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Old 11-07-2012, 06:30 PM   #21
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Are you sure you didn't mean to post that on another site?

This is the boating site ...
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Old 11-07-2012, 07:40 PM   #22
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Rick,
AFT end .. not rear end. What have you been think'in lately?
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Old 11-08-2012, 06:24 PM   #23
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I run the same engines on a lighter boat and consistently get around 2 gallons per hour at 12-1500 RPM. That includes some generator time so I think my numbers are pretty much the same as Daddyo's
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Old 11-09-2012, 08:43 AM   #24
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Unless you have some sort of fuel management system, I don't see how one can possibly determine how much fuel was used on a short trip. Fuel gauges are notoriously inaccurate so unless one started with absolutely full tanks and filled them after ther trip or measured with a stick or sight gauges, the fuel use can only be estimated.

Current and wind play a big part in the amount of fuel used to get from point "A" to point "B". RPM plays a big part in GPH.

Fuel use on a single trip is only important if you run out of fuel before the end of the trip. Meaningful fuel use figures can only be obtained over a long period unless there is a fuel management system in use.

Here are my figures: Note that I used a lot of fuel the first couple trips before I learned to slow down and enjoy the ride. This still skews my average fuel burn figures which you can see are about 1.9 GPH. This is typically at 2K RPM which translates to 7 knots over water.
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File Type: pdf High Cottton Fuel Use.pdf (19.2 KB, 96 views)
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Old 11-09-2012, 08:51 AM   #25
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.... or measured with a stick or sight gauges, the fuel use can only be estimated.
I have inch marks on my sight tubes, and after every trip I take a measurement. I have a spreadsheet that models my tank dimensions and has been accurate to within a couple of gallons every time I add fuel. Even with the inaccuracies in the readings, the actual re-fuels are added back in as correction factors and the overall accuracy improves. My numbers are very consistent so far.
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Old 11-09-2012, 08:56 AM   #26
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I have inch marks on my sight tubes, and after every trip I take a measurement. I have a spreadsheet that models my tank dimensions and has been accurate to within a couple of gallons every time I add fuel. Even with the inaccuracies in the readings, the actual re-fuels are added back in as correction factors and the overall accuracy improves. My numbers are very consistent so far.
I would love to have sight tubes and have considered adding them, but haven't gotten around to it. My tanks are rectangular so the calculations would be pretty simple.
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Old 11-09-2012, 12:46 PM   #27
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yep...any way to accurately measure fuel such as sight tubes, measuring stick, sounding tape or even just fill it to the same point in the filler neck and see what it takes next time you fuel....easy peezy.

sure there are a few assumptions about possible errors even then.... but after getting to know your boat a lot of the assumptions are quickly eliminated.
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Old 11-09-2012, 02:03 PM   #28
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yep...any way to accurately measure fuel such as sight tubes
Each inch in mine equates to about 8 gallons (actually 8.023 gallons). I have a graduated rule behind each sight gauge so I can get with better than a half inch on each side. So my margin of error might be 4-5 gallons overall (assuming two 200 gallon tanks).
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Old 11-09-2012, 04:08 PM   #29
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Each inch in mine equates to about 8 gallons (actually 8.023 gallons). I have a graduated rule behind each sight gauge so I can get with better than a half inch on each side. So my margin of error might be 4-5 gallons overall (assuming two 200 gallon tanks).
Why not remark them to represent gallons, not inches?
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Old 11-09-2012, 04:13 PM   #30
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I don't think this has been mentioned but measuring fuel consumption w a dip stick or sight tubes is best done by starting w empty tanks and filling 5 or 10 gallons at a time marking or measuring the fuel on the stick/tube. This method takes care of any shape of fuel tank and single gallons can be interpolated between major marks such as 5/8ths" per gallon or whatever. Depending on the sight tube or dip stick location laterally on the tank even variable list could be accounted for but usually tubes and holes will be inboard so the boat should be listing or not listing in the same way when fuel measurements are made. But any such measuring system taking this much effort should only apply to boats going far offshore or doing predicted log races. If normal people need to compute accurately their fuel consumption they may need to find a boat that burns enough less fuel so they wouldn't need to think about such things.
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Old 11-09-2012, 05:05 PM   #31
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Why not remark them to represent gallons, not inches?
They're already marked in inches.

I had to do the calculations initially without draining the tanks. Knowing the shape and dimensions I could know what an inch drop in fuel level meant.
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Old 11-09-2012, 05:21 PM   #32
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SS, would you care to share the spreadsheet so we can have a template to work from?
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Old 11-09-2012, 07:28 PM   #33
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.............. measuring fuel consumption w a dip stick or sight tubes is best done by starting w empty tanks and filling 5 or 10 gallons at a time marking or measuring the fuel on the stick/tube. ....................
If we're going to get down to the nitty and the gritty, we should also realize that on many boats, the last few gallons of capacity cannot be used because the fuel pickup not at the very bottom of the tank. So, you might put 100 gallons in n empty tank but run out of fuel after using just (for example) 95 gallons.
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Old 11-09-2012, 11:49 PM   #34
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Rwidman,
Well you can't see the unusable amount in the sight tubes and you can't determine where it is without running out of fuel. But all this measuring very carefully and computing the last drop of fuel won't gain anything for 97% of us so perhaps out efforts would be better served talking about smaller boats that burn less fuel. Full disp will work but there's hardly enough to go around. But measuring fuel in the tank to determine burn rate won't require using the very bottom of the tank anyway and anyone running that close to the bottom of the tank shouldn't be in that situation. But running on the last 2" of fuel in the tanks thinking about the height of the sump tubes could be a sobering thought.
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Old 11-09-2012, 11:56 PM   #35
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My gauges show empty (0 inches) even when there is some fuel remaining and available.
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Old 11-10-2012, 12:15 AM   #36
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Our Cheoy Lee 46 has twin FL120s, and probably weighs in around 60K lbs (the original CL manual states a dry displacement of 50K lbs). After spending the summer cruising and working and measuring the tank before and after fills and usage, we averaged 2.5 gph at an average of 1400 rpms and about 7 kts.

One our last crossing of the Strait of Juan de Fuca we were trying to get across a little quicker due to a small craft advisory. So I kicked it up to 1650 rpms for 5 of the 8 hours, and that increased the gph for that trip up to 3.9.

Still very happy with our boats fuel economy. Now to paint the bottom and get a little better mileage next summer...
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Old 11-10-2012, 12:30 AM   #37
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Why not remark them to represent gallons, not inches?
The previous owner of our boat who had the entire fuel system replaced-- tanks, hoses, valves--- had the sight tube on each of the four new saddle tanks calibrated in gallons. Very handy to keep track of fuel usage during a trip.

A nice feature of our tanks is that they gravity feed from the bottom so we can, in fact, use every drop that's in each tank. The tank we don't want to run dry is the center day tank as that's the tank the engines and generator normally feed from. But it has an electric sender with a fuel gauge on the helm console so we know how much fuel is in that tanks at any given time. Even though that tank, too, feeds from the very lowest point in the tank we don't let it get much less than a quarter full before transferring new fuel in from the saddle tanks.
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Old 11-10-2012, 01:46 AM   #38
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For me, fuel burn is only a consideration regarding range. I'm not interested in bragging about my mpg, just want to know if i can get there and back. Measuring fuel 'depth' in inches, liters, meters or gallons is fine I guess, but I sprung for a Floscan that measures gallons used, and I don't have to go poking about in the bilge to see what is left...it's accurate within tenths of a gallon for fuel sent to the the engine.
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Old 11-10-2012, 02:19 AM   #39
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Haven't fine-tuned my Floscans yet. They read about 30-to-40 percent high. I'm lazy.

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Old 11-10-2012, 08:04 AM   #40
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For me, fuel burn is only a consideration regarding range. I'm not interested in bragging about my mpg, just want to know if i can get there and back. Measuring fuel 'depth' in inches, liters, meters or gallons is fine I guess, but I sprung for a Floscan that measures gallons used, and I don't have to go poking about in the bilge to see what is left...it's accurate within tenths of a gallon for fuel sent to the the engine.
Range is my consideration as well although fine tuning RPM for economy could be nice.

Because I have only electrical fuel gauges to measure level, I am not comfortable trying to extend my range and I may take on fuel when I don't really need to. I think the most I've ever used was 2/3 of my capacity before filling up.

I looked into a fuel use monitoring system, but for my diesel. it would cost in the neighborhood of $1K and I figured it wasn't worth the investment for me.
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