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Old 28-05-2016, 06:09   #1
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High Diesel Fuel Burn

Hi Guys,

I have a vexing problem.

We are experiencing high fuel burn. We usually burn about .4 gallons an hour but yesterday we burned about 1.1 gallons an hour! We ran at our normal cruising speed and used our normal RPM range. We had adverse current of about 1 to 1.5 knots for about 3 hours during an 11 hour day. The rest was going with the current or in slack.

We recently had a new bottom put on and the prop just had propspeed applied. Also, we calibrated our fuel tank by sucking down the fuel to the point the pickup stopped sucking in fuel then calibrated the tank by pouring in 5 gallons at a time - we have a Hart Tank Tender. We really don't make any smoke at all - maybe just a puff of white smoke at startup but she seems to be burning very clean.

Here are the things I have looked at:

1) My initial assumption was we have a crack in the tank - Upon inspection of the bilge there is no evidence of a crack. The bilge under the tank is dry, there is no diesel smell, and our oil bilge pads (which sit in the only place where the diesel could go) has no evidence of diesel smell either.

2) Leaking past rings - we just had our oil inspected by Blackstone before leaving. There is no evidence of combustibles or coolant in the oil.

I'm out of ideas? Maybe fouled injectors? Bad timing of the valves?
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Old 29-05-2016, 17:43   #2
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Re: High Diesel Fuel Burn

If you maintained the same rpm as prior runs and you think you used more than twice as much fuel. That's tough to do. Any engine I've run would be near max rpm, hp, and showing signs of overheating. Unless you raised the rpm to maintain speed while running against current, I think your measuring system is off.
I think an 8 gallon difference could get lost in the tank shape and natural errors in the tank minder.
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Old 29-05-2016, 18:39   #3
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Re: High Diesel Fuel Burn

Go to wide open throttle and see what you max RPM is. My fuel burn went up significantly during a cruise to Florida two years ago. I was heavily loaded for cruising, ran against some current and hadn't noticed the subtle increase in my fuel burn. About 2/3 of the way back after going from an average of .65 to 1.0 off shore, I opened the throttle wide open and couldn't get but 3100 RPM where I previously ran around 3500 WOT. I have always cruised at 2750 RPM and 6.2 knots and I was doing around 6 knots. Also had a lot of soot build up on the stern and just attributed that to a lot of motoring. I didn't really notice the smoke, but it must have been smoking to generate the soot.

I put new injectors on both my Yanmar and my Northern Lights generator in Mobile. That didn't increase the RPM at WOT very much. We pulled the boat and discovered that the MaxProp had some play in it and that was making me over pitched and instead of running 75-80% at 2750, I was way higher, thus more specific fuel consumption. I lowered my cruising RPM to 2500 and my old fuel economy was much closer. I am pulling next month and having my prop reconditioned and a new bottom job.

You said you just did some prop work. I would look there and check your injectors.

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