Should fuel tanks be full?

We have 2, 112gal diesel tanks. I haven't needed to put fuel in them for a long time. Both tanks are a bit over 1/2 full and racors don't show crud or water. Motor starts quick every time and runs great. We burn very little fuel and do use it at least 2-3 times per month.

- Is the consensus to keep tanks always full?

Having full tanks is less important in a warm climate than a cold one. A fuel tank is a little like a lung. As the day heats air in the tank expands and some leaves via the vent. When it cools at night the vent brings in cold, moist night air, causing a water buildup. It also brings in the bugs that can live with water at the tank bottom. This is the same way water tanks get contaminated.
If you use a fuel conditioner, that should kill the bugs and make separating the water in your primary filter easier.

Lepke said:

Having full tanks is less important in a warm climate than a cold one. A fuel tank is a little like a lung. As the day heats air in the tank expands and some leaves via the vent. When it cools at night the vent brings in cold, moist night air, causing a water buildup. It also brings in the bugs that can live with water at the tank bottom. This is the same way water tanks get contaminated.
If you use a fuel conditioner, that should kill the bugs and make separating the water in your primary filter easier.

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Good to know as I've always wondered if I needed fresh diesel all the time in the tanks. I have used conditioner and run one tank at a time so fuel is recirculated and cleaned to some degree.

Look at Rod Collins’ Marinehowto.com website scroll to “condensation in marine fuel tanks”. He tried to create it but couldn’t. So just replace the deck filler O rings regularly and you’re good to go

Just because he couldn't doesn't mean something known to occur doesn't.

Condensation in tanks is certainly possible (especially with large tanks). But unless the tanks are mounted in an area where they see significant temperature swings (keep in mind that the tanks will change temperature fairly slowly) and end up breathing a whole lot of air (and then get the right conditions for moisture to condense on the walls), it's unlikely that you'll get a significant amount of water into the tanks from condensation.

If temperature swings are the culprit, are not boats whose tanks are mostly below the water line pretty much insulated against significant temperature swings? And how fast and how much of a swing is needed to cause condensation. Count me a skeptic.

catalinajack said:

If temperature swings are the culprit, are not boats whose tanks are mostly below the water line pretty much insulated against significant temperature swings? And how fast and how much of a swing is needed to cause condensation. Count me a skeptic.

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Exactly. I'd think the worst-case would be tanks next to the engines and hull sides in cold weather. Running the boat and then letting it all cool would cause significant heat cycles, and 1 wall of the tank would be exposed to outside temperature, which could potentially cause condensation when the temperature drops further overnight.

Flatswing said:

Look at Rod Collins’ Marinehowto.com website scroll to “condensation in marine fuel tanks”. He tried to create it but couldn’t. So just replace the deck filler O rings regularly and you’re good to go

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Don L said:

Just because he couldn't doesn't mean something known to occur doesn't.

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I really appreciate Rods articles but do feel there is a (rare) flaw in the one referenced. Rod had no fuel in the tank he experimented with IMO that is a critical variable. If any moisture dud condense it could easily have evaporated when temps warmed.
A layer of diesel fuel in the tank would be a better test so any condensate sinks to the bottom and is unable to evaporate.
My opinion... opinion only... is condensation is an overblown concern IN MANY LOCATIONS / CLIMATES. My diesel boat is stored on the hard in the relatively dry & cold NE. The mass of fuel buried in the bilge has a significant time lag vs the exterior environment. Cold air in our area are relatively dry (lo dew pt) thatched will need Tobe present for an e tended time before the tank liquid & vapor contract and pulls in (then dry) air.
When it warms is when the environment can hold more moisture (higher dew pt ) and then the tank is warming and "exhaling"
ALSO... If moisture build up were a problem for me or in my environment, I'd see it in the Racors in the spring...never have moisture unless it's from a bad source after a fill.

I've been working on and and around boats as well as running recreational boats since 1970 and have never seen condensation caused water in a fuel tank.

I have seen it in shore based equipment fuel tanks because those do experience dramatic daily temperature swings. Had it happen to both a tractor and a pickup when I lived in the foothills where daily temperature swings were significant and night humidity was high. The tractor after sitting all winter under cover would need to have the water from it's fuel tank drained before spring startup.

I can think of one boat fuel tank set up that could be at risk. That would be a metal boat with skin tanks that are above the waterline so the outboard sides are exposed to sun induced heating and daily exterior temperature swings. Maybe engine space heating and cooling well. That would be similar to a farm tractor fuel tank heating and cooling daily.

I do think flush deck fills and vents on the sides of the hull are where the water gets in recreational boat fuel tanks. Work boats with fills and vents well above the deck don't see water in the tanks unless they were unlucky enough to get a bad load of fuel.

Flatswing said:

Look at Rod Collins’ Marinehowto.com website scroll to “condensation in marine fuel tanks”. He tried to create it but couldn’t. So just replace the deck filler O rings regularly and you’re good to go

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rslifkin said:

Condensation in tanks is certainly possible (especially with large tanks). But unless the tanks are mounted in an area where they see significant temperature swings (keep in mind that the tanks will change temperature fairly slowly) and end up breathing a whole lot of air (and then get the right conditions for moisture to condense on the walls), it's unlikely that you'll get a significant amount of water into the tanks from condensation.

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catalinajack said:

If temperature swings are the culprit, are not boats whose tanks are mostly below the water line pretty much insulated against significant temperature swings? And how fast and how much of a swing is needed to cause condensation. Count me a skeptic.

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On the gulf cost temperature swings can be extreme.

I literally freaked out when we first got the boat and I saw the condensation in the engine room one morning.

With cold water and a warm humid day my first thought was the water pump hose had broken and sprayed water everywhere.

It only took once, then I put an electric radiator down there for the winter and spring, no more problems.

I keep tanks completely full when storing boat, and use a fuel polisher monthly. Before a trip we drain all Raycor bowls as a habit.

Was in a plane crash from water in the fuel, kind of has my attention.now.

The normal Racor filters should be enough assuming you keep an eye on them and drain the bowl as necessary.
I also have a polishing system too. Why, if I refuel and spend the night docked, fuel polishing may prevent a problem before it happens.

Sandpiper has 2 X 300 gallon steel fuel tanks in the engine room. It is moored in the PNW.

I've not filled the tanks full in 20+ years, usually put in around 400 gallons every spring. I leave minimal fuel in the tanks (15 to100 gallons) through fall, winter and spring and maintain a 50 degree engine room temperature. Less fuel allows easier fuel polishing and constant temperature reduces fuel tank breathing. Sandpipers fuel tanks have sumps where the fuel is drawn. From the sumps, the fuel is routed to Racor 500's, then the engine and generator.

I test for water every month during winter with a dip stick and KolorKut and polish the fuel right before spring.

I have never detected water in the fuel in 23 years.

syjos said:

Sandpiper has 2 X 300 gallon steel fuel tanks in the engine room. It is moored in the PNW.

I've not filled the tanks full in 20+ years, usually put in around 400 gallons every spring. I leave minimal fuel in the tanks (15 to100 gallons) through fall, winter and spring and maintain a 50 degree engine room temperature. Less fuel allows easier fuel polishing and constant temperature reduces fuel tank breathing. Sandpipers fuel tanks have sumps where the fuel is drawn. From the sumps, the fuel is routed to Racor 500's, then the engine and generator.

I test for water every month during winter with a dip stick and KolorKut and polish the fuel right before spring.

I have never detected water in the fuel in 23 years.

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Does your boat still have the original tanks? Mine does, and I think they must be very similar, but mine are listed as 200 each. I sample the sumps now and then, and get nothing but good fuel. I leave them half full or thereabouts so I have fuel for heating over winter.
Also have never seen any water in the sumps or the filter bowls.

Bmarler said:

Does your boat still have the original tanks? Mine does, and I think they must be very similar, but mine are listed as 200 each. I sample the sumps now and then, and get nothing but good fuel. I leave them half full or thereabouts so I have fuel for heating over winter.
Also have never seen any water in the sumps or the filter bowls.

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Yes, my tanks are original. Bluewaters came equipped with a variety of tank sizes and location. The boats were semi-custom and buyers were able to configure them to suit their needs.

For example, Sandpiper was ordered with the stairs from the pilothouse to the stateroom deleted. The second head was also deleted.

PhilPB said:

We have 2, 112gal diesel tanks. I haven't needed to put fuel in them for a long time. Both tanks are a bit over 1/2 full and racors don't show crud or water. Motor starts quick every time and runs great. We burn very little fuel and do use it at least 2-3 times per month.

- Is the consensus to keep tanks always full?

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I try to run my down tanks when I am doing local day trips and have even taken the boat out intentionally on rough days with less than full tanks to agitate any sediment on in the tanks, then I change out the primary filter. This is very local water that I could easily anchor in and recover in case I manage to plug the filter and need to change it immediately and bleed the engine. Prior to a longer trip, when my family is onboard, I'll be sure and top off the tanks to minimize and sloshing around of fuel and sediment.

This is all easier said than done because I burn so little fuel and the tanks rarely get below 3/4 between taking longer trips. If I ever need to replace my fuel tanks, I would go with substantially smaller fuel tanks and use the space for larger water tanks.

Thread drift....

I would likely do the same when fuel tank replacement time comes. I find endurance is limited more by potable water and holding tank than fuel tanks.

Gdavid said:

If I ever need to replace my fuel tanks, I would go with substantially smaller fuel tanks and use the space for larger water tanks.

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Don L said:

Just because he couldn't doesn't mean something known to occur doesn't.

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There are situations where condensation can occur and cause issues, but mostly it's an old wife's tail propagated on forums. There are thousands of boat owners out there who don't keep their tanks full, including off season with no issues.

Full and you wont get a bite in the butt when you refuel.
Mid season level, might have to polish the fuel if you don’t have a low spot with drain.
I have yet to see a predicted price.

Fletcher500 said:

There are situations where condensation can occur and cause issues, but mostly it's an old wife's tail propagated on forums. There are thousands of boat owners out there who don't keep their tanks full, including off season with no issues.

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Tell that to the mung in my fuel filter separator

Don L said:

Tell that to the mung in my fuel filter separator

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Sounds like you have an issue that most other people on here don't have.

Flatswing said:

Look at Rod Collins’ Marinehowto.com website scroll to “condensation in marine fuel tanks”. He tried to create it but couldn’t.

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I agree.
Our tanks are rarely over half full and in seven years of ownership we have never had a drop of water show up in our tri - monthly drain 1/2 a litre from tank crud sump check or seen anything in the 1000fg bowls.

So just replace the deck filler O rings regularly and you’re good to go

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Yep
Even better is get rid of deck fillers and have them a few feet up in the cabin side's

rslifkin said:

Condensation in tanks is certainly possible (especially with large tanks). But unless the tanks are mounted in an area where they see significant temperature swings (keep in mind that the tanks will change temperature fairly slowly) and end up breathing a whole lot of air (and then get the right conditions for moisture to condense on the walls), it's unlikely that you'll get a significant amount of water into the tanks from condensation.

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It's interesting
As stated earlier we've had zero sign of even a droplet of water
But out tanks get pretty toasty mounted in the ER either side of the engine.

Also Cummins PT fuel system appears to let fuel do a lap around the engine before returning that now HOT diesel to the tanks.

I'd call that a fairly significant temp swing.

Simi 60 said:

It's interesting
As stated earlier we've had zero sign of even a droplet of water
But out tanks get pretty toasty mounted in the ER either side of the engine.

Also Cummins PT fuel system appears to let fuel do a lap around the engine before returning that now HOT diesel to the tanks.

I'd call that a fairly significant temp swing.

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I think the more important part is whether the tanks get exposed to exterior cold at night, etc. If the tank walls don't get below the dew point of the air in the tanks, then there's no condensation.

I also think the issue is over-played. Back when I owned a Clipper (CHB) 34, my tanks were stainless and definitely not below the waterline. I also never had them more than half full, because we'd have never used enough fuel to justify it, not travelling long distance much. Also, as we never got to use her as often as I'd have liked, she sat for quite long periods. Here, especially in winter in Queensland, there are big swings in temp from daytime to night. Yet I never got more than a teaspoonful of water out of the separator under the primary filter.

However, I must say that the set-up of the fuel drain from the tanks, being from right at the bottom of the tanks, which were interconnected so as to always self-level, may well have helped. Effectively, we were polishing the fuel each time the engine was running, so there was no tank bottom build-up of crud or water, which would have ended up in the separator anyway, so can't ever have been much.

I'm not sure why more boats don't have the fuel pick-up from the tank bottom, (I believe Nordhavn do this also), as surely the common system of fuel pick-up via a long tube down from the top of the tank just predisposes to crud/water build-up, because that tube ends well above the tank bottom. Gets stirred up in a seaway = clogs primary filters..?

PhilPB said:

We have 2, 112gal diesel tanks. I haven't needed to put fuel in them for a long time. Both tanks are a bit over 1/2 full and racors don't show crud or water. Motor starts quick every time and runs great. We burn very little fuel and do use it at least 2-3 times per month.

- Is the consensus to keep tanks always full?

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You will never get consensus on this forum on something like that.
If what you are doing works for you then by all means keep doing it.

Peter B said:

I'm not sure why more boats don't have the fuel pick-up from the tank bottom, (I believe Nordhavn do this also), as surely the common system of fuel pick-up via a long tube down from the top of the tank just predisposes to crud/water build-up, because that tube ends well above the tank bottom. Gets stirred up in a seaway = clogs primary filters..?

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Or at least if using dip tubes, make a sump or low spot in the tank and extend the dip tube down into it for that reason. I agree that I'd rather have the junk in the filters than sitting in the tank and accumulating.

Don L said:

Tell that to the mung in my fuel filter separator

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Fletcher500 said:

Sounds like you have an issue that most other people on here don't have.

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I totally agree that if you regularly have stuff in your fuel filter separator, than you have something else going on! Depending on what it is, you could have a failed O-ring on the deck filler, if gunky and brown, you may have a horticulturist's dream going on in your fuel tanks(s). Fuel additives combined with a full tank cleaning/fuel polishing may correct it.

Our boat is 41 years old. We've not had any issues in the two years we've owned it, and we don't always keep the fuel tanks full. The previous owners owned her for 32 years, and never had a problem. Regular sumping of the tanks from the lowest point rarely yields more than maybe 1/4 teaspoon of water, and never any deposits in the fuel filter bowls.

Maybe we've just been extremely lucky.

Test for water in fuel

I apologize if someone has already mentioned the following. I may have missed them in the thread….

1. During the times of year with significant daily temperature swings I keep a 75w incandescent light bulb or a “golden rod” dehumidifier in the engine room. It eliminates sweating on all the metal parts.

2. Have you used Kolor Kut to monitor your tanks? It’s a yellow water reactive paste that you smear on your fuel dipstick. It turns red when it comes in contact with water. Good product. Available at most marine supplies or on Amazon.

EVERFOR said:

I apologize if someone has already mentioned the following. I may have missed them in the thread….

1. During the times of year with significant daily temperature swings I keep a 75w incandescent light bulb or a “golden rod” dehumidifier in the engine room. It eliminates sweating on all the metal parts.

2. Have you used Kolor Kut to monitor your tanks? It’s a yellow water reactive paste that you smear on your fuel dipstick. It turns red when it comes in contact with water. Good product. Available at most marine supplies or on Amazon.

View attachment 137489

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Using light bulbs to warm engine rooms is not the safest or economical choice. Use your Golden Rod instead. If you use a light bulb, mount it inside a coffee can to protect against breakage and burns. A thermostatically controlled engine room heater set at 40 to 50 degrees uses less electricity and maintains a constant temperature which reduces tank breathing.

I test monthly with KolorKut