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Old 30-12-2019, 08:51   #1
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Fuel Consumption Q - please help my friend

I am the shore contact for my friends on an Atlantic crossing. Due to low winds, they are having to motor their Sailboat (with Yanmar engine). They are worried about running out of fuel and sent me the following question. I have been unable to find answer. Can you help?

Here it is:

"Does a diesel engine running at 2000rpm use less fuel when not under load (e.g. the propeller not engaged) than it does when it is? We have yanmar engine but i would imagine its a general question. I would guess it uses a bit less fuel, which would be good as half our engine hours were for charging and not under load. Just trying to get an idea as we don't have fuel guage."
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Old 30-12-2019, 08:53   #2
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Re: Fuel Consumption Q - please help my friend

I canít directly help except to say that if the engine is primarily being used for recharging and they have no fuel gauge then they should examine their power requirements to save fuel for when they need it to maneuver.
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Old 30-12-2019, 08:57   #3
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Re: Fuel Consumption Q - please help my friend

Yes it will use quite a bit less fuel, how much less depends on how much the load normally is.
If they are running it for battery charging, I would run it at probably 1000 RPM though and not 2000, but the fuel consumption won’t be much different.

On edit, I would guess that running just to charge fuel consumption would be half what it is when underway.
But that is a wild guess

The fuel burn in neutral will be just enough to overcome the engines internal friction at the set RPM plus any accessories that need to be driven, like water pumps and alternator of course.

If they are needing to run it to make way, one assumes slack water. I’d run it at no more than 1500 or so if I was trying to maximize range and time wasn’t as important as range
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Old 30-12-2019, 09:00   #4
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Re: Fuel Consumption Q - please help my friend

Usually a fuel tank has two extra fittings for a generator to be hooked up. If so one can be removed and any kind of stick can be used to determine fuel level.
It’s actually usually more accurate than a gauge
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Old 30-12-2019, 09:32   #5
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Re: Fuel Consumption Q - please help my friend

Thanks everyone - passed along the messages and they now have a solid plan!
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Old 30-12-2019, 10:26   #6
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Re: Fuel Consumption Q - please help my friend

The Yanmar consumes diesel based on how much power it generates, the formula is approximately 200 grams/hour/hp. Smaller engines consume 220g, larger engines a little bit less than 200g. If you are using the engine to charge only, you would typically use as much fuel as a generator (may be a little bit more because alternators are approximately 60% efficient vs. proper high voltage generators at 80-90% efficiency. I would say you use 2-3 hp for just turning the engine and gearbox and 1-2 hp for the alternator or about for 4 hp. The fuel consumption is this case is 800 g per hour or about a liter or 1/4 gallon per hour. Under load, the consumption will be based on how much hp is needed to drive the boat given size, speed, propeller, etc. and can change from up to 16 hp for small Yanmars to 110 hp for large Yanmars, respectively many gallons per hour.

Bottom line is, assume 1/4 gallon per hour for charging and 3/4 gallon under way at speeds up to 1.1x waterline. At higher speeds the consumption increases rapidly.
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Old 30-12-2019, 13:08   #7
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Re: Fuel Consumption Q - please help my friend

Hi, SLL, here's another thought. If they have a jerry jug aboard, then they could pump out 20 l. into it. That's their emergency reserve. They can use the engine oil removal pump to suck it out.

If they do not know their normal usage of fuel per hour, it is going to have to be guess work, and they will need some fuel at the time of landfall.

We used to never run the engine till the boat speed got below about 2 knots. And there is a light air game, which we have played at sea, which is to see if we can get her moving at all. Then steer at ~60 deg to the wind, and she'll build up to whatever she can do, maybe as little as .3 knot. Then keep her moving. The direction doesn't really matter because you're going so slowly anyway, and the wind always comes back. In a day, or two, or three. Take down or furl the sails to keep them from slatting. We've gone down to the 3rd reef, just for the roll damping effect, big long lazy swells do roll the boat. It is a time to chum, and see who comes. Remember the wind ALWAYS comes back, eventually.

What A64 wrote seems correct to me. Our engine uses less when just charging batteries, and uses a great deal more at 2400 rpm than at 2000 rpm.

My advice to your friends is to not worry about going slowly. Run the engine in gear at about 2/3 total revs, for the charging periods, if they want to change which spot of windless ocean they're in, and only use the fuel for charging, especially if they have no way to create the reserve.

It is kind of contrary to the ideals of modern life, but conserving the fuel is really the safest thing for them to do. Accept the no-wind situation. Play with it. They can stay frustrated with it if they want; but they can also change how they think about it. Read my signature line. Soon enough they will have plenty wind.

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Old 30-12-2019, 14:37   #8
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Re: Fuel Consumption Q - please help my friend

Answer is yes.

If they want best fuel economy under power, don't exceed 4.5 kts.
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Old 30-12-2019, 15:45   #9
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Re: Fuel Consumption Q - please help my friend

I bet it wouldn't be as much less in neutral compared to in gear as some believe. I know from years of record keeping that my now dead diesel generator used about 1/4 gph no matter what the load was.

But i don't really understand the original question in regard to light air conditions.
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Old 30-12-2019, 16:16   #10
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Re: Fuel Consumption Q - please help my friend

For a comparison an idling OTR trucks on average burns .8 GPH at idle.
When on the highway they average 6 MPG or about 11 to 12 GPH
Now a sailboat charging batteries won’t be that big a difference, but as long as your not running the engine at cruise RPM to charge batteries, there will be a significant difference, there will be a difference even at cruise RPM, but not as big a difference.
The extra power extracted from the engine to drive the boat is a lot of power and of course that is fuel burn.

Not sure why your generator didn’t show much difference, my Nexgen says it’s fuel burn is one quart an hour lightly loaded and a half gallon loaded up. I’ve not measured it though, that’s just what the manual says.
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Old 31-12-2019, 09:33   #11
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Re: Fuel Consumption Q - please help my friend

Low revs = low fuel consumption. You can charge the batteries at 1000-1200rpm.

If you are using a voltmeter for SOC, the batteries are always going to appear drained, since you will always be running your autohelm, electronics, etc. If you are really concerned...turn everything off, save power, let the batteries settle for a couple hours, then check the voltage.

Part way through a crossing you should be able to turn off your chartplotter/electronics/AIS. A handheld GPS will give you all the nav data you need. If you have AIS, run it only at night.

And it goes without saying...shut of any refrigeration. There will be cold drinks when you get there.

SAVE YOUR FUEL.

By now your crew likely is out of fresh food. Time for the beans+rice and baked potatoes. Most crew lose weight (as much as 20 pounds) on a crossing. Urge them to eat...it will help their emotional well being and their mental abilities as well. Best of luck to your crew.
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Old 31-12-2019, 09:41   #12
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Re: Fuel Consumption Q - please help my friend

Quote:
Originally Posted by JPA Cate View Post
Hi, SLL, here's another thought. If they have a jerry jug aboard, then they could pump out 20 l. into it. That's their emergency reserve. They can use the engine oil removal pump to suck it out.
If they have a jerry jug aboard, hopefully it was already full of diesel when they started the trip!
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Old 31-12-2019, 10:42   #13
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Re: Fuel Consumption Q - please help my friend

Quote:
Originally Posted by a64pilot View Post
Usually a fuel tank has two extra fittings for a generator to be hooked up. If so one can be removed and any kind of stick can be used to determine fuel level.
Itís actually usually more accurate than a gauge
Old school and assuming room for a straight shot into the tank. Helps to mark the stick when filling the tank since many are irregular in shape.
I wouldn't trust a fuel gauge any farther than I could throw it.
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Old 31-12-2019, 11:44   #14
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Re: Fuel Consumption Q - please help my friend

FWIW, our 6.5kva genset (Kubota 3-cyl eng)burns 0.25gph under load(10-18 amps). Our 55hp Yanmar (4JH-TE) burns considerably more fuel the higher the RPMs get, but as best as I've been able to gage it, 2000rpm=5.9kts@1.71lph (baseline). 2250rpm=6.5kts@2.38lph(+39%). 2500rpm=6.9kts@3.22lph(+89%). 2750rpm=7.3kts@4.26lph(+250%. 3000rpm=7.7kts@6.86lph(+401%) Even if the numbers are not exact, you can see the trend!
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Old 31-12-2019, 16:49   #15
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Re: Fuel Consumption Q - please help my friend

Before doing anything else find a piece of scrap lumber and make a dip stick. at worst you can mark calibrations from bottom to full level by assuming inches parallel consumption. you results will be more accurate than any computations based on engine hours.
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