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Old 09-08-2019, 15:40   #1
txg
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Running Diesel tank empty?

Like most boats we have a rather inaccurate fuel gauge and carry extra fuel in jerry cans. We did several trips >1000nm this year and when it was lighter winds it was kind of hard to plan on our fuel consumption. We know that our fuel consumption averages about 2L/hour, but there are big differences between motorsailing down wind and waves or motoring into a swell without any wind. So we tend to stay on the safe side and stop the engine as soon as we believe we could be down to an emergency reserve.



Our Vetus diesel has a factory-installed electric fuel pump, so bleeding the engine is done by simply switching it on and waiting for a minute while listening to the alarm sound. No hand pumps or bleed screws. The engine is a Mitsubishi S4L, so a standard old-school diesel without fancy common rail or turbocharger stuff.



For the future, i would like to know if there is anything wrong about just using up all the fuel in the tank till the engine stalls and then refill with one last leftover jerry can (20L ~ 50nm range for emergencies and docking).



When i tried to google this problem, only car-related answers came up. There it said that the injectors can be damaged and fuel filters clogged when the tank is run empty completely because of debris in the tank. I know that our tank is absolutely clean as it gets a regular inspection and we used a lot of fuel this year so far compared to the size of the tank. In the last 6 months, the engine ran for 330 hours which equals to about 5.5 full tanks used.



Is there anything i am missing regarding engine damage? The process of refilling and bleeding the engine would take about a minute on our boat and we are talking about being far offshore on the open ocean, so there are no traffic problems or anything like that.
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Old 09-08-2019, 16:01   #2
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Re: Running Diesel tank empty?

Ya donít do that. The diesel is the lubricant for the fuel system including your injection pump and running it dry is running it dry.
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Old 09-08-2019, 16:13   #3
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Re: Running Diesel tank empty?

It's good way to keep the tank clean. For what it's worth, it's not uncommon to run a fuel tank dry in a piston plane. However, you can switch tanks when the engine or the psi needle starts to flicker. Plus the engine starts right up again. I avoid running out of fuel in the boat for many reasons
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Old 09-08-2019, 16:19   #4
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Re: Running Diesel tank empty?

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Originally Posted by rbk View Post
Ya donít do that. The diesel is the lubricant for the fuel system including your injection pump and running it dry is running it dry.
When the fuel runs out, most diesels tend to stop running.
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Old 09-08-2019, 17:12   #5
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Re: Running Diesel tank empty?

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Originally Posted by S/V Illusion View Post
When the fuel runs out, most diesels tend to stop running.
Yes as they start sucking air trying to push it through injectors and the flywheel keeps the mechanical pump moving as it all slows to a stop as all fuel is spent and more or less run dry, literally. Not saying its going to cook the pump first time, but its not good for the longevity, good way to cause undue wear and tear. Would you drain the oil from the sump with the engine running and when its done turn the engine off?
Honestly I'm not really sure why this is even a question? Why you wouldn't just tip a jerry can in when you think you're getting low vs purposefully running it dry, you don't even have to stop, just do it underway.
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Old 09-08-2019, 17:27   #6
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Re: Running Diesel tank empty?

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Yes as they start sucking air trying to push it through injectors and the flywheel keeps the mechanical pump moving as it all slows to a stop as all fuel is spent and more or less run dry, literally. Not saying its going to cook the pump first time, but its not good for the longevity, good way to cause undue wear and tear. Would you drain the oil from the sump with the engine running and when its done turn the engine off?
Honestly I'm not really sure why this is even a question? Why you wouldn't just tip a jerry can in when you think you're getting low vs purposefully running it dry, you don't even have to stop, just do it underway.
The short time involved makes it a trivial concern but I admire your fastidiousness. Stuff happens and running out of fuel is among those events.
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Old 09-08-2019, 17:39   #7
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Re: Running Diesel tank empty?

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It's good way to keep the tank clean. For what it's worth, it's not uncommon to run a fuel tank dry in a piston plane. However, you can switch tanks when the engine or the psi needle starts to flicker. Plus the engine starts right up again. I avoid running out of fuel in the boat for many reasons
First off, the vast majority of all piston engines on airplanes are gas and this is a discussion about diesels. Second, as someone with several thousand hours of flight time who flew professionally for many years.....are you freaking kidding me! No, its not at all common among safe, professional pilots to run a tank dry. I've done it exactly never, and if i ever do it will because of a major failure on my part! I only hope your flying days are behind you, if they aren't I say with all the good intentions in the world toward your passengers...they should be!
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Old 09-08-2019, 17:43   #8
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Re: Running Diesel tank empty?

txg ... don't run the engine dry, much less when you are out there away from help. Definitely not a good idea. it is great that you don't need to bleed the engine if accidentally you run out or suck air and stall, but doing it on purpose is not what I would do.

Your problem rather than to stall or not to stall, is one of fuel storage and a reliable gauge.

I don't know your boat and how much space is available but perhaps you need to consider installing a second fuel tank and a transfer pump or a valve to switch tanks. The valve to switch tanks needs to be double so that you also switch the return line.

And a good quality reliable gauge.

The idea that you can keep the fuel tank clean buy using it all up is misguided. The sucker does not go all the way down anyway so you never use the tank up completely. PLus to pump potential rubbish in the filters is a poor notion of cleaning tanks.
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Old 09-08-2019, 18:17   #9
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Re: Running Diesel tank empty?

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Originally Posted by S/V Illusion View Post
The short time involved makes it a trivial concern but I admire your fastidiousness. Stuff happens and running out of fuel is among those events.
Yes but doing it purposefully several times a week/month? Then you have the bleeding while cranking it dry until fuel begins to flow. You do what you want with your engine, money and safety; no matter how you look at it, its just bad practice.
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Old 09-08-2019, 18:20   #10
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Re: Running Diesel tank empty?

I can add that we don't have a fuel gauge and the dip stick is a PITA to access. I record fuel ups and hours and knowing your burn rate its easy to figure out how much you have left to within 5%-10%. When we fuel up its a game, how close are our calculations (rounded to whole numbers), closet wins and all are usually within a gallon up or down.
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Old 09-08-2019, 18:25   #11
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Re: Running Diesel tank empty?

Even if you run until the engine quits your tank is not empty. Pickup tubes don't usually go all the way down. Avoid running dry. Why would you?
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Old 09-08-2019, 19:30   #12
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Re: Running Diesel tank empty?

Why would anyone ever think about doing that?

If you don't know your fuel consumption for sure, then you're missing something. I hear this kindas BS all too often: "My engine use is different, sometimes I'm going slow, other times I'm at 80% WOT. Therefore it's always different."

Nonsense!

You use your AVERAGE over a long period of time to find out.

If you add a diesel heater, you LEARN what it uses in addition to your average fuel consumption.

I motored up the coast from SF to BC, 1500 nm. I haven't had an operating fuel gauge since I bought this boat in 1998. I wouldn't want one. I LOG all my refueling figures and stick 'em in my spreadsheet which calculates fuel consumption and how much I need to completely refill next stop, based on engine hours only.

This is rather simple.

Varying engine use, I personally believe, is pure hogwash. If you're running your diesel at 1000 rpm because you're motor sailing downwind for days on end, make a mental adjustment: your fuel is gonna last "a little bit" longer. And also, when bashing upwind against current at 80% WOT, your engine simply DOESN'T KNOW or care what the boat is doing. All your engine is doing is using the amount of fuel it consumes PER HOUR, not per mile. That's why miles per gallon makes NO SENSE on a boat.

Running your tank empty deliberately makes no sense at all.
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Old 09-08-2019, 20:36   #13
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Re: Running Diesel tank empty?

IMO I can't see anything wrong with what you want to do.
Your pump will hardly be "dry" as no fuel no motor go & it's not as if the tiny film in the lap fitted parts instantly disappears & you get metal on metal contact.
Use uncommon sense.
I wouldnt do it on ours coz it's a PITA to bleed but if you only take a minute go for it.
Obviously others think different, only you can decide.
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Old 09-08-2019, 21:08   #14
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Re: Running Diesel tank empty?

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Originally Posted by redneckrob View Post
First off, the vast majority of all piston engines on airplanes are gas and this is a discussion about diesels. Second, as someone with several thousand hours of flight time who flew professionally for many years.....are you freaking kidding me! No, its not at all common among safe, professional pilots to run a tank dry. I've done it exactly never, and if i ever do it will because of a major failure on my part! I only hope your flying days are behind you, if they aren't I say with all the good intentions in the world toward your passengers...they should be!


You apparently only flew larger aircraft, likely with paying passengers.
Itís very common to run a small piston engine dry before switching tanks, itís one way to know for sure that the fuel you have in the other tank is all there is. People have been known to convince themselves there is fuel in the other tank, when there isnít much, run it dry and there is no question.
It hurts nothing at all, the engine just begins to windmill, you switch tanks and it almost immediately picks back up.
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Old 10-08-2019, 01:16   #15
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Re: Running Diesel tank empty?

I doubt that you are going to harm the fuel pump by running the tank dry but it's a bad practice anyway unless you do it at the dock.
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