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Old 09-06-2014, 14:07   #16
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Re: Fuel Consumption 4-108

A64,

Just trying to understand, roughly speaking, you are saying that a 100hp engine producing 50 hp will burn about twice as much fuel as a 50 hp engine running at rated rpm?

Do I have that right?
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Old 09-06-2014, 14:55   #17
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Re: Fuel Consumption 4-108

No, what I am saying is that a Perkins 4-108 making say 30 HP will burn X amount of fuel regardless of what is done with that HP, or maybe said another way that if 30 HP pushes one boat at 6 knots and another at 3 kts, their fuel burn will be the same, so hull shape, speed etc. are irrelevant when it comes to fuel burn.
A properly propped engine will have the same fuel burn at the same RPM regardless of what boat it's in.
We have to use RPM as we don't have a torque meter, if we did then we could calculate actual HP
Since we have to use RPM, you have to assume properly propped, and that is your variable, not the boat
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Old 09-06-2014, 15:00   #18
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Re: Fuel Consumption 4-108

Ok, thanks.
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Old 09-06-2014, 15:06   #19
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Re: Fuel Consumption 4-108

depends on condition of injector nozzles also.
for a 22.000lbs boat, fin keel, 40 feet, I was doing 1gal/hour at 1600 rpm 4.5knots, with a 4108 of 1976. I changed the nozzles, all the engine gaskets, seals and bearings, clean the oil system, water system, changed all parts not in good shape, rebuilt the transmission. Seems Im doing almost half now at same rp but need to test more as I just overhauled the machine.
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Old 09-06-2014, 16:23   #20
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Re: Fuel Consumption 4-108

25,000 lbs boat, 2,600 rpm, 7kts smooth water just under 1 gph. runs nice at that rpm.
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Old 09-06-2014, 16:52   #21
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Re: Fuel Consumption 4-108

If you were interested this was beat up quite a bit a while ago here.

Gallons per Hour . . . [Sitemap] - Cruisers & Sailing Forums

I'll stand by my 1/2 gallon/10hp/hour rule of thumb.
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Old 11-06-2014, 14:49   #22
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Re: Fuel Consumption 4-108

From Perkins
Normal Operating Fuel Consumption
4,000 rpm 1.59us gal 6.0 litres per hour
3,600 rpm 1.22 gal 4.6 litres ph
3,000 rpm 0.95 gal 3.6 litres ph
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Old 11-06-2014, 19:14   #23
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Re: Fuel Consumption 4-108

Thanks for that start....from there...


From Perkins
Normal Operating Fuel Consumption
4,000 rpm 1.59us gal 6.0 litres per hour
3,600 rpm 1.22 gal 4.6 litres ph
3,000 rpm 0.95 gal 3.6 litres ph

4,000rpm is 47 net shaft hp or propeller law shaft hp
1.59/47 is .034 gal/hp/hr

3,600 rpm is 45 net shaft hp or 35 propeller law shaft hp
1.22/45 is .027 gal/hp/hr Or. 1.22/35 is .035 gal/hp/hr

3,000 rpm is 40 net shaft hp or 20 propeller law hp
0.95/40 is .024 gal/hp/hr or 1.22/20 is 0.061 gal/hp/hr

http://www.cruisersforum.com/forums/...4&d=1293458663

Here it is graphically, .05/hp/hr sounds pretty good rule of thumb.

http://www.universal-diesel.no/files...0%20Range4.pdf

You may also like this
http://svrainshadow.com/?p=719
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Old 11-06-2014, 19:23   #24
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Re: Fuel Consumption 4-108

i can't believe people actually run their 4-108's at those rpm's. i normally run at 1700; i once ran it at 2200 for about a minute to make a bridge, but i kept saying to myself 'this is stupid this is stupid this is stupid'....
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Old 12-06-2014, 04:51   #25
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Re: Fuel Consumption 4-108

I originally ask this fuel consumption question so I could estimate fuel needs to take Vigah from KY to gulf next year via the Tombigbee. She has been out of the water for years and I neglected to run fuel consumption curves back when. If memory serves me our top end is 2800rpm and I normally cruised at 1800 - 2200 giving 6 knts or so. Heck my tach stops at 4000 rpm. From my USPS training years ago, we were told to normally cruise about 2/3 full throttle or about where I did. The increased rpm did little for speed as I approached hull speed, just increased fuel consumption.

From what I gather .7-.9 gph seems about correct.

I was going to ship her straight to the gulf until I got an estimate of $7000! I just paid $900 to move her 100 ft to firmer ground and the Ohio river is only 15 miles away, so ...
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Old 12-06-2014, 11:13   #26
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Re: Fuel Consumption 4-108

Regarding running at higher RPM's.
I do not think that there is a problem running a 4108 at higher RPM's.
I have a 25,000lb boat 4108 perkins 2.1 walters VDrive.
17" 3 blade Max VP prop set at 10 degs, 5.5" of pitch.
WOT in gear 3100 rpm.
2,500 rpm is 6.5 knts 175 deg temp
2,600 rpm is 7 knts 180 deg
2,800 rpm is 8 knts 195 deg

The engine sounds fine at these higher RPM's but engine temp get high.
I cruise at 2,600 RPM but am thinking about changing to a larger Heat Exchanger. At lower RPM we do not get the HP required or the HP the 4108 is capable of delivering.
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Old 12-06-2014, 16:21   #27
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Re: Fuel Consumption 4-108

wainui, i've read around that the heat exchanger supplied with the 4-108 is barely adequate. somewhere i read a suggestion that the 4-154 heat exchanger is larger and an easy fit, but i don't know for sure.

it could be that sendure makes a larger one for the 4-108 and i think in your case it might be worth exploring. as for me, i'm too chicken to go much past 1700....
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Old 12-06-2014, 17:04   #28
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Re: Fuel Consumption 4-108

I ran the one in my 44 footer up to 2600 normally although usually at maybe 2400. I think there were two versions of heat exchangers.. I never experienced overheating, even pushing the boat in high current situations. My boat didn't have any other exchangers on the transmission, or no oil cooler etc.
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