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Old 05-04-2012, 18:45   #1
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Fuel Consumption Confusion

Hi folks:
Once again I ask for assistance. This time it regards diesel fuel consumption at different RPM's. I read an old discussion (2005) but it didn't clear things up for me. I have a 42' LOA Passport sloop with Yanmar 4JH2E (about 50 HP). I look at the performance and fuel consumption graphs in the Yanmar manual and they tell me that when I'm running my engine at 2200 RPM I'm developing about 27.5 HP and the fuel consumption (the DIN62708 rating) comes in at about 177 g/HP-h. Now, if I divide 27.5 into 177 I get a figure of 6.4. What does that number mean? Maybe I shouldn't be dividing those numbers, but then, what does the g/HP-h value represent? I am assuming it stands for gallons per horsepower hour.
I know I'm not burning 6.4 gallons per hour. Is this number high because it does not calculate the return flow to the fuel tank? Am I wildly off base here?
What I'm really trying to figure out is how much fuel I will burn at 2000, 2500 and 2900 (recommended RPM setting) RPM's.
I just made a crossing from Sabah, Malaysia to Singapore into the SW monsoon start and it was a brutal sail/power and I was down to my last gallons (I carry 75US inboard) when I faced crossing the Singapore Straits Fairway, where the big boys are bumper to bumper. A thunderstorm was approaching (and they are colossal in these parts) and I knew viz was going down to zero in the rain but all I could think about was if I turn back I won't have enough fuel to make another run at crossing the Fairway.
I did make it across and the rain missed me but my nerves were shot. If I could get a better handle on this fuel consumption deal I will live a little longer.
The only way I can think of figuring this out is to use a metered amount of fuel in a jerry can with the fuel line and fuel return line both inserted in the jerry can and do some test runs for 30 minutes at different RPM's and in calm waters, no currents and make notes. Tell me there is an easier way.
As always, thanks for any help.
Pete
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Old 05-04-2012, 18:58   #2
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Re: Fuel Consumption Confusion

g/HP-h stands for grams/(horsepower*hour). The specific gravity of diesel fuel is 3200 grams/Gallon. To get fuel consumption per hour, multiply your rating by the horsepower, or (177 g/HP-h) * (27.5 HP) = 4867.5 g/h
To convert to gallons, divide by 3200: 1.5 Gal/h

Does this seem more reasonable?

Keep in mind that the power curve in the manuals is theoretical; the actual power developed may be different.
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Old 05-04-2012, 22:06   #3
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Re: Fuel Consumption Confusion

Quote:
Originally Posted by rigormortis View Post
Hi folks:
I'm developing about 27.5 HP and the fuel consumption (the DIN62708 rating) comes in at about 177 g/HP-h.


What I'm really trying to figure out is how much fuel I will burn at 2000, 2500 and 2900 (recommended RPM setting) RPM's.
My guess.
Performance and fuel consumption graphs in the Yanmar manual give you the maximum power that that engine can produce at given rpm. The load on the engine will vary, (weather, loading, etc) and so the fuel consumption at a given rpm.
Unless you have accurate sensor in the flow and return diesel line only experience will let you guess au much fuel you may use.
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Old 05-04-2012, 22:14   #4
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Re: Fuel Consumption Confusion

Apples and oranges.

Your specific fuel consumption is how much horsepower your engine is producing at the rate at which the fuel is being burned. This information is usually available from the engine manufacturer.

Your horsepower is your horsepower regardless of how much fuel you are burning.

The horsepower vs RPM curve is a close estimate of how much horsepower your engine is producing at a given RPM....provided you have a properly pitched propeller. This means your engine can reach its maximum governed RPM. And this means by not having too much pitch nor too little pitch...with respect to the accuracy of the curve.

For example if your engine can reach its maximum governed RPM with 18 inches of pitch but cannot with 19 inches of pitch, then 19 inches is too much pitch and 17 inches is too little pitch. You could pitch it to 17 inches and of course there would be no harm. Too much pitch and you would never be able to reach maximum RPM in order to burn the carbon out of the combustion chamber....which is a different topic.
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Old 06-04-2012, 03:23   #5
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Re: Fuel Consumption Confusion

Ahhhh, Ziggy. Grams/Hp-hour. Now I understand. Thank you. The performance curves for the 2000-2900 RPM are pretty much flat so would appear to be little difference in fuel burn in that range. Now it comes down to the speed I get at the RPM settings I use in that range and that, of course, brings into play the figures that David M is talking about. I don't know the pitch of my 2-bladed fixed prop and I guess I don't care now.
I should have known the little 'g' was grams.
And another thing--the fishing boats in these parts (and all other parts I've been to) chug along at 2-3 kts doing their thing for days. Then they boot it home when the holds are full or the bellies are empty. They don't seem to care about the 'damage' low RPM does to a diesel. And they are in it for the long haul. Maybe we agonize over the small bits too much.
Thanks again, folks.
Pete
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Old 06-04-2012, 04:02   #6
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Re: Fuel Consumption Confusion

There's nothing wrong with low revs just if there's some reasonable load as it's case sailing few knots.
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Old 06-04-2012, 04:05   #7
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Re: Fuel Consumption Confusion

remember that adding a knot generally doubles fuel consumption.
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Old 06-04-2012, 05:20   #8
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Re: Fuel Consumption Confusion

Quote:
Originally Posted by rigormortis View Post
The performance curves for the 2000-2900 RPM are pretty much flat so would appear to be little difference in fuel burn in that range.
Most manufacturers, including Yanmar, show two power curves: maximum power and a theoretical propeller curve. The maximum is just that, the max power the engine is rated to develop at a given RPM. The propeller curve is the power required to turn a theoretical propeller perfectly matched to the engine (it's just an equation, not based on data). The curve for your real propeller and boat will be somewhere in between. So you can think of these two curves as giving you an upper and a lower bound on the horsepower and thus fuel consumption at any rpm.

Quote:
And another thing--the fishing boats in these parts (and all other parts I've been to) chug along at 2-3 kts doing their thing for days. Then they boot it home when the holds are full or the bellies are empty. They don't seem to care about the 'damage' low RPM does to a diesel. And they are in it for the long haul. Maybe we agonize over the small bits too much.
When they race home at high rpms at the end of the day, that tends to clean out any carbon deposits they may have accumulated at 2-3 knots.
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Old 06-04-2012, 06:04   #9
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Re: Fuel Consumption Confusion

It's not about fuel efficiency of the engine, its' about the fuel efficiency of the boat! Who cares what the engine efficiency is anyway? What you want to know if how far a gallon will allow you to go.

I can motor at 6.5 knots and burn 1.2 gph to do it. The engine is efficient at this speed. That's 5.42 km/gal.

Or I can motor at 5 knots and burn 0.7 gph to do it. The engine is not as efficient if I do this. That's 7.14 km/gal which is 31% better running at that engine efficient speed , or 31% better range.

To the OP run that engine at about 1800-2000 rpm and 4.5-5 knots and you will have better range. You could run slower and go farer but go nuts probably. Every few hours run the engine up to 3000 rpms for a couple of minutes.
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Old 06-04-2012, 06:20   #10
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Re: Fuel Consumption Confusion

I have 3,000 hours on a 100hp Yanmar Turbo Model 4JH3-HTE. I have logged fuel consumption every morning for the last 6 years of our Circumnavigation. I enter daily fuel and hour data in an Excel model that predicts fuel usage and fuel on arrival for passages.

I can tell you that based on the design of our boat, design of the AutoProp, our load, and normal RPM's at 1800-2000 that we have averaged 3.3 liters/hour over the 6 years. This includes motoring and motor sailing at about 6.5kts. That said, the Yanmar Turbo Model 4JH3-HTE will burn 4 gallons per hour at 3600 RPM with the same load...more than 4 times as much!

You need to record the same information as we have to understand your engine's best performance. You need an accurate method of determining your consumption. I use a dip stick and I stick it daily. It will take you some time to calibrate your dip stick, but it will be worth it.

I hope this helps you.

Bill
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Old 06-04-2012, 06:29   #11
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Re: Fuel Consumption Confusion

Quote:
Originally Posted by svBeBe View Post
I have 3,000 hours on a 100hp Yanmar Turbo Model 4JH3-HTE. I have logged fuel consumption every morning for the last 6 years of our Circumnavigation. I enter daily fuel and hour data in an Excel model that predicts fuel usage and fuel on arrival for passages.

I can tell you that based on the design of our boat, design of the AutoProp, our load, and normal RPM's at 1800-2000 that we have averaged 3.3 liters/hour over the 6 years. This includes motoring and motor sailing at about 6.5kts. That said, the Yanmar Turbo Model 4JH3-HTE will burn 4 gallons per hour at 3600 RPM with the same load...more than 4 times as much!

You need to record the same information as we have to understand your engine's best performance. You need an accurate method of determining your consumption. I use a dip stick and I stick it daily. It will take you some time to calibrate your dip stick, but it will be worth it.

I hope this helps you.

Bill
Wow! 3.3 liters/hour! I have the very same engine and the very same prop, and I don't think that I have seen less than 4 or 5 liters/hr. What speed do you get at 1800RPM? I have always been happy to get anything less than 6; maybe I need to move the bar up a notch.

Over the years, I find myself motoring slower and slower. It has taken me time to realize that the "diminishing returns" effect of more RPM starts much sooner than you would think. My normal motoring speed used to be 2700 RPM, which in the next season became 2500, and now it's 2200 - 2300. At 2700 in flattish water I get 9 knots; at 2200 I get between 7.5 knots and sometimes nearly 8 knots for much less effort and probably half the fuel consumption.

You've now inspired me to throttle back even more. I think I might go crazy if it means making only 6.5 knots, but let's see.
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Old 06-04-2012, 06:32   #12
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Re: Fuel Consumption Confusion

Hi,

With accurate tank sensors one can go by gph/hp. With big engines a flow meter can be mounted in the line and an accurate readout taken from any nmea / dedicated instrument.

If the engines / flow are big enough, I would go the meter way.

If the tanks are deep enough, I would go the gph/hp way.

b.
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Old 06-04-2012, 07:09   #13
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My experience with that engine is on average 3 litres / h at around 2000 rpm falling to 2.5 at 1800 this is a across several French 40 footers albeit lighter then your passport.


I use 3 in my range calculations ( 4 for worst case analysis)
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Old 06-04-2012, 07:15   #14
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Re: Fuel Consumption Confusion

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Originally Posted by goboatingnow View Post
My experience with that engine is on average 3 litres / h at around 2000 rpm falling to 2.5 at 1800 this is a across several French 40 footers albeit lighter then your passport.


I use 3 in my range calculations ( 4 for worst case analysis)
Dave
French 40-footers will weigh about half or less than what my boat does (44,000 lbs light ship). Are you sure they had 4JH3 HTE's? 100 horsepower is a lot for a 40-footer.

I use fuel for generating electricity and heating besides propulsion, so it's pretty hard for me to measure consumption.

In any case, the point is well taken that I have still not gotten to the "sweet spot" in consumption. Next time I'm out I will experiment with a 2000 RPM cruise.
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Old 06-04-2012, 07:21   #15
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Re: Fuel Consumption Confusion

There are really two issues to consider. The first is the engine efficiency which the manufacturer measures using a dynamometer. This graph is called the brake specific fuel consumption, BSFC, graph and is a family of curves for a range of rpm versus horsepower settings.

At each RPM there is a range of mechanical output, horsepower, that can be realized. For example, at 2500 rpm the engine could be generating 30 hp or 40 hp or the max hp (at rpm) or anything in between.

There is an optimal setting of rpm and horsepower for the engine where the most amount of mechanic energy is created from each drop of fuel. Any other setting of rpm and HP and you get less HP. Running at this optimal setting may or may not be able to be realized with your boat.

Thus the second issue, how this mechanical energy is coupled into the water to propel the hull. Putting the energy into the sea is a function of the wetted hull parameters, rpm, and the propeller parameters.

Think of this part analogous to setting the throttle at 2000 rpm and going 6 knots on calm water and 5 knots with a head wind. The head wind case is equivalent to a second hull having more resistance or drag where the propeller efficiency may be degraded by slipping more.

If one had the time and patience you would empirically vary the engine rpm and propeller parameters, pitch and diameter, to find a setting that gives the most miles per gallon for your hull.

Without option of varying propeller parameters I would simply vary the rpm until I found the rpm that gives me the most miles traveled per gallon. It may be more valuable to plot miles traveled per gallon for a range of rpm. This plot will let you select rpm setting that trades off mpg for speed. You would select an rpm setting where you would go 2 knots faster for only 0.2 gallons less per mile for example.

Mariners talk about fuel consumption in terms of gallons per hour. This measure is not relevant to getting the most miles per gallon.
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