Nomad,
That's the specs that Yanmar calls out for with the current prop and what I use at this time.
If you're over propped one can get instrumentation that will tell them the HP.
Re-Pitching prop for speed or economy
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Nomad,
That's the specs that Yanmar calls out for with the current prop and what I use at this time.
If you're over propped one can get instrumentation that will tell them the HP.
Smitty,
Your statement about aircraft engines is not correct, the ratio is not necessarily tight, nor does the variable speed prop raise or lower the rpm to match power needed. There's a LOT more to it.
I'd bet that a marine diesel would certainly burn fuel based on RPM, if it's working at all, i.e. pushing the boat. Sure, with not load it's not working at all.
If a variable prop or different pitches have absolutely no value, what are they there, and what do some operators chose different pitches?
I certainly don't know the answers and may or may not do anything, but my post was for curiosity .
But I'd bet my bottom dollar that if you took two identical boats and engines and loaded one with a LOT more weight, the heavier one would require more power and more fuel to push it the same speed. So do you under pitch it? Or can you operate a lighter boat with a larger pitch? Don't know, but we know it's being done. So, what are the limits?
"I'd bet that a marine diesel would certainly burn fuel based on RPM, if it's working at all, i.e. pushing the boat. Sure, with not load it's not working at all."
The governor on the diesel adjusts the fuel load on the engine based upon how its loaded. The prop transmits the power to the water - if the prop is too aggressive (high pitch) then the engines governor will call for too much fuel at too low an rpm and the overload condition is in place. The best route to take it to pitch your boat so that your highest load will still allow your engine to reach max rated rpm +3-5%. Then at every other selected rpm below that your engine will be below an overload condition.
"But I'd bet my bottom dollar that if you took two identical boats and engines and loaded one with a LOT more weight, the heavier one would require more power and more fuel to push it the same speed. So do you under pitch it? Or can you operate a lighter boat with a larger pitch? Don't know, but we know it's being done. So, what are the limits?"
Heavier boat requires less pitch so that the engine is not overloaded. These boats that are weighted differently no longer are identical so they require different props to make sure that the engines can reach full rated rpm +3-5%.
The limits are most easily identified by reaching that full rated rpm +3-5% but can also be measured with EGT , fuel flow gages and boost gages - those many folks do not have.
Besides adding weight to two identical boats to change the load on the engine there are many other ways:
- hotter and more humid weather
- growth on the hull
- growth on the running gear
- props that have defects and are unbalanced
- heavy seas and weather
- running in fresh water vs sat water
Any of these can add load to the boats work load and often a couple of these or more work together to achieve an overload state that did not exist a few months or weeks prior.
The governor on the diesel adjusts the fuel load on the engine based upon how its loaded. The prop transmits the power to the water - if the prop is too aggressive (high pitch) then the engines governor will call for too much fuel at too low an rpm and the overload condition is in place. The best route to take it to pitch your boat so that your highest load will still allow your engine to reach max rated rpm +3-5%. Then at every other selected rpm below that your engine will be below an overload condition.
"But I'd bet my bottom dollar that if you took two identical boats and engines and loaded one with a LOT more weight, the heavier one would require more power and more fuel to push it the same speed. So do you under pitch it? Or can you operate a lighter boat with a larger pitch? Don't know, but we know it's being done. So, what are the limits?"
Heavier boat requires less pitch so that the engine is not overloaded. These boats that are weighted differently no longer are identical so they require different props to make sure that the engines can reach full rated rpm +3-5%.
The limits are most easily identified by reaching that full rated rpm +3-5% but can also be measured with EGT , fuel flow gages and boost gages - those many folks do not have.
Besides adding weight to two identical boats to change the load on the engine there are many other ways:
- hotter and more humid weather
- growth on the hull
- growth on the running gear
- props that have defects and are unbalanced
- heavy seas and weather
- running in fresh water vs sat water
Any of these can add load to the boats work load and often a couple of these or more work together to achieve an overload state that did not exist a few months or weeks prior.
"Your statement about aircraft engines is not correct, the ratio is not necessarily tight, nor does the variable speed prop raise or lower the rpm to match power needed. There's a LOT more to it."
Please describe what you are referring to as a "normal" air/fuel ratio and how far of a range that ratio can reasonably work without engine damage during normal use.
Please describe what you are referring to as a "normal" air/fuel ratio and how far of a range that ratio can reasonably work without engine damage during normal use.
Jeff F
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I don't think that Yanmar can tell you this, unless that's your WOT rpm. i.e. you're way overpropped. Current prop is only relevant when combined with reduction gear, boat weight and design, bottom condition etc etc.
There are prop calculators that might give an approximation, and fuel consumption also gives a pretty good proxy for HP. But accurate fuel consumption is harder to come by than you might expect given the need to calibrate on both supply and return. EGT is also useful but I doubt that it alone is going to tell you HP.
That Yanmar "spec" that you refererence is pure marketing bs. Most knowledgeable mechanics- Tony Athens the former boatdiesel guru for example would tell you to run at 400 rpm or more below rated rpm. 80% continuous load for a high output engine like yours is 55 hp per liter which will wear out your engine faster.
But it is all a continuom- more load = less life. It is your money to burn.
David
FF
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"Please describe what you are referring to as a "normal" air/fuel ratio and how far of a range that ratio can reasonably work without engine damage during normal use."
For aircraft ,(3350's) we would set a cruise RPM ,and manifold pressure.
Then lean the mixture (there was a lever) for a 200 RPM drop.
The throttle (air volume) would be increased to gain back the RPM.
The CHT rose and the plugs all looked OK , this was the eng. mfg method for long range cruise.
Can't be done on a diesel , but diesels do have BMEP graphs that show where the engine is most efficient. Its never full throttle.
If the RPM and load are in the bulls eye , you have the right prop.
Engines wear out from piston miles so a lower RPM will allow the engine to last more hours.
As Long as the engine is IN the bulls eye range .
Running a 300Hp engine at 35 hp may shorten the life by over half, depends.
For aircraft ,(3350's) we would set a cruise RPM ,and manifold pressure.
Then lean the mixture (there was a lever) for a 200 RPM drop.
The throttle (air volume) would be increased to gain back the RPM.
The CHT rose and the plugs all looked OK , this was the eng. mfg method for long range cruise.
Can't be done on a diesel , but diesels do have BMEP graphs that show where the engine is most efficient. Its never full throttle.
If the RPM and load are in the bulls eye , you have the right prop.
Engines wear out from piston miles so a lower RPM will allow the engine to last more hours.
As Long as the engine is IN the bulls eye range .
Running a 300Hp engine at 35 hp may shorten the life by over half, depends.
Thank you FF - I have not been around aircraft engines for well over 20 years and do not want to pretend I am current at all.
What I really seek is the real air fuel ratios that are currently run in reference to the analogy.
What air fuel ratios do they typically run?
What are the extreme limits for air/fuel ratio before damage is imminent?
FWIW - whenever I have been involved with gas engines that have pistons there was a very narrow applicable A/F ratio and lean conditions often led to a very short life.
FF,
That's correct and with aircraft there are many different configurations, turbo'd (and different turbochargers) and normally aspirated. High and low compression engines. And with all of them the mixture, manifold pressure and RPM can vary depending on the goal. Can operate them way rich with fuel and way lean... depending on the goal. The point being is that there are a LOT of variables, and different configurations. And all these variables have limits, that if exceeded could blow the engine in a matter of seconds. But, we don't do that.... usually.
And, I'm sure marine diesels have their differences, but seem to operate in a much more predictable environment, with fewer variables. I find it very interesting and enjoy learning more about what can and can't be done with them.
Also, I'm not convinced that the limits aren't so strict that we only have one choice. Just trying to figure it out.
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Gotcha, and agreed.
I don't want to take over here, but want to present one more thought experiment using your specific case. Think it's at the heart of Seevee's question.
If you wanted to fully optimize for operation at 17 knots and pledged never to try to exceed that, what would the optimum rpm be? Basically you can adjust the gear ratio through prop changes to lock into that optimum rpm.
Here's the range I think you have. You can gear down (underprop) to the point at which you're running at 2600 rpm, which is the maximum spec for continuous operation. Or you can gear up such that the load is still within the motor's power curve, but closer to the limit. Let's call that 2100 rpm.
Where is the 'sweet spot'? Are there tradeoffs in motor longevity or fuel consumption? Going for 2100 introduces risks. An extra load or adverse conditions might do harm. Your 2300 seems very safe from that perspective. So let's limit the practical target range to 2300 - 2600. Why would 2300 be any better than 2600? Is is possible that 2600 might in fact be better in terms of motor health, or even fuel consumption? Is there any downside apart from noise? Maybe a true expert can answer.
I'll shut up now
I honestly have no clue. There has to be some wear related to RPM alone if you were running at 2600 and 17 knots. There also must be an efficient spot but I simply do not know. I do feel "good" about where my engine is running...but that is simply emotional....knowing I am getting better performance without overstressing the engine. But I don't know the answer to your question.
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I will say that a prop governor and diesel governor actually work similarly...spring pressure and flyweights. Also, you can overload a gasoline engine with a constant speed prop by too much throttle(manifold pressure) and not enough RPM....basically like lugging a car in too high of a gear.
Anyway, there are diesel powered airplanes...so you can chew on that for awhile!!!
<<Diamond Aircraft revolutionized the general aviation market with being the first to introduce Diesel piston singles and twins. Together with Austro Engine we invested millions to develop and build our own eco-friendly, fuel-saving and powerful aircraft diesel engines.
The 170 hp AE300 Jet-A1 fuel engine – installed in our DA42 and DA40 - with state of the art common rail fuel injection is one of the most efficient engines in General Aviation.
Why the top-of-the-line AE300 is the better choice than a conventional gasoline engine:
Less than a half fuel burn
Worldwide operability due-to multi-fuel certification (Jet-A, Jet-A1, TS-1, RT, JP-8, and No. 3 Jet fuel)
Lower fuel costs compared to Avgas (average relation from Jet-A1 to Avgas 1:2)
Unique high altitude performance provided by the integrated turbo charger
Easy engine management by an electronic controlled system (ECU) with integrated single power lever design
Tremendously lower maintenance costs compared to conventional gasoline engines>>
I think that second to last bullet point is important...single lever control. The computer figures out what pitch the prop needs to be at...also protecting the engine from the human operator.
Anyway, there are diesel powered airplanes...so you can chew on that for awhile!!!
<<Diamond Aircraft revolutionized the general aviation market with being the first to introduce Diesel piston singles and twins. Together with Austro Engine we invested millions to develop and build our own eco-friendly, fuel-saving and powerful aircraft diesel engines.
The 170 hp AE300 Jet-A1 fuel engine – installed in our DA42 and DA40 - with state of the art common rail fuel injection is one of the most efficient engines in General Aviation.
Why the top-of-the-line AE300 is the better choice than a conventional gasoline engine:
Less than a half fuel burn
Worldwide operability due-to multi-fuel certification (Jet-A, Jet-A1, TS-1, RT, JP-8, and No. 3 Jet fuel)
Lower fuel costs compared to Avgas (average relation from Jet-A1 to Avgas 1:2)
Unique high altitude performance provided by the integrated turbo charger
Easy engine management by an electronic controlled system (ECU) with integrated single power lever design
Tremendously lower maintenance costs compared to conventional gasoline engines>>
I think that second to last bullet point is important...single lever control. The computer figures out what pitch the prop needs to be at...also protecting the engine from the human operator.
Baker,
A bit off topic....
I'd love to have diesel power in my plane, but it's just cost prohibitive. The conversion (but not available) is way more than the plane is worth so if it burned no fuel it would still be too expensive for my use. But, I like to see it develop. Have a good friend that is CEO of Delta Hawk that is developing diesels for planes, but limited options for now.
Also, I'm not a fan of the single lever control... loose too much control, and doesn't give enough variables for me. I like to set the power settings appropriate for the mission, and the single level control doesn't do that well.
Now, for the boat, I'd also like control over what's happening. And, another point, the ability to continue to be able to operate with a failure of a system, within reason....
As for the boat, unless we get a variable pitch prop, we need to choose one pitch. And I'm trying to figure out what options are out there. And I can certainly understand to not get too far out of spec from the responses here.
Jeff F
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Emotions matter, and I think you're justified in feeling good about your setup. This is close to home for me as I have deliberately underpropped in my new configuration. My own sense is that anything in this range is perfectly acceptable, and trying to refine it further is just tinkering. Seems like setting for 2600 is fine but silly because you permanently lose the ability to go faster on demand.
Thanks again for your comments. They've helped me in my theoretical examination of my setup.
dhays
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One of the things that will be interesting to check when my new engine is installed (hopefully this week) will be if I reach max RPM. The engine is rated for 3000 rpm. At the purchase sea trial max rpm was 2925. The boat wasn't fully loaded. I would say about 1/2 fuel and 1/2 water plus 4 adults. The bottom and running gear were clean.
This means that the boat was slightly over propped, and would be even more so at a full load. I am not too concerned as most of the time I am only running the engine at about 50% of its rated hp. I try to run it at about 75% of rated hp for about 10-20 minutes periodically.
So I am not working the engine terribly hard. Even so, if I ever needed to have my prop worked on, I would consider seeing if it could be repitched to add another 100 rpm at max throttle.
Delfin
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Other than a new CAT 3306 and a genset, Delfin was an empty shell when I purchased her. The CAT technician came out to certify the engine to start the warranty as well as to allay a concern I had on the diameter of the dry stack exhaust pipe. After running around outside Long Beach for a couple of hours, I was told that the engine reached full rpm and that the stack was fine. 60,000 pounds of interior later, the 2200 rpm engine would only make around 2025 rpm at WOT. Since we ran typically at 1300 rpm I wasn't too concerned about overloading, so let it be. Fast forward 6 years and I had the wheel removed to be tuned and flattened a bit. The end result was an increase in fuel efficiency of a tiny bit, lower EGT by around 150 degrees, a smoother feel, slower idle speed through the water, 200 rpm more at the same speed through the water and a rough spot - typical of this model CAT, I have been told - became almost unnoticeable. I assume, but have no clue, that any perceived increase in fuel efficiency at the speeds we run at is due to less energy creating heat and more energy turning the wheel.
I had been told that slight over propping was fine for a slow turning full displacement vessel like Delfin, and had dividends in fuel economy. Having removed most of that over-propping, I can say that my experience was just the opposite. The engine is happier with a flatter prop and runs cooler, so I am going to assume that any additional wear due to spinning a bit faster over its lifetime would be offset by running cooler during that lifetime.
Not a big proponent of the over-propping trawler engines argument anymore, and I am pretty sure there is no merit to over-propping a planning hull, regardless of what % of WOT you run at.
I had been told that slight over propping was fine for a slow turning full displacement vessel like Delfin, and had dividends in fuel economy. Having removed most of that over-propping, I can say that my experience was just the opposite. The engine is happier with a flatter prop and runs cooler, so I am going to assume that any additional wear due to spinning a bit faster over its lifetime would be offset by running cooler during that lifetime.
Not a big proponent of the over-propping trawler engines argument anymore, and I am pretty sure there is no merit to over-propping a planning hull, regardless of what % of WOT you run at.
Your experience is due to two competing forces I suspect:
The engine burns a little (a few percent) more fuel at the higher rpm to go the same speed once you flattened the prop. But your prop is in a more efficient spot- less slip. So your fuel economy is slightly better.
That is something not discussed so far on this thread- props can sometimes operate more efficiently with less pitch and it can be more of a factor than engine efficiency.
That is one reason why I would never intentionally over prop a boat.
David
The engine burns a little (a few percent) more fuel at the higher rpm to go the same speed once you flattened the prop. But your prop is in a more efficient spot- less slip. So your fuel economy is slightly better.
That is something not discussed so far on this thread- props can sometimes operate more efficiently with less pitch and it can be more of a factor than engine efficiency.
That is one reason why I would never intentionally over prop a boat.
David
Delfin
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Sounds plausible...
Nomad Willy
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Delfin,
Well said.
I've always felt my engine prop combination performed best underpropped about 100rpm. That is 100rpm over rated power rpm. Have been about 70-80 rpm overpropped for some time and don't like it. Wife thinks hauling the boat for about $500 to change prop pitch a bit is stupid.
Something that's never been mentioned (I think) on this forum is that w the very high side loads on the piston and cylinder wall (a considerable area) create a lot of drag at high loads that could be alternating efficiency thinking in traditional ways. Any internal friction must be considered regarding efficiency.
I've always thought the overpropping choice was full of holes.
Nomad Willy
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I agree,
Just like air spilling out beyond the wingtips water spills off the end of the prop blades. At high angle of attack w wings and w high pitch on a boat prop. But if one continues this concept of less tip loss w less pitch one would need more blade area (and it's drag) would perhaps shoot down the efficiency.
Delfin
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As one who thought over-propping made sense, reality corrected me. Or at least reality as I experienced on my particular boat.
Jeff F
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I'll buy that, and am interested in the prop side of things too. Big part of the overall equation.
WRT the parasitic fuel cost of underpropping I can't help but think that it's truly miniscule, like maybe a fraction of a percent. Could we take high idle fuel consumption as a proxy for this cost, or more specifically the delta in RPM at high idle? I don't have instrumentation to measure this, but anecdotally it takes very little fuel to run the motor at 2500 rpm in neutral...
Nomad Willy
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One other benefit of underpropping is a nice low speed at idle around the harbors.
