Do you leave your tank(s) full or emply in winter?

[Don S/V ILLusion]

For those who live in the northern lats, I've done what I consider to be a reasonable amount of due diligence and found arguments from so-called experts on both sides of the issue which leads me to believe no one really knows if a partially empty or empty tank(s) promotes condensation. I'd like to know if anyone has found any reliable information regarding tank condensation fact or fiction. Notwithstanding a recent post referencing Dave Pascoe's concept, can someone point to an actual study or other data?

We should all know that the single more prominent source of water in tanks is via the fill with a bad O ring. My q is related specifically to condensation.


Without seeing any convincing evidence either way, I have always filled my tanks prior to winter and added the recommended biocides and cetane booster only because I assumed it can't hurt and I consider having "old" fuel to be preferable to water.

Other than person experience, is there anything definitive on which you base a decision?

 

[Ross]

I base my choices on personal experience and discussions with farmers

A diesel powered combine is worth at least as much as most of our boats and some of the large tractors are perhaps worth even more. So I top off my diesel tank, drian my water tanks and pump my holding tank dry. I add anti-freeze to the toilet and to all of the plumbing and pump it through the system. In the spring I flush the water systems and make them ready and start my engine and flush out the winter anti-freeze. I leave the batteries connected just as the farmers do.

 

[Rich Stidger]

I don't have a study to poin to, but.....

when we took delivery of our sailboat it had been sitting in the yard for about 9 months- from July thru the next May. There was about 1/4 tank of diesel from the factory.

During that first summer we were plagued with water in the fuel. I purged about one oz of water from the filter for every hour run. In total we removed at least 20 ounces of water.

The fill o-ring was fine and we are still using the same o-ring 11 years later.

Maybe the cap was loose, but I never observed that.

If not condensation, then what?

BTW, like Don I top off my diesel and add a shock portion of biocide prior to winter storage here on Narragansett Bay

 

[Ross]

Rich, since I doubt that the pick-up tube is in the

deepest part of the tank I expect that there is still a substantial amount of water in the tank. Clean dry fuel doesn't grow microorganisms so the need for biocides would indicate water in the fuel.

 

[s sauer]

Water in the Tank

I've cut open my poly fuel tank to install an acess hatch and clean it thoroughly. There was no water but there was some sludge and an overall dirt film on all the inside surfaces and the gauge float mechanism. Once cleaned, wiped down with alcohol satuated rags and dried, we started over with about 1/2 tank of fresh clean fuel, replaced the OEM fuel lines with new CG aproved fuel lines and new filter elements.

The pick-up tube is about 3/8" to 1/2" above the bottom of the tank and cut off at a 45 degree angle, so it will never pick up all of what's on the bottom, whether dirt contamination or water.

This poly tank is in contact with the hull which is in contact with the water around the boat. The respective water and air temperature can't change fast enough to allow condensation to occur inside this tank. I have replaced the fuel filler 'O' ring after water did enter from that filler location flush with the deck. I now only fill with enough diesel fuel (maybe 1/2 tank) for my typical needs and try to get down as low as possible at the end of the year so I can start with fresh fuel with cetane boost at the beginning of the season.

 

[MoonSailer]

Year Round in the Water

My boat is sailed year round and kept in the water. But in 5 years of use I have kept the tank less than half full usually about 1/4 or less and have never had any water in my filters. Pascoe points out the physics involved. I actually drug out a handbook of chemistry and physics before reading Pascoe and came to the same conclusion. Bad thing happen when you store diesel for a long time. For most sailboaters a full tank menas old fuel. My tank is about 20 gallons and would be enough fuel for me for two years of normal use. Now I add 5 gallons of fresh fuel whenever I get between 1/4 and empty. I do add biocide.
I think that the full tank ideal comes from gasoline. Gasoline is highly volatile and expands and contracts much more than air or diesel. Just look at a 1/2 full plastic jug of gasoline. It looks like it is going to explode when it is hot and collapses when it's cold. I do keep my outboard's tank full with the vent closed after observing the tank collapsed one cold morning. BUT we are talking diesel fuel. An empty tank is the best tank for winter storage. To test the condnsation theory leve an empty jug in your garage and see if it fills up with condensation after a month or so. I have NEVER observed a container filling itself with condensation.

 

[Big Windy]

Leave Them Full

If your boat is outside and sits in the sun...leave the tanks full. It is no myth that condensation forms from the constant heating and cooling that occurs every day the sun shines then goes in and things cool down. If the boat is inside, no problem leaving them partially filled.

 

[MoonSailer]

Big Windy Stop Blowing

Big windy instead of making empty statements make some arguments. Pull out a book of physics and give some facts!!!!! I can assert that pigs can fly using their big ears and gas out their butts. But that doesn't mean pigs can fly. Give some facts and reasons not empty assertions. Explain the physics involved in tanks filling with condensation water.

 

[Don S/V ILLusion]

condensation theory

Putting aside the kitchen experiments and urban legends, I can infer that no one can point to specific studies under conditions similar to what we experience on boats except for the obvious that water can form from condensation with wide temp swings. As oil companies and home heating oil tank mfgs have arguably some liability to avert, it would be expected they too would jump to the obvious conclusion that water from condensation can form and is bad if it does.

What seems to be lacking is these warnings is the ability of the marginally coherent person to block the vent line in winter to minimize if not eliminate the vapor transport route thereby minimizing or eliminating the formation of all except an insignificant amount of condensation regardless of the level of fuel contained in the tank.

I guess this a largely academic exercise but it's raining (again) here and with the price of fuel as it is, I'd like to get a better sense if it's worth spending money to buy what will eventually be old fuel by the time I use it.

 

[Ross]

Don and everyone else, I think we can conclude that

the tank should either be completely full or completely empty. A partially full tank is subject to problems.

 

[RichH]

Dont put a valve in the vent line !!!!!!

.... unless you want a smaller tank. If the tank is 'vacuum rated' (nobody build a boat fuel tank that's vacuum rated) then its ok as the wall strength will prevent the possible slow implosion.

Fuel oil is produced at such extreme high temperature that its essentially totallly 'dehydrated'. Water vapor will continue to 'quilibrate' into the oil until equilibrium is reached if the oil is in contact with water vapor from the atmosphere.

Water soesnt magically condense in empty tanks ... if so then for water all we'd have to do is install *empty* tanks and they would magically fill with water.

Water that does 'condense' inside a tank only means that the oil has become saturated (contaminated) with water (vapor) .... its the water from the contaminated oil condensing from the mixture, etc. etc. .... doesnt happen with 'dry' fuel. If you dont want water in your oil, buy 'fresh' fuel that hasnt become saturated, keep the fill port's 0-ring intact, (if you must) put a canister of desiccant on the vent line to trap/adsorb any moisture entering the tank by equilibrium. No, the tank is NOT 'breathing' - its simple equilibrium drive of the water INTO the oil and it doesnt matter how full the tank is as if there is dry fuel in the tank equilibrium will drive the water vapor into the oil. Its all just simple physics/chemical kinetics.

 

[Jim]

I have hear both sides and done both

no difference but now we have to deal with ULSD fuel. What is the impact of old fuel?

 

[sailaway78]

ULSD

I cannot comment on the life of ULSD. Whether it gets old or stale sooner is a question for someone else to answer. I do think the cetane rating of ULSD is lower, and I have begun using a cetane booster for my 3gm Yanmar. Does it help, don't know, but it gets the cetane rating up to what Yanmar recommends.

 

[GuyT]

Water will condense

on ANYTHING that is below its dew temperature.

Fresh air is introduced into the tank every time the diesel temp is higher than the outside air temp. This is because the warmth is creating a low pressure system within the tank and the colder air outside (being a higher pressure) is drawn in.
Now if this fresh supply of air has water vapor in it, it will condense when the temperature inside the tank gets below the water vapors dew point. This cycle repeats itself many times over the winter period.

If the tank is full, not as much air can be drawn in. No vapor - no condensate.
If the tank is empty, no thermal mass(fuel) is present to create a low pressure to draw in the new vapor.

Seems to me that the half full tank is the worst case.

Don, I havent seen a case study of the process but I will keep my eyes open.
I'm still trying to figure out how water vapor condenses and penetrates through PVC wire (with no pressure differential)which was posted here 6 months ago.

 

[MoonSailer]

Guy T Colder Air has Less Water

Guyt using your argument that it is warmer air inside the tank...That means that warm air holds more water vapor. Cold air has less water vapor. So in your senario the warm air leaving the tank would carry away more water out of the tank than the cold air will bring into the tank. We have established in past posts that diesl fuel contains water in the fuel and that the warmer the fuel the more water it can contain. The same with air. So in my simple mind warm moist air leaving the tank is replaced by colder drier air with the net result being less water in the tank.... Explain how I am wrong please.

 

[Rick Webb]

I Know That Pigs do Fly

Anything will with enough thrust coming out it's ass.

When they land they always fill all of their tanks up to prevent condensation on the walls of the tanks.

BTW they use JP-4 which is much closer to diesel than it is to gas. Diesel whil tough to light with a match does in fact burn I've seen enough smoking holes to prove that. Gas itself is real hard to light as well it is the vapors that ignite real easy.

 

[Ross]

Moonsailor, I am bewildered by how complicated this question is being made.

During the day when the humidity is 100 percent at any temperature it feels muggy. In the evening as the air cools fog forms. The moisture in the air is condensing on nuclei in the air. This fog dampens everything it touches. If you take a beer mug out of the freezer it immediately collects a coating of moisture that quickly freezes. If you have a container with 2 gallons of gin in it that has been in the freezer, the moisture in the air will condense on the gin and will continue to do so until the gin warms to the dew point temperature. So if you don't want you gin watered down and you keep it in the freezer, drink it quickly. ;D

 

[Bob16841]

Condensation Experience

MoonSailer...we are trying to have an adult conversation and discussion...your comments make it difficult.

I looked at the site you posted...it's all Barbra Streisand (BS). In 1969 I started flying. To this day, we drain the fuel sump (s) in the bottom of every tank (jet fuel or av gas, makes no difference) prior to flight. Why you ask...because there is WATER in there from condensation! I'm not telling you what's in some book, this is the way it is in the real world, I see it every day.

 

[Don S/V ILLusion]

GuyT -re: wire condensation

I picked up on your comment regarding insulated wire condensation with some interest as I have had (for decades) various wires (jacketed, coax and pvc insulated) buried in the ground within a drained, relatively dry conduit, at least as dry as possible given it's in the ground which connects my ham radio station to the tower some 200 ft away.
My observation, for what it's worth, is that many of the cables degraded from moisture within the cables rather than wicked in from the terminals making me conclude the jackets were not impermeable to condensation as was advertized annd required replacement periodically.
As it relates to this topic, the moisture, regardless of how it got there, would be considered trivial in a tank as the volume would likely measure in microliters.
for what it's worth...

 

[Ross]

A little while ago I put a tray of ice in a stainless steel

bowl on my postage scales. It weighed 1 pound 5 ounces. when the ice has melted and I have dried the outside of the bowl I will post the weight of the bowl and the melt water. ;D