Shaft Brake

[bottomscraper]

Although in my gut I agree with SPIBob there is one other thing to consider. If the prop is locked there will be torque tending to heel the boat. Depending on what tack you are on this torque would either cause the boat to heel more or less. Heeling a little more could lengthen the water line increasing boat speed. On the other tack the opposite would happen. But then again with more heel the effective sail area decreases...



Rich

[darmoose]

Rich,

All very interesting, but not changing the final analysis a bit. The resultant torque and potential heeling of the boat is merely one of those other factors that i mentioned that are accounted for in the workings of the shaft brake.

It is not the measure in foot pounds or any other measurement that matters for us to reach our conclusion, for we can slow the shaft an infinite number of times,and no matter how slightly we wish to slow the shaft, it will always require a little "squeeze" and hold from Super O.J., the boat will slow, and the wind will remain constant.

This then is the answer. Unless you can show that the shaft in our experiment with O.J. can be slowed by loosening his grip, you must recognize that this conclusion can not be overcome.

respectfully

darrell

P.S. Ken, sorry for misleading you.

[Oswego John]

Calling all CDers,

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.

Thanks in advance

Darrell
s/v Mystic Rose

*****************************************************

Hi Darrell,

The above is your original post of this years ongoing saga of the locl/no lock prop question. I went back and reread this years replies to it. Based solely on what is stated in your post, are you sitting down, I have to fully agree with you in what is included in your initial statement.

Not meaning to take anything away from or lessen its impact, I have to mention that it is a very academic statement that would be difficult for anyone to argue with. In the same vein, I could say that today is Saturday and there would be someone, somewhere, who would rightfully argue the point that today is the day after Friday, and others who would insist that it is really the day before Sunday. Applying force to the prop shaft WILL slow it down, agreed.

Leaving your oriiginal premise for this year behind as settled (as far as I am concerned), I think that others as well as myself have entered the discussion with the main thought of how anything that is said would apply to lock or freewheeling the prop. In all fairness to the discussion, all other thoughts and variations of circumstances should be discounted from the basic proposition mentioned in your post. That is not to say that they aren't valid interjections that don't apply to a deeper discussion of the subject. Some of the other responders have brought up exellent variations on the original theme.

A prime example of this is I feel you and I are both speaking, relatively, about the same subject. The difference being that you flavor your thoughts with orange juice and I flavor mine with apple juice. Your position, a valid one at that, is speaking about slowing the rotating shaft in varying degrees by the use of a proportionate force. My thoughts were more toward the degree of force necessary to ultimately slow down the shaft rotation to the point of locking it.

Not that I am the slightest bit offended by it, but you did take one of my statemente and misconstrued its meaning by segregating it out of context with its full application. It was the part where I was mentioning 21 fp/s. You disagreed with me and stated that even one fp/s would slow the shaft speed some. I fully agree with you on that point.

My statement was that it would take at least 21 fp/s to OVERCOME the 20 fp/s being applied to the shaft via the moving water on the prop, eventually bringing it to a stop.

I certainly hope that you have no misgivings about my mention of the Greek vacuum cleaner salesman named Archie. Sometimes I have an uncontrolable penchant for yanking other peoples chains.

Have an enjoyable weekend, Darrell

O J

[D Rush]

Since this topics seems to rear it's head periodically, perhaps it should include a warning label.
"Warning: propeller spin / drag topic"

[Parfait's Provider]

"You CAN get sucked into the vortex."

[Oswego John]

AVAST

Aw, that's okay. It's a nasty day today. I sorta enjoy a little diversion. Besides, I'm getting tired of Freecell..

Waiting around for Seattle at Green Bay later.

O J

[bottomscraper]

Since this topics seems to rear it's head periodically, perhaps it should include a warning label.
"Warning: propeller spin / drag topic"

Maybe we can get Cathy to add a new top level discussion:

• Cape Dorys
  About This Board
  Buy/Sell CD Boats and Gear
  Propeller Spin Drag Vortex... Warning Enter At Your Own Risk!

[darmoose]

O.J.,

I do not wish to offend you, or slight you, or have you think that i am not listening to your comments. That also goes for John Vigor as well as any others that are posting and contributing to this thread.

I do not understand what that was all about (your post on Saturday at 1:55 pm) With out addressing your post point by point let me just say if you are not clear in any way about what i am doing, here it is right up front.

I am prooving that a locked propeller provides more drag or resistance to a moving sailboat (under sail or being towed) than a propeller allowed to rotate at any RPM from 1rpm to whatever RPM would represent a completely freewheeling propeller.

Respectfully, i can only surmise that you missed the point of my post of Saturday at 2:36 am.

What this subject has needed for over two years is very simply a way to measure drag or resistance as it applies to the propeller.
I have found a way to do just that.

As i said in setting up the perameters when i was explaining the shaft brake, you must understand and agree to two simple things.

1. A force (drag or resistance), if it cant be measured directly, can be measured be the force it takes to overcome it.

2. We are operating in a sailboat that is under sail with a steady wind of say 15kts. This means that whatever we do to the prop with our shaft brake, the boat will continue along (perhaps a little slower).

Drag comes from the wind pushing the boat through the water. The water, because the boat is moving through it, provides some resistance to the boat, its hull, and all of the appurtances on the boat that are under water.

While the hull and all of the appertances are creating drag on the boats movement through the water, we are going to only concern ourselves with the propeller for this discussion.

As the propeller is pulled through the water, if it is not locked, the water will cause it to spin(due to its design with the angled blades). The force with which the propeller spins is transfered directly to the shaft.

So, if we can measure the force with which the shaft is turning, we are in effect measuring the drag or resistance the propeller is creating on the boats movement through the water.

We can measure the force with which the shaft is turning with a shaft brake. Remember that a force can be measured by the opposing force that is necessary to overcome it.

While our shaft brake (imaginary) cannot tell us the specific foot pounds of the force being applied, it can readily tell us if the force we are applying needs to be increased or decreased to change the RPMs of the shaft.

So, as we apply pressure to the shaft with the brake, we will slow the RPMs, and as we reduce pressure applied to the shaft with the brake, the RPMs will increase. Because it takes increased pressure on the shaft, applied by our brake to slow the RPMs, we know that the slower the RPMs, the greater the resistance, the greater the drag.

The minimum drag caused by the propeller will occur when the force being applied to the shaft by the brake is zero and the propeller is allowed to freewheel.

The maximum drag created by the propeller and measured by the brake will occurr when we take the RPMs of the shaft from 1RPM down to ZERO and stop it completely.

Every time we wish to reduce RPMs we will "squeeze" or increase the pressure on the brake, and every time we wish to increase the RPMs of the shaft we will reduce the pressure on the brake.

Remember, all the while we are increasing or reducing pressure on the brake, the boat is being forced thru the water by the strength of the wind.

Every time we apply more pressure to the brake and slow the RPMs of the propeller, the drag created by the propeller being pulled thru the water increases and the boat slows (and visa versa)

The operation of the shaft brake is clearly understood. How it works is simple and clearly understood. And whether one needs to squeeze or release the brake to increase or decrease the RPMs of the shaft is obvious, inherent, and instinctive.

No one would release the pressure on the brake thereby reducing the force being applied to the shaft and expect the shaft and propeller to slow down or stop.

Anybody that would can speak up.

Respectfully

Darrell Randolph

[Neil Gordon]

If the prop is locked there will be torque tending to heel the boat. Depending on what tack you are on this torque would either cause the boat to heel more or less. Heeling a little more could lengthen the water line increasing boat speed. On the other tack the opposite would happen. But then again with more heel the effective sail area decreases...

Rich... You have added multiple dimensions to this debate, for which you will either be ... well, you know.

Okay... what he said... so you have to factor in left handed or right handed prop, wind speed, reefs, jib size or furler furls, etc., etc.

Are you sure whether you lock props or not is dependent on whether you have a wheel or a tiller?

[M. R. Bober]

Once you open a can of worms the only way to close it, is to use a bigger can. It's the law!

Mitchell Bober
Sunny Lancaster (much quieter than Sunny Annapolis) VA

[Al Levesque]

I have only quickly scanned the comments so far but it seems there is one repetitive error. A rotating prop has different dynamics than one that is stalled. The shaft that is locked before the boat is in motion will be locked and held easily. Think of the big rock on the side of the hill with a wedge under the edge to keep from rolling.

[bottomscraper]

Darrell,

Although I really like the simplicity of your solution I think it has one major flaw that makes it incomplete. Your solution would be adequate in a closed system. The problem is we don't have a closed system. Your solution focuses on one small part of a much larger system. We can't ignore the boat heeling and most of all the boat speed! Although my gut tells me a free wheeling prop will offer less resistance the engineer in me says not to jump to conclusions. I have been busy mindlessly painting walls for the past few days. My mind keeps returning to this problem and the more I think about it the more convinced I am that we will not come to any conclusion with thought experiments. We need a test tank!

http://en.wikipedia.org/wiki/Ship_model_basin
http://en.wikipedia.org/wiki/David_Taylor_Model_Basin
http://web.mit.edu/towtank/www/

[Derek Matheson]

Darryl:

First, let me apologize for the pain of boredom about to be inflicted . . .

First, your basic assumptions are incorrect, so all further conclusions are incorrect.

Assumption number 1:

"1. A force (drag or resistance), if it cant be measured directly, can be measured be the force it takes to overcome it."


Force can never be measured directly. The only direct measurement that can be taken is that of length. All other measurements are derived.

Next, the "drag" of the propeller has been defined here as the force that will restrain the boat from going forward. This drag is an axial force along the length of the boat, not a torsional (rotational) force attempting to twist the propeller shaft. The drag and the torque are related by the propeller blade design, but this is very complicated, to say the least.

Further, your original question, related to the braking torque required to slow the shaft versus the braking torque to stop the shaft. This is related to the static and dynamic co-efficients of friction of the material used in the brake. Usually, it takes more torque to slow a shaft that to keep it stopped, as the static coefficient of friction is higher than the dynamic co-efficient of friction for all materials I know of.

Finally, as the axial force (drag) is what is being questioned, this is what needs to be measured. You need to measure the force along the length of the shaft, not the braking torque of the shaft.

This question has been studied before, and emperical testing done. See figure 7 in this paper . . . http://www.goshen.edu/physics/PropellerDrag/thesis.htm . Although the fluid used is air, the principles apply here equally.

I'm locked until further notice . . .

Derek M.

[minke]

O J,

.....
1. Drag or resistance can be measured by the force it takes to overcome it.

.....

The force with which the shaft is spinning is indeed related to
the force required to stop it from spinning. The force of the
spinning prop on the boat is not the same.

Those of you who, like me, are failures in life and have to at
least occasionally get on airplanes, may find favor with the
following idea:

Look at the giant winglets on the ends of the wings of e.g.
an Airbus A320. They are added surface area. They are
added resistance. But, if you start to go fast you use less fuel
because you actually have less resistance since you are no
longer dragging those gigantic vortices behind you. (They
were originally added for safety but succeeded because
of their efficiency.) Would you say that by adding resistance
it makes sense that the result would be less resistance??
Well, it may not make simple sense but apparently is true.

Does this argument apply to the spinning prop issue? The
answer is a certain maybe. An empirical study is called for.

I like to lock the prop to get rid of the vibrations.

[Sea Hunt]

Although I have read all of the posts on this subject and the prior posts from last year, I have not previously posted on this subject. The reason is simple enough.

I do not have any degrees in engineering - mechanical, hydrodynamic, structural, electrical or other. I barely obtained a high school diploma (technically it was a "GED") with an average grade of C-. I also do not yet own a sailboat with which to test the hypotheticals (or theorems - your choice of words) advanced in this thread and prior threads.

However, I did obtain a C+ in reading, and with that achievement I have read a lot. I recently read Mr. Vigor’s book "The Seaworthy Offshore Sailboat" (Ed. 2001). I was very pleasantly surprised by Mr. Vigor’s simple but elegant analysis and solution to the prop spinning versus locking issue. It is found on page 71. I am now in my second reading of this book and highly recommend to all.

Why do I not just quote from Mr. Vigor's book you ask

What would be the fun in that I respond.