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Old 07-20-2020, 08:57 AM   #1
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How low to run fuel tanks?

I have two 250 gal diesel tanks. I'm gonna be doing the the South Florida Mini Loop. I see a ridiculously cheap Diesel dock. If I don't fuel before then, I should be able to make it there with at least 50 gal in each tank.

Is there any reason to fear running an old tank down to 1/5 full? I'm wondering about impurities (altho thats what the racors are for), and also the lower fuel level sloshing around allowing some air into the fuel line?

To be clear, I'm not trying to cross an ocean with a tight fuel window. If I burn more fuel than expected, I can always stop at a closer marina -- so I don't need any preparedness scolding. I'm just asking about functionality concerns. Thanks
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Old 07-20-2020, 09:21 AM   #2
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Hi ERTF


You don't say what the tanks are made of and how old they are. I've seen a lot of threads on this forum discussing the need to replace rusting tanks on some makes of older boats. This would imply to me risk of enough scale and sediment to clog Racors or any good filtering system - especially in rough seas.



You also don't mention whether your have one or two engines or whether the tanks are separate or connected. These are things I would take into consideration when trying to calculate my "get to the nearest fuel dock" strategy should the experiment you're considering fail.



20% has always been my threshold, even though I have plastic tanks, very good Vetus separators and have never had any sediment or bio issues. If your boat is new to you and you want to test your 20% threshold idea, obviously, it's great to do something like this in an area where there fuel docks are not scarce. For the duration of the experiment, you also might consider having 20gal. of diesel in plastic fuel jugs tied down in the aft deck, just in case.


Finally, I would suggest using a dipstick or some other visual method to assess your fuel levels prior to and during this experiment (assuming you have tank access for this). You might learn that your fuel gauges are not accurate during the course of this experiment!



I'm just sharing my thoughts on this subject. I'm sure there are members of this forum who have had extensive experience with this and will jump in with more informed info. than I am able to provide.
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Old 07-20-2020, 09:23 AM   #3
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Quote:
Originally Posted by ERTF View Post
I have two 250 gal diesel tanks. I'm gonna be doing the the South Florida Mini Loop. I see a ridiculously cheap Diesel dock. If I don't fuel before then, I should be able to make it there with at least 50 gal in each tank.

Is there any reason to fear running an old tank down to 1/5 full? I'm wondering about impurities (altho thats what the racors are for), and also the lower fuel level sloshing around allowing some air into the fuel line?

To be clear, I'm not trying to cross an ocean with a tight fuel window. If I burn more fuel than expected, I can always stop at a closer marina -- so I don't need any preparedness scolding. I'm just asking about functionality concerns. Thanks
Not a lot of history here. But I can tell you I was in a similar situation with really old diesel that probably had a ton of crud in it. Getting rid of it would have been difficult. I was headed on a 500 nm trip down the Pacific Coast and I decided to just go for it. I have a vacuum gauge and dual Racor's so I could hot-swap if needed.

Sounds like your situation is better on all counts - no indication of prior issues, friendlier waters than California coast.

Good luck!

Peter
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Old 07-20-2020, 09:31 AM   #4
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Twin engines. 30yrd old steel tanks with a connecting hose with closeable valves. Sight gauges, so I know how much is really in there.

Tanks haven't been much below half full in the 18 months I've owned the boat. I did clog a racor on my generator a few months ago right after I was offshore in some kinda rough stuff. But that was an old filter, and the mains had no issue. (Also, I have a dozen spare racors on hand.)
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Old 07-20-2020, 09:33 AM   #5
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I don't run mine less than 1/3. I have two tanks with a passive cross-over. A little list could drain one tank into the other. Same if the diesel return favors one over the other. As weight in one tank increases, so does the list and the list is exacerbated, further draining one tank into the other. Suck air and the engine dies.

Leave tanks full enough so there is no chance that once could have a condition where it sucks air. Just my humble opinion.
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Old 07-20-2020, 09:35 AM   #6
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In some ways it actually would seem like a good idea to periodically burn off as much fuel as reasonably possible in the bottom half of the tank.

Any additives you guys reccomend to avoid bio / water issues?
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Old 07-20-2020, 09:35 AM   #7
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Can you hot-swap the Racors, like mvweebles mentions? (i.e. is it a dual-filter setup so all you have to do is flip a valve to change to the fresh filter?).
With older diesels, there can be risk of getting air in the line if the filter is not changed in the correct manner. I wouldn't want to try this in rough weather!
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Old 07-20-2020, 09:36 AM   #8
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Yes each engine has dual filters.
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Old 07-20-2020, 09:40 AM   #9
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Any additives you guys reccomend to avoid bio / water issues?
I use the first 3 on the list with every visit to the fuel pump. The last one looks interesting, however I've never used it.

https://www.defender.com/product.jsp?id=3863005

https://www.defender.com/product.jsp?id=1380818

https://www.defender.com/product.jsp?id=3862862

https://www.defender.com/product.jsp?id=2793436
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Old 07-20-2020, 09:41 AM   #10
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Yes each engine has dual filters.
Happiness...
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Old 07-20-2020, 09:54 AM   #11
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FWIW,

There's absolutely NO down side to running your tank down to 1/5th full, or even 1/16th full, as long as you don't run out.

Your fuel pickup is in the same position whether the tank is full or empty so you really don't pick up any more crud regardless.

I typically run my tank down to 1/5th, as long as I'm SURE that I have a place to fill it then. In areas where I'm not familiar, I'll be mush more conservative, even topping off at the 1/2 mark if fuel availability is not for sure.
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Old 07-20-2020, 10:02 AM   #12
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With 4 tanks and a fuel transfer pump we empty each annually by pumping to another. I'd advise OP to install a transfer pump and isolating valves that allow draw and return from and to either tank.

The cross over should be a good place to use transfer pump to draw from. That way the tanks can be totally emptied and assess crud buildup on bottom, if any.
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Old 07-20-2020, 10:30 AM   #13
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FWIW,

There's absolutely NO down side to running your tank down to 1/5th full, or even 1/16th full, as long as you don't run out.

Your fuel pickup is in the same position whether the tank is full or empty so you really don't pick up any more crud regardless.

I typically run my tank down to 1/5th, as long as I'm SURE that I have a place to fill it then. In areas where I'm not familiar, I'll be mush more conservative, even topping off at the 1/2 mark if fuel availability is not for sure.
^^^ That is what I thought. It was beginning to sound like some have floating pickups that always draw top of fuel avoiding the crud below.
So except for drawing air one should be able to drain the tank with no difference of the fuel quality being drawn. Avoid crud at all times by using additives.
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Old 07-20-2020, 10:35 AM   #14
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The good thing about low fuel levels is that junk t the bottom may get sloshed around more to get cleaned by the filters especially if you have engines that return lots of fuel to the tank. A free fuel polishing.
Keep watch on the filter or gauge so you can change them at any stop.
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Old 07-20-2020, 10:51 AM   #15
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I'll run mine down to about 1" on the dipstick if travelling in calm water. I switch at about 2" remaining if offshore, but it is hard to get a good "dip" reading with the sloshing.

I have screwed up and air loaded the engine offshore. No fun repriming while boat rolls and engine room is hot as heck. That's a mistake I do not want to repeat.

With sight level tube you can run them pretty dang low. Just be aware that some tanks the pickup will catch air even with with some fuel showing on the sight tube.

Guess you will find out!! Make sure you know how to reprime you motors.
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Old 07-20-2020, 10:56 AM   #16
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Ideally, all tanks would be designed with a baffled sump and the pickup part-way down in the sump. Then you'd be able to safely run them down to the last couple of gallons even in fairly rough conditions. Unfortunately, setups like that are rare.

The other option to reduce unusable fuel (if it's a concern) would be a buffer tank. Fuel gets pumped from main tank to buffer tank (with overflow back to main tank) to keep the buffer tank full. Then the engine draws from the buffer tank. That way a momentary slosh away from the pickup won't cause the engine to suck air.
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Old 07-20-2020, 11:09 AM   #17
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It's a good time of year to try as seas are mild now. Be sure to spend a night at Faro Blanco iMarine Resort in Marathon. The Keys are amazing this time of year. Easy time to troll for some Mahi Mahi on the Atlantic side as well.
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Old 07-20-2020, 11:45 AM   #18
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I have two 250 gal diesel tanks. I'm gonna be doing the the South Florida Mini Loop. I see a ridiculously cheap Diesel dock. If I don't fuel before then, I should be able to make it there with at least 50 gal in each tank.

Is there any reason to fear running an old tank down to 1/5 full? I'm wondering about impurities (altho thats what the racors are for), and also the lower fuel level sloshing around allowing some air into the fuel line?

To be clear, I'm not trying to cross an ocean with a tight fuel window. If I burn more fuel than expected, I can always stop at a closer marina -- so I don't need any preparedness scolding. I'm just asking about functionality concerns. Thanks
To me the number isn't how far down you're running, but it's what percentage extra you have for the day in question. I don't care what happened a week ago. I care on this day. Do you have 10%, 20% or 30% extra for the final run to the cheap fuel. You have 50 gallons extra in each tank so 100 gallons and if you're only using 100 gallons this final day that's 50% extra but if your final day is to use 1000 gallons, then you don't have adequate room for error.

Now, it seems like you feel it's a bit risky. I must ask if you'd take on risk to save a few cents a gallon. Please I hope not. Only make a run like this if you're personally sure it's safe.
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Old 07-21-2020, 03:57 AM   #19
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If you have spare Racor primary filters why not go for it? Best to find out under ideal conditions if you can draw your tank(s) right down or not, rather than find out they are full of crap in emergency conditions. My only caveat would be whether you have calibrated your fuel gauges. (electric? dipstick, sight gauge?) Hate to think you have 50 gals left but actually only 10. Best to run 1 tank down at a time to ensure you have enough fuel to reprime if you start sucking air/crap.
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Old 07-21-2020, 04:02 AM   #20
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I am lucky. My fuel supply is a bottom feeder, no dip tube. Water and crud is continuously removed and filtered out. I have opened my bottom drains valves (38-year-old boat) and found NO water and just a few, barely discernible flecks of crud. But, most boats are dip tube fed. I have been in very rough conditions many times, no filter clogging from "stirred up" junk lurking in the bottom. Plus, no need for those monster dual Racors!
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