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Old 13-11-2013, 16:08   #31
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Re: Leave tank full for winter?

So would the best thing to do to avoid condensation just be to block the vent so that no air changes throughout the day? That would seem to limit the water that can be lost to the fuel and wouldn't make any difference how much fuel you leave in the tank.
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Old 13-11-2013, 16:14   #32
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Re: Leave tank full for winter?

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Originally Posted by jkleins View Post
So would the best thing to do to avoid condensation just be to block the vent so that no air changes throughout the day? That would seem to limit the water that can be lost to the fuel and wouldn't make any difference how much fuel you leave in the tank.
You could potentially split the tank by doing so. This is why products like H2Out exist.
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Old 13-11-2013, 17:16   #33
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Re: Leave tank full for winter?

I personally believe the condensation theory is an old wives (sailors, mechanics, etc) tale. We used to be told that all the time with our cars but I do believe that most of the water getting into the tank comes from:
a) the fuel. I think this happens a lot.
b) the fuel fill gasket on the deck fitting. This one is probably the worst and most occurring option.

When my brother had oil heating he used to fill his tank once a year (outside tank), in the fall. He never had problems with water condensate in his fuel.
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Old 13-11-2013, 17:49   #34
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Re: Leave tank full for winter?

My belief is that it does not matter how you store diesel as long as you do not allow rain or some other direct contaminant to enter.

-My aluminum tanks were 20% full for 10 years while on the hard.
-Tanks vents were open to the atmosphere.
-No fuel additives have ever been added to these tanks.
-Fuel outlet fittings are on the absolute bottom of the tanks.
-No water has ever been found anywhere in the fuel system.
-No debris have ever been found anywhere in the fuel system.

I was paranoid about the age of the fuel so I replaced it before launching. A friend happily burned the old fuel in is truck. I will never give away old diesel fuel again.

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Old 14-11-2013, 07:36   #35
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Re: Leave tank full for winter?

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Originally Posted by valhalla360 View Post
This is where my theory that condensation is a myth comes in.

At 50% humidity normal pressure and temperature of 5C, 1 cubic meter of air contains approximatley 2.5 grams of water. So assuming you have a large 500 gallon tank half full there is around 1 cubic meter of air space.

If we assume the air is completely replaced once a day for a 180 days and every last molecule of water is condensated out, you get around 450 grams of water from condensation.

This is overstating the condensation by a large amount.
- With a small vent, maybe 25% of the air is turned over each day due to thermal expansion of the airspace. This is the only source of additional water, so we are down to around 112 grams of potential water from condensation.
- If the outside air averages around 50% relative humidity, cooling first raises the relative humidity before creating condensation. Even when condensation is created some of the moisture always remains in the air. That further reduces the amount of condensation created. Probably by at least another half. so we are down around 60 grams of potential water from condensation.
- In the morning, a large percentage of the condensation will still be on the walls and ceiling of the tank (at very cold temperatures, the water will form frost and almost none will make it into the fuel. As it warms, this moisture will largely evaporate, so you are down to maybe 10 grams of potential water from condensation.

10 grams is still likely a worst case senario as most boats have much smaller tanks and even in Michigan we only store the boat for 5 months at a time. Further south it would be even less time.

So while a dry tank would have those 10 grams evaporate, even if there was fuel, it's not likely enough to overwhelm your water seperating filter.

This leads me to the belief that water in fuel problems are far more likely to be coming from other sources than condensation such as a leaky fuel fill.
The empty tank condensation theory for inside cold storage is probably a myth and so I agree with you Valhalla in that narrow example.

I did some calculations for a empty 100 gallon tank, used 90 days (spring and fall) for condensation and a 20 deg C at 75% RH. I let half of the water condensate and got roughly 2 gms per day which could turn into 180 gms in 90 days or 0.05 gallons of water (if all my sums are correct.) An insignificant amount of water is accumulated.

The amount of accumulated water could change dramatically if there is constant air circulation into the tank.

For example, if the boat is stored outside, air is blowing past the vent may cause a constant circulation and replenishment of moist air in the tank. The replenishment could result in 10's of air changes per hour in the tank while near the dew point. If the tank is colder (sustained by the thermal mass of the fuel) then the replenishment and subsequent condensation by the cold tank could easy result in a dramatically amplified accumulation of water.
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Old 14-11-2013, 07:56   #36
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Re: Leave tank full for winter?

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The empty tank condensation theory for inside cold storage is probably a myth and so I agree with you Valhalla in that narrow example.

I did some calculations for a empty 100 gallon tank, used 90 days (spring and fall) for condensation and a 20 deg C at 75% RH. I let half of the water condensate and got roughly 2 gms per day which could turn into 180 gms in 90 days or 0.05 gallons of water (if all my sums are correct.) An insignificant amount of water is accumulated.

The amount of accumulated water could change dramatically if there is constant air circulation into the tank.

For example, if the boat is stored outside, air is blowing past the vent may cause a constant circulation and replenishment of moist air in the tank. The replenishment could result in 10's of air changes per hour in the tank while near the dew point. If the tank is colder (sustained by the thermal mass of the fuel) then the replenishment and subsequent condensation by the cold tank could easy result in a dramatically amplified accumulation of water.
Though this physics class is amusing I don't think anyone bright would want to test it. The good news is you will get maybe a mile offshore before you have to drain the system and bleed the lines. Hopefully that will get you back to the dock before having to do it all again. Heaven forbid you start a major passage by sail and have to fire up the iron horse 200 miles or more out and go through the exercise over and over again. Your call.
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Old 14-11-2013, 08:06   #37
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Re: Leave tank full for winter?

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Though this physics class is amusing I don't think anyone bright would want to test it. The good news is you will get maybe a mile offshore before you have to drain the system and bleed the lines. Hopefully that will get you back to the dock before having to do it all again. Heaven forbid you start a major passage by sail and have to fire up the iron horse 200 miles or more out and go through the exercise over and over again. Your call.
You may have missed all or some of the previous 34 posts in this thread???
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Old 14-11-2013, 10:52   #38
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Re: Leave tank full for winter?

Lake Superior:

Considering the small size of the vent, relatively long hose and the fact that it should be the only opening (assuming the fuel fill is closed and seals), I doubt wind blowing over the vent opening is going to change the air over several times per hour. I would expect very little change over beyond what thermal expansion causes. My 25% assumption is probably on the high side.

Sailing Cowboy:

And that's how this myth is perpetuated. We resort to an attempt instilling blind fear into the reader rather than discussing factual information.

I may have made a math mistake or got one of my assumptions wrong but if the real problem is leaky fuel fills, wouldn't it make more sense to point people in the direction of solving that problem?

A better question may be why manufacturers don't solve the problem:
- A small sump with a drain would make it easy to drain off any water in the spring.
- It shouldn't be hard to design a bladder that absorbs the expansion while limiting the introduction of new air. There are a few complications to be accounted for but nothing expensive or overly complicated (yeah I know if it's marine it's expensive).
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Old 14-11-2013, 11:16   #39
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Re: Leave tank full for winter?

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Originally Posted by valhalla360 View Post
Lake Superior:

Considering the small size of the vent, relatively long hose and the fact that it should be the only opening (assuming the fuel fill is closed and seals), I doubt wind blowing over the vent opening is going to change the air over several times per hour. I would expect very little change over beyond what thermal expansion causes. My 25% assumption is probably on the high side.
Think of a manometer measuring air speed past the tank vent opening (orifice.) The air flow gives rise to a decent pressure differential in the manometer or as a consequence air flow in the vent hose. If the air flow speed is fluctuating, as is normal in the ambient environment, this will give create a pumping effect. Air will flow in and out of the vent hose and perhaps the tank depending on the length of hose and space in the tank. The more the space in the tank the more air that will be pumped.

So if the vent hose is short enough, and many are fairly short, then lots of liquid water could be made.
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Old 14-11-2013, 12:04   #40
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Re: Leave tank full for winter?

Other facts that need consideration in this discussion.

ULSD Diesel is not the same as the dyno-diesel of 10 years ago. First it's hydro-craked to remove the sulfur. It is now more then ten times hygroscopic then the older diesel.

Fuels are bulked transferred in the same truck as E-10 gasoline, without cleaning in between.

A study by the University of Idaho determined the LSD Diesel began degradation after 28 days. ULSD diesel is even worse.

ULSD has shelf life under normal conditions of 90 days.

After said period it will have more diffused water, more oxidation, less cetain value, more gums and varnish. This leads to lower btu values, coking in the valves, and ring lands.

My Dickinson oil stove is my Canary in the coal mine. When my on-board fuel starts aging. The stove burns with less heat and and higher carbon and soot build up.

This how I deal with the issues now. I have 500 gallons in 2 tanks. I only keep on-board what I can use in 90 days. I have an on-board fuel polishing system that I run every 60 days. I treat my fuel with Standyne Performance additive, I treat my fuel with TCW3 2 cycle oil at 200:1.

Lloyd
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Old 14-11-2013, 12:33   #41
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Re: Leave tank full for winter?

Here is an article by an expert:

The Myth of Condensation in Fuel Tanks by David Pascoe: Boat Maintenance, Repairs and Troubleshooting
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Old 14-11-2013, 13:05   #42
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Re: Leave tank full for winter?

Quote:
Sailing Cowboy: Though this physics class is amusing I don't think anyone bright would want to test it. The good news is you will get maybe a mile offshore before you have to drain the system and bleed the lines. Hopefully that will get you back to the dock before having to do it all again. Heaven forbid you start a major passage by sail and have to fire up the iron horse 200 miles or more out and go through the exercise over and over again. Your call.
If these scenarios happen then it is pure sloth. There is no excuse for lack of maintenance IMHO. Having run into a similar situation while offshore (not my boat) I know that fuel and tank have to be kept clean and if there is water in the tank the source has to be found and corrected. There is thread after thread on this site of people running into fuel problems. There is just no excuse not to have a clean tank, clean fuel and clean filters. There are however lots of "hasn't happened to me so...". They should add the word "yet" to those sentences.

I don't believe that any appreciable amount of water comes from condensation. There is just not that much air circulating in and out of the tank. The vent is there specifically to stop the tank from collapsing, bursting and to help prevent oil spills.
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Old 14-11-2013, 13:21   #43
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Re: Leave tank full for winter?

Experts....

I am a licensed API tank inspector and have been inside over 1000 oil tanks. I frequently--though not always--see corrosion and rust streaking underside of the roofs due to condensation. In fact, the API inspection procedure requires that the roof and rafters be examined for soundness before that tank can be entered. On several inspections I have found rafters on the tank floor. So yes, condensation is a myth.

What is true is that the amount of water generated with this mechanisim should never be enough to generate measureable free water. The fuel will be able to absorb the water when temperatures rise, unless water is entering the tank from another source. But there is enough water entering to increase corrosion. The extent to which this happens is VERY situation specific and a few ancidotal case studies on either side of the debate do not prove or disprove the mechanism, since that amount is vanishingly small.

I have also observed that a silica gel filter can actually reduce the water content of an unused tank during the winter, removing a few grams of moisture. The tanks with the vent filters were noticable clearer, contained less water (tested by Karl Fisher) and had lower corrosion rates. Free water? No, we never saw persistant free water in any case.
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Old 14-11-2013, 13:39   #44
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Re: Leave tank full for winter?

If the boat is in the water and the tank is in the keel, temp changes in the tank on a daily basis will be minimal if you discount fuel return from a running engine. Vent size can be tiny - ours is 3mm diameter hole in a 25 mm pipe which is only uncapped whilst filling the tank.
If you want to fill the tank with argon for long term storage ( inert gas plus no moisture) , get a mig or tig regulator and a small argon cylinder. Cool the argon tank with ice in a garbage can so the gas is colder than the air in the tank and will therefore fall to the bottom of the tank.
Knowing the fuel tank capacity, and the flow rate on your argon gauge, calculate how long it will take to fill the tank. You can trickle the argon into the tank to displace the air in there. You are left with dry inert gas and no possibility of corrosion or condensation.
You should be able to hire a cylinder and regulator if you do not already own one.
IMPORTANT: Argon can suffocate you, make sure your work space is well vented and that you do not over fill the tank and thus fill the boat with argon. A person on deck watching could be a good safety measure.

Regards,
Richard.
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Old 14-11-2013, 16:21   #45
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Re: Leave tank full for winter?

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I have also observed that a silica gel filter can actually reduce the water content of an unused tank during the winter, removing a few grams of moisture. The tanks with the vent filters were noticable clearer, contained less water (tested by Karl Fisher) and had lower corrosion rates. Free water? No, we never saw persistant free water in any case.
This sounds true to me. I bet that some of the water vapor actually came from the fuel. Were these tanks in general much larger than our normal cruising yacht fuel tank...30 - 200 gals. I would suppose they were from the description of the "rafters". Maybe been in service for a long time, out in the elements?
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