( authors, engineers, airplane pilots, helicopter pilots, scientists, and other assorted authorities) have steadfastly refused to "see the light".
DRAG OR RESISTANCE IS MEASURED BY THE FORCE NECESSARY TO OVERCOME IT.
As evidenced by the workings of a simple shaft brake on a sailboat, if you want to slow down the RPMs of a freewheeling propeller, you must apply some pressure (force) to the shaft, and the more you wish to slow the propeller the more pressure (force) you will need to apply.
Then, when it finally comes to the point of bringing the propeller to a complete stop, you must again increase the pressure (force) to accomplish this.
The "proplocker" crowd would have you believe you could relieve or release some pressure (force) from the shaft and stop the propeller. After all "a locked or stopped propeller creates less drag (resistance) than a spinning propeller", and therefore requires less force to effect. I dont think anybody reading this believes that a shaft brake works like that.
So where are all those "proplockers" to set us straight. It may be that they grow tired of this controversy. Thats easy to do when one realizes the error of ones way. I remember though, that a little more than a year ago, when we first started debating this issue, that this question of what to do with ones propeller to effect the least resistance through the water was a dilema that went unresolved down through the millenia to that very day. Indeed, serious articles and scientific studies were devoted to one side or the other. August universities did laborious experiments and came up with the wrong answers.
Our very own helicopter enthusiast and renouned author John Vigor has written seriously in his books (wonderful books, i might ad) of the virtues of proplocking. I would hope he would correct this for the benifit of future sailors, if he gets the chance.
Anyway, i believe that this evidence is undeniable and absolute. I welcome any readers to show me the error of my way. Step forward, dont be shy. In the absence of rebuttal, i think we must declare this controversey resolved.
A freewheeling or spinning propeller creates less drag (resistance) than a locked propeller.
In the words of that most famous of proplockers "EUREKA, I AM RIGHT, I AM RIGHT, I AM RIGHT".
Another year, another chapter in the ongoing, perplexing discussion of locking one's prop or not. I have serious doubts whether I should reenter the debate. I, too, am neither an author, engineer, airplane or helicopter pilot, scientist or any other sort of an authority on the benefits of prop locking or not.
Not meaning to be offensive in any way, but it matters less than a hill of beans to me whether one should lock one's prop, or not. I sail a Typhoon with an outboard hung on the stern. When I sail, I merely tilt the engine up out of the water.
As far as my professionalism in delving into this scientific enigma, I have to say, up front, that I too am a tadpole. I ask myself, " Am I worthy to reenter the fray"?
About a year ago, I had a perfectly splendid watertight, bulletproof method of explaining and solving , once and for all, the merits and folly of prop locking. It had to do with the mechanics of a pinwheel. I have no way of knowing whether you even considered the theorem I presented. You certainly didn't acknowledge this most excellent example. To say that I was utterly miffed and cut to the quick by your disallowance of my offering would be an understatement.
Upon rethinking the statements being made on shaft braking, please tell me what Eureka vacuum cleaners have to do with prop locking. I googled this to no avail. The only hint of my search being some dude in Eurasia running wildly through the agora yelling "I've found it, I've found it". It was some Greek named Archie something or other. He never did say what he found.
In truth, I am leaning, somewhat, toward your hypotheses. I do have some reservations about one or two of your theories. Namely:
[quote]
As evidenced by the workings of a simple shaft brake on a sailboat, if you want to slow down the RPMs of a freewheeling propeller, you must apply some pressure (force) to the shaft, and the more you wish to slow the propeller the more pressure (force) you will need to apply. [unquote]
I believe that I understand what you are trying to convey to us by the above statement. I think that a better way to express this would be rather than say"the MORE you wish to slow the propeller . . ." substitute the word FASTER for the word MORE.
[quote Darrell]
Then, when it finally comes to the point of bringing the propeller to a complete stop, you must again increase the pressure (force) to accomplish this. [unquote]
*****************************************************
This, too, doesn't sit well with me. Thinking back to my high school physics, I seem to recall some laws that state that a body in motion, in this case a rotating shaft, can be measured by weight x time x distance. For sake of argument, let's say that the 1" shaft (3.14+" dia.) is rotating with a force of 20 foot pounds (fp) per second. All other things remaining equal, a counter or reverse force of 21/ fp/s would overcome the force and start to slow the shaft down. In time, the shaft will eventually come to a halt. If a counterforce of 30/fp/s were applied to the shaft, it would come to a halt sooner. A 50 fp/s counterforce would halt it much sooner.
I contend that a higher counterforce is necessary to overcome the initial inertia of the rotating shaft's force in order to begin to slow the speed of rotation. Here's where I differ the opinion.
As the speed of the shaft's rotation lessens, LESS counter effort is necessary to continue in slowing the shaft's rotation speed and lessen its momentum until that time when the shaft is barely spinning. At this point, very little effort is, in comparison, needed to finally stop the rotation completely.
As long as the boat is moving and the prop is unlocked, there will be a hydroforce tending to rotate the prop. Whatever that force measures, not considering mechanical friction, any braking counterforce greater than the hydroforce affecting the prop will maintain zero shaft rpms.
My disclaimer is that I, after all, am a scientific tadpole and my thoughts are subject to debate.
I am merely the messenger. don't kill the messenger with subjective criticism. Whatever you do, please don't clutter this discussion with facts.
O J
Last edited by Oswego John on Jan 12th, '08, 00:08, edited 2 times in total.
First let me say that i welcome your critique and questions. I fully understand that this discussion of whether to lock or not to lock ones prop when sailing is of no importance whatsoever. If it werent for the fact that people sincerely differ on this matter, it wouldnt be worth the slightest consideration.
What intrigued me, however, from the very beginning was that credible people, on and off of this board, legitimate scientists, engineers, academics, and sailors over many decades and even centuries couldnt seem to agree on this controversey. Wherever i read or researched, i found theories, flawed studies, and unconvincing arguments abound. This was the epitimy of a real inigma.
The ferver of the discussions during our previous threads for the past eighteen months or so was very exciting, and i know that i am not the only one who thought so. But, all through every bit of that i felt that i knew the answer. I struggled mightily to explain and persuade and convince others that i was right, mostly my main adversary, John Vigor, who is very gifted and persuasive himself.
The wealth of ideas and rebuttals flying around, i think, caused the discussion to get very complicated, and sometimes more heated than it should have, and this turned some people off. Unfortunately, i did my share, maybe more than my share, to fan the fires. I know though that my comments were always born from my frustration in not coming up with the absolutely irrefutable proof that would resolve this question for everyone and for all time. I meant no malice.
I did pay attention to your pinwheel theory. When you first hinted at it i thought now this is worth thinking about, but i got confused by the back and forth motion, and i was so focused on trying to understand this helicopter autorotation business that i let it go. I am sorry if you felt slighted.
All thru this discussion, i have enjoyed everyones humor, wit, clever twistings of phrases, and the everpresent little digs. I am guilty of attempting to keep pace. "Eureka" is a gentle prodding of J. V. as he used this exclaimation to pronounce his discovery.
Sensing that this topic was a little overdone for many, i have refrained from mentioning it for quite some time, except to respond on occasion to some abstract reference. I have, however, continued to give it considerable thought. A short while ago i came across the idea of a shaft brake, and the more i thought about it, the more i realized that the entire explanation of this controversy is contained in the working of a simple shaft brake. "Unbelievable", after all that has been said on both sides of this argument, to think that this simple mechanism answers all the questions is indeed hard to swallow, but it is true.
But, how to explain it so that everyone can see. I tried to foster a discussion of how a shaft brake works, and how it would affect a freewheeling shaft, thinking that as it becomes obvious how the brake works, it would become equally obvious that as one puts the brake through its full range of applications, from a fully and freely spinning shaft to a completely stopped shaft it would be recognized that there truly is nothing else to consider.
Then SPIBob wrote his remarkable and extremely clever recounting of his "recurrent dream", yet another takeoff from our friend John Vigor. Every word a fountain of truth, wisdom, and scientific fact.
We all know how a shaft brake works. It works just like your car brakes work, by constricting and applying pressure to the wheel, or in our case the propeller shaft.
So, to make my case, and to answer your questions O. J. I must get your agreement on a couple of things.
1. Drag or resistance can be measured by the force it takes to overcome it.
2. The sailboat that our shaft brake is installed in is sailing along under a relatively constant wind of say 15kts. (this provides a reasonable force to create a freewheeling propeller rotation)
Now, lets make believe that your hand is the shaft brake, and to start you have your arm outstretched in front of you, and your hand is wrapped loosely around the propeller shaft (we"ll also pretend that you are Superman so you cant get hurt) The boat is sailing along at a relatively constant speed, and the shaft and propeller are freewheeling.
Now i ask you to slow the shaft a little. You naturally respond with a little squeeze to counter the force causing the shaft to spin which is the freewheeling propeller. The shaft slows perceptively (in direct proportion to the amount of pressure you apply). By the way, the boat also slows down slightly.(see SPIBobs post) Hold that squeeze.
Heres a good place to address something you said in your post that i believe is incorrect. When you were recounting one of your lessons from high school physics, you indicated that it would take a force of 21fp/s to begin to slow a shaft spinning with a force of 20fp/s. I dont think this is true. I think any force, even 1fp/s, will begin to slow down the shaft, dont you?
Anyway, back to our experiment. I now ask you to again slow the spinning shaft even more, and you dutifully and instinctively respond with another little squeeze and hold. This squeeze is a little more forceful than the first, indicating that the opposing force you are trying to overcome is greater than before. Once again the shaft slows, the propeller slows, and the boat slows. Meanwhile the wind remains constant.
We could do this an infinite number of times, and every time i asked you to slow the shaft even more, you would squeeze tighter and hold. And if every time we did this, we measured the ever increasing force you applied with your hand to the shaft to overcome the rotation of the shaft, we would be proving that the resistance being offered by the propeller being dragged through the water is increasing as the propeller is slowed and thereby slows the boat a little more.
Finally, i would ask you to stop the shaft completely. And you would know instantly that you will need to squeeze mightily and hold to bring that shaft to a halt. And if we measured the force you would be applying, it would be greater than any force you applied prior, because that propeller being locked would be creating the greatest drag of all, and it would be trying to spin and rip that shaft from your hand, and the boat would once again slow, while the wind remains steady.
We have now reached a state of maximum drag with our propeller locked. The really neat thing about using the operation of this simple shaft brake to analyze propeller drag is that it takes into account all factors. By that i mean that whatever affect any of the other things that have been discussed on this board like "lift" or "turbulance" or anything else might have on the results, the shaft brake measures it all. There is absolutely no room for doubt.
And so, we have demonstrated by measuring the counter force of your grip that drag increases as the shaft and the propeller slow, and ultimately that drag reaches its maximum when the shaft and propeller is stopped.
Propeller drag is indeed linear. It ranges from least resistance in the freewheeling mode and gradually moves to greater resistance as the propeller is slowed, and ultimately reaches maximum resistance in the locked mode.
I truly welcome all critiques to this alalysis and conclusion. I dont see how this could be wrong.
Is there anyone out there that would say that to slow the shaft and eventually stop it, as i asked O.J. to do, they would have loosened their grip?
Good Night To All
darrell
P.S. OJ, i am gonna take another look at your pinwheel analogy. If i can get my mind around it this time, i will have some questions for you no doubt.
Well, I did mention that there could be no discontinuity in the drag/force curve.
However, kudos are well-deserved for getting me sucked into the discussion again without even realizing what the real question was. JV must be rolling on the floor at least by now if not from the start.
FWIW: I was towed a few years ago and the tow operator requested that I lock the shaft with a pair of visegrips.
