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Old 10-15-2014, 01:26 AM   #81
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I can't believe, when faced with solid written evidence that propping to put your engine in its lower specific fuel consumption area at cruise speed can be trumped by "feeling" the engine is "happier". Sounds like something my wife would say...
The relevant information is that the engine is running cooler and consuming the same or lower fuel per knot of speed. Can you explain how this is trumped by theoretical evidence that this isn't the case when it is measurable in the real world? The "feeling happier" comment in my case is derived from pretty intimate awareness of the sound of the engine before de-tuning the prop and the sound after de-tuning. It could be complete b.s. of course, but then again, I have the empirical evidence of cooler temps and what looks like the same fuel consumption based on the Floscan to validate that purely subjective observation.
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Old 10-15-2014, 06:08 AM   #82
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The sound of a "happy" engine is largely related to the individual boat's peculiar harmonics/vibrations. For instance, my JD4045 sounds/feels happier at 1100 RPM ("no-wake" speed) rather than minimal idle of 750 because there is less vibration because the engine runs smoother. "Happy" cruise is up to 1800 (1.7 GPH) as the engine runs smooth operating at 45 percent capacity. There is minor vibration of the propeller shaft between 1900 and 2100 RPM so I avoid that. Wide-open is at 2400 (100 percent, 4 GPH) with minimal speed gain compared to a "high cruise" at 2200 (attaining hull speed at 75 percent load, 3 GPH), but 2200 approaches double the fuel consumption for a one-knot increase and increased engine noise.
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Old 10-15-2014, 06:56 AM   #83
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>The relevant information is that the engine is running cooler and consuming the same or lower fuel per knot of speed.<

Cooler running is by definition less efficient , the engine converts fuel to HEAT and heat is what spins the prop.

>The "feeling happier" comment in my case is derived from pretty intimate awareness of the sound of the engine before de-tuning the prop and the sound after de-tuning. The "feeling happier" comment in my case is derived from pretty intimate awareness of the sound of the engine before de-tuning the prop and the sound after de-tuning.<

Just as your car sounds like it is working hard going up a hill and sounds happier with less load on the way down.

The fuel consumption for a few hundred RPM change will probably be hard to find a few % is usually within the error rate of many flowscans.

A few hundred hours may tell,

Remember what is called overproping is not in any way overloading ,

it simply matches the engine power curve and efficiency to the vessels actual needs .
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Old 10-15-2014, 08:30 AM   #84
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Quote:
Originally Posted by Brooksie View Post
I can't believe, when faced with solid written evidence that propping to put your engine in its lower specific fuel consumption area at cruise speed can be trumped by "feeling" the engine is "happier". Sounds like something my wife would say...

Over propping is simply the installation of a fixed pitch prop that will not let the engine achieve the manufactures rated max RPM range on that specific platform.

My wife like most woman is very perceptive, if your wife has a "feeling" perhaps she should be in as an adviser. Men tend to be.... how do I say this .... hardheaded and stubborn as to their beliefs.

I can dig up evidence to support most any belief, the reliability or accuracy is always what needs to be questioned.


I can only offer the "facts" as to how they effect me but I am not a fan of gross over propping weather in an aircraft or a boat.

Large slow turning non-turbo diesels are going to be fine with a small amount over propping when not run on the pins for long periods of time, this is my educated opinion. Turbos will get into high EGT's in this condition as any truck driver can tell you.
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Old 10-15-2014, 09:06 AM   #85
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We all can hear that an engine at idle is noisier at higher rPM.
I think intake air noise will be more at higher RPM. Pumps and alternators turning faster and trannys too should be a bit noisier.
Not sure about exhaust because while pops are faster they have less force I suspect.

But we are usually not talking about doubling RPM just a 10 or 20 % change. So the noise should change little as well.
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Old 10-15-2014, 09:12 AM   #86
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Detroit is now building their large truck engines to be run at about 1000 RPM in cruise.

When these find their way into large boats , the lack of engine noise will become remarkable!!

Just dont go out in a lightning storm.
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Old 10-15-2014, 09:56 AM   #87
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FF what ever you're talking about I'd like to know but I suspect it's some monster engine the size of my house

Pics and specs please sir.
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