Propeller Pitch
- Thread starter El and Bill
- Start date
Amen to that Roger!
Another thing that you have to take into consideration is the engine driving the prop. It could be that Suzuki chose the grear ratio they did because they wanted to put the power curve of the motor in a different place than say Honda or Yamaha chose.
Anyway this has certainly been an interesting thread!
Another thing that you have to take into consideration is the engine driving the prop. It could be that Suzuki chose the grear ratio they did because they wanted to put the power curve of the motor in a different place than say Honda or Yamaha chose.
Anyway this has certainly been an interesting thread!
dogon dory
New member
Sorry, Folks - Post Deleted By Author
First thing out of the box, let me qualify my stake in this discussion. I really thought my simplistic comment earlier put the common boat owner's needs in the proper perspective. That comment being:
But I admit without reservation that I was out-englished by a biologist (we'll get to the science teacher in due time) when Roger reduced the concept to its purest form:
I think the quest for perfection here is not worth the perceptible difference between it and near-perfection. As for me, I have limited resources at the moment, and finite ambition as well, so I am content to try the options at hand with the collection of props I already have, and select what seems to work the best. But that's just me this week. I certainly encourage others so inclined to refine the science.
So, now that you know I am not arguing, nor trying to disprove or dissuade, let's continue.
"Whatya know" is right! I don't dispute the math that I probably couldn't work out anyway, and I trust that the gallons per minute/prop rpm relationship is linear throughout the entire range so that at any given motor rpm the gpm calculation is the same for both models. Therefore I accept this:
Now, if I were sitting in Joe's science class having this discussion, I would raise my hand and seek some further clarification here:
Furthermore, being the consummate class smart aleck, I would try to steer away from the intellectual discussion to focus on this comment:
Dan - If they do not make the prop you need, might you consider a ProPulse adjustable? At least for the sake of experimentation before putting the hammer to a piece of metal that costs more to replace. Or if not the four bladed ProPulse, maybe a conventional four blade prop? It is my simple minded understanding that a four blade on these small outboards produce a result comparable to a three blade with a 1" higher pitch.
And by the way, you're welcome!
Ain't this fun?
TyBoo":3s94mbh2 said:
But I admit without reservation that I was out-englished by a biologist (we'll get to the science teacher in due time) when Roger reduced the concept to its purest form:
rogerbum":3s94mbh2 said:
I think the quest for perfection here is not worth the perceptible difference between it and near-perfection. As for me, I have limited resources at the moment, and finite ambition as well, so I am content to try the options at hand with the collection of props I already have, and select what seems to work the best. But that's just me this week. I certainly encourage others so inclined to refine the science.
So, now that you know I am not arguing, nor trying to disprove or dissuade, let's continue.
"Whatya know" is right! I don't dispute the math that I probably couldn't work out anyway, and I trust that the gallons per minute/prop rpm relationship is linear throughout the entire range so that at any given motor rpm the gpm calculation is the same for both models. Therefore I accept this:
So, while not dismissed, the engineer is relieved of further input on this particular aspect of the discussion. Feel free to contribute to the rest of it.
Now, if I were sitting in Joe's science class having this discussion, I would raise my hand and seek some further clarification here:
Wouldn't a physically larger propeller have also a larger surface area in contact with the water? And because it is doing more work in less time, create more force between the water and the surface area in order to achieve the same result? And wouldn't those factors reduce the "relatively minor" downside to an even less consequential consideration?
Furthermore, being the consummate class smart aleck, I would try to steer away from the intellectual discussion to focus on this comment:
The potshots were not at all sideways. The hit to the Nerd was intended to be direct, and Ford just happens to be the best means to get to him. He likes that, you see, and responds well to it. Other outboard brands took no hit at all - I simply stated industry accepted fact. As for Greg, it was not so much a potshot as an attempt to help him open his mind a little so that he doesn't give up boating when his trusted Yamaha wears out as any piece of machinery ever made eventually will when used as intended. And just acknowledging that shows my great respect for Mother Nature.
Dan - If they do not make the prop you need, might you consider a ProPulse adjustable? At least for the sake of experimentation before putting the hammer to a piece of metal that costs more to replace. Or if not the four bladed ProPulse, maybe a conventional four blade prop? It is my simple minded understanding that a four blade on these small outboards produce a result comparable to a three blade with a 1" higher pitch.
And by the way, you're welcome!
Ain't this fun?
dogon dory
New member
Sorry, Folks - Post Deleted By Author
Dan-
Sounds like you're "swearing off" on propeller discussions! Is there a group you can join like AA? I think I'll join you! Maybe we can get Mike to join too! Then we can sit around and find something else to divert our attention to like discussing whether twins or a single with a kicker is better.
Too bad this whole thing is so complex that it's hard to come up with solid, simple, easy to follow answers. But it's just that problem, exactly, that leads to these endless discussions. I'll try my best to bow out here.
Mike-
As far a friction with the water is concerned, it might seem that the larger propeller's greater surface area turning at a slower speed would be an equal trade off with the smaller one turning at a faster speed, but unfortunately friction in water goes up exponentially with the speed as opposed to arithmetically. For example, to put it in easier terms, if you double the speed, the friction goes up to four times* as great, not two times as great.
So the smaller, faster turning propeller winds up wasting more energy through friction to try to move the same amount of water. Since the smaller, faster turning one wastes a little more energy in friction, it can't move as much water, and the thrust forward to the boat is a little less, and the boat is a little slower, but not much.
In addition, my somewhat cavalier comments about the "potshots" were just another attempt to humor the "dry" discussion along, just as yours were!
* I don't remember if the exponent here is 2 or 3 or some other number, but this illustrates the idea.
Peace on the Propeller Front.
Joe.
Sounds like you're "swearing off" on propeller discussions! Is there a group you can join like AA? I think I'll join you! Maybe we can get Mike to join too! Then we can sit around and find something else to divert our attention to like discussing whether twins or a single with a kicker is better.
Too bad this whole thing is so complex that it's hard to come up with solid, simple, easy to follow answers. But it's just that problem, exactly, that leads to these endless discussions. I'll try my best to bow out here.
Mike-
As far a friction with the water is concerned, it might seem that the larger propeller's greater surface area turning at a slower speed would be an equal trade off with the smaller one turning at a faster speed, but unfortunately friction in water goes up exponentially with the speed as opposed to arithmetically. For example, to put it in easier terms, if you double the speed, the friction goes up to four times* as great, not two times as great.
So the smaller, faster turning propeller winds up wasting more energy through friction to try to move the same amount of water. Since the smaller, faster turning one wastes a little more energy in friction, it can't move as much water, and the thrust forward to the boat is a little less, and the boat is a little slower, but not much.
In addition, my somewhat cavalier comments about the "potshots" were just another attempt to humor the "dry" discussion along, just as yours were!
* I don't remember if the exponent here is 2 or 3 or some other number, but this illustrates the idea.
Peace on the Propeller Front.
Joe.
El and Bill
New member
Dan, and all the others in the pub --
This has been a most useful discussion, for those in the pub and those joining. We can each, individually, select those comments that best pertain to our boat's needs -- Thanks from us, and, we think, from new and future owners for the thoughtful and experienced responses -- El and Bill
This has been a most useful discussion, for those in the pub and those joining. We can each, individually, select those comments that best pertain to our boat's needs -- Thanks from us, and, we think, from new and future owners for the thoughtful and experienced responses -- El and Bill
dogon dory
New member
Sorry, Folks - Post Deleted By Author
dogon dory
New member
Sorry, Folks - Post Deleted By Author
dogon dory
New member
Sorry, Folks - Post Deleted By Author
I think it's time to reopen this subject and add some new ingredients to the mix.
I am still thrashing with thrashing trying to come up with the right prop combination for my CD22 with 90 Honda VTEC. What's confounding the situation is way the ECM manages fuel flow. I went through this whole thread and there really weren't any comments about fuel flow, versus slip at various rpm settings. I recall reading a book about the Wright brothers and when they had figured out lift and control they then tackled propellers and then I think it was Orville who wanted to quit as it seemed unsolvable.
So last night I plotted graphs of fuel flow on various boats using the 90 VTEC on the Honda web site against my own experience. When I plotted the points, 3 of the boats almost formed a single curve, and the other two boats were almost on a single curve. The first 3 were planing hulls and other two were essentially catamarans (a Glacier Bay and a pontoon boat). My fuel flow was like the twin hulls up to 3500 rpm (just on step at 10 mph) and then went rapidly above the planing hulls and stayed there.
My fuel flow was essentially the same with either a 15 pitch or 17 pitch Solas 4 blade. The reason for the spike in fuel flow is exiting the lean burn mode (like opening the other two barrels on a Holley 4 barrel). In the past with carbureted engines and many fuel injected, fuel flow was a nice almost straight line. With computers making the decision it went non linear. My engine with the load and prop combination came out of lean burn at 3500 RPM, the planing hulls at just above 4000 and the twin hulls at 4500. I then started looking at the prop pitch and diameter on the twin hulls. The pontoon boat was using a 14" 11P prop and the Glacier Bay was was twin engine boat so not a good subject to look any further.
Based on what I saw, the lean burn feature kicks out based on engine load. I don't know how it measures load but on the old 4 barrels it was vacuum. I looked at prop slip and found the pontoon boat had a large amount of prop slip over the entire rpm range, anywhere from 46% down to 24% at the high rpm range. My prop show about 55% slip at which point passing through 3500 rpm it started to bite the water better and slip dropped to 12%at WOT. At about the same time it started to bite the water, the engine exited lean burn and fuel flow went from 2.1 gph at 3500 rpm to 6.2 gph at 4500 rpm and speed went from 10 mph to 16.5 mph.
Based on this I am seriously considering changing the techique I use to determine WOT. Rather than trim the motor up until the prop ventilates and then drop it a little, I will stop WOT throttle testing, when trimming the motor stops increasing the speed. The next prop I intend to test is a 3 blade 14 inch in diameter prop of 13 pitch. I'm taking Dan's lead on that on bigger diameter and try to ease the engine load at the midrange rpm where I typically cruise so I can postpone the opening of the other "two barrels".
Things I have learned:
a. Prop slip generally decreases as rpm increases. As you reduce prop slip engine load can increase.
b. Fuel consumption is not linear as rpm increases.
c. I'm glad Orville didn't quit, but I wish he had studied marine propellers.
Tom
I am still thrashing with thrashing trying to come up with the right prop combination for my CD22 with 90 Honda VTEC. What's confounding the situation is way the ECM manages fuel flow. I went through this whole thread and there really weren't any comments about fuel flow, versus slip at various rpm settings. I recall reading a book about the Wright brothers and when they had figured out lift and control they then tackled propellers and then I think it was Orville who wanted to quit as it seemed unsolvable.
So last night I plotted graphs of fuel flow on various boats using the 90 VTEC on the Honda web site against my own experience. When I plotted the points, 3 of the boats almost formed a single curve, and the other two boats were almost on a single curve. The first 3 were planing hulls and other two were essentially catamarans (a Glacier Bay and a pontoon boat). My fuel flow was like the twin hulls up to 3500 rpm (just on step at 10 mph) and then went rapidly above the planing hulls and stayed there.
My fuel flow was essentially the same with either a 15 pitch or 17 pitch Solas 4 blade. The reason for the spike in fuel flow is exiting the lean burn mode (like opening the other two barrels on a Holley 4 barrel). In the past with carbureted engines and many fuel injected, fuel flow was a nice almost straight line. With computers making the decision it went non linear. My engine with the load and prop combination came out of lean burn at 3500 RPM, the planing hulls at just above 4000 and the twin hulls at 4500. I then started looking at the prop pitch and diameter on the twin hulls. The pontoon boat was using a 14" 11P prop and the Glacier Bay was was twin engine boat so not a good subject to look any further.
Based on what I saw, the lean burn feature kicks out based on engine load. I don't know how it measures load but on the old 4 barrels it was vacuum. I looked at prop slip and found the pontoon boat had a large amount of prop slip over the entire rpm range, anywhere from 46% down to 24% at the high rpm range. My prop show about 55% slip at which point passing through 3500 rpm it started to bite the water better and slip dropped to 12%at WOT. At about the same time it started to bite the water, the engine exited lean burn and fuel flow went from 2.1 gph at 3500 rpm to 6.2 gph at 4500 rpm and speed went from 10 mph to 16.5 mph.
Based on this I am seriously considering changing the techique I use to determine WOT. Rather than trim the motor up until the prop ventilates and then drop it a little, I will stop WOT throttle testing, when trimming the motor stops increasing the speed. The next prop I intend to test is a 3 blade 14 inch in diameter prop of 13 pitch. I'm taking Dan's lead on that on bigger diameter and try to ease the engine load at the midrange rpm where I typically cruise so I can postpone the opening of the other "two barrels".
Things I have learned:
a. Prop slip generally decreases as rpm increases. As you reduce prop slip engine load can increase.
b. Fuel consumption is not linear as rpm increases.
c. I'm glad Orville didn't quit, but I wish he had studied marine propellers.
Tom
:beer That's probably what it is. I found reference to it in the shop manual. The shop manual is very detailed, but doesn't say anything about how it works.
The science experiment continues. It gives me a reason to take the boat out when I don't need any more fish and its raining.
Tom
The science experiment continues. It gives me a reason to take the boat out when I don't need any more fish and its raining.
Tom
You have to look at the hull form --the C Dory planes at a much lower speed than the majority of boats tested--and thus other boats are not good comparisons. The Glacier bay and Pontoon boats are both non planing--displacement cats--no relevance to the C Dory.
The prop slip is less at higher speeds because the boat is planing more effeciently.
What is your WOT? Your boat should be proped so that it will reach WOT--if not you you may lug the engine at lower speeds--in the curve where the boat is just starting to plane.
If you have a flow meter, it will pay for itself and this is the best way to determine what the effeciency of the boat is--miles per gallon. Gallons an hour are rarely relevant.
The prop slip is less at higher speeds because the boat is planing more effeciently.
What is your WOT? Your boat should be proped so that it will reach WOT--if not you you may lug the engine at lower speeds--in the curve where the boat is just starting to plane.
If you have a flow meter, it will pay for itself and this is the best way to determine what the effeciency of the boat is--miles per gallon. Gallons an hour are rarely relevant.
Tyboo? I can resist no longer...
What is the best prop for my CD-22 with a Honda 90?
It has a blue stripe (the boat, not the Honda).
It has power tilt (the Honda, not the boat).
If you can answer that I will contact you about my yellow car. It don't run anymore.
You coming to NC next weekend? If so I'll buy you a beer. Perhaps two if they have a keg.
I am old & single and confess I spend an hour or so every evening on this site.
It almost feels like "home, West Virginia". Of course I am from PA so I get lost a lot.
Don't think John Denver ever owned a C-Dory anyhow. If he did it probably had the wrong prop.
My hat is off to you gentlemen; this is quite a website!
Al :beer :beer
What is the best prop for my CD-22 with a Honda 90?
It has a blue stripe (the boat, not the Honda).
It has power tilt (the Honda, not the boat).
If you can answer that I will contact you about my yellow car. It don't run anymore.
You coming to NC next weekend? If so I'll buy you a beer. Perhaps two if they have a keg.
I am old & single and confess I spend an hour or so every evening on this site.
It almost feels like "home, West Virginia". Of course I am from PA so I get lost a lot.
Don't think John Denver ever owned a C-Dory anyhow. If he did it probably had the wrong prop.
My hat is off to you gentlemen; this is quite a website!
Al :beer :beer