Does anyone have fuel consumption rates at various rpms for 8V71 naturals?
They are in a 53MY classic.
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Thread: fuel consumption 8V71 naturals
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02-01-2009 04:58 PM #1Senior Member
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fuel consumption 8V71 naturals
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02-01-2009 07:29 PM #2Senior Member
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Re: fuel consumption 8V71 naturals
Ron, I haven't plotted a full fuel curve yet (and may never have the patience to do so). However, if I hold the speed down to hull speed of about 9.6 knots, I burn just under 10 gal/hr. For my 53MY, that's just about 1450 RPMs (give or take a few depending on hull cleanliness). If I run hard (2000+ RPMs) I figure I'm burning 2 1/2 times that much. Needless to say, I usually take the slow lane and enjoy the ride/view!
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02-01-2009 11:32 PM #3Senior Member
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Re: fuel consumption 8V71 naturals
serene warrior, thanks for the reply is the 10gph @ 9.5 knots for both engines? I would expect it to be and that seems consistant with my experience on a recent trip.
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Re: fuel consumption 8V71 naturals
hull speed on the 53 is a little under 9.6 actually, closer to 9 kts.
fuel burn goes up exponentially above hull speed and if you push even just 1/2 knot faster you will burn a lot more fuel, as much as 2 or 3 gph.
and if you're running in shallow water, under 8 or 9', expect to loose as much a full knot, which will obviously affect your fuel burn...Pascal
Miami, FL
1970 53 MY #325 Cummins 6CTAs
2014 26' gaff rigged sloop
2007 Sandbarhopper 13
12' Westphal Cat boat
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02-02-2009 10:29 AM #5Senior Member
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Re: fuel consumption 8V71 naturals
Fuel consumption is not "exponential" but cubed:
Propeller power varies by the cube of the rpm. Consequently, twice the rpm requires 8 times the power (and fuel)....But twice the RPM is not quite twice the speed..cavitation losses,etc....
Or, equivalently, running at 10% more RPM means about (1.1)(1.1)(1.1) more fuel consumption or about 33% more fuel....
Another rule of thumb: 18 diesel HP burns about 1GPH of fuel for a modern diesel, more like 16 HP for older 2 cycle Detroits . (I think those are the correct numbers...?? hmmmm, that seems about right since I believe my 12KW ONAN has about a 20HP diesel engine and uses about 1GPH..)
If you can find the DD diesel Specifications charts for your engine , "fuel- propeller load" gives the approximate fuel consumption as long as your turn up rated 2300 RPM at WOT.
I have found these to be a bit optimistic...actual fuel consumption might be 10% more...
I believe somebody here has posted a source....Last edited by REBrueckner; 02-02-2009 at 10:43 AM.
Rob Brueckner
former 1972 48ft YF, 'Lazy Days'
Boating isn't a matter of life and death: it's more important than that.
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Re: fuel consumption 8V71 naturals
Ron,
Back in April 2008, purchased a 1969 53' MY with 8-71N. We traveled from Maryland to North Carolina... I ran the boat at 1150 - 1200 RPM, full fuel and water. Props are Mich. 28 X 28 and averaged out at 1.5 Mile per Gal a fuel!! This is the base line I have. (Full tanks when leaving Chestertown MD, topped off in CoinJock NC, and took on only 163 gals.
Mike,
SeaWhisperer
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02-02-2009 10:46 AM #8Senior Member
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Re: fuel consumption 8V71 naturals
exponential means e^x (base e to the power "x") which turns out be a much faster increase than the cubed relationship I posted....
Rob Brueckner
former 1972 48ft YF, 'Lazy Days'
Boating isn't a matter of life and death: it's more important than that.
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Re: fuel consumption 8V71 naturals
Wow..."cubed," "exponential," "base e to the power x!"
I have trouble dealing with the math involved in figuring out the MPG involved in, "we sailed 500 miles and we put in 500 gallons of fuel..."
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02-02-2009 12:32 PM #10Senior Member
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