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Tuna! AKA Papermaker
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While researching fuel tank corrosion, I found this interesting article written by a boat surveyor. I know that every year there is discussion on this.

The Myth of Condensation in Fuel Tanks

by David Pascoe



Frequently we hear it said that the cause of water in fuel tanks is due to condensation. I have long doubted this assertion but the issue has come up so frequently that I was finally motivated to try prove to the point. The basis of my belief or assumption is that:

There isn't enough air volume within a tank to hold much vapor.

On average, tanks are half full, further reducing volume

The amount of water vapor in air is very small, even at 100% humidity

Conditions aren't right to cause condensation in a fuel tank
Research produced the following values for the maximum amount of liquid water in air at the following temperatures:

30C/86F30 grams/cubic meter20C/50F17 grams/cubic meter10C/13F9 grams/cubic meter
There are 28 grams per ounce, so 30 grams = 1.07 oz; 17 grams = 0.6428 oz.

A cubic meter equals 264 gallons of liquid volume, therefore:
A 200 gallon tank = 0.76 cubic meter.

At 86F, an empty 200 gallon tank could contain 22.8 grams of water vapor, or 0.81 oz.

At 50F, an empty 200 gallon tank could contain 12.92 grams of water vapor, or 0.46 oz.
Note that this is the maximum amount of water vapor that a completely empty tank could contain, in neither case a full ounce of water.
In order to condense water out of the atmosphere a surface must be much colder than the air. The problem for the condensation in tank theory is; how do we end up with a fuel tank that is much colder than the air? One way would be to have a very cold day that suddenly warms up dramatically, but when does this ever happen? The weather can turn cold very fast, but does not suddenly get very warm.
Aluminum is second only to copper for rapid heat transfer properties; it will therefore adjust to atmospheric temperature changes quickly. Gasoline and diesel fuel, like water absorb [sic] heat and cold slowly. Thus one might expect to see sweating on the outside of a tank as the day warms up from cold mornings, but do we? Well, I can say that after 35 years of inspecting boats, I've rarely seen tanks sweating. Note: Sweating may be likely to occur with boats in very cold waters when warm days are encountered.
This issue first came up a number of years ago over a question of whether internal engine rusting could be due to condensation caused by sudden temperature changes as from day to night and vice versa. Since that time, inspection of hundreds of engines showed that rust only occurs on the underside of valve covers due to water contamination of the oil. Very few engines have rusty undersides of valve covers, thereby proving the point that ice cold engine blocks in the morning don’t sweat at it warms up during the day. If that is true, then how could it be true that fuel tanks sweat?
My answer is that they don’t and these calculations prove it. My initial assumptions were correct. You do not need to store or lay up your boat with full fuel tanks. If you are getting water in your fuel, it is getting there some other way.
Contaminated Fuel
Years ago we had serious fuel contamination problems due to underground steel storage tanks that rusted and leaked. Today all tanks are fiberglass, so this no longer happens (that I know of). However, those underground tanks do have fill plates on the ground surface (usually the parking lot) that can leak just like your boat deck plate. As the marina pumps its tanks nearly dry before the next fuel delivery, those who buy fuel from the near empty tank are the ones that are going to get the water (because it's pumped from the bottom of the tank). This despite the fact that the dock fuel pump has a water separating filter. I've opened the panels on occasion and have found the sight bowls completely filled with water, so at this point the water is being passed on to the customer. Next time you buy diesel, ask to see the filter at the pump! You have to remove the lower pump panel to see it.
However, it is important to note if you're getting water from your fuel retailer, chances are that it won't be a small amount. Most likely it will be a lot and your filters will fill up and engines crap out post-haste.
Leaking Fill Caps
By far the most common cause of contaminated boat tanks are deck fill caps that leak. Most of these things are stupidly mounted flat on decks which may puddle with water. The cap has a tiny little O-ring that is supposed to seal and keep water out. DOES IT? I wouldn't depend on one of these things unless I could prove that it doesn't leak. Check the condition of the o-ring and weather it is sealing.

One way to check positively is to clean the o-ring seat thoroughly; next apply some black or any color paint to the o-ring and screw the cap in place, tight. Then remove it and see if the paint has been completely transferred to the ring seat. If not, you now know where the problem is.

Another problem is the simple failure to seat the cap fully after refueling. This actually happens a lot, so check to see if the cap is loose.
The Tank Vent
Improperly located fuel tank vent fittings are one of the top causes of water getting into tanks. When this is the cause, if you are a salt water boater, then it will be salt water in your tank. A fuel tank vent fitting on the side of the hull should be angled down and aftward. If angled in any other direction, you've got a problem that needs fixing. Watch out for deteriorated plastic and zinc alloy fittings; some of these things deteriorate incredibly fast.
The vent line should have a riser loop on the inside. That is, it travels upward first, then downward. If not, that is another potential problem.
Check the Fuel Gauge Sender
One final possibility is the fuel gauge sender plate on top of the fuel tank. These are often made of steel or have steel screws that can rust away, a situation I've seen several times. Is water puddling on the tank top? Test all screws with a screw driver to make sure they are securely seated.
 

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Also watch out when you do your wash down because you usually spray water everywhere and most people do not cover the vent tube outlet before washing the boat down. Most vents do not have a vertical section of tube inside the boat to keep water from running on into the tank. Check yours out.
 

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Any Pilot will attest that Water will condensate in fuel tanks. Granted the fuel tank's temperature can very as the airplane reaches different altitudes and reach differant temperatures.

But for those of us that trailer a boat, do we not travel through varying climates; over coastal passes.....from a warm shop or garage to a cold coastal bay? Or the reverse going home?

All I know is that my tank condensates.
 

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Air is warm during the day, gas and air expands, vapors are developed and fuel vapors (like butane ) and air goes out

At night fuel and air contract, vapors condense air goes in with moisture, leaving some behind, repeat for 60 days and you have ounces if not pound of water in your fuel tank.

Stabil will just keep deposits soluble, it doesn't have enough isopropyl alcohol to keep all the moisture in solution.
 

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Air is warm during the day, gas and air expands, vapors are developed and fuel vapors (like butane ) and air goes out

At night fuel and air contract, vapors condense air goes in with moisture, leaving some behind, repeat for 60 days and you have ounces if not pound of water in your fuel tank.

Stabil will just keep deposits soluble, it doesn't have enough isopropyl alcohol to keep all the moisture in solution.
Yepyep
 

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My first thought when I saw the path he was going down is that he wasnt going to take into consideration that the tank is a vented system. Thus air can move into and out of the tank after it's moisture content is reduced. Big difference compared to when you calculate the moisture content based on a closed system tank. And if that was the case, you wouldn't have to worry about moisture in your gas because your motor wont run if the tank cant vent.
 

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My first thought when I saw the path he was going down is that he wasnt going to take into consideration that the tank is a vented system. Thus air can move into and out of the tank after it's moisture content is reduced. Big difference compared to when you calculate the moisture content based on a closed system tank. And if that was the case, you wouldn't have to worry about moisture in your gas because your motor wont run if the tank cant vent.
ALSO he is forgetting that Ethanol in fuels openly attract and bond with water vapor, so even if condensation isnt visible, water can still contaminate your fuel.... if you have 10% ethanol, in 100 gallons so 10 gallons pure alcohol... 1% or .1 gallons of water is all it takes 12.8 OZ of water (coke can) to create a phase shift of the entire 100 gallon tank... rendering it USELESS!!!!!!!
 

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Once again lots of hypothesis about condensation, but no actual data by anyone.

Spend the time to calculate the actual volume that can possibly be transported through a vent tube with each possible expansion/contraction based on temperature change..... That is... When the fuel in the tank expands as well as the gas vapor due to heating, what is the volume that cannot be contained by the tank and is expelled to the environment? With the idea that tanks "breathe" then this is the available volume that would in turn be able to by drawn back into the tank- This exchange will only happen ONCE per day---expand with heat in the morning, be drawn back in the evening/night. Average temperature change from High to Low in Oregon from November through May is around 15 degrees.

Next be sure to consider what the relative humidity is, and how much water vapor can be carried by the given volume of air. Remember, you are only calculating the EXPANDED vapor as that is all that is replaced in the tank, not the open volume of the tank...... Here's some help.... 3 cubic feet of air equates to about 20 cubic gallons. 20 cubic gallons of air can hold max water vapor of about 1/20 of an ounce of water at 100% humidity.

Next up is factoring how much of the water vapor can actually come out of the gas form via dewpoint on the open surface area of the tank. Difficult to absolutely control, so easiest (although not exactly right) to just assume that 100% of the water vapor comes out of the gaseous form.

The point of all this is that there is a lot of supposition about the transfer of water from the air INTO the gas tank. In order for this to take place there needs to be a physical reason for the tank to actually CAUSE condensation. The tank walls need to be below dewpoint (like a cold glass of water in a warm room) to pull the water vapor from the air. There is no physical scientific way that the water can transfer from the air vapor to the tank without the basis of condensation via the tank wall being lower temperature than the ambient air temperature.

Now.....there is also one other way that water vapor can get into the tank via osmosis instead of vapor exchange. The ability for water to come out of the vapor still relies 100% on condensation on a tank wall.

Water intrusion via condensation is possible, but the degree and amount that is attributed to it is probably overstated to a large degree. If the max the above volume of air (20 cubic gallons) can hold is 1/20 of an ounce, and ALL of it condenses out every day- (so the relative humidity in the tank goes to 0%) then over the course of 180 days you would have 9 oz. of water. Water in fuel tanks of BOATS are more likely caused by contaminated fuel or from leaky caps or vents than condensation.

In an earlier thread, you can see further detailed information on all the mathematics used to reach the conclusions, including statistical references for the average temperature, dewpoint, humidity volume at 100% per given volume of air and other info. I've looked into this alot as a sort of hobby question...any info that contradicts this would be welcome, but so far nothing has come up except Urban Legend rationalization.

The pictures are no longer in the thread below, but the info is. Boat stored outside, at the coast, about 1000 feet from the Ocean. Lots of humidity all winter. Point being..... 150 gallon tank stored with only 20 gallons all winter (like always) and no water present in the spring.

http://www.ifish.net/board/showthread.php?t=148177&highlight=condensation
 

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Tuna! AKA Papermaker
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Discussion Starter #10
:yeahthat: Thanks
I posted this as information from a professional who surveys boats for a living. Found it when researching a problem.
But like many threads, was overtaken by hearsay and Isay. :bricks:
Glad to see someone using science.
 

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Leaking Fill Caps
By far the most common cause of contaminated boat tanks are deck fill caps that leak. Most of these things are stupidly mounted flat on decks which may puddle with water. The cap has a tiny little O-ring that is supposed to seal and keep water out. DOES IT? I wouldn't depend on one of these things unless I could prove that it doesn't leak. Check the condition of the o-ring and weather it is sealing.



In my years of boating, the statement above defines the only issues I have ever had with water in my fuel cell. Get rid if the plastic fill cap that is placed flat on your gunnel and like magic.... problem is gone. In the 4 boats I have owned, the only time I ever had an issue was when I had a filler cap on my boat that was put in this way. Many of us on this board had the issue. The 0.46 of a quart is nothing. It's the gallon plus that will kill ya.
 

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Nearly a dozen boats over the past 20 yrs.....NEVER stored them with full fuel tanks or bought stabilizers for the fuel.....and have never noticed condensation or water problems in the fuel....no carb rusting problems...no engine failures...no difficulty getting my boats operational in the spring....every boat I have owned has been stored outside.

I guess I got lucky???
 

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While researching fuel tank corrosion, I found this interesting article written by a boat surveyor. I know that every year there is discussion on this.

The Myth of Condensation in Fuel Tanks

by David Pascoe

Seems like we need to contact the MythBusters on this one!!:jester:

David Pascoe is a sharp guy and I always look forward to reading his articles but I can't help thinking he's wrong on this one. If putting a cold glass of water on the coffee table will leave a puddle of water (and a scowling wife) then it doesn't seem too far fetched to believe that a less than half full tank of gas in a boat might cause some water to develop. Still, I have never seen a tank sweat on a boat either and I've been in hundreds of bilges.

I should have paid more attention in science class :tongue:
 

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Seems like we need to contact the MythBusters on this one!!:jester:

David Pascoe is a sharp guy and I always look forward to reading his articles but I can't help thinking he's wrong on this one. If putting a cold glass of water on the coffee table will leave a puddle of water (and a scowling wife) then it doesn't seem too far fetched to believe that a less than half full tank of gas in a boat might cause some water to develop. Still, I have never seen a tank sweat on a boat either and I've been in hundreds of bilges.

I should have paid more attention in science class :tongue:
The first thing you should ask is what is the temperature differential between the cold glass of water and the ambient air temperature in the room? If a glass with ice in it is 40-45 degrees, then your typical temperature differential is going to be at 25-30 degrees. Humidity forms on the glass and around the bottom of the glass due to this differential. The reason you probably have never seen tanks in boats "sweat" is because they typically heat and cool at about the same rate as the ambient air temperature. The only time they WILL sweat would be in the morning as an outside air temperature rises rapidly and they do not quickly catch up with the ambient air temperature. In this instance they could sweat until their relative temperature catches up.

As Andycoho pointed out, this also explains more thoroughly why planes experience condensation. It is not simply the change in temperature at altitude, but the RAPID change in temperature upon descent FROM altitude. Being typically metal skinned with tanks located generally in the wings, the tank shell will frequently be very cold then brought into a much warmer environment as the altitude drops. Just like bringing a cold glass into a warm room.

As far as mythbusters goes..... I submitted it to them last year and got no response. I'd really be interested to find out the reality.
 

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Once again lots of hypothesis about condensation, but no actual data by anyone.
Average temperature change from High to Low in Oregon from November through May is around 15 degrees.
only on the west side of oregon.....
 

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only on the west side of oregon.....
Actually, that is the mean average for the ENTIRE state.

If you really want to split hairs, then you can see that the mean differential in Bend during the winter is 20 degrees per the Central Oregon Development Center. The mean average differential goes up during the summer significantly, but the topic of discussion is winter storage and condensation so the summer temp differential is moot.

Since you felt inclined to bring up "west side of Oregon" then talking about the East side of the state brings up some other significant data that actually leads to refuting condensation even more.... humidity and dew points. Sisters and Bend vary significantly in rainfall average per year with.. Around 12" for Bend and around 4" for Sisters. Compared to the West side of the state where Portland has around 36" of rain a year.

The dew point is also lower over East on average at about 34 degrees versus 45 degrees in the West portion of the state. When below freezing there is NO condensation as well.

It is a well know fact that the Eastern portion of the state of Oregon is "drier" than the west with significantly less humidity. Less moisture in the air means less chance of condensation. With only about a 5 degree average temperature differential from high to low compared to the West side, I don't follow any logic that it makes a whole lot of difference.

I'm certainly open to anything that shows different than I have found, I just have yet to see anyone that has made an argument based on science as opposed to supposition.
 

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Just curious...

If a water glass, or a fuel tank, has condensation on the outside, does that mean there is condensation on the inside?

Or vice versa.... when would there be condensation on the inside of the tank or glass, but not on the outside?

Like others, I've never had a water-in-fuel problem (knock on wood), but now that this topic has been "spoken out loud", I hope it will not become a self-fulfilling prophacy (knock on wood). Thanks a lot - now I've one more thing to worry about.
 

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The condensation on any item is formed by it being below the dew point of the ambient air temperature it is in, and the ambient air having enough humidity to condensate. A glass of water that is room temperature will not form condensation on the outside. A glass of water that is chilled, and filled with ice and is a temp of 35 degrees with a dewpoint of 40 degrees will form condensate on the outside until it warms to above the 40 degree mark.

Your fuel tank is the same thing. Say the dewpoint is 40 degrees again. If your boat is stored outside in 34 degree weather, then brought into a garage that is 60 degrees, condensate would form on all surface areas that are below 40 degrees until they warmed to above that same mark.

So....if the tank is condensating on the outside, then YES it could also be doing the same on the exposed surface areas on the inside. The factor that would have to be determined would be what the humidity in the tank is, and how much the open space is occupied by humid air and not by fuel vapor. This goes back to how much actual amount of water vapor could come out of the gaseous form in a short period of time.

My background on condensate comes from working with boilers in domestic and hvac heating systems. Condensate has very particular characteristics and the mixture of condensate and flue gases can destroy boilers in short periods of time. Improperly designed systems can literal produce gallons of water inside the combustion chamber.
 

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I store the fishing boat at my grandparents in bend.... no rot of wood floors in 20 seasons... no fuel problems, ect.. the AVERAGE may be 20 degrees in bend in winter but it can vary over 50 at any given day any time a year.... I grew up in Johnday and Seneca where it could be -20 or colder in morning but 30 by mid day if clear and sunny..
 

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I think it must be Gremlins...
 
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