posted 11-17-2011 12:14 AM ET (US)        

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We have shown that the propeller, in order to generate thrust must accelerate or move a mass of water astern. Now the propeller, being shaped like a screw might conceivably, as it rotates, slide through the water as a machine screw would into a nut, without displacing any water aft. If this happened the propeller (and the boat) would, in one revolution of the shaft, advance an amount equal to the propeller pitch. This would be called zero slip. But in order to produce a thrust, we must accelerate or move some water aft, and therefore it is apparent that the propeller will not advance the full amount of its pitch in each revolution, but will advance some lesser amount, depending upon how much water it accelerates astern in the process of producing enough thrust to offset the resistance of the boat to being driven ahead.

If the boat were tied to a dock, the propeller would not advance at all but would generate maximum thrust because full engine power would go into accelerating water astern. This would be called operation at 100% slip.

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Cf.: http://www.vicprop.com/propeller101.htm#3

This is actually one of the better explanations of why a propeller must have some SLIP in order to produce thrust. I've seen other explanations that simply say a propeller must have some SLIP to product thrust, but don't offer much explanation.

The explanation of propeller pitch is generally likened to the rate of advance of a screw in a solid whose threads have the same pitch as the propeller blades. However, here is a definition that actually defines pitch in terms of advance when slip is zero:

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Propeller pitch is a linear dimension usually expressed in inches, feet, millimeters, or meters, and is equal to the advance of the propeller in one revolution at "zero slip". It is exactly the same thing as the pitch of a machine screw if one imagines the propeller to replace the screw and the water to replace the nut into which the screw is threaded.

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Again, from: http://www.vicprop.com/propeller101.htm#2

This is really a circular definition--perhaps fitting for a propeller. We can't determine the slip until we know the pitch. We define the pitch as the advance when there is no slip. How can we know one without the other?

In the real world, we see this same problem when real world data of boat performance is used in calculations like those performed by the PROPELLER CALCULATOR. We can input propeller shaft speed and boat speed, and we can calculate pitch or slip, but the value we get for either one will depend on the value we use for the other.

posted 11-17-2011 12:25 AM ET (US)            

With propeller A said to be of pitch 20, a boat goes 42-MPH with an engine speed of 5,700-RPM and a gear reduction of 1.86:1. This calculates to a slip of over 27-percent.

With propeller B said to be of pitch 16, the same boat goes 42-MPH with an engine speed of 5,700-RPM and a gear reduction of 1.86:1. This calculates to a slip about 9-percent.

Which propeller is better? Which propeller has its pitch marked more accurately?

I think the answers are:

--the two propeller are both producing the same thrust, because they both take the same horsepower from the motor and produce the same boat speed; and,

--it is impossible to say which propeller has its pitch more accurately marked, because we really cannot measure the pitch or the slip from the test data.

posted 11-17-2011 09:27 AM ET (US)            

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[T]he throttle is wide open but the engine is turning only about [three-quarter] of the normal [RPM] at wide open....

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This is a very interesting observation. By inference, we can assume that the load on the engine was greater than it would be in the normal case, that is, when the boat is moving through the water.

The engine speed is assumed to be proportional to the load. Under the normal load, the engine can accelerate to its normal full-throttle speed. In the test condition, with the boat held stationary, the engine must encounter a greater load, and it cannot accelerate to its usual speed.

posted 11-17-2011 02:20 PM ET (US)            

With propeller B said to be of pitch 16, the same boat goes 42-MPH with an engine speed of 5,700-RPM and a gear reduction of 1.86:1. This calculates to a slip about 9-percent.

posted 11-18-2011 01:06 AM ET (US)         

.70 or .75 diameter of the prop seems to be the "standard" point for measuring blade angle using a chord line from leading edge to trailing of the blade.

More accurate pitch measurements can be made with a scanning apparatus, and there are in fact standards for propeller pitch measurements, at least for larger boats. There does seem to be some standards related to measuring propeller pitch, specifically ISO 484.

http://books.google.com/books?id=8w09O4hooWUC&lpg=PA22&ots=AejFXlqzEV& dq=propeller%20nominal%20pitch%20standard&pg=PA22#v=onepage& q=propeller%20nominal%20pitch%20standard&f=false

http://books.google.com/books?id=1-QONfkgc78C&lpg=PA71&ots=cCQz-ZiZE9& dq=prop%20scan%20nominal%20pitch&pg=PA71#v=onepage& q=prop%20scan%20nominal%20pitch&f=false

posted 11-18-2011 10:54 AM ET (US)            

Calculating propeller slip is a very useful tool when trying to fit a propeller to your boat. Every model of propeller will have a range of slip that is typical. If you find, when testing a propeller, that the slip falls outside this normal range, either above or below it, it should be viewed as a notice that something is amiss.

Calculating propeller slip is easy. The formula for calculating propeller slip is simple arithmetic. Engine speed is divided by gear ratio to get propeller RPM which is multiplied by pitch to get theoretical boat speed. Propeller slip is the difference between this speed and the actual boat speed expressed as a percentage.

You can do this math long hand or with a pocket calculator but there is also ContinuousWave's handy Propeller Calculator that uses this same formula.

http://continuouswave.com/cgi-bin/propcalc.pl

The value of the slip calculations is no in any way affected by how a particular propeller's pitch is measured or if you make any adjustment to the input pitch, such as adding one inch. You can subtract one inch of pitch just as easily or add 3-1/2" if that makes you happy....so long as you are consistent and make that same adjustment with all your calculations.

posted 11-18-2011 12:00 PM ET (US)         

Here is a link describing some approaches:

posted 11-18-2011 01:02 PM ET (US)         

Structural or nominal pitch: determined by the propellers physical dimensions.

Effective or experimental pitch: The distance the propeller actually moves forward under certain condition in one revolution o the shaft.

Dynamic pitch: The value of effective pitch that would produce zero thrust.

Some pertinent quotes:

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In propeller operation it is its effective pitch, rather than its structural pitch, in which we are most interested. As shown later, the effective pitch of a propeller varies with conditions of operation, being sometimes less and sometimes more than the structural pitch which has but one value fixed by its dimensions.

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The effective pitch, or forward travel of a propeller per revolution, as just stated varies under different conditions of operation. It will be shown later, that as effective pitch increases, thrust decreases and finally becomes zero. The propeller then goes through the air smoothly, as a screw with no slip, without disturbing the air and without imparting velocity to the air particles.

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The dynamic pitch and pitch ratio of a propeller is greaterf than the nominal or structural pitch and pitch ratio. In other words* the actual forward travel of a propeller through the air for no thrust is greater than its travel calculated as a screw passing through a solid, unyielding material. It is for this reason that the screw theory of the propeller is abandoned.

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Dynamic pitch and pitch ratio can only be determined by experiment, where special facilities are available, whereas nominal pitch or structural pitch and pitch ratio can be determined by measurements described later on the propeller itself. For this reason the values for pitch and pitch ratio usually given are nominal values, and are always so understood unless otherwise specified.

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So some key points here, are that:

1. Propellers stated pitch is usually the nominal pitch since this can be measured. THis is most likely how marine outboard propellers are rated by the manufacturer.

2. Slip can be the difference between effective pitch and nominal pitch or effective pitch and dynamic pitch.

3. Slip need only be positive to produce thrust when compared to dynamic pitch, however when compared to nominal pitch it may be negative.

posted 11-18-2011 01:40 PM ET (US)            

Mention of the ISO 484 is interesting too. Prop Scan is a proprietary propeller measuring tool that measures a propeller against the ISO 484 standards.

http://www.props.com.au/inspection_system/default.htm#presentation

I have never had a propeller measured using Prop Scan. I would like to do so some day. I did find one example of outboard propellers being measured by Prop Scan and interestingly enough they were 14-1/4" x 17" Stiletto Advantage propellers which have been much discussed recently. There has even been an accusation that Stiletto somehow, for some reason, gives their propellers an inaccurate nominal pitch.

Here is what the owner if these propellers reported:

"Prop Scan measured the pitch of my props at three different radii (.5, .7, & .9) of each blade. Both the design mean and measured mean were ~.9" less than the pitch size stamped on the wheel. Closest to the hub (.5) had the most pitch, followed by the far outside measurement (.9), with the middle measurement (.7) having the least amount of pitch."

http://www.thehulltruth.com/boating-forum/ 119400-prop-ventilation-problem-cure.html

posted 11-19-2011 10:40 AM ET (US)            

As I recall, a few years ago at a boat show there was a fellow with a exhibit booth from a local propeller shop. He had a PropScan. I asked him about doing a scan on an outboard propeller, but he was not too keen on the idea. He said he didn't work on outboard propellers. I suspect he figured the price he was charging for the scan would be too high compared to the cost of an outboard propeller. I think the cost of the scan was in the $100 to $200 range, as best as I can recall.

The report of the Stiletto propeller measured pitch being less than the marked pitch is quite interesting. As Tom notes, an accusation was made that the pitch marked on the Stiletto propellers was under the real or effective pitch, and now we hear an actual measurement shows the marked pitch was greater than the measured pitch.

posted 11-20-2011 11:51 AM ET (US)         

If the case of a propeller the force is generated by a foil moving through a liquid or gas and is primarily related angle of attack which changes the momentum of the medium, which would be the water column you reference for a marine propeller.

The main difference between air and water is the compressibility of the medium, however marine propellers aren't run through a perfectly in-compressible liquid because entrained air in the water, in fact in case of a surface piercing marine prop the medium is closer to 50/50 air/water and they start to function more like airplane props.

posted 11-20-2011 02:53 PM ET (US)            

Few airplane propellers operate when compressibility is an issue. Tip speeds can be transonic even on small airplanes, but for all practical purposes the equations describing airplane propellers assume incompressible flow.

Here is a word about "slip." Its an unfortunate word, because it assumes something is slipping. Lets start on land. A tire develops frictional force on the road. The reason it can do so is because the road is connected to the earth. A road has, essentially infinite viscosity. Momentum is always conserved. Forces from the tire are transmitted to the earth. When you accelerate your car you make a momentum change of the earth, equal and opposite to the momentum change of the car.

Now lets go to sea. The water, unline the asphalt, is not "connected" to anything, or isn't connected much, anyway. To be sure, water has nonzero viscosity. That means when you pull on the water like with an oar, the water is attached to more water, and so on, until the force acts on the bottom of the lake. Well, that's true, but only for certain masses and speeds is it measurable. If you are a tiny bug, water viscosity is a factor. If you are a boat going 25 mph, water viscosity is pretty much irrelevant. Thus the propeller has no way to "hook up" with the lake bottom and thus change the momentum of the earth. A propeller is not a "screw" because it operates where viscosity effects are tiny. It's like a rocket in space, really. So how to generate force? Newton's third law comes to the rescue. It says you get force from momentum change, so you eject water sternward, chaning its momentum, and you get force from that. Its all about pumping water backward.

"Slip" is just an artificial comparison of your speed and the speed of the water column being pumped sternward. Free propellers create momentum change through lift, and at a given boat speed relative to the propeller speed, the angle of attack becomes zero and thus the momentum change is zero. However, If I enclose a propeller in the boat and feed it enough water (a jet), there is no reason the boat can't go faster than the water stream.

posted 11-21-2011 11:50 AM ET (US)            

I'm thinking that there may be a component of lift to the thrust generated by a prop, but I'm not proficient enough in fluid dynamics to know if this is a possibility. Perhaps things are always turbulent enough that thrust is generated purely by the amount of water that is displaced.

posted 11-21-2011 01:10 PM ET (US)         

The better explanation for how a foil creates lift or thrust even if it's measured angle of attack through it's chord line is zero, is the shape of the foil is asymmetric, usually curved on one side and flat on the other. The curve on the top changes what's known as the effective the effective angle of attack. This occurs because of the Coandă effect.

The foil of a outboard marine propeller is very thin (unlike an airplane wing), so even if it had some asymmetry the effect would be extremely weak.

Either way the lift or thrust generated by a foil is directly related to the effective angle of attack alone.

posted 11-21-2011 04:17 PM ET (US)            

L = 1/2 rho v^2 A Cd(alpha)

whhere rho is density, v is speed through the medium, Cd is the coefficient of lift, A is the area (usually the wing area, or it can be the frontal area projection), alpha is the angle of attack. Since alpha is dimensionless, (degrees or radians) it is not multiplied per se but is a function argument to Cd. Nonetheless Cd(alpha) is considered to have a linear range, intersecting zero lift and zero alpha.

So yes the blade shape plays a role, but the role would be to influence the shape and steepness of the lift curve. Regardless, there is some point of zero lift, defined by that as zero alpha.

The Bernoulli issue is often debated and discussed. Many physical phenomena are understandable in more than one way. Bernoulli's principle can be thought of as an explanation why some wing shapes are very efficient in accelerating air or water downward or sternward. Flat plates are less efficient, but they do create some lift if held in the correct angle. Wings that use Bernoulli effect create more momentum change for a given drag. Airplanes with flat plate wings have a hard time flying, although some can manage.

Why marine propellers do not appear to have classic wing shapes, i.e. greater radii of curvature on the leading or upper surface, must be a design compromise of their small size and the consequent need to make rapid blade incidence changes from hub to tip to try and maintain a constant pitch along a given radius. I honestly don't have a better explanation; maybe someone does.

posted 11-21-2011 04:28 PM ET (US)            

"Is the thrust a prop develops simply a function of the water column it can move or is there a component of lift similar to an airplane wing ?"

Yes and yes. Both thrust of the propeller and lift of the wing are dependent upon accelerating air or water downward or sternward.

For very small and slow flying things, like birds possibly, viscous forces in air may contribute to lift, so the total lift would not be entirely newtonian, i.e. dependent on a momentum change of the air downward. Viscosity effects are important at very low Reynolds' numbers (a product of size and speed). Inertial effects, i.e. acceleration of the water or air column, dominate in the world of powerboats and airplanes.

posted 11-21-2011 04:47 PM ET (US)            

Slip is two different quantities:

1. The difference between the speed of the water column accelerated backward and the theoretical pitch of the propeller times rpm.

2. The difference between the speed of the water column and the boat speed.

These are two different things entirely. Slip #1 is dependent on the propeller design. Ideally it would be zero. A perfect propeller would push a water column at a speed equal to its pitch times RPM.

Slip #2 has nothing to do with the propeller and everything to do with the boat.

Because we don't usually measure water column speed (we measure boat speed, propeller pitch and RPM), we combine slip #1 with slip #2 and get

3. The difference between the theoretical pitch times RPM and the boat speed.

Confusing as can be, because two entirely different processes are responsible.

posted 11-22-2011 06:52 AM ET (US)            

Does the design of the "torpedo" or other parts of the lower unit affect slip? In other words, does the design of the lower unit and the flow of water around it before it gets to the prop affect slip. If so, how do we take that into consideration?

Thanks,

posted 11-23-2011 07:29 AM ET (US)            

Also, with all that in mind, and what has been written in this thread, how would the design of systems like the Volvo IPS drive and conversely the Arneson Surface Drive have on slip.

Thanks!

posted 11-23-2011 07:39 AM ET (US)            

I note that on airplanes the more common arrangement of the propeller is for it to be in front of the engine and the engine nacelle, whereas on boats with outboard engines or stern drives the propeller is aft of the gear case. There are a few planes with rear-facing propellers, which I believe are referred to as pusher-propellers, but they seem to be a very small minority of propeller-driven planes.

posted 11-24-2011 08:38 AM ET (US)            

http://www.quincylooperracing.us/subpage26.html

The term might mean the same to some commercial boats too, who knows, tractor might mean alot of things, but I was referring to outboards.

posted 11-25-2011 09:50 AM ET (US)            

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The term "tractor drive" lower units for outboards is not a term that was made up by the author of that article in the Orlando Sentinel.

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No one said that the author made up that term. I said it was not now in common use to mean an outboard engine with a forward facing propeller.

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The literal meaning of the word "tractor"--I looked this up--is "pulling."

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posted 12-04-2011 04:36 PM ET (US)            

Pushers avoid the drag of having the airframe in the slipstream. However the incoming air is turbulent. The effects may be close to a wash.

Every so often a new pusher design promising increased efficiency and speed is offered. Few have gained general acceptance. Problems with pushers (in airplanes) include difficulty in finding a place to discharge exhaust, compromised engine cooling, ground clearance issues, and for singles balance problems when the engine is located in back. (None of these problems exists in boats.)

Problems with tractor propellers in boats include increased vulnerability to damage, more difficulty in securing antiventilation (though in some configurations the boat bottom provides antiventilation), having to abandon prop hub exhaust, and maybe some problems I have not thought of.

There were some early outdrives and I think an outboard or two that used tractor props. None were successful, though probably for other reasons.