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Old 03-30-2016, 11:03 AM   #61
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A boat's fuel consumption is most often expressed in gph just as cars are expressed in mpg. The point of it being that the most useful expression for boats is gph because fuel consumption is usually used to compare one boat to another and gph is also almost immediatly put to navagational use. But fuel burn specifics is usually used for comparision to other boats. Some use mpg but it's just a carryover from our extremely automotive culture.
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Old 03-30-2016, 11:14 AM   #62
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Dougcole wrote;
"But isn't knowing your average rate of fuel burn over a long period at many different rpm, speeds, conditions, of the greatest value? That's how boats are run pretty much all of the time. For me there is very little value in knowing how much I'm burning in flat water at 1873 rpm as opposed to how much I burned per hour at normal speeds and conditions over the course of 300 or more gallons."

Doug if you cruise at 1873rpm there is great value in knowing what your fuel burn is at that speed. But if you cruise at 2850rpm what you burn at 1873 is probably of little value .. unless you often do something at 1873 rpm. What is of most value is what your boat burns at it's design speed.
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Old 03-30-2016, 11:24 AM   #63
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A boat's fuel consumption is most often expressed in gph just as cars are expressed in mpg. The point of it being that the most useful expression for boats is gph because fuel consumption is usually used to compare one boat to another and gph is also almost immediatly put to navagational use. But fuel burn specifics is usually used for comparision to other boats. Some use mpg but it's just a carryover from our extremely automotive culture.
The reason why we use GPH instead of MPG is because water is a moving medium. The same boat at the same power setting with the same load could get VERY DIFFERENT MPG while burning the exact same GPH due to current of other factors(wind). We could go even further and go pounds per hour...which is what we do in airplanes sense fuel density varies with temperature but the weight of it does not.
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Old 03-30-2016, 11:30 AM   #64
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A boat's fuel consumption is most often expressed in gph just as cars are expressed in mpg. The point of it being that the most useful expression for boats is gph because fuel consumption is usually used to compare one boat to another and gph is also almost immediatly put to navagational use. But fuel burn specifics is usually used for comparision to other boats. Some use mpg but it's just a carryover from our extremely automotive culture.
It may be most useful for you. I find gph if you don't give me the corresponding speed to be totally useless. If I have speed and gph, then I had nmpg, which is what I really want in planning long trips and fuel stops. It's not for me a carryover from any other culture. It's a matter of knowledge and planning. I want to cover 250 nm today and I have a 950 gallon tank. So with a 10% reserve, I know I must average .3 nmpg. Based on history, I know I can do that at 26 knots.

We do want to know usage at all speeds and distance we can cover at all speeds. We do not talk gph ever without relating that to a specific speed. Once that is done, you can call it whatever you want but nmpg is what we're really after. The combination of usage and speed. In our boating, usage without speed means nothing to us. We might use 9 gph and we might use 117 gph. The key to us is knowing how far it will take us.

Also we do find instantaneous fuel usage to be very valuable. We see value in knowing at all speeds and not an average, because we don't run averages, we run specific speeds. We'll run at 5-10 knots through the canals and at 26-28 knots on the Great Lakes.

One other place this does help is in double checking our loads and seeing that the numbers we're getting electronically are reasonable.

Now, I'm not saying any of this applies to you.
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Old 03-30-2016, 12:03 PM   #65
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It may be most useful for you. I find gph if you don't give me the corresponding speed to be totally useless. If I have speed and gph, then I had nmpg, which is what I really want in planning long trips and fuel stops. It's not for me a carryover from any other culture. It's a matter of knowledge and planning. I want to cover 250 nm today and I have a 950 gallon tank. So with a 10% reserve, I know I must average .3 nmpg. Based on history, I know I can do that at 26 knots.

We do want to know usage at all speeds and distance we can cover at all speeds. We do not talk gph ever without relating that to a specific speed. Once that is done, you can call it whatever you want but nmpg is what we're really after. The combination of usage and speed. In our boating, usage without speed means nothing to us. We might use 9 gph and we might use 117 gph. The key to us is knowing how far it will take us.

Also we do find instantaneous fuel usage to be very valuable. We see value in knowing at all speeds and not an average, because we don't run averages, we run specific speeds. We'll run at 5-10 knots through the canals and at 26-28 knots on the Great Lakes.

One other place this does help is in double checking our loads and seeing that the numbers we're getting electronically are reasonable.

Now, I'm not saying any of this applies to you.
NMPG might be "what you're after", but there is absolutely no way to know that without knowing the current. At 26kts, a 1.5-2.0 knot current is not a huge percentage of your speed. But at 7 knots, it is. Most of the people on here are going 7 knots. And when they know how much their boat burns and how long they have been burning it, they know how much is left in their tank.
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Old 03-30-2016, 12:20 PM   #66
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Let's face it. If you don't want to run out of fuel you really need to know both your fuel consumption and your speed. Only then will you be able to adjust your RPM to get home without running out. Knowing your typical mpg in the conditions where you boat will allow you to do trip planning based on your range (with a safety margin for currents, etc).

I can't imaging planning a trip that is near the range of your vessel without knowing both. In my case that would only likely be for ocean crossing. On my delivery from Ketchikan to San Francisco last year I had enough fuel on board to do a round trip. Range wasn't a concern.

Of course if you're going to run a generator underway you'll have to factor that into your fuel consumption rates too.

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Old 03-30-2016, 12:21 PM   #67
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Whether you use gph or mpg may depend on why you are on the water. If you are there to enjoy the journey instead of the destination you may be more interested in gph but if you just want to get somewhere mpg may be a better measure. For instance if you are just enjoying a sunny day on the water with friends and family, six knots is just as enjoyable as ten knots and the gph ($ph) is much less. However, if you need to get to the next harbor 100 miles away before sundown you will pick the highest speed that gets you there before dark with sufficient fuel. That requires you to know mpg versus speed.

On this last point I may be a little weak but I think a planeing hull gets almost the same mpg regardless of speed once it is on a plane, even though gph is much higher as speed increases. In this case mpg is a better criterion for planning a trip than gph.

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Old 03-30-2016, 12:46 PM   #68
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NMPG might be "what you're after", but there is absolutely no way to know that without knowing the current. At 26kts, a 1.5-2.0 knot current is not a huge percentage of your speed. But at 7 knots, it is. Most of the people on here are going 7 knots. And when they know how much their boat burns and how long they have been burning it, they know how much is left in their tank.
We are capable of factoring current into our calculations. Those using gph sure need to factor in current as well if they're wanting to know what range they have left.

And while many here are going 7 knots, quite a few exceed that, at least some of their time. And others are capable of exceeding it if needed. I see this scenario. Let's say I normally go 7 knots but boat it capable of a 12 knot cruise at much higher fuel usage. The forecast turned negative and conditions are expected to worsen soon. I am 300 nm from a port. Can I speed up to 12 knots, do I have the range. Well, I get .8 nmpg at 12 knots vs. my 3 nmpg at 7 knots. I only have 300 gallons of fuel remaining. I can't go 12. What about 10? I get 1.2 nmpg at 10, so 10 it is.

Both gph and nmpg are useful and we keep tables with all the data in them. But talking about normal cruising, many cruise at various speeds and get much different usage at those different speeds.
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Old 03-30-2016, 12:55 PM   #69
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We are capable of factoring current into our calculations. Those using gph sure need to factor in current as well if they're wanting to know what range they have left.

And while many here are going 7 knots, quite a few exceed that, at least some of their time. And others are capable of exceeding it if needed. I see this scenario. Let's say I normally go 7 knots but boat it capable of a 12 knot cruise at much higher fuel usage. The forecast turned negative and conditions are expected to worsen soon. I am 300 nm from a port. Can I speed up to 12 knots, do I have the range. Well, I get .8 nmpg at 12 knots vs. my 3 nmpg at 7 knots. I only have 300 gallons of fuel remaining. I can't go 12. What about 10? I get 1.2 nmpg at 10, so 10 it is.

Both gph and nmpg are useful and we keep tables with all the data in them. But talking about normal cruising, many cruise at various speeds and get much different usage at those different speeds.
I fully agree....with Brittania also. GPH is stated just as a means of how efficient the boat is. If you are looking at a Nordhavn, you have a pretty good idea of cruising speed...although adjusting speed can have a SIGNIFICANT impact on range....even going from 9kts to 7kts. But I guess one of my points above was the title of this thread..."Fuel Consumption at low speed".
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Old 03-30-2016, 01:00 PM   #70
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On this last point I may be a little weak but I think a planeing hull gets almost the same mpg regardless of speed once it is on a plane, even though gph is much higher as speed increases. In this case mpg is a better criterion for planning a trip than gph.

Paul
Actually our experience with planing hulls somewhat supports what you're saying. With boats that get all the way on top of the water, generally smaller and faster boats, I've found that from the time they reach plane until their ideal cruising speed, somewhere around 70-80% load, there is very little difference in nmpg. Only above that speed do they see a major degradation. An example is a 44' boat that gets between 0.66 and 0.71 at all speeds between 17 and 37 knots, but then between 37 and 42 drops to 0.58 nmpg.

Now in bigger or heavier boats that plane (perhaps semi-planing in the minds of some), we find a rapid drop off until they are on plane but then a gradual drop off in nmpg all the way up their curve. An example is a boat that from 19 knots to 27 knots (19, 23, 25, 27) gets 0.27, 0.24, 0.20, 0.18 nmpg.

Then boats like gas powered outboards are entirely different. A 39' CC, gets the same nmpg at 11 knots and 43 knots. It has a bell curve in between going up by 25% and then coming back down with best economy between 26 and 33 knots.

Jet drives and surface drives also take your theory even further as they have bad fuel numbers getting on plane and then they actually improve through a large part of their curve.

I'd say, everyone use what works for them. For us, it's nmpg at each speed.
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Old 03-30-2016, 02:57 PM   #71
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I just wish my 50 foot full displacement trawler, with little bitty engines, would get 3 NMPG. I'd be happy as a 2 pekkerd billy goat.
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Old 03-30-2016, 04:50 PM   #72
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So the fuel dock was cheating stealing people's money everyone who filled up? Free fuel for the charter boat the price to keep it a secret?
That's the message I got!
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Old 03-30-2016, 05:42 PM   #73
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NMPG might be "what you're after", but there is absolutely no way to know that without knowing the current..
Well, actually there is! but you'll have to spend a few bucks if you want to know NMPG in real time, with no mental gyrations.

These screens are not at the same throttle settings or the same sea state. The fuel burn screen is accurate at any setting and the NMPG is instantaneous since the system is coupled to my GPS. The algorithm is quite simple when fuel burn & distance info are available (and accurate) in real time.

These 3 screens are but a sample of the information that is available with the Maretron FFM100 Fuel Management System. I realize that the newer boats with all electronic engines have all this info but my engines are mechanical and I wanted to know their capabilities with respect to fuel usage.
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Old 03-30-2016, 05:54 PM   #74
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My old Floscan is connected to a GPS. It shows both gph and mpg over the ground.
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Old 03-30-2016, 07:06 PM   #75
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Havent we already been over how inaccurate this stuff is ?
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Old 03-30-2016, 07:18 PM   #76
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Havent we already been over how inaccurate this stuff is ?
I think someone claimed that, but properly set up and used, I think they're pretty accurate. The inaccuracy accusations must have come from the anti-technology or anti-electronic engine groups.
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Old 03-30-2016, 07:20 PM   #77
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Baker,
Get a grip ... the current is usually fly crap even w my 6 knot boat.
Sure is easy to tell when you've got one foot in the cockpit.
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Old 03-30-2016, 07:43 PM   #78
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Greetings,
Mr. mb. "...the current is usually fly crap..." Hah! Tell THAT to the folks that transit Hell's gate in NYC or the St. Lawrence River above Quebec City.
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Old 03-30-2016, 07:56 PM   #79
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Greetings,
Mr. mb. "...the current is usually fly crap..." Hah! Tell THAT to the folks that transit Hell's gate in NYC or the St. Lawrence River above Quebec City.
Wifey B: And wouldn't fly crap mean it's really bad? really sh..ty? Just asking. I don't like fly crap and definitely not ignoring it.

Now as to current, what mb was trying to say but using a defective analogy, he did say "usually" and most of the time we're not doing the two areas you mentioned. Have done Hell's Gate but not made it to or above Quebec City yet.

Let's toss out somewhere though that is more common for east coast cruisers, the Gulf Stream. It's average speed is 3.5 knots and that's a lot to slow boats, even to fast ones. I love the speed we can get in it going north. Plan on using it in about ten days.
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Old 03-30-2016, 08:00 PM   #80
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I think someone claimed that, but properly set up and used, I think they're pretty accurate. The inaccuracy accusations must have come from the anti-technology or anti-electronic engine groups.
Two different things: These are flow meters added to mechanical engines, they have the subtraction error issue at low flow.

Nothing like electronic common rail or electronic unit injector engines, where the built in algorithms have proven quite accurate. No subtraction error there.
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