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Old 06-15-2014, 12:20 PM   #1
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Diesel engine fuel use

belowi is an example of a typical diesel engine fuel use vs RPM chart. Often people use the more frequently available top curve to estimate fuel use.

That is incorrect and over estimates fuel use because that curve shows the maximum power the engine is capable of producing at any speed and the fuel required to do so. The lower curve, the prop demand curve, shows the fuel used by a typical propeller loaded to achieve the rated MAX RPMs
Prop demand curve is what owners will see in their boats if propped properly.

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Old 06-15-2014, 12:36 PM   #2
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That chart shows the relationship of power output to engine speed (RPM).

More useful is rate of fuel consumption compared to boat speed when measuring fuel use.
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Old 06-15-2014, 12:46 PM   #3
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With modern engines one advantage is the computer based fuel burn and boost and load information generated real time. While this is not always 100% accurate it does allow one to make their own fuel burn curve vs boat speed. Even if it is off a few % it beats guessing.
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Old 06-15-2014, 01:28 PM   #4
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Indeed the built in fuel flow meters are great as are added flowscans.
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Old 06-15-2014, 01:32 PM   #5
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Cummins performance curves

The standard performance curves for my boat don't match actual fuel consumption on our boat. The twin Cummins 210s burn 3.5 gph which is what one engine burns as represented by the curves. The difference is the engines are not pulling the 161 HP each that are in the curves.
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Old 06-15-2014, 01:42 PM   #6
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Each boat has its own hp required vs speed curve. The engine builders approximate this using an exponential curve, using an exponent of 2.5, 2.7 or 3.0. I think Cummins uses 2.7. This is a fair approx for displacement hulls that approach but not reach hull speed. Bump into the hump, curve is garbage. The Yanmar curves are developed mathematically, and are basically worthless as they don't give gph or bsfc with the curve (might be elsewhere??).

If you have electronic engines with gph indication, you can plot the curve yourself. Us with old mechanical engines, either install meters, or do various runs and dip the tanks. I love my old mech 6cta, but that gph data sure would be nice.

Cat and Cummins give lots of data with their spec sheets, you can actually calc their efficiency at various points without picking numbers off a curve. Yanmar makes it hard. Same with Volvo.
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Old 06-15-2014, 02:37 PM   #7
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David: The only place I see 161 HP on the curves is max rated. As explained above your boat will follow the prop demand curve which is the lower curve. Does that fit your data better??
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Old 06-15-2014, 02:39 PM   #8
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Ski: Cant you find curves for your mech 5.9s???
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Old 06-15-2014, 03:41 PM   #9
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Yea, I can get the curves, no problem. But better than that they give tabular data.
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Old 06-15-2014, 04:24 PM   #10
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I certainly understand bayview's point and I do know how to read the lower prop curve. From many years of watching these forums, I have found that owners always and I do mean always report lower fuel consumption than the prop curve for their engine would indicate.

Why? Well the prop curve may be inaccurate, particularly at the low sub displacement speed values that most members of this forum cruise at. But also owners are optimistic in their numbers. Some are guesses, some are real fill to fill measurements but ignore time idling in and out of the harbor, etc. The only owner data that I would pay attention to is Flowscan or electronic engine display data. But even Flowscans require calibration to be meaningful.

But here is a rule of thumb, oft repeated by me on this and other forums: a displacement hull boat will require about 1.5 hp per 1,000 lbs of displacement to push it to its displacement speed (1.34*sqrt(lwl)). That number is fairly solid and consistent. Bebe's book reported fuel consumption curves of a dozen passagemakers and they averaged that value with little variation.

Semi-displacement boats require more power, maybe 2-2.5 hp per 1,000 lbs. The flat aft sections that make it go faster don't let the water flow over it as cleanly as a true displacement hull.

And once you have figured how much power is required then you have to convert that to fuel burn. Modern, high output turbocharged engines make 16-20 hp per gph, the lower number is applicable to the lower power settings. Older engines like the Perkins, Lehman and Detroits make 14-17 hp per gph.

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Old 06-15-2014, 10:21 PM   #11
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David wrote;
"But here is a rule of thumb, oft repeated by me on this and other forums: a displacement hull boat will require about 1.5 hp per 1,000 lbs of displacement to push it to its displacement speed (1.34*sqrt(lwl)). That number is fairly solid and consistent. Bebe's book reported fuel consumption curves of a dozen passagemakers and they averaged that value with little variation."

That's great. It's spot on re my Willard. I'm going to learn this rule of thumb and keep it forever. Actually I already know it ... as 4hp per ton. So why would there be a 2nd rule that states the same thing w half the weight (or twice the other way around)? But I like having both as boat weight is most often expressed in thousands of pounds for pleasure boats. Strange that I haven't seen your/the 1.5 rule before. Thank's David.
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Old 06-16-2014, 07:27 AM   #12
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Quote:
Originally Posted by djmarchand View Post
I certainly understand bayview's point and I do know how to read the lower prop curve. From many years of watching these forums, I have found that owners always and I do mean always report lower fuel consumption than the prop curve for their engine would indicate.

Why? Well the prop curve may be inaccurate, particularly at the low sub displacement speed values that most members of this forum cruise at. But also owners are optimistic in their numbers. Some are guesses, some are real fill to fill measurements but ignore time idling in and out of the harbor, etc. The only owner data that I would pay attention to is Flowscan or electronic engine display data. But even Flowscans require calibration to be meaningful.

But here is a rule of thumb, oft repeated by me on this and other forums: a displacement hull boat will require about 1.5 hp per 1,000 lbs of displacement to push it to its displacement speed (1.34*sqrt(lwl)). That number is fairly solid and consistent. Bebe's book reported fuel consumption curves of a dozen passagemakers and they averaged that value with little variation.

Semi-displacement boats require more power, maybe 2-2.5 hp per 1,000 lbs. The flat aft sections that make it go faster don't let the water flow over it as cleanly as a true displacement hull.

And once you have figured how much power is required then you have to convert that to fuel burn. Modern, high output turbocharged engines make 16-20 hp per gph, the lower number is applicable to the lower power settings. Older engines like the Perkins, Lehman and Detroits make 14-17 hp per gph.

David

But reporting lower fuel consumption than the prop curve could be because most boats are overpowered and as Ski noted the curves assume the engine at max power is still operating at displacement speed.
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Old 06-16-2014, 07:59 AM   #13
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Marco:

Go back and read the first post. That curve is the prop curve. Diesel manufacturers offer two curves: the wot, max HP curve and the prop curve which is well below the wot curve except at max rpms where they are the same.

I was speaking of the prop curve.

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