To fill or not to fill....(the diesel tanks)

[Nicholson58]

Originally Posted by Lee Jerry

We are (or at least I am) only referring to partially filled fuel tanks here.

When there is a difference in pressure inside of the tank and outside then there is a transfer of air to equalize the pressure. That's the purpose of the vent. This can be in either direction - expelling air as the tank contents heat up (and expand), for example, or into the tank as the contents cool (and contract). Or into the tank as the ambient pressure increases, say with a high pressure system, or out of the tank as the ambient pressure decreases, say with a low pressure system. Or into the tank as the fuel level drops (as it is pulled out of the tank for consumption in the engine) and out of the tank as the tank is refilled with fuel. Those are ways for air to get into the fuel tank.

Air has some amount of moisture in it all the time and will have some corresponding dew point. The relative humidity then varies with the temperature. So whether you consider it warm or not, or moist or not, is not really germane. Air, and therefore whatever moisture is in it, go into and out of fuel tanks. Also, note that condensation can occur on cold surfaces (i.e. metal) before the ambient air temperature reaches the dew point as the surface (may) cool faster than the ambient air and then the cold surface cools the air in the immediate vicinity. (Dew can form without fog.)

The physics inside the tank are exactly the same as those outside the tank (and on the car), but with two notable differences: the tank contains an air / fuel vapor mixture rather than just air (so possible slightly different chemistry); and the conditions of the tank, most notably the temperature, could vary from "ambient," simply by being a lag in getting to equilibrium due to thermal mass and such.

But I noticed you didn't answer the question, so I'll ask again. If there is no condensation in partially filled fuel tanks, why does MaineSail drain his fuel tanks for the winter?

The day-night here in the eastern Caribbean varies about five degrees. The decks do not even accumulate dew. The tanks are below waterline so their daily temperature swing is nil. You do not get tank condensation here.

If you get water in your tanks it is most likely from a faulty deck port O ring. This was so well known where we started in Michigan that they handed out free replacement O rings at the pump. We occasionally get water in the fuel at the pump but I always use a Baha filter. My polisher removes dirt to 1 micron and also water. We don’t get water in the tanks.

As I noted in my previous post, old diesel turns to gunk, turds in jello. If you don’t periodically pull your tanks low there will be some very old fuel blended with new. Oxygen and heat make it into long chain molecules like varnish that’s been around just long enough to make lumps. I had 250 gallons of it to pump and dump.

[Lee Jerry]

Originally Posted by Nicholson58

The day-night here in the eastern Caribbean varies about five degrees. The decks do not even accumulate dew. The tanks are below waterline so their daily temperature swing is nil. You do not get tank condensation here.

I agree, those aren't conditions where condensation is much of a concern. But not everybody lives there...


I also said up-thread that IMHO condensation (in fuel tanks) is generally not a concern "in season" (warm temperatures and regular use) but can be for longer term storage (or light use).

[merrydolphin]

I always keep mine full. To have back up. You never know! And to keep airspace out.

[Nicholson58]

If you think condensation can occur then start with an empty, dry tank and conduct the experiment. No fuel required for this test. All sorts of closed vessels around. Bet nothing happens!

[Dockhead]

Originally Posted by Lee Jerry

We are (or at least I am) only referring to partially filled fuel tanks here.

When there is a difference in pressure inside of the tank and outside then there is a transfer of air to equalize the pressure. That's the purpose of the vent. This can be in either direction - expelling air as the tank contents heat up (and expand), for example, or into the tank as the contents cool (and contract). Or into the tank as the ambient pressure increases, say with a high pressure system, or out of the tank as the ambient pressure decreases, say with a low pressure system. Or into the tank as the fuel level drops (as it is pulled out of the tank for consumption in the engine) and out of the tank as the tank is refilled with fuel. Those are ways for air to get into the fuel tank.

Air has some amount of moisture in it all the time and will have some corresponding dew point. The relative humidity then varies with the temperature. So whether you consider it warm or not, or moist or not, is not really germane. Air, and therefore whatever moisture is in it, go into and out of fuel tanks. Also, note that condensation can occur on cold surfaces (i.e. metal) before the ambient air temperature reaches the dew point as the surface (may) cool faster than the ambient air and then the cold surface cools the air in the immediate vicinity. (Dew can form without fog.)

It's not indeed irrelevant whether the air is warm or not -- warmER is what is meant here. Condensation occurs when the temperature of the air falls below the dew point. So for condensation to occur, the air has to start out warmer and moister, and get cooler.

But how on earth can that happen inside your fuel tank? If the fuel tank is colder than the outside air, then it cannot breathe in -- it will breathe out as it warms and expands. It cannot be getting colder and breathing in when it's warmer outside. And that's if your fuel tank even changes temperature at all between day and night -- mine doesn't, and I bet the ones that do are rare if they exist at all -- maybe on some catamarans with fuel tanks above the waterline.

Air doesn't just "go into and go out of" fuel tanks with a single long thin vent, unless you're blowing it in with a fan. The tank might conceivably "breathe" if the temperature of the tank fluctuates making the air inside expand and contract -- and this is the mechanism posited in the myth about fuel tank condensation. This is however busted -- as the breathing would be in the wrong direction, as discussed.

Nor is it irrelevant how moist the air is -- without a very large temperature difference, the warmER air has to be near saturation (100% relative humidity) for condensation to occur.

Originally Posted by Lee Jerry

. . . The physics inside the tank are exactly the same as those outside the tank (and on the car), but with two notable differences: the tank contains an air / fuel vapor mixture rather than just air (so possible slightly different chemistry); and the conditions of the tank, most notably the temperature, could vary from "ambient," simply by being a lag in getting to equilibrium due to thermal mass and such.

If your tank changes temperature at all from day to night, it will get warmer in the daytime as the temperature inside the boat gets warmer, and will get colder at night as the temperature inside the boat gets cooler. The temperature cannot change randomly.

Yet another point -- some years ago when we had this discussion before, someone did the math on how much water could be brought into a fuel tank based on a tank somehow breathing in the right direction (which it can't, but for the sake of argument), and it amounted to three drops a year or something like that in a 400 liter fuel tank, in conditions of 100% humidity every day and cool nights every night. I'll try to find that old post.

Originally Posted by Lee Jerry

But I noticed you didn't answer the question, so I'll ask again. If there is no condensation in partially filled fuel tanks, why does MaineSail drain his fuel tanks for the winter?

MaineSail can answer himself if he's around, but he has stated that he tries to keep fresh fuel in his boats. Certainly it's not to prevent condensation, which he has stated many times is a myth.

[Lee Jerry]

Originally Posted by Dockhead

It's not indeed irrelevant whether the air is warm or not -- warmER is what is meant here. Condensation occurs when the temperature of the air falls below the dew point. So for condensation to occur, the air has to start out warmer and moister, and get cooler.

But how on earth can that happen inside your fuel tank? If the fuel tank is colder than the outside air, then it cannot breathe in -- it will breathe out as it warms and expands. It cannot be getting colder and breathing in when it's warmer outside. And that's if your fuel tank even changes temperature at all between day and night -- mine doesn't, and I bet the ones that do are rare if they exist at all -- maybe on some catamarans with fuel tanks above the waterline.

Air doesn't just "go into and go out of" fuel tanks with a single long thin vent, unless you're blowing it in with a fan. The tank might conceivably "breathe" if the temperature of the tank fluctuates making the air inside expand and contract -- and this is the mechanism posited in the myth about fuel tank condensation. This is however busted -- as the breathing would be in the wrong direction, as discussed.

Nor is it irrelevant how moist the air is -- without a very large temperature difference, the warmER air has to be near saturation (100% relative humidity) for condensation to occur.

I don't know what "the myth" is or what it says. The tank "breathing" is not required for what I am talking about (other than allowing the air into the tank in the first place).

It's really fairly simple: A partially filled fuel tank has a volume of air above the fuel. This air has some quantity of water (moisture) in it. If this air is cooled to the dew point, this moisture will start to condense out of the air, typically on the tank sides (and not as fog). Some of this condensation may make its way into the fuel (before it warms up and evaporates back into the air). That's it.


Does this happen often? Probably not. But it can at times.

If the air is warmer (which you didn't define) to start, might that make the conditions for condensation more likely occur? Sure, perhaps. If the air is moister (which you didn't define) to start, might that make the conditions for condensation more likely to occur? Sure, perhaps. But neither is required. The dew point could be reached regardless of the starting point (within reason) IF the changing conditions are right.

Are there times (seasons, for example) when the conditions for condensation to occur are less likely? Of course (like summer). More likely at times? Of course. Are there arrangements when the conditions for condensation to occur are less likely? Of course, like with the boat in the water. More likely at times? Of course, like a boat on the hard (and not temp controlled).

MaineSail can answer himself if he's around, but he has stated that he tries to keep fresh fuel in his boats. Certainly it's not to prevent condensation, which he has stated many times is a myth.

He stated that about condensation in an EMPTY tank is a myth, NOT a partially filled tank. Perhaps he thinks differently today, but ten years ago when he wrote the article that you referenced, he said the attached - the first paragraph of the article. I call your attention to the second sentence: "The fuel...never got a chance to absorb moisture by sitting in the tank over the winter."

[s/v Jedi]

Actually, the mechanism of expanding and contracting fuel causing air movement via the vent takes moisture out of the fuel and moves it out the vent into the atmosphere.

If you are smart, you have no bacteria in the tank, no water in the tank and you use the enzyme based treatment when fuel is expected to be stored in the tank for more than a couple of months. This removes any water and when there is no water, there can be no bacteria.

For direct experience, it’s not just Rod Collins writing about this myth; I’m pretty sure I read an article in Practical Sailor or similar magazine that said exactly the same and they had tested it. The myth was busted years ago and still pushing it today is remarkable (note my use of words has been influenced by the admins )

[team karst]

Some basics:
One cubic meter of Air at 100% RH and 30 deg C holds 1.2oz of water.
What is not so basic is the flow rate into YOUR tank and the thermodynamics of condensation. Liquids also have a RH vs temperature vs ppm to study as well.

Maybe just study a free venting water tank that you also have. See how long it takes to refill itself. [emoji41]

[donradcliffe]

If I was spending hurricane season in the Caribbean, part of my hurricane prep would be to have 60 hours of motoring fuel in the tanks at all time.

[Simi 60]

We have 7000 litres of tankage
When we got her tanks were half full
Diesel was discoloured, owner guessed 10 years

Ran her for six months on that fuel before diluting with new
zero water, zero bug, a few flecks of rust
Changed original filters and they were clean.

7 years later we still have zero water, zero bug and I really should change the filters - check them and they are spotless so keep using them.

No biocide ever used.
Tanks are often at 50% for half the year.
We cruise in sub tropical to tropical climate, same latitudes as Florida to Carribean

I believe our results are mostly due to NOT having deck fillers

[Lee Jerry]

Originally Posted by s/v Jedi

Actually, the mechanism of expanding and contracting fuel causing air movement via the vent takes moisture out of the fuel and moves it out the vent into the atmosphere.

Ummm...out of the FUEL? I assume that is a typo or mis-speak. Otherwise you will have to elaborate. I've never heard of that.

If you are smart, you have no bacteria in the tank, no water in the tank and you use the enzyme based treatment when fuel is expected to be stored in the tank for more than a couple of months. This removes any water and when there is no water, there can be no bacteria.

For direct experience, it’s not just Rod Collins writing about this myth; I’m pretty sure I read an article in Practical Sailor or similar magazine that said exactly the same and they had tested it. The myth was busted years ago and still pushing it today is remarkable (note my use of words has been influenced by the admins )

What / which myth?

[Papasail]

My 2 cents are that fuel prices will spike exceedingly high this summer. I would stay permanently topped off.

[Simi 60]

I'm surprised no one has pulled out magnets yet.

[s/v Jedi]

Originally Posted by Lee Jerry

Ummm...out of the FUEL? I assume that is a typo or mis-speak. Otherwise you will have to elaborate. I've never heard of that.



What / which myth?

Air introduced into the tank is cold so low moisture and air expelled is warm and high moisture. When one considers that for the other way around moisture is absorbed into the fuel from the air, then in real life moisture will be pulled out of the fuel by the dry air

Which myth? I recommend you read the thread and the piece from Rod Collins before asking

[Lee Jerry]

Originally Posted by s/v Jedi

Air introduced into the tank is cold so low moisture and air expelled is warm and high moisture. When one considers that for the other way around moisture is absorbed into the fuel from the air, then in real life moisture will be pulled out of the fuel by the dry air

No. Just no.

Which myth? I recommend you read the thread and the piece from Rod Collins before asking

I don't believe anyone has "pushed" that myth in this thread. Certainly not me. That's what I asked. So maybe you should read the thread.