posted 09-05-2008 05:08 PM ET (US)        

My thesis is that fancy-schmancy propellers are not much different, where it counts, from old tried-and-true propellers. Before everyone reaches for their keyboards, I am *not* talking about race boats or very high speed operation, where granted a very small change in "something" underwater can win or lose the race.

I am talking about normal, everyday service. Like 20-35 mph, where almost everyone cruises.

I am *not* talking about a comparison of propellers where one has a different "real" or "effective" pitch than another, so the test shows differences. That is easy to spot.

I am stating that in the normal cruise range, efficiency (defined as miles per gallon) will not change more than 5% between equivalently pitched props. By "equivalently pitched" I mean resulting in the same speed on the same hull at the same RPM. Obviously, if the props are pitched differently, one will need a higher RPM to cruise at the same speed, and I will bet, further, that that one will most often use more fuel.

So if we leave pitch out of the equation, I maintain no propeller design will produce much difference.

I cite to the aviation experience for one, where the Wright Brothers propeller was almost as efficient as one you buy today. No one has, despite a lot of trying, come up with much better.

I maintain, for example, the 2-blade Mercury bronze props of the 1960's will hold their own for efficiency with anything today.

Any scientific hypothesis begs to be proven wrong, and I would be happy to declare it false on proper evidence.

posted 09-05-2008 08:12 PM ET (US)            

Please define what a "fancy-schmancy propeller" is.

Please define what an "old tried-and-true propeller" is.

posted 09-06-2008 07:47 AM ET (US)         

Consider that when you change from a 3 blade prop to a 4 blade prop of the same basic pitch, RPM's drop, often significantly, because there is usually more surface area spinning 4 blades, although not necessarily proportional. This change represents a 33% increase in the number of blades, and a bit less than that percentage in increased surface area assuming 4 blades have smaller per blade surface area. Personally, I experienced a 300 RPM drop changing from a 23 pitch, 3 blade Mercury Mirage Plus to a 22P 4 blade Mercury Bravo I. Even though the pitch was less on the Bravo I's RPM still went down significantly primarily because of the increased surface area and perhaps influenced by some smaller factors as well.

posted 09-06-2008 10:38 AM ET (US)            

Okay, let me simplify this:

Today and tomorrow, with fuel and emissions and C02 issues on the rise, I believe that the name of the game will be fuel efficiency in mid range cruise. Small planing vessels will continue to exist, but they will be optimized for fuel efficiency, not top speed. Best efficiency, from the hull, occurs just over transition, at some mid range in the 20's (for small planing boats under 30' or so).

I have said, despite being called a heretic, that the best mid range fuel efficiency occurs when the engine speed is kept as low as possible. That means as large propeller diameter as practical, and as much pitch as possible so that the engine can still plane the boat (or a multi gear transmission if necessary) but forget setting pitch to get to top RPM as that is inconsistent with low engine speed at mid range cruise.

Now my argument is: what prop *design* will give peak efficiency at mid range cruise? This is *not* an idle question, if you believe what is going to happen to boating in the future.

By "design" I do not mean merely what diameter and pitch. I mean what design elements other than those specifications improve economy.

I challenged the propeller gurus here to explain how design affects this type of performance. I do believe that the propeller "tests" that I have seen on this forum and others support the proposition that the design, other than the diameter and pitch, as almost no effect on mid range fuel economy.

Apparently I am right.

posted 09-06-2008 01:38 PM ET (US)            

This has the potential to be an interesting conversation. Please do not side track it.

Again, I am asking you to define what a "Designer propeller" is.

Define what a "fancy-schmancy propeller" is.

Define what an "old tried-and-true propeller" is.

We are waiting for your answers.

quote:

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I have said, despite being called a heretic, that the best mid range fuel efficiency occurs when the engine speed is kept as low as possible.

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Apart from that sentence not making any sense, I do not believe anybody has ever called you a heretic. If I am wrong, please cite even a single instance of anybody calling you a heretic.

Reviewing the thread, I see you are wavering about what your thesis is. Could you firm it up a bit so we all know what your point is?

Since when is a five percent improvement in [ fill in: speed, fuel economy, acceleration, whatever your performance goal is. ] Insignificant? I would LOVE to see a five percent improvement in fuel economy on my boat for example.

posted 09-06-2008 05:59 PM ET (US)            

quote:

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By "equivalently pitched" I mean resulting in the same speed on the same hull at the same RPM.

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If you have two props with the same pitch resulting in the same speed on the same hull at the same RPM, the props will be identical. They will have the same slip % and should have the same efficiency.

posted 09-06-2008 09:02 PM ET (US)            

Well, not exactly.

It is a common misconception, however.

Remember, except for WOT, having equivalent RPM and boat speed does not mean the propellers are identical at all. What you don't measure directly is manifold pressure or throttle setting, which relate to torque. It may take a different amount of torque to turn two different props the same speed, although the same boat speed is produced. The throttle setting does have a big effect on fuel flow, of course.

Also remember, "slip %" is an amalgamation of hull and propeller factors. It is not an index of propeller efficiency. And there is no way to directly measure it anyway, because no one knows the "real" pitch of the propeller, only what is stamped on it by the manufacturer. I guess one could try to measure the speed of the water column ejected sternward, but I have never seen that done.

Designer fancy schmancy propellers are in my opinion the various series of pretty looking propellers offered for sale by touting some design characteristic other than merely diameter and pitch. Some are said to have lift, some are said to be good on small boats, some good on big boats, some just good on everything. I looked at the Mercury website and found several series of very pretty propellers, although the exact differences in the design were not clarified. Perhaps our propeller gurus could clarify it, because I don't sell propellers and can't speak for the manufacturers.

"I have said, despite being called a heretic, that the best mid range fuel efficiency occurs when the engine speed is kept as low as possible."

What about that sentence does not make sense to you? I believe it scans well, is in acceptable if somewhat vernacular english, and is in proper grammar. One could question the term "mid range"; perhaps "midrange" or "mid-range" would be better. The dependent clause "despite being called a heretic" might in formal use be better rendered "despite *having been* called a heretic," although the use of present tense is, in the present context, possibly even better and less ponderous.

A heretic is one who espouses heresy. Heresy is spoken or written words that are anathema to the established dogma. I have said, contrary to the established dogma, that the practice of propping for WOT RPM at the mid or high end of the manufacturer's specified range, results in less midrange fuel economy because of engine friction. Many have joined the chorus of denouncing this idea. Therefore, I accept the implied title of heretic.

So, what does that have to do with the physics? And this issue is purely a matter of physics.

posted 09-06-2008 11:03 PM ET (US)            

The 5 sets of props I own:

Enertia 19’s
Tempest 19’s
Mirage Plus 19’s
Rev 4 17’s
Rev 4 19’s

Now when I take all 5 sets to the beach and make the same run up and down the ocean, THEY DO DIFFERENT THINGS. Some are faster than others, some are smoother, some get better mpg at cruise or WOT than the others and some has a bit of each…One is clearly the winner on mph WOT by 3 mph.....

posted 09-07-2008 09:29 AM ET (US)            

I run the rev 4 17 as my everyday prop

posted 09-07-2008 10:16 AM ET (US)            

All the questions about the question remind me of my physics students. If they are having problems understanding the concept, they ask the question be repeated, hoping I will give them the answer in the question.

"
I guess you think that all new the new props are fancy and it’s just some major marketing scam put out by the prop mfrs."

Yes, that is my hypothesis. Manufacturers are driven to produce "new models" for marketing reasons. Certainly. Is that surprising? So, is the hypothesis correct?

Two different propellers, especially when they differ in diameter and pitch, certainly can differ in WOT speed, midrange economy, and other factors. I am interested in isolating the differences, if any, that result from design characteristics OTHER than diameter and pitch.

So what design characteristics, if any, differentiate
Enertias from Tempests from Mirages from Rev4's, if they have equivalent diameters and pitches? When we get these characteristics defined, we can move forward to design the experiment that will validate or disprove the hypothesis.

posted 09-08-2008 09:39 PM ET (US)            

Socratic or Scientific?

posted 09-09-2008 11:01 AM ET (US)         

In reality, most of us use our boats in a variety of conditions. We often compare propellers on boats to the tires on our automobiles. I do not suppose there are many automobiles or trucks in northern Michigan sporting mild weather touring radial tires you may find on south Florida automobiles, or SUV's with 160 MPH speed rated tires you might find on a corvette. Interchanging summer and winter rated tires, or tirs of other uses has become impractical.

posted 09-09-2008 01:07 PM ET (US)            

quote:

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I think the simple response to this hypothesis is, well, it just doesn't really matter, unless of course, all boaters were relegated to simple cruising under the most ideal of conditions

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posted 09-09-2008 07:33 PM ET (US)            

Enertia’s
Tempest’s
Mirage Plus’s
Rev 4's

and probably a lot more than I can recall..

Secondly, of course you would not pull skiers with a big pitch prop. Pitch down, gear down.

Says nothing whatever about whether you should have an "Enertia" or a "Mirage" or a "Rev."

Thats my point. Not the diameter and pitch, but the "design" if it truly exists. Manufacturer's planned obsolescence, or actual value?

You know, Cadillac convinced a lot of people they needed tail fins on the 1960 deVille.

posted 09-09-2008 08:10 PM ET (US)         

Instead of offering any direct evidence to support your hypothesis, you have asked the forum here to go out and conduct these experiments to refute a theory that has no data to support its basis. Reminds me of a professor dictating an assignment to his or her students. I enjoy learning, but I'm done with school.

posted 09-10-2008 10:19 AM ET (US)            

Now we are getting somewhere.

Which accelerate better and why? (Assuming in the comparison we are not just comparing different pitches.)

Which "hold" better in rough water (I assume that means resistance to ventilation) and what design characteristics are involved? Is this subject to documentation or is it anecdotal?

Smoother is usually associated with more blades. Easy to understand that one.

Which maneuver better in reverse and why?

And the original premise was:

"I am talking about normal, everyday service. Like 20-35 mph, where almost everyone cruises."

So, who has the slightest rational thought about the effects of a "chopper" versus a "cleaver" versus a "round-ear" (if that is really a "versus") versus other design characteristics?

"Merc offers alot of props that are "taylored" or "designed" for certain requirements and applications. I don't believe they are doing it strictly for marketing or the need for something new every year."

Well, why don't you believe that? Seems reasonable to me.

posted 09-10-2008 07:25 PM ET (US)            

Revolution"
Four-blade stainless steel design
"Brings power, top speed and lift with incredible traction and acceleration "

Fury
Increase both acceleration and top speed; the large blade area produces a better hole shot, and the high-performance blade geometry creates better top speed

Enertia
Large diameter and patented high-output blade geometry to move larger boats with less effort
Improves acceleration and top speed on larger high-horsepower boats

Mirage
Large diameter and patented high-output blade geometry to move larger boats with less effort
Improves acceleration and top speed on larger high-horsepower boats

Laser
High-rake design for greater lift on 15-18 foot outboard- and 18-22 foot Alpha sterndrive-powered boats
Thinner blades reduce drag and increase top speed while improving handling

Vengeance
Precision handling and acceleration - better than most three-blade aluminum propellers
For outboards 25-250 hp and Alpha sterndrives

Tempest
Ventilated for increased acceleration and unparalleled speed for large and heavy loads
Higher rake and bigger cup for better handling and lift
Large 14-5/8" diameter for high engine height installations

Now my head is *really* spinning. All the propellers are the "best"!

posted 09-10-2008 07:44 PM ET (US)            

quote:

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How to separate the salesman's hype from reality?

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That's easy. Go run them.

posted 09-11-2008 08:35 AM ET (US)            

The pitch designation is not very consistent. I can show you curves of boat speed as a function of engine speed which are almost identical, yet they are produced by propellers with pitch markings of plus or minus two inches. In some families, the "gain" is really just in having a propeller with more pitch than it has been designated compared to your existing propeller.

posted 09-11-2008 04:27 PM ET (US)            

I have not heard one bit of theory.

Nor any experimental results, at all.

Jimh, I cannot imagine a lower pitch prop getting better fuel economy than a higher pitch prop at mid range cruise.

If that has happened, I would like to see the fuel flows.

posted 09-11-2008 04:31 PM ET (US)            

The "test-drive" is a salesman's dream.

Hopefully, not everyone in the boat universe should have to test drive propeller.

Science works by generating hypotheses, testing them, refining them, and using them to predict things.

Now, I have a hypothesis, but it is not what the manufacturers say. There could be a lot of other hypotheses on these elements of propeller design. In addition, there is a great deal of testing in various sources, anecdotal and otherwise.

Yet no one seems to have a clue. No synthesis of information. No theory of how it works. Just "go run it."

posted 09-11-2008 06:00 PM ET (US)            

I CAN tell you this; THEY ALL FEEL DIFFERENT when rigged with any of the combinations listed above.

Why don't you come up with some results from your testing? It seem's your the one who came here spouting your theories. Most of the scientific types I have known would have been testing different propellers in a well organized DOE. That way, your could prove to us that they are NOT DIFFERENT.

Quit your nagging please.

posted 09-11-2008 08:21 PM ET (US)            

There aren't too many dealers that
A) will lend you one prop to test,
B) lend you a bushel basket full of props to test,
C) spend any time with you testing,
D) know what the differences between the props are,
E) have every style prop in every size and pitch or
F) all of the above.
Not enough profit in it.

Also, I think the differences will all result in some kind of compromise. If you get more top end you will lose hole shot. If you want a better hole shot you will lose top end. If you want better handling you will lose either top end, hole shot or both. If you want to have a lot of snacks in the boat and pull a 208 pound skier like me you're going to lose top end. If you want good economy you will lose hole shot and top end.

Coop's post from the Mercury website about the different props is hilarious. All the props are best. They almost all claim the same things.

The more choices there are the more people will think one is better than the other.

I wish I could find it but jimh made a funny but true statement some time ago about the proportion of money spend to the percieved effect. I wish I could find it. Maybe he'll repeat it.

posted 09-11-2008 09:02 PM ET (US)            

quote:

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There aren't too many dealers that ...

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No, there aren't. Who said anything about going to a dealer? In the last twenty years I have bought exactly one propeller from a dealer. It came on my brand new 1990 Johnson 150.

Boy, some of you are dense. What do you think a discussion area like contiunuousWave is all about if not a place to share experiences and offer each other advice, maybe loan a prop for testing or sell a used one that is not longer being used.

Where does this fear of trying something new come from? I just don't understand that.

And for anybody who cannot afford a new propeller: powerboating is definitely NOT the hobby for you.

As to Mercury's marketing being somewhat ridiculous, that is very true. I've made the same observation myself several times over the years; a lot of fluff that does not say much. There are some here who just regurgitate this stuff and offer it up as "advice" but if you want to know how a propeller will work for you, run it on your boat.

posted 09-11-2008 10:24 PM ET (US)            

And where do you get your bushel basket of props if not a dealer? "A" new propeller? Coop is talking about dozens of props and their real differences.

Also it isn't the fear of trying something new. Often it is the cost. There are probably 10 to 30 different props for my boat. Yeah, and Coop's point is what is the real difference?

His point is that the differences are minimal in the same effective pitch.

Like I said before it is mostly trial and error. Expensive trial and error. Lucky for me I ain't a tweeker. I have 3 props and use one 95% of the time.

posted 09-11-2008 11:15 PM ET (US)            

In this case the 16" Vengeance proved to have a greater actual pitch than the 17" Trophy Plus. Performance-wise, the vented 4-blade Trophy had much better acceleration, slightly less top speed, and a much smoother ride (can't really explain that one, but it was abundantly obvious in testing).

Though as I said I don't have fuel flow data available, I can say that my average fuel consumption per hour or per mile has not changed significantly between the two props.

posted 09-11-2008 11:34 PM ET (US)            

To be honest, I am not sure what cooper1958nc is talking about. For any given boat/motor there are not "dozens of props", there are a few good candidates.

Here is how it goes. Somebody has a Boston Whaler with an outboard motor or two. They are curious if there is a better propeller than what they have now so they start a thread to discuss it. It there are hundreds of such threads of discussion here on CW.

Owners of similar boat chime in with their experiences, much as Bob has just done. The advantages and disadvantages of the different props are discussed, performance data is shared and debated and sometimes a decision is made to try a different propeller. Sometimes that proves to be a good move and the results are shared as well.

posted 09-12-2008 12:17 AM ET (US)            

It was a disaster. With the larger, heavier boat, the Trophy ventilated wildly, produced very poor acceleration, and it seemed as if no throttle setting could produce a constant boat speed. I couldn't believe the difference in results as compared to my boat.

quote:

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From all of this, a body of experience is built. A collection of experiences that anybody with the ability to search the archives can now access. It's a pretty good system.

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I couldn't agree more.

posted 09-12-2008 10:53 AM ET (US)            

I just don't get compromising on your source of drive force and connection to the water with all the other money that is tied up in your boat.

posted 09-12-2008 02:13 PM ET (US)            

The whole purpose of inductive reasoning is to understand basic principles.

The "line" seems to be: there are no principles. Just go and endlessly test different propellers without having the slighest understanding of why any design is likely to help or hurt a given situation.

In the middle ages, people lacked theores, by and large. Someone would put an apple in the window to see if anything happened. Maybe it did,maybe it did not. Anything was possible.

The design of a propeller is part of the design of the package. The contribution of different design elements to performance goals is not a chance event.

Or is it?

Mr. Clark is, I think he will admit, in the business of selling propellers, so his bias should be clearly stated, and discounted, at the outset.

Having said that, I am sure he has some knowledge of these things.

So, lets start:

1. What are the variables in propeller design? I can think of several. The obvious: diameter and pitch. Rake. Blade area and blade count. Materials and thickness. Blade shape, including cupping.

What are the others?

2. Now for these variables, how do they affect performance?
I can start: diameter -- moves a larger water column, increases efficiency with increasing diameter at middle speeds. Etc.

pitch -- moves a smaller column water faster. Changes gearing. Etc.

Rake -- Bow lift (?). Stern lift (?). Nice styling (?).

Cupping -- increases effective pitch at higher RPM. May decrease tendency to cavitate.

Blade area -- increasing decreases efficiency, but decreases cavitation tendency at high blade loadings

The above is a start. Lets tap the knowledge base around here and see what is there.

posted 09-12-2008 10:54 PM ET (US)            

quote:

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Lets tap the knowledge base around here and see what is there.

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posted 09-13-2008 11:06 AM ET (US)            

quote:

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Since the general adoption of the screw propeller for marine propulsion, it has proven an exceedingly attractive field for the inventor. The number of inventions and patents that have been taken out covering every individual item of the instrument is myriad, and one can hardly suggest anything concerning any part of the propeller which he would not find had already been suggested or patented by some one in the past.

For many years after its adoption for the propulsion of ships the seeming vagaries in the performances of screw propellers in actual service or propulsion cast a great mystery over it and over the laws governing its action.

The greater part of this mystery is, however, not due to the propeller, but can be directly attributed to the carelessness with which trials of ships have been conducted and with which the data of performances have been collected. The major part of the remainder of the mystery is due entirely to the effect of variations in hull form with the changing character of the flow of water to the propeller accompanying these variations, and the resultant effect on the propulsive efficiency; and to incorrect estimates of effective horse-powers required for given speeds, these estimates of power having been based on frictional and residual resistances of the bare hull of the vessel, the malign influence of appendages fitted to the hull not having been apreciated and, therefore, having been entirely neglected.

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From page five of

Screw Propellers and Estimation of Power for Propulsion of Ships.
Also Air-Ship Propellers

posted 09-13-2008 11:29 AM ET (US)            

quote:

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So, lets start:

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No let's not. We are 59 posts into this thread and it is not going anywhere as you refuse to make your point. If you want to discuss the various design elements of propeller, that's great. I would enjoy a thread where that is the topic is discussion; I might learn something new.

If you would like a discussion along those lines, then start a new thread with a title like:

"What are the Different Design Elements of an Outboard Propeller and how to they Affect its Performance?"

...or perhaps a more concise topic of discussion such as:

"Propeller Rake: What Does it Do for You?"

Bt this thread was started by you and the title is:

"Designer propellers: what gains?"

If you have refuse to tell us what you mean by that. How can we have a discuss it?

You have rambled on about this and that but the one consistent theme I pick up from you is:

If something cannot be explained, it does not exist.

That's not only illogical, it is silly.

posted 09-13-2008 11:48 AM ET (US)            

Here's the real problem with this topic: Cooper proposes a thesis/hypothesis without any supporting facts and then challenges anyone to prove it wrong. This flies in the face of classical rhetoric as well as the scientific method, where the proponent of a thesis/hypothesis has the obligation of proving it up factually, in a manner which can be tested and confirmed by others. Cooper has it backwards. Here's an example of what I mean:

My thesis is that fancy-schmancy astronomy has it all wrong, and the sun actually revolves around the earth (or that the world rests on the back of a huge turtle, etc)...now you prove me wrong.

Nope, that's not the way it works, Cooper. Advance your hypothesis, support it with facts which can be verified by others, and let those who disagree state their position. It's impossible to deal with the other way around, and you should know better as a retired engineer.

Tony

posted 09-13-2008 11:49 AM ET (US)            

quote:

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Mr. Clark is, I think he will admit, in the business of selling propellers...

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Again, you are wrong. I am in the business of designing and building homes and improvements to homes.

One of my recent hobbies, an offshoot of my love of boats and Boston Whalers in particular, is the study of propellers, particularly propellers for outboard powered boats like my own.

This interest came about after purchasing my Revenge 25 Walk Around four years ago. Subsequent to the purchase of this boat, I never gave any great thought to the propellers on my boats apart from making sure they were stainless steel and appropriately pitched.

With the purchase of the Revenge 25, fuel economy suddenly took a higher priority. Additionally, a new boat always brings a certain curiosity about speed and what the ultimate top speed of one's boat might be.

At this point I really started to follow discussions of propellers more closely researching what had been written about different props here on the forum. Having never been a believer in aftermarket propellers being anything more than merely less expensive, I was surprised to consistently hear about these propellers called Stiletto. I decided I wanted to buy a pair for myself and see how great they are.

To that end I contacted my find who had an account with Precision Propeller as he had offered them to me at a deep discount. In the end he suggested I just open my own account with Precision and contacted them on my behalf to set it up.

So I opened my own account with Precision and can now offer a good deal on any Stiletto or Turbo prop to folks here on the forum. I have done so numerous times. My goal is to be helpful and perhaps learn something new from the feedback offered with each new trial. It has worked out well for everybody.

posted 09-15-2008 05:19 PM ET (US)            

quote:

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Mr. Clark is, I think he will admit, in the business of selling propellers...

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