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When is diesel fuel too old?

jviss

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Feb 5, 2004
4,627
Tartan 3800 Westport, MA
Yea, I'll be sure to tell my buddy with the Grand Banks 43 Eastbay that he should pump out his 450 gallons of diesel at the end of the season and bring it home. :)

Seriously, I do what works, and has worked in my experience, in my climate, for 21 years. I try to fill it close to full for winter storage, and put in the correct dose of biocide. I monitor my water separator and filter. I've never had water accumulate in the separator, and never had a problem with growth in the tank. I have a 38 gallon tank now, had a 32 gallon one perviously.

I think a lot of it is the quality of the fuel one puts in, and the environment - climate, etc. I'm in cold New England. I think that matters. I know MaineSail is, too, and while he hasn't had a problem, I don't know that he's done it other ways, too.
 
Oct 26, 2010
1,431
Hunter 40.5 Beaufort, SC
We sail all year long down here in South Carolina so emptying the tank would be somewhat of an inconvenience and cut into sailing time. However, I have installed a fuel polishing system with a pickup only about 1/4 inch off the bottom at the lowest point and a return at the other end of the tank to recirc fuel when polishing. Hopefully this will keep the biomass down. @ontherocks83 your picture of the tank from the PO looks like the bottom of my tank before I replaced it too. I plan on emptying the tank and cleaning it manually when on the hard for bottom paint about every 2 to 3 years to stay ahead of the crud buildup.
 
Jun 7, 2016
312
Catalina C30 Warwick, RI
Yea, I'll be sure to tell my buddy with the Grand Banks 43 Eastbay that he should pump out his 450 gallons of diesel at the end of the season and bring it home. :)
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I get what your saying, but just because it is hard to do on big boats doesn't change that it works. On something of that size I would say a quality fuel polishing system would be more practical and doable. The big point (I believe) is that the idea of topping off your tanks to prevent moisture build up is a myth and overtime causes your tanks to gum up.
 
Dec 28, 2015
1,357
Laser, Hunter H30 Cherubini Tacoma
That seals it for me!;) To paraphrase an old financial commercial "When Maine Sail talks, people listen"

In my previous post, I was speculating based on the statement in the pdf file but nothing beats a real world demonstration. Thanks again to Maine Sail. Hope he recovers quickly and is back into providing us the best in advice :thumbup:
A better study would have been if a partially filled tank collects water.
 
Oct 26, 2010
1,431
Hunter 40.5 Beaufort, SC
The big point (I believe) is that the idea of topping off your tanks to prevent moisture build up is a myth and overtime causes your tanks to gum up.
I believe that there is some "middle ground here" that has to be considered. If a tank is completely full, there is little free volume for air (and thus moisture) to enter the tank as it "breaths" so if you are not going to empty your tank completely (as @ontherocks83 and Maine Sail do) then keeping it full is better than 1/4 to 1/2 full where there is ample air exchange PLUS fuel to absorb the moisture that goes get in. Realistically, unless you keep your tank full all the time and fill it up each time you use it you will get moisture in the fuel during the months you use it. For me in South Carolina where the humidity is very high accumulating some moisture in the fuel is difficult if not impossible to prevent.

So I'm with @jviss - Keep the tank full or near full, add BIOFOR and STARBRITE, check the Racor for water and in my case, run the fuel polishing system whenever the engine is running (to power it and not drain the battery) PLUS plan on cleaning out the tank when on the hard for bottom paint AND MOST IMPORTANTLY - sail your boat and HAVE FUN!
 
Jun 7, 2016
312
Catalina C30 Warwick, RI
I believe that there is some "middle ground here" that has to be considered. If a tank is completely full, there is little free volume for air (and thus moisture) to enter the tank as it "breaths" so if you are not going to empty your tank completely (as @ontherocks83 and Maine Sail do) then keeping it full is better than 1/4 to 1/2 full where there is ample air exchange PLUS fuel to absorb the moisture that goes get in. Realistically, unless you keep your tank full all the time and fill it up each time you use it you will get moisture in the fuel during the months you use it. For me in South Carolina where the humidity is very high accumulating some moisture in the fuel is difficult if not impossible to prevent.

So I'm with @jviss - Keep the tank full or near full, add BIOFOR and STARBRITE, check the Racor for water and in my case, run the fuel polishing system whenever the engine is running (to power it and not drain the battery) PLUS plan on cleaning out the tank when on the hard for bottom paint AND MOST IMPORTANTLY - sail your boat and HAVE FUN!
I see your point, but (and I could be wrong) what if you look at the amount of fuel in your tank as the size of a sponge. The bigger the sponge the more water it can collect. For example if you have a drop of fuel in a tank vs a gallon of fuel, then which will absorb more moisture and deposit it into the tank? Without the fuel there is nothing to attract or hold onto the moisture. I like MikeHoncho's idea of experimenting with how much a partially filled tank collects water and adding to that, a full tank of fuel. However I doubt Maine Sail (and I can't blame him) would want to go through with that experiment since he's already proven his point.
 
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May 17, 2004
3,476
Beneteau Oceanis 37 LE Havre de Grace
I see your point, but (and I could be wrong) what if you look at the amount of fuel in your tank as the size of a sponge. The bigger the sponge the more water it can collect. For example if you have a drop of fuel in a tank vs a gallon of fuel, then which will absorb more moisture and deposit it into the tank? Without the fuel there is nothing to attract or hold onto the moisture. I like MikeHoncho's idea of experimenting with how much a partially filled tank collects water and adding to that a full tank. However I doubt Maine Sail (and I can't blame him) would want to go through with that experiment since he's already proven his point.
I would think of the fuel less like a sponge and more like a one way trap. Diesel will absorb water but the water will tend to sink to the bottom. So the next time there’s condensation more water just comes in and sinks again. The amount of water would be proportional to the surface area of the empty part of the tank and fuel surface. A 100% full tank like jviss’s has virtually no surface area for condensation, so it’s not a problem. A 100% empty tank like yours and Maine’s has no fuel to trap the moisture down, so also not a problem. But the intermediate conditions seem to be the ones to avoid.
 
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Likes: TimFromLI
Oct 26, 2010
1,431
Hunter 40.5 Beaufort, SC
@ontherocks83 - you miss my point. I agree with you that a completely empty tank (as Maine Sail) tested might very well be the best solution, but that doesn't cut the mustard down here where we sail all year long. My last post was along the lines of @MikeHoncho in that a partially full tank might be the "least desirable" solution. We don't store our boat for the winter so it is always in the water and ready to sail on a nice winter day (sometimes the best sailing here). If you can't empty it to reduce the "sponge" as you refer to it, so reduce the free volume of air available for "exchange" into the tank volume. Wish Maine Sail was available to weigh in on this but we'll have to wait on his recovery for that.
 
Jun 7, 2016
312
Catalina C30 Warwick, RI
@ontherocks83 - you miss my point. I agree with you that a completely empty tank (as Maine Sail) tested might very well be the best solution, but that doesn't cut the mustard down here where we sail all year long. My last post was along the lines of @MikeHoncho in that a partially full tank might be the "least desirable" solution. We don't store our boat for the winter so it is always in the water and ready to sail on a nice winter day (sometimes the best sailing here). If you can't empty it to reduce the "sponge" as you refer to it, so reduce the free volume of air available for "exchange" into the tank volume. Wish Maine Sail was available to weigh in on this but we'll have to wait on his recovery for that.
You make a good argument and I am curious to what the final answer is.

Going to an extreme between what you and Davidasailor are saying, then a partially filled tank would eventually become completely full having filled up with water from condensation, where a full or empty tank would not. I wish I had 3 extra fuel tanks I could put in my shed attic and give it a couple year experiment.
 
Oct 26, 2010
1,431
Hunter 40.5 Beaufort, SC
Interesting discussion. One thing to consider between Maine's experiment and the leaving fuel in the tank.

1. In a completely empty tank even if water "condenses" on the tank walls at night when things cool down, the water will evaporate when the tank warms up during the day. Think a pan of water sitting in your garage. Put a cup of water in a pie plate and come back a week later. The water will be gone as it eventually evaporated into the atmosphere. I suspect that is the mechanism in an empty tank over a period of time. As we say in engineering - the gozzouta equals the gozzintu.

2. In a tank with fuel in it, as water vapor enters the tank and condenses, some is absorbed by the fuel in the tank and "sinks" to the bottom, being non-soluble in the fuel. Maybe only a teaspoon but some water accumulated. Its a one way trip to the bottom of the tank. There is no opportunity for the water to evaporate with the fuel on top of it. The next day the cycle repeats itself. I doubt there would be enough to "fill the tank" but enough to allow algae growth and accelerated corrosion in the bottom of the tank (which happened in my old tank)

So how do we deal with this. When putting it in storage for the winter (that is if you do) emptying the tank is probably the best solution. If you can't, or don't, layup your boat for the winter, then it would seem keeping your tank full then would be the next best thing so as to reduce the free volume of air for exchange and thus the amount of water vapor that might be introduced to feed the moisture cycle. Also, the addition of a moisture trap filter on the vent line seems like a good idea and I think Maine Sail has mentioned this before.

Just my engineering/thermodynamics speculating here and since I didn't sleep in a Holiday Inn last night its just that - speculation.
 
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Dec 28, 2015
1,357
Laser, Hunter H30 Cherubini Tacoma
Interesting discussion. One thing to consider between Maine's experiment and the leaving fuel in the tank.

1. In a completely empty tank even if water "condenses" on the tank walls at night when things cool down, the water will evaporate when the tank warms up during the day. Think a pan of water sitting in your garage. Put a cup of water in a pie plate and come back a week later. The water will be gone as it eventually evaporated into the atmosphere. I suspect that is the mechanism in an empty tank over a period of time. As we say in engineering - the gozzouta equals the gozzintu.

2. In a tank with fuel in it, as water vapor enters the tank and condenses, some is absorbed by the fuel in the tank and "sinks" to the bottom, being non-soluble in the fuel. Maybe only a teaspoon but some water accumulated. Its a one way trip to the bottom of the tank. There is no opportunity for the water to evaporate with the fuel on top of it. The next day the cycle repeats itself. I doubt there would be enough to "fill the tank" but enough to allow algae growth and accelerated corrosion in the bottom of the tank (which happened in my old tank)

So how do we deal with this. When putting it in storage for the winter (that is if you do) emptying the tank is probably the best solution. If you can't, or don't, layup your boat for the winter, then it would seem keeping your tank full then would be the next best thing so as to reduce the free volume of air for exchange and thus the amount of water vapor that might be introduced to feed the moisture cycle. Also, the addition of a moisture trap filter on the vent line seems like a good idea and I think Maine Sail has mentioned this before.

Just my engineering/thermodynamics speculating here and since I didn't sleep in a Holiday Inn last night its just that - speculation.
I sent MS a message on this a while back and he was not interested. Water won’t evaporate when it’s at the bottom of the tank with diesel on top. The sloshing of the diesel onto fuel tank walls that are condensing increases the potential. Empty tanks decrease the potential of condensing due to its ability to heat sink.
 
Oct 26, 2010
1,431
Hunter 40.5 Beaufort, SC
I sent MS a message on this a while back and he was not interested. Water won’t evaporate when it’s at the bottom of the tank with diesel on top. The sloshing of the diesel onto fuel tank walls that are condensing increases the potential. Empty tanks decrease the potential of condensing due to its ability to heat sink.
@MikeHoncho - That was what I was trying to say. "even if" was applied to the empty tank so "even if" it did condense onto the walls (of an empty tank) it will eventually evaporate. Bullet 2 says it can't evaporate if there is diesel on top of it. I think we agree Nes Pa?
 
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Oct 26, 2010
1,431
Hunter 40.5 Beaufort, SC
Is the above contradictory? If the water is non-soluble in the fuel, how can the fuel absorb it?
Good point @jviss - probably a poor choice of words for non-soluble. Diesel fuel is highly hygroscopic (How does water get into my fuel tank) and does absorb water plus it can get into your fuel from contamination in the tanks where you refuel. I'm not sure of the "mechanics" nor do I wish to spend time discussing it any more, but it does sink to the bottom where, as Maine Sail said and we can all agree, it does not evaporate out. So again I ask. If you are not putting your boat to bed for the winter, which many of us in the Southern States do not, and emptying the tank it is not practical on a daily basis how do we keep the water and the "diesel bugs" out. Filtration for the bugs, fuel treatment, a "water trap" on the filter checked frequently, and keeping the tank mostly full seems to be the best compromise. I seem to remember Maine Sail writing (but don't quote me on it since I'm getting long enough in the tooth to confuse things) that he has a onboard fuel polishing system and wouldn't have a sailboat without one. I don't know his position on a vent filter of some sort though. We could always "use a lot of fuel" but then that kind of defeats the purpose of a sailboat doesn't it.:biggrin: Good discussion though.
 
Jun 7, 2016
312
Catalina C30 Warwick, RI
Is the above contradictory? If the water is non-soluble in the fuel, how can the fuel absorb it?
Fuel can retain water but once it hits a saturation point it settles to the bottom of the tank. It's not soluble in the sense that water does not dissolve in fuel it remains separate. Think oil and vinegar dressing, they can mix but will eventually settle separately.
 

jviss

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Feb 5, 2004
4,627
Tartan 3800 Westport, MA
It's not soluble in the sense that water does not dissolve in fuel it remains separate.
That's not entirely so. A small amount of water can be in solution in diesel, in the ppm range, and some can be in suspension as well. The water in suspension will eventually settle to the bottom.
 
Jul 7, 2004
8,016
Hunter 30T Cheney, KS
Diesel fuel has a very long life. Don't worry about it.

Do be concerned about bacteria growth in the tank and use a biocide. BioBar JF is one of the best biocides.
:plus: I use BioBar and StarBrite Star Tron Enzyme fuel treatment. I also have an H2Out desiccant filter in the fuel vent line to trap incoming moisture. I get a season out of the desiccant crystals before I need to recharge them. They turn pink when saturated.
 
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Sep 25, 2008
6,315
Alden 50 Sarasota, Florida
While opinions seem to vary -
1. Over time, diesel loses much of its volatile components. That why we can smell diesel. The longer (e.g., 6 months or more), it is sits, the greater fraction of volatiles is lost.
2. over time in the presence of any moisture, the volatile components solubilize in the water at concentrations in the high ppm range thereby losing more combustion characteristics, and
3. over time, water if present can emulsify in the diesel which results in entrained water in filters in addition to other problems such as ‘growth’ on the tank walls.

Old diesel is problematic. Your choice…