"Worth it" to top off diesel for the winter?

[Old Swampy]

I was told by a professional diesel technician that it is best to have room in the tank for fresh fuel at the beginning of the season. He said that old fuel was bad for diesel engines; gum builds up. So, that seems to favor empty tanks. It means getting rid of fuel, which I do by putting it in my home furnace fuel tank. Yes, I believe in condensation, the amount of which will depend on the size of the space in the tank, the change in temperature in each air exchange, the humidity of the ambient air, and the number of cold-warm cycles. Now how would you get rid of the accumulated moisture before filling the tank? Polish a small volume of fuel repeatedly.

[Dockhead]

Originally Posted by S/V Alchemy

This is true, too, because the tank side of the boat can heat up in the winter sun, and then chill at night. This promotes air movement (think a really slow bellows). Air movement is how moist air gets in and out.

But if the tank heats up, it will breathe out, not in.

Think about it - to get condensation from breathed-in air, the tank would need to cool off during the day when the outside air is warmer. But it's on the contrary warming up.

This is an old wives tale, which in my opinion has been debunked by MaineSail by empirical testing.

I leave my tank about half full.

Never a drop.

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[varkpilot]

I became a pilot (aircraft) back in the early '60s. One of the items stressed was to always fill the tanks at the end of the flying day. The Air Force did it for the fighters I flew, too. Later, I got lazy with my own Cessna 421, a larger twin-engined plane. One day, while on a cross-country, I switched tanks and the left engine quit. Long story short, the reason was fuel contaminated with water. It just flat drowned the engine. I went back to filling the tanks (the plane had four tanks) before I put it away for the day. That was 100LL aviation fuel, so I'm not sure that makes a difference. The fighters flew with a mixture of avgas and kerosene, a little closer to diesel, but as I said, the Air Force filled the tanks prior to putting them away for the day. For whatever all that's worth.

[OrangeCrush]

Originally Posted by Dockhead

But if the tank heats up, it will breathe out, not in.

Think about it - to get condensation from breathed-in air, the tank would need to cool off during the day when the outside air is warmer. But it's on the contrary warming up.


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As OP I'm obviously not the authority here, but it seems to me that an empty tank could breathe in and out endlessly without any condensation. The condensation would occur because as the boat heats up, the thermal mass of a large volume of diesel lags behind. At one point, the air is warmer than the surface of the fuel and condensation forms. Sorta like advection fog, or how on a humid day the toilet tank in a bathroom can have condensation on it in the morning. Is this incorrect?


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[Cotemar]

Originally Posted by OrangeCrush

As OP I'm obviously not the authority here, but it seems to me that an empty tank could breathe in and out endlessly without any condensation. The condensation would occur because as the boat heats up, the thermal mass of a large volume of diesel lags behind. At one point, the air is warmer than the surface of the fuel and condensation forms. Sorta like advection fog, or how on a humid day the toilet tank in a bathroom can have condensation on it in the morning. Is this incorrect?

OrangeCrush = Brilliant

Top Off Fuel tank = Good
Empty Fuel tank = Good

Half Empty Fuel tank = BAD

[Cheechako]

I'm on the fence about topping off and wouldn't worry about it if I were you. First; you are not living aboard so not heating the cabin much right? That minimizes condensation. Second; many boats carry too much fuel for their usual use. Your fuel gets old. Why add more fuel to get old?
What might tip the scales for me is if I had plain steel tanks, possible rust might occur in tanks that are not near full.

[markpierce]

Perhaps condensation is a major worry in tropical climates, mostly east of the Rocky Mountains as far as the USA goes.

[Dockhead]

Originally Posted by OrangeCrush

As OP I'm obviously not the authority here, but it seems to me that an empty tank could breathe in and out endlessly without any condensation. The condensation would occur because as the boat heats up, the thermal mass of a large volume of diesel lags behind. At one point, the air is warmer than the surface of the fuel and condensation forms. Sorta like advection fog, or how on a humid day the toilet tank in a bathroom can have condensation on it in the morning. Is this incorrect?


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Yes, it's incorrect, but you're asking the right questions.

The toilet tank has condensation on it because you've flushed it and it has refilled with cold water, colder than the dew point of the wet morning air.

Condensation in your fuel tank can't work the way you say -- although it's an intelligent and creative theory. Condensation can't form just from air being in contact with something cooler. It has to be SATURATED air, or close enough to being saturated that it becomes supersaturated when it's cooled by the cooler object.

The tank doesn't just breathe in and out like a dog -- it will breathe in as it cools, and out as it warms. It will not take in the warm saturated air which could cause condensation, because it is breathing out at that precise time.

Besides that, the very thermal mass you talk about will prevent the tank from heating or cooling much and thus prevent much breathing. I've measured my own tank -- I've never seen the temperature vary by more than 2 degrees C even the outside temperature is varying by 10C or more. So only extremely tiny amounts of air are breathed in and out. And in any case, the fuel will always be warmer than the cool air being breathed in at night anyway.

It just cannot work like this -- it's an old wives' tale, and if you read the MaineSail articles linked to above, you'll see the empirical proof. At least with respect to boats. I make no claims with regards to underground storage tanks etc. which might have different factors at play.

People get water in their tanks from other sources -- mostly bad o-rings in deck level fillers (which are the work of the devil), or badly placed tank vents. Then when they find that water, they grasp at straws to figure out where it could have come from, and the old wive's tale about condensation is a comforting myth. Which gets handed down through the generations.


Diesel fuel does hold a certain amount of water and will tend to absorb water from moist air until it's saturated. Also modern diesel fuel oxidizes and throws sediment with time. See:

http://www.bp.com/content/dam/bp-cou...age-diesel.pdf

Therefore notwithstanding stories about using WWII era fuel, it is not desirable to keep modern diesel fuel in tanks for more than 6 months or so. This is another reason not to top off your tanks in the fall -- that will leave you with a whole tank load of old, stale fuel in the spring. Far better to leave the tank mostly empty, so that the old, stale fuel will be diluted with fresh fuel when you start using the boat again.


Also, how you handle your fuel is, in any case, no substitute for keeping your tanks clean, and opening them up and inspecting them on a regular basis. All diesel fuel has at least some water in it. Why, during WWII, diesel-electric submarines used to ballast their partially empty fuel tanks with seawater. They would just draw the fuel from above the water level, and centrifuge the hell out of it. If you do get a little water in your tank, it's not actually a big deal if you are inspecting your tank often and get it out before fungus starts to grow.

[bkrizmanic]

Originally Posted by Sailmonkey

I don't buy into the condensation thing. There's just not enough air in there, and not enough exchange of air in the tank to keep adding water.

If it makes you feel good, schlep Jerry jugs to the boat, but I personally wouldn't bother.

As far as marine diesel. Other than being pink, there shouldt be a difference (unless you've found something different). We haven't had dyed fuel in the boat in four years (I buy diesel 12 gallons at a time from the corner store and siphon it into the boat)


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The only difference with dyed fuel is Federal Tax, it is a big no no for trucks to run red diesel.
As far as my experience I try to top off but sometimes don't get to it and have never had any problems in the spring.

[boeing1]

I am old school,I top off my tank

[Celestialsailor]

Originally Posted by Sailmonkey

I don't buy into the condensation thing. There's just not enough air in there, and not enough exchange of air in the tank to keep adding water.

Maybe you should...I just emptied 2 gallons of water out of my 50 gallon tank and spent 3 hours dismantling the Racor and cleaning it.

[Celestialsailor]

[QUOTE=Dockhead;2255241]But if the tank heats up, it will breathe out, not in.
Think about it - to get condensation from breathed-in air, the tank would need to cool off during the day when the outside air is warmer. But it's on the contrary warming up.
/QUOTE]

Actually, the tank does not know day or night. It heats up for awhile then cools off dropping moisture out of the air in the tank. That and constant humidity changes add to the problem. Because one person claims they have proven the dilemma one way or the other does not apply to everyone's situations. Someone in the Keys would be totally different then say Toronto Ontario or Seattle or here in Mx.

[SteveInMD]

Even if you buy fuel at a dock that sells lots of fuel it's not uncommon to occasionally get small amounts of water in the fuel when you buy it. I believe that's more often the source of water than condensation. If you have ever seen the "tank bottoms" that are pumped out of large scale storage tanks you would understand how it ends up in your fuel. This is why we have filters on our fuel systems. If you maintain your filters it will more than take care of the any condensation or water purchased along with the fuel.

[Steven UK]

I haven't read every post so forgive me if I am going over old news or may be just confirming what others have said.
I work in the ag machinery industry mostly engineering but I have done some training in Asia. I always taught that diesel tanks should be kept full because if the breathing issue tank breathes in when the cold of night hits then the moisture condenses and runs down the sides of the tank, if it's one night or one week not a problem but an extended length of time then there is a problem the bigger the air space then the bigger the problem. Also the lamina between fuel and water is where the microbes, fungal and cooties grow.

That being said I have had machinery on my farm and never bothered filling them as I always recommended in my training... do as I say not what I do! What a $&@? Hypocrite.

Hope this helps

[ramblinrod]

Originally Posted by valhalla360

The theory is by staying full, you keep water from condensing out of the air by eliminating the air space.


But if you run the calculations on how much water can condense out of the available air space, the amount that could condense over the course of a winter is on the order of a few drops.

If you are getting noticeable amounts of water in the fuel tank, it's likely the result of a leak allowing rain water in. Often the fuel fill is on a level section of deck and if it doesn't seal well, rain water can find it's way in.

The issue is that the fuel (and especially fuel vapours) expand and contract with temperature, so air is being repeatedly pushed out of and sucked back into the tank with each temperature cycle, typically, warm in the day, air is pushed out, cool at night, air is sucked back in.

The greater the volume of air/fuel vapour and wider the temp swing, the greater the air change. do that every day for six months on a 1/4 full tank and you'll have a lot more than couple drops of water in there.

For gas powered engines, never fill the tank to the top of neck when it is cold. On a warm day with sun on the tank side, the fuel can expand and spill out the fill cap.

But yes, if you are getting significant amounts of water in the tank, check the seal on the diesel fill cap.