I've recently bought a 49 yachtfish with twin deisel 871 325hp naturals. I'm bringing the boat up the east coast and am trying to extimate gas consumptioin. I've had 3 different numbers from 3 different people. PS engines in very good shape and boat is a 74.
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MPG - Anyone have experience on 871's
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Re: MPG - Anyone have experience on 871's
it all depends on what speed you want to cruise at !
hull speed (around 9kts) should result in about 9 to 10 GPH. or just under 1 MPG. slow down a little more, 8kts... and you should get a good 1 mpg. anything above hull speed and you're going to use a lot of fuel pushing a big wave.
i dont' know if the 49YF will get on plane with naturals. if it does, that will probably drop your mileage to .5 mpg...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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Re: MPG - Anyone have experience on 871's
At 1400rpm with 8v71n's our 53'MY burns about 10-gal/hr combined. The numbers have been checked over a 700-mile run down the Pacific coast from Anacortes, WA to San Francisco. We averaged about 9 - 10 knts depending on sea conditions. Most of the trip the wind wave hts were about 3 -4', on top of 3 - 5' swells.
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Re: MPG - Anyone have experience on 871's
Stay off plane and you'll do a little better than 1 MPG. If it planes, it'll be less than 0.5 MPG. If you run at 7-8 knots, you can get almost 2 MPG, but it's not good to run the engines that slow all the time without running them up to operating temps for 30 minutes per day.
Doug Shuman
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12-18-2007 06:14 PM #5
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Re: MPG - Anyone have experience on 871's
your gas milleage should be infinite.....
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Re: MPG - Anyone have experience on 871's
Sky Cheney
1985 53EDMY, Hull #CN759, "Rebecca"
ELYC on White Lake--Montague, MI
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12-18-2007 09:41 PM #7
Re: MPG - Anyone have experience on 871's
I've attached a set of power/fuel curves for 8V71Ns. Hope this helps.
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Re: MPG - Anyone have experience on 871's
I've seen those curves for various DD engines and I am embarrased to admit that although I spent years building performance gas engines and personally installed/tested them on dynos and printed out the data, I don't have a clue what that lower DD curve is telling me. I'm assuming that the BHP and SHP are simply what they sound like - the difference between the power available at the flywheel and the power available at the propshaft. But what does the lower curve tell me and how is it useful?
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12-19-2007 07:55 AM #9
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Re: MPG - Anyone have experience on 871's
Use the lowest curve of the four to determine fuel consumption versus RPM via the right hand fuel consumption data. It's the "fuel consumption- propeller load" we all care about.
The upper pair of curves are HP versus RPM, using the left hand HP data.
My DD chart says: (regarding the lowest of the four curves) "PROPELLER LOAD- Indicates horsepower absorbed by a typical propeller and the corresponding fuel consumption throughout the speed range."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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12-19-2007 08:00 AM #10
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MPG - Anyone have experience on 871's
There was a thread last winter with MANY posters about this subject. Most owners posted that in the 45 to 58 foot range they got about a mile per gallon at around eight knots...turbos,naturals,all seemed pretty close. In another thread a theoretical claim was made that turbos should be about 10% better but even if true this could be masked by many other factors. No one claimed better mileage at higher RPM's and speed. About a half mile per gallon in the 1800 RPM range is typical; run at 2000 RPM and it's lower still.
Rob Brueckner
former 1972 48ft YF, 'Lazy Days'
Boating isn't a matter of life and death: it's more important than that.