How sailboats sail

[darmoose]

Neil,

I am sorry that I must conclude that you and John just simply refuse to see what is clear, and right in front of you.

Too bad for all the readers on this board. I apologize to them all that I have tried this hard (and taken too much of their time) in an apparent hopeless cause.

I only ask that they remember that I did not start this.

Darrell

[darmoose]

Darrell,

The boat goes from here to there. It's forward motion is measured in knots. The prop and shaft spin around. That speed is measured in RPM. Note that the boat's speed doesn't require the boat to have RPM and the prop's speed doesn't require forward motion. Having noted that, I conclude that the boat can go faster/slower and the prop can go faster/slower, each independently of the other.

I've often noted while tied in my spip that no matter how fast the prop turns, the boat's speed remains zero.

Neil,

In the context of our discussion, which is a freewheeling, or locked propeller, you could not be more wrong with your above conclusion.

The fact is that every boat has a built in relationship between the RPMs that the prop will do when freewheeling, and the speed that the boat is sailing through the water.

This is so because each boat has its own amount of friction built into its drive train, unique from any other boat. But what is important here is that for any given speed through the water (unless the friction is changed), the propeller will freewheel at the same RPM. You can sail at five knots a dozen times and the freewheeling RPMs of the propeller will be relatively the same each time.

Your conclusion is wrong and misleading, and your statement regarding your boat speed while being anchored shows either how little you understand about this subject, or to what lengths you will go to distort the facts and mislead those who may be trying to determine what the truth of this matter really is (I note there are almost 4000 viewings to date).

While I am sorry to speak so bluntly, it simply must be said.

Darrell

[Neil Gordon]

Darrell,

Where in your analysis have you considered this:

"A driving propeller is cupped away from the direction of rotation so that the angle of attack at the leading edge is low, and increases towards the trailing edge of the chord. A driven propeller is cupped the other way. They are markedly less efficient when being used for the opposite purpose."

[Amgine]

Neil: in what way does reverse come into this question? unless you've figured out how to sail your Cape Dory in reverse, I mean.

[darmoose]

Darrell,

Where in your analysis have you considered this:

"A driving propeller is cupped away from the direction of rotation so that the angle of attack at the leading edge is low, and increases towards the trailing edge of the chord. A driven propeller is cupped the other way. They are markedly less efficient when being used for the opposite purpose."

Neil

I have considered ALL of the possible forces, all of the idiosecrencies of the propeller, and any other factors you wish to bring up when I said earlier that there are NO forces at play that are not acting on the rate of spin of the propeller when freewheeling at any given speed.

What you continue to fail to see is that the freewheeling propeller will spin at a speed that is relative to the boats sailing speed dependent only on the amount of friction built into that particular boats drive train.

So, if we affect the speed of the spin by slowing the propeller by slowing the shaft, there must be some change in speed of the boat (you like to think it speeds up, in fact it slows down), which I can prove by simply releasing the shaft which causes the propeller to return to its higher rate of spin which can only be caused by the boat speeding up.

Now, if you dont believe my analysis, I will be happy to discuss it with you, or John Vigor or anyone else, provided you do me the courtesy of addressing the examples I have described, and telling me where you think I am wrong and why.

I would appreciate your doing that for once, if you dare.

Darrell

[Neil Gordon]

Neil: in what way does reverse come into this question? unless you've figured out how to sail your Cape Dory in reverse, I mean.

I was just asking. The comment was made in the context of using a spinning prop as a generator, vs. towing a prop/generator designed for that purpose.

Stan asked a question that wasn't answered (or I missed the answer). Does a freewheeling prop spin in forward or reverse?

[Neil Gordon]

Darrell,

>>I have considered ALL of the possible forces, all of the idiosecrencies of the propeller, and any other factors you wish to bring up when I said earlier that there are NO forces at play that are not acting on the rate of spin of the propeller when freewheeling at any given speed.<<

I addressed that one. There are forces that act on the prop if it's spinning and forces that act on the prop if it's locked. There are also forces that act on the water flowing past the prop and there are forces on the boat that are transferred from water to prop to hull. Your conclusions are based on intuition. I'm just asking if someone can explain the physics.

>>What you continue to fail to see is that the freewheeling propeller will spin at a speed that is relative to the boats sailing speed dependent only on the amount of friction built into that particular boats drive train.<<

I've acknowledged any number of times that the speed at which a prop will spin is directly related to the speed of the boat. Boat goes faster, prop spins faster. You seem to have added an extra step though... boat goes faster, prop spins faster, therefore boat goes even faster! If that was true, I'd only use sails to get the boat started, then use the ever faster spinning prop to propel the boat to hull speed.

>>So, if we affect the speed of the spin by slowing the propeller by slowing the shaft, there must be some change in speed of the boat ...<<

There's no logical conclusion either way, just intuition. That's why I'm so interested in understanding the science.

>>... (you like to think it speeds up, in fact it slows down), which I can prove by simply releasing the shaft which causes the propeller to return to its higher rate of spin which can only be caused by the boat speeding up.<<

The propeller spins because the boat is moving and water is flowing past the prop. In a gross example, if the boat is going 10 knots with a locked prop, what happens to the boat if the prop is released? Yes, it spins. Now if I'm right and the boat slows (in this extreme example) to 5 knots, I'm right. If the boat speeds up to 15 knots, you're right. But the fact that the prop spins proves nothing

[darmoose]

Neil,

>>I addressed that one. There are forces that act on the prop if spinning, and forces that act on the prop if it's locked. There are forces that act on the water flowing past the prop, and there are forces on the boat thar are transferred from the water to prop to hull. Your conclusions are based on intuition. I'm just asking if someone can explain the phisics?<<

I think this discussion is about drag created by a prop, either freewheeling or locked. There is only one moving part (the prop) and no matter what forces you wish to discuss, they have only one place to effect a result, that is the prop itself, and if freewheeling, its rate of spin. If you wish to add other appendages or discuss hull drag, I think you are "off topic".


>>I've acknowledged any number of times that the speed at which a prop will spin is directly related to the speed of the boat. Boat goes faster, prop spins faster. You seem to have added an extra step though...boat goes faster, prop spins faster, therefore boat goes even faster!<<

If you believe that the rate of spin of the prop is directly related to the speed of the boat (as you say, and as I do), why have you stated previously that if I mechanically slow the prop by applying friction to the shaft, that the boat will SPEED UP?

I have not added another step, as I say that the boat slows as I slow the prop, and then speeds up as I release the prop. The fact that the prop speeds up as I release it proves my point because only the boat speeding up can explain the prop speeding up.


>>There is no logical conclusion either way, just intuition. Thats why I'm so interested in understanding the science.<<

Now, there's progress. Up til just now, you claimed that if I slowed the prop the boat had to speed up.(incorrectly, I might add) The science is Newtons 3rd law of physics.. for every action there is an equal and opposite reaction. The boat moving forward forces water against the prop, the prop, if allowed to freewheel pushes back and transfers the energy into spinning, thereby keeping the forces in balance. If the prop is locked, as the boat moves forward water is pushed against the prop, and since there is no place to transfer the energy (like spinning) the energy goes into simply pushing against the prop and slowing the boat (which happens to be attached to it) That is the science.


Your last paragraph, I cant even begin to address, but let me be clear, as always, I never claimed that the prop merely spinning proves anything.


Neil, your claims of intuition only and no science are a bit ludicrous, especially when the only "evidence" presented for locking ones prop are irrelevant analogies of helicopters and motorcycles and such. The science is very clear (if you will just study and understand it), the evidence is mounting. John was simply wrong in trying to transfer what he saw in a helicopter to our sailboats. It is time for knowledgeable sailors to abandon that position.


Darrell

[Neil Gordon]

>>There is only one moving part (the prop) ...<<

No, because the water is moving, as well. Perhaps each and every molecule needs to be considered a moving part.

>>... and no matter what forces you wish to discuss, they have only one place to effect a result, ...<<

No, because in addition to water flow against the prop, there's water flow behind the prop and against the rudder. The turbulance of that flow changes with changes in props spinning or not.

>>If you wish to add other appendages or discuss hull drag, I think you are "off topic".<<

I'm not adding appendages; Alberg already did that. I'm only suggesting that the whole environment needs to be taken into account.

>>If you believe that the rate of spin of the prop is directly related to the speed of the boat (as you say, and as I do), why have you stated previously that if I mechanically slow the prop by applying friction to the shaft, that the boat will SPEED UP?<<

Becasue you take cause and effect and confuse it with effect and cause. If the boat is moving and you unlock a prop, the prop will spin. The rate of spin will be determined by the speed of the boat. If you plot both the speed of the boat and the speed of the prop, the lockers would say that as the prop speeds up, the boat slows down. It's the escalator analogy I posted some time ago... step on the up escalator... you speed up (a lot) but the escalator slows down (a little).

>>... and since there is no place to transfer the energy (like spinning) the energy goes into simply pushing against the prop and slowing the boat ...<<

But when the wind comes up against the sails and there's no place for the energy to go (like sails flogging), all of the energy makes the boat go faster.

>>... the only "evidence" presented for locking ones prop are irrelevant analogies of helicopters and motorcycles and such.<<

Except that your "the energy needs to go somewhere" explanation should apply to helicopters, etc., as well. Why wouldn't a helicopter fall faster with its rotors spinning?

[darmoose]

Your conclusions are based on intuition. I'm just asking if someone can explain the physics.

There's no logical conclusion either way, just intuition. That's why I'm so interested in understanding the science.

Neil,(and others)

Since you and many others are interested in the science and physics, here is a research paper describing "tank tests" and an exhaustive study that was done by the Strathclyde University in Glasgow, Scotland and published in 2007 in the prestigeous Ocean Engineering journal.

[/url]http://eprints.cdlr.strath.ac.uk/5670/[url]

When you get to the site, simply click on the PDF symbol and you can read the entire research study. It comes complete with charts, graphs, pictures, references, engineering formulas, and conclusions. It is quite well done and represents the latest in research of this subject.

Incidently, it states that propeller drag for a locked propeller can be as much as 100% more than that of a freewheeling propeller. The really valuable thing, for purposes of our discussion is that the entire study is aimed at sailboats like ours.

Enjoy and let me know what you think.

Darrell

[Neil Gordon]

>>The really valuable thing, for purposes of our discussion is that the entire study is aimed at sailboats like ours.<<

I noticed that they simulated "sailing" using a fin keel and a prop on a strut. As some of us may have noticed, we have full keels with attached rudders.

I'm curious if their simulation allowed for leeway. As we know, boats don't exactly go in the direction the pointy end faces. That also means that the water flows unequally and at different pressure down either side of the hull, perhaps flows through the prop aperture and in any case hits the prop at an angle rather than straight on.

Even if I accept their findings, I'm not convinced. There's lots of math and measurements of drag. I'd like to see a direct test of speed, which is what we're talking about here. Apply an equal force to equal boats that simulate sailing conditions, let one prop spin and lock the other and measure which boat gets from here to there the fastest.

[darmoose]

is a 42 page report covering "tank tests" and detailed engineering studies of exactly the question of propeller drag (locked vs. freewheeling). The report is objectively written and includes graphs, charts, pictures, engr. formulas, and is fully referenced. It was published in late 2007, and represents the latest scientific research on this subject.


The report can be accessed (and printed), see the URL in my above post. Here are some quotes from the report....


"Adding to this confusion is the myth of common currency amoung many yachtsman that the practice of locking a fixed blade propeller to prevent rotation results in less drag than would allowing it to freewheel"


"The experimental results confirm that a locked propeller produces greater drag than does a freewheeling screw (up to 100% more drag was observed, this being at the higher speeds). Furthermore, for the freewheeling case, the magnitude of the hydrodynamic resistance is significantly affected by the amount of frictional torque on the shaft, low torque being accompanied by low drag."(this means that the more friction you have in your drivetrain the slower your gonna sail)


The report shows that for moderate displacement hulls the parasitic drag for a locked prop is 14.8%, while for the frewheeling prop it is 7.3%. For heavier displacement hulls the numbers are 3.4% and 1.7% respectively.


All of this to support and prove what most thinking sailors already know intuitively. The energy from the water rushing against the propeller can go either into spinning the propeller or into slowing the boat (those are the choices).


One can only hope that the perpetraters of the "myth" can finally see the light and admit with dignity their folly. Course they still have their helicopters, motorcycles, escalators, and windmills to fall back on.


What I suggest in the meantime is if you can get any "proplocker" in a race, you oughta have easy pickins with a 1/4 to 1 MPH advantage.


Darrell

P.S Did I mention "EUREKA"

[Amgine]

So, Darmoose, your transmission uses a shaft-driven lubrication system, yes?

[Neil Gordon]

The "lockers" in the Boston Harbor Island Regatta and racing across Buzzards Bay have outperformed the spinners consistently. This is measuring results only for Cape Dory vs. Cape Dory.

Btw, there have been other scientific studies, as referenced here by John Vigor, that show exactly the opposite results from the test you reference. I'm not surprised, as there are more variables involved than appear at first blush.

[darmoose]

So, Darmoose, your transmission uses a shaft-driven lubrication system, yes?

Amgine,

Yes, my Volvo manual recommends that when sailing and the engine is off, I should FREEWHEEL (for several reasons obviously) ha=ha

I recognoze that others need to lock their props for reasons having to do with their transmissions (that has always been a given in this discussion.

Neverthe less, it is proven that as far as the drag question goes, one will experience considerably less drag while freewheeling.

Thanks for the question, Amgine, and good luck.

Darrell