fuel consumption 8V71 naturals

[ron6785]

Does anyone have fuel consumption rates at various rpms for 8V71 naturals?
They are in a 53MY classic.

[SereneWarrior]

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!

[ron6785]

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.

[Pascal]

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...

[REBrueckner]

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....

[Avenger]

Fuel consumption is not "exponential" but cubed:

Huh?

[SeaWhisper]

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

[REBrueckner]

exponential means e^x (base e to the power "x") which turns out be a much faster increase than the cubed relationship I posted....

[MikeP]

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..."

[ron6785]

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, thanks for the insight. We are liveaboards so the boat is pretty well maxed out weight wise. I seem to approach the oft quoted 1mpg at about 8.8-9 knts. Assuming no serious current, seas,etc.