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GPH vs MPH

Old 09-24-2020, 05:55 AM
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Default GPH vs MPH

Why does GPH get so much attention? Isn't MPH a more important practical piece of information? I want to know how far I can go per gallon and find the GPH pretty useless for my purposes. I understand both are just fuel burn calculations but you never hear of a car GPH only MPH.
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09-24-2020, 06:01 AM
NedLloyd
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It is just because that is what the marine industry has always used and is a static unchanging number.
“X” GPH at “XXXX” RPM.

Working in MPG your number at a given RPM is always changing based on current, wind and other variables.

Say you are traversing a river with a 4 knot current, your MPG will be very different depending on whether you are going up river or down river. You GPH will always be the same.

you’re the chief engineer on a larger vessel and the captain asks “how much fuel do we have left”. As the chief engineer you can certainly answer “20 hours”. You really have no idea how far that will get you, you don’t know if you are bucking the current or have it with you, or if you have a 20 knot headwind or tail wind.

MPG is not a number the industry can work with because it doesn’t tell you anything useful.
Old 09-24-2020, 05:58 AM
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MPH is a speed measurement, GPH is a range measurement. GPH is akin to MPG, not MPH.
Old 09-24-2020, 06:00 AM
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I think he meant MPG
Old 09-24-2020, 06:00 AM
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Or GPM
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Old 09-24-2020, 06:01 AM
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It is just because that is what the marine industry has always used and is a static unchanging number.
“X” GPH at “XXXX” RPM.

Working in MPG your number at a given RPM is always changing based on current, wind and other variables.

Say you are traversing a river with a 4 knot current, your MPG will be very different depending on whether you are going up river or down river. You GPH will always be the same.

you’re the chief engineer on a larger vessel and the captain asks “how much fuel do we have left”. As the chief engineer you can certainly answer “20 hours”. You really have no idea how far that will get you, you don’t know if you are bucking the current or have it with you, or if you have a 20 knot headwind or tail wind.

MPG is not a number the industry can work with because it doesn’t tell you anything useful.

Last edited by NedLloyd; 09-24-2020 at 06:11 AM.
Old 09-24-2020, 06:33 AM
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Originally Posted by NedLloyd View Post
It is just because that is what the marine industry has always used and is a static unchanging number.
“X” GPH at “XXXX” RPM.

Working in MPG your number at a given RPM is always changing based on current, wind and other variables.

Say you are traversing a river with a 4 knot current, your MPG will be very different depending on whether you are going up river or down river. You GPH will always be the same.

you’re the chief engineer on a larger vessel and the captain asks “how much fuel do we have left”. As the chief engineer you can certainly answer “20 hours”. You really have no idea how far that will get you, you don’t know if you are bucking the current or have it with you, or if you have a 20 knot headwind or tail wind.

MPG is not a number the industry can work with because it doesn’t tell you anything useful.
Something doesn't make sense on what you are saying. GPH will increase of MPG does. If consumption goes up you are using more fuel per hour. GPH is something like KWH for your house and that will change as you consume more. That is why the industry uses it. However, if you know how much fuel you have and are a constant speed you can tell how far you can go based on MPG. GPH will only tell you how long you can go.
Old 09-24-2020, 06:43 AM
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Ned is 100% correct. When I'm calculating my fuel burn to get home when I'm 70 miles offshore, I'm using the GPH calculation to give me an accurate estimate of how much fuel I'll need. Last weekend we were 100 miles off of GE Inlet in New Jersey. I know that my range is 130 miles out tops for a Canyon run. Conditions were favorable, with a 2-3 foot sea at 9 seconds offshore, (but I knew that inshore the intervals were going to be tighter/choppier). I knew the course for my trip home would involve a following sea. Taking those conditions into consideration, I knew my boat would be able to cruise at 2250RPMS with a fuel burn of 35-38 gallons per hour, (about 26 knots). I knew my trip home would be almost 3 1/2 hours, and that I'd should calculate a fuel burn of about 133 gallons for the ride home. My boat has a 450 gallon tank, (although the Smartcraft displays are set at 420 gallons). I had burned about 160 gallons by 2PM, (according to the Smartcraft), and knew I needed 130-150 gallons to get home (depending on conditions). When the boat was filled up the following week by my fuel supplier, he delivered 316 gallons. 160 gallons out + 135 gallons in equals 295 gallons. I always figure 10 gallons of fuel burn to and from the Bell Buoy. Add that 20 gallons to 295 gallons and you come up with 315 total. Dead on.

Keep in mind that due to wind and current, fuel consumption changes. A couple weeks ago, I was coming in on a dead calm/flat sea the entire 90+ run home. I've probably never made a Canyon run out and back in such conditions. The ocean was like a lake. At that same RPM of 2250 the boat was doing 28 knots. So at the same fuel burn, I was covering more ground.

Last edited by cobraarvey; 09-24-2020 at 06:48 AM.
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Old 09-24-2020, 06:46 AM
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Originally Posted by davepjr71 View Post
Something doesn't make sense on what you are saying. GPH will increase of MPG does. If consumption goes up you are using more fuel per hour. GPH is something like KWH for your house and that will change as you consume more. That is why the industry uses it. However, if you know how much fuel you have and are a constant speed you can tell how far you can go based on MPG. GPH will only tell you how long you can go.
you are not considering that on the water speed through the water rarely equals speed over the bottom.

If I am going up river at a speed of six knots and burning ten gallons per hour and that river is flowing out at a current of six knots I am burning six gallons an hour and I am getting zero miles per gallon.
GPH is constant, MPG is all over the place.
Old 09-24-2020, 06:51 AM
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Originally Posted by nedlloyd View Post
it is just because that is what the marine industry has always used and is a static unchanging number.
“x” gph at “xxxx” rpm.

Working in mpg your number at a given rpm is always changing based on current, wind and other variables.

Say you are traversing a river with a 4 knot current, your mpg will be very different depending on whether you are going up river or down river. You gph will always be the same.

You’re the chief engineer on a larger vessel and the captain asks “how much fuel do we have left”. As the chief engineer you can certainly answer “20 hours”. You really have no idea how far that will get you, you don’t know if you are bucking the current or have it with you, or if you have a 20 knot headwind or tail wind.

Mpg is not a number the industry can work with because it doesn’t tell you anything useful.
spot on !!!!
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Old 09-24-2020, 06:56 AM
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Originally Posted by NedLloyd View Post
you are not considering that on the water speed through the water rarely equals speed over the bottom.

If I am going up river at a speed of six knots and burning ten gallons per hour and that river is flowing out at a current of six knots I am burning six gallons an hour and I am getting zero miles per gallon.
GPH is constant, MPG is all over the place.
GPS speed could be used. It is independent of current. That shows you actual distance being covered.
Is the mpg shown on our modern consoles not calculated like that? So I know based on my fuel left exactly where I can reach. GPH tells me how long I can go for but not how far...

Last edited by busanga; 09-24-2020 at 07:02 AM.
Old 09-24-2020, 07:02 AM
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Originally Posted by davepjr71 View Post
Something doesn't make sense on what you are saying. GPH will increase of MPG does. ,,,.
RE-Think about that.
Old 09-24-2020, 07:03 AM
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Originally Posted by busanga View Post
GPS speed could be used. It is independent of current. That shows you actual distance being covered.
It is not the distance being covered, but the SPEED OVER GROUND.
Old 09-24-2020, 07:09 AM
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Well, you probably have never flown an airplane.
You calculate fuel in gallons per hour at different RPMs
Calculate your air speed at a given altitude, subtract for wind, get your true SOG, calculate your travel time, calculate your fuel burned, and allow for your reserve.


Your problem is you're not doing any calculations. Fine and dandy untill you start fighting current, or heavy seas, or having to zigzag to maintain forward progress. Then all of a sudden you're going to be real interested in gallons per hour. The fuel the motor burns at a given rpm is pretty much a constant...... Your progress toward your destination ........is not. Except in calm conditions with no current.
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Old 09-24-2020, 07:11 AM
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I use GPH on displacement hulls, MPG on planning hulls.
Old 09-24-2020, 07:11 AM
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Originally Posted by Absolute View Post
It is not the distance being covered, but the SPEED OVER GROUND.
yes correct, ...so my statement stands. Once you know sog you know how far you can go. GPS calculates sog hence our computers calculates mpg which is easier to work with for the average boater.
Old 09-24-2020, 07:11 AM
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Originally Posted by busanga View Post
GPS speed could be used. It is independent of current. That shows you actual distance being covered.
Is the mpg shown on our modern consoles not calculated like that? So I know based on my fuel left exactly where I can reach. GPH tells me how long I can go for but not how far...
ok,... so this can all be made as complicated as hell.

So you are headed out to the canyon, you know you are burning ten gallons an hour. All your fancy GPS & electronics calculate that you are getting 1.5 MPG. Then you enter the gulf steam and are bucking the current. You are still burning ten gallons an hour, but your fancy electronics all of a sudden start telling you that you are only getting 1.25 MPG and you are sitting there wondering what the hell just happened.
Again, GPH is constant, MPG is all over the place.
Old 09-24-2020, 07:12 AM
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I prefer the GPH . On water there are an infinite amount of variables . GPH is an aprox usage at economical rpm in a probable best case scenario . It's up to me to take in wind , current , cargo , and my lead foot in consideration .
Old 09-24-2020, 07:13 AM
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Originally Posted by busanga View Post
yes correct, ...so my statement stands. Once you know sog you know how far you can go. GPS calculates sog hence our computers calculates mpg which is easier to work with for the average boater.
The speed over the ground (SOG) is constantly changing.
Yes, all your electronics can correct for that in the moment, so your display of MPG will always be changing. And again, GPH is constant.

ok,.... so Yes, your “average boater” who doesn’t know enough to understand what he is looking at likes to see MPG, because he likes to think it is just like driving a car.
If you want real information that is meaningful and you can do something with over the long haul you want GPH.

Last edited by NedLloyd; 09-24-2020 at 07:18 AM.
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Old 09-24-2020, 07:15 AM
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Originally Posted by NedLloyd View Post
ok,... so this can all be made as complicated as hell.

So you are headed out to the canyon, you know you are burning ten gallons an hour. All your fancy GPS & electronics calculate that you are getting 1.5 MPG. Then you enter the gulf steam and are bucking the current. You are still burning ten gallons an hour, but your fancy electronics all of a sudden start telling you that you are only getting 1.25 MPG and you are sitting there wondering what the hell just happened.
Again, GPH is constant, MPG is all over the place.
no argument there..GPH is constant but if current is opposite me at same speed. So sog is zero. If I don't know that how do I know that burning 10 gph won't get me anywhere in an hour. But if I know sog is zero I know I will not get anywhere
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Old 09-24-2020, 07:15 AM
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One reason mpg on a boat is not used much is prop slip is so high and the MPG numbers are so low they are depressingly low for most powerboating.
And people travel longer distances in cars while boats it is more about how many hours they go boating before returning to the dock then drive home.
So if they know they will be out boating 5 hours, they can more easily figure how much gas they need.
So if I am out 5 hours and I know I burn typically 20 gallons per hour, a quick figuring tells me 100 gallons is a good number to have in the boat.

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