How low to run fuel tanks?

The friendliest place on the web for anyone who enjoys boating.
If you have answers, please help by responding to the unanswered posts.
See posts with no answers
E

ERTF

Senior Member
Joined
Aug 16, 2017
Messages
245
Location
USA
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
 
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.
 
Last edited:
ERTF said:
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
Click to expand...

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
 
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.)
 
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.
 
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?
 
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!
 
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.
 
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.
 
Seevee said:
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.
Click to expand...

^^^ 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.
 
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.
 
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.
 
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.
 
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.
 
ERTF said:
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
Click to expand...

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.
 
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.
 
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!
 
catalinajack said:
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!
Click to expand...
Not sure I understand the connection between bottom draw valves reducing need for dual Racors. I have bottom draw valves too and wouldn't be without dual Racors even though I've never had a fuel issue.

To the OP: you're over thinking this. Assuming you have saddle tanks, they are most likely tapered meaning 50-gallons remaining is pretty deep in the tank. Running most of the fuel out and turning it over is a good thing.

Have a good trip. Your risk profile by running light is not significantly changed. You have dual Racors and placed to duck into if you get nervous.

Good luck. Wish I were along for the ride.

Peter
 
One thing to consider is the relationship of the height of fuel to the engine. If a normal level in the tank is more or less even with the engine, there is not a lot of vacuum needed to move fuel. When the fuel pump is working harder to raise the fuel up several feet there is more probability for air to get pulled into the lines. Not from the pickup, but from any fittings or fuel filter o-rings that are suspect. They may not leak externally while the engine is off, but apply more than normal vacuum and they will allow small amounts of air to enter, which eventually builds up and shuts down an engine. This may seem like air was sucked into the pickup, but it’s just small amounts accumulating over time. I’ve also had fuel lift pumps fail when running low.
 
No worries

Rather than worry, if possible, just stop and partially refuel some place short of the cheap fuel location. Just put enough fuel in that you will be comfortable with your arrival fuel. You may not save as much money, but perhaps not worrying will be worth the difference.
 
ERTF said:
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
Click to expand...

Well you don't want to suck air:banghead:
I refilled at 25%
 
It is difficult to provide an absolute response to this. But in my opinion there are a lot of factors to take into account.


1) How far between fueling stations.
2) What are the chances that your passage between one area and another will take longer than anticipated. For example if rough weather were to blow in and you burn twice as much fuel as predicted or the weather prevents you from entering the port for fuel until after the storm passes.
3) Can you replace filters while underway if they become clogged, especially if you consider the potential for sediment being churned up.


These a few of the major issues that need to be taken into account. Here in the PNW, weather can change dramatically in a very short period of time. Even here in the Puget Sounds, getting into a port under severe weather can be a challenge. Even more so when you are worrying about if you have enough fuel or will the filters get clogged.



I rarely ever allow my tanks to fall below 1/2 to 1/3.
 
Star Tron, available at West Marine, liquifies and the dead bio growth collects in the filters, I got a bad load years ago and the dual Racors saved the day, I went through 8 to 9 each engine. One only needs Startron when leaving the boat for extended periods, or believing the bottom of the tank is suspect. All diesel fuel has a component of water and therefore subject to algae.
 
Carry plenty of spare filters and go. You have already said if you get tight on fuel you can stop at a closer marina. You should not use the bottom of your tank as a collective for contamination, it will just get higher and higher in the tank. Your filters are the antidote. Just don't run out!
 
catalinajack said:
But, most boats are dip tube fed.
Click to expand...

What leads you to this conclusion?

For the PO, what engines? Want to know to get an idea of the return rate.
 
Where on the South Florida Mini Loop is this ridiculously cheap Diesel dock?
 
You must log in or register to reply here.

Similar threads

D
Haunted Fuel Tank
Replies
6
Views
740
Island Cessna
Island Cessna
T
Best way to purge new fuel lines
Replies
1
Views
340
Larry M
Larry M
PhilPB
Fuel Filters
Replies
15
Views
2K
jleonard
jleonard
PhilPB
Diesel fuel return
Replies
13
Views
1K
Circumnavigator
Circumnavigator
C
Fuel Tank Mystery
Replies
17
Views
1K
Greg QS
Greg QS

Latest posts

Back
Top Bottom