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Old 29-09-2011, 14:28   #1
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Gallons per Horsepower-Hour

I have a fuel flow graph for my motor, Volvo Penta 2003, 3 cylinder diesel. I can not figure out/dont know if it is possible to convert G/hph or g/kwh to a normal gallons per hour.

Any help is greatly appreciated.

Thanks,
Jeff
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Old 29-09-2011, 14:37   #2
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Re: Gallons per horsepower hour

My 50 HP uses 96 oz. per hour. So that would be about 2 oz per HP per Hour.
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Old 29-09-2011, 14:41   #3
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Quote:
Originally Posted by Thank you dad
I have a fuel flow graph for my motor, Volvo Penta 2003, 3 cylinder diesel. I can not figure out/dont know if it is possible to convert G/hph or g/kwh to a normal gallons per hour.

Any help is greatly appreciated.

Thanks,
Jeff
Rule of thumb is 1 GPH for every 40 HP. Adjust that for however hard / rpm you're running.
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Old 29-09-2011, 14:43   #4
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Re: Gallons per horsepower hour

Had one of those 2003's in my CS36. About 1/2 gal per hour.
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Old 29-09-2011, 15:33   #5
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Re: Gallons per horsepower hour

The correct rule of thumb is 1 gal per 20 horsepower per hour, which is consistent with the curves on my Beta 50, and every other diesel I have looked at up to 1000 hp. Gasoline engines are not as efficient.
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Old 29-09-2011, 16:39   #6
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Re: Gallons per horsepower hour

Gallons / Hour with a Yanmar 3gm30

Gallons / Hour Discussion

Gallons per Hour . . .

Perkins 6.354 Diesel Engine Fuel Economy

fuel consumption
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Old 29-09-2011, 17:47   #7
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Re: Gallons per Horsepower-Hour

Marine diesel engines range from about 15 hp per gph for a NA diesel operating at the lower ends of its operating range (down to about 50% rpm) to 20 hp per gph for a modern, common rail, turbocharged engine operating heavily loaded.
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Old 29-09-2011, 20:13   #8
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Re: Gallons per Horsepower-Hour

The use of Gal per Hp hr is the proper way to determin fuel consumption. There are a lot of variables and at the end of the day How much HP you use will determine the gal/liters used. Simply stating "I use 1 gal per hr" is very useful but not really accurate. RPM, Load, all make a difference. If a 50 hp , 3600 rpm max engine is running at 70 % max throttle it may only make 30 - 40 hp. Gear reduction, pitch, are all part of this. Then .. after figuring all this out it is... what it is.
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Old 29-09-2011, 20:20   #9
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Re: Gallons per Horsepower-Hour

If my engine was using 2 1/2 to 3 Gals per hour, according to the 15 yo 20 HP rule, I would throw it over he side.
My neighbors motorsailer with 110 HP uses less than 3 per hour, still way too much as he only can get 1 KT faster than me.

THAT MAY BE TRUE, BUT THERE'S NO SANE REASON TO RUN THE ENGINE @ 3600 rpm's WHEN YOU CAB REACH HULL SPEED AT 2100. Excessive RPM's for no noticable gain, just more froth at the stern and squat at the stern.

Sorry about the caps, for those that get excited about them...
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Old 29-09-2011, 21:05   #10
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Re: Gallons per Horsepower-Hour

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Originally Posted by Thank you dad View Post
I have a fuel flow graph for my motor, Volvo Penta 2003, 3 cylinder diesel. I can not figure out/dont know if it is possible to convert G/hph or g/kwh to a normal gallons per hour.

Any help is greatly appreciated.

Thanks,
Jeff
What your graph shows is the efficiency of the engine across its load range. It's telling how many GPH to engine will burn for every HP it produces. To convert to GPH, just multiply the G/hph times the HP at that point on the graph. Does that make sense?
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Old 29-09-2011, 21:22   #11
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Re: Gallons per Horsepower-Hour

TU Dad,

I think that gallons per horsepower-hour is the same thing as saying gal/HP/hr, or gallons/hr per each HP expended. So for example, if the gal/HP-hr = 0.05, and you were pulling 16 HP, then you would use .05 * 16 = 0.8 gallons/hr of fuel consumed.

As for your other question, there are 0.746 kW in 1 HP. So, to convert 0.05 gal/HP-hr to gal/kW-hr:

0.05 gal/HP-hr * [1 HP/0.746 kW] = 0.05/.746 = 0.060724 gal/kW-hr

This makes sense, because 1 kW is more power than 1 HP, so you would expect the specific fuel consumption to be higher.

Hope this helps.
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Old 30-09-2011, 02:05   #12
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Re: Gallons per Horsepower-Hour

While this doesn't directly answer your question it may be food for thought.

There are two separate issues that need to be addressed with the goal being to optimize fuel consumption, engine efficiency (shaft output horsepower/fuel flow rate) and boat speed versus the amount of push required.

Consider a graph that shows fuel consumption versus boat speed. To go faster requires more horsepower and the fuel consumption gal/hr increases. When hull speed is approached this curve becomes steep...lots more power is required for a small increase in speed.

However, the faster you go the less time it takes to go from point A to point B.

So one could make a second plot that shows fuel consumed to go a fixed distance at different speeds. This second plot combines the changes in the efficiency of the engine running at different RPM and output horsepower with horsepower required to drive the boat through the water at each speed.

With the propeller matched to the transmission and boat there may exist an optimal throttle setting which gives the best fuel consumption for distance traveled at a reasonable cruising speed. The reason for this is the way an engine functions from a thermodynamic perspective.

Your engine manufacturer has generated a set of curves called the brake specific fuel consumption, BSFC. It is a set of closed curves plotted on RPM versus crankshaft output. For example, you can run an engine at a fixed RPM and output a range of shaft power. Anyway, this chart shows where the engine sweet spot is or the best efficiency, the highest HP/fuel flow rate ratio.

Combining the BSFC information with your power versus speed for the boat will allow you to figure out how efficient you are. However, power versus speed is difficult to obtain unless you instrument the shaft.

This brings us to your fuel flow meter. My guess is it is showing fuel consumption for an estimated curve through the BSFC plot or more simply for a constant engine efficiency and it really doesn't have much to do with the reality of the boat performance.

I would do a series of experiments going from point A to point B on calm water at 5 or 6 rpm settings and record the time to get from point A to point B (or read the speed from the GPS in no current water and calculate the time to go 1 mile). Record your fuel flow meter reading at each speed. Create a plot showing fuel consumed for one mile versus speed or time to go 1 mile and pick the best speed.

To estimate fuel consumed (incorrect by a constant factor but doesn't matter) multiply your fuel flow reading by the time it takes to go one mile at each rpm setting.
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Old 30-09-2011, 10:09   #13
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Re: Gallons per Horsepower-Hour

The ratio is not fixed but pretty close to 200 grams per hp per hour for most engines in their typical use. The hp per revs can be read for each engine's specs.

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Old 30-09-2011, 10:24   #14
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Re: Gallons per Horsepower-Hour

But isnt gal per hp per hr, based on the HP you are USING rather than the rated HP of the engine? ie: if you have a 50 hp diesel, but only 25 hp is required to keep you to hull speed.... My 51 hp Perkins reliably burned .6-.65 gal an hour in a 44 ft boat. My 85 hp Nanni burned about .75-.8 gal per hour in a heavy 47 foot boat.
Just keep track of your usage in a small tablet. You should find that it's very consistant, at least if the way you operate your boat is consistant... and most people are... You have an engine hour meter right?

Here's a teezer for the group!: If a boat is motoring at hull speed (not SOG)against 5 knots of current, will it burn more fuel?
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Old 30-09-2011, 10:43   #15
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Re: Gallons per Horsepower-Hour

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Originally Posted by Cheechako View Post
Here's a teezer for the group!: If a boat is motoring at hull speed (not SOG)against 5 knots of current, will it burn more fuel?
No such thing as a dumb question...do you mean to get from point A to point B or are you talking about burn rate (gal/hr)?? Yes to the first and no to the second.
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