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.

 

[ron6785]

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

Sea Whisperer, what speed were you maintaining at those rpms and what were the sea conditions?

 

[ron6785]

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..."

Mike, thanks for the info can you tell what typical speed you maintain. One of the problems of course that unless your off shore its difficult to get any kind of accurate reading unless you have flow meters. I don't even try if I'm in the ditch.

 

[MikeP]

We cruise normally at around 9.5K and she seems to do around 1 NMPG based on fuel filling. I do not have flow meters. At 10K she does around .8 NMPG - again, just based on refueling and getting the fuel gauges to the same readings they were at the last refueling. These are 8V71TI engines...

 

[REBrueckner]

MikeP's figures look very reasonable...in a 45 to 55 foot twin screw heavy planing hull, like Hatts, it's REALLY hard to get 1 NMPG over 8 or 9 knots. That's why real long range boats, like Nordhaven typically have a single engine and cruise around 6 knots, maybe 7 knots or so...for the reasons stated in earlier posts: speed takes a lot of power and that means a lot more fuel consumption.

I've tried slow cruising and besides the fact my engines don't seem to like such slow running as much as 10 or 11 knots, I get really bored: 10 knots gets you where you are going in about 6/10 or 60% of the time at 6 knots...on a half hour run, nobody cares much but if you are running say 100 miles, 16.67 hrs (at 6 knots) vs 10 hs (at 10 knots) is really tedious!!! (Besides I need to walk my dog.)

 

[SeaWhisper]

Sea Whisperer, what speed were you maintaining at those rpms and what were the sea conditions?

I averaged around 8.9 to 9.0 kts. First Couple of days heading south on the Chesapeake, seas were 3 to 4'. Once we reached Norfolk VA. we continued down the ICW.

Latested up date... I topped of the boat last night. Most of our running is ICW and out to Cape Lookout. Most of the running is 1200 rpm's with the normal run up for temp on the last mile home. Average this time was round 1.28 mpg... with about 20 hours of gen time. Hope that helps. Like most... everyone else, I plan 1 for 1 on fuel burn.

Mike,

SeaWhisper

 

[TedZ]

I love it when we are bored.

Exponential indeed means some power to e, a natural logarithm with a value of about 2.7. It is not true to say something that is exponential - most people simply mean it is not linear - grows faster or slower than cubed or squared or anything else. Remember there are fractional exponents, i.e. x to the 1.2 for example. So something that is exponential, without further amplification, could be any rate of growth under the sun including a cubed function.

Thought your collective lives would not be complete without this info! :--)

Ted

 

[MikeP]

Thanks Ted, your info has improved my life exponentially!

 

[stormchaser]

We cruise normally at around 9.5K and she seems to do around 1 NMPG based on fuel filling. I do not have flow meters. At 10K she does around .8 NMPG - again, just based on refueling and getting the fuel gauges to the same readings they were at the last refueling. These are 8V71TI engines...

What are your RPM readings for each of those speeds?

 

[MikeP]

Here's my rpm/speed chart (clean bottom/shafts and fairly smooth water)


RPM Knots

IDLE 5
750 6
1000 7
1250 9
1500 10
1750 11
2000 14
2150 16
2300 18

The above readings are taken from the lower helm tachs/GPS HOWEVER, for references purposes, below I have listed the tach readings at several RPM settings at upper and lower helm and as measured at the same time with a photo tach at the engine. These readings were done at the slip under no load conditions. This shows the error of the tachs as opposed to the actual engine RPM. The tachs aren't all that much out of agreement with each other/actual until the RPM gets near WOT - I should have done another check at around 2200 and at the moment can't think why I didn't.


Port Low Star Low Port FB Star FB PHOTO Port PHOTO Star


1000 1050 1050 1050 1045 1042
1500 1500 1500 1500 1549 1533
1725 1750 1750 1750 1784 1784
2375 2380 2400 2350 2489 2511


HMMM...the data didn't align in the actual posting. Don't know why, it was aligned before I hit post. Well, you should be able to figure it out...Port low is the port lower helm, Port FB is the port flybridge. PHOTO is the photo tach reading

 

[ron6785]

MikeP's figures look very reasonable...in a 45 to 55 foot twin screw heavy planing hull, like Hatts, it's REALLY hard to get 1 NMPG over 8 or 9 knots. That's why real long range boats, like Nordhaven typically have a single engine and cruise around 6 knots, maybe 7 knots or so...for the reasons stated in earlier posts: speed takes a lot of power and that means a lot more fuel consumption.

I've tried slow cruising and besides the fact my engines don't seem to like such slow running as much as 10 or 11 knots, I get really bored: 10 knots gets you where you are going in about 6/10 or 60% of the time at 6 knots...on a half hour run, nobody cares much but if you are running say 100 miles, 16.67 hrs (at 6 knots) vs 10 hs (at 10 knots) is really tedious!!! (Besides I need to walk my dog.)

REBrueckner, thanks for the info I tend to cruise at about 9knts and occasionally faster although with naturals weight loaded like I am to try to faster means I'm just pushing water up front and fuel out the back. I'm not sure a 53 with naturals would plane unless she was stripped and Jupiter wer aligned with mars.