Leave tank full for winter?

[LakeSuperior]

Originally Posted by DeepFrz

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.

How do you know how much air is circulating? Do you have a measurement or are you making it up? Many times this stuff turns out to be counterintuitive. As you seem very positive that you know the answer, is "not much" equal to 1 tank exchange per minute, hour, day, month??

[LakeSuperior]

Originally Posted by garyohv

Here is an article by an expert:

The Myth of Condensation in Fuel Tanks by David Pascoe: Boat Maintenance, Repairs and Troubleshooting

I started to read this article in the past and the author (expert?) doesn't have a clue about heat transfer. I stopped reading when I realized he didn't know what he was talking about.

[Sailing Cowboy]

Originally Posted by valhalla360

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).

Not instilling fear or reinforcing a myth, just giving an example of what has happened on occasion in my business dealing with multiple boats in a REAL situation. Has nothing to do with dirty tanks and lines as Deepfrez suggested. It has do with real water in a real carburetor. Not once not twice but probably 15 times over 12 years. Only happened in the spring, when we put the boats in the water and always a slip up by the guides for using a half filled gas tank that was not filled up in the fall. Maybe the northern climate and the cold winter made it worse than a dryer climate, who knows. But having been stuck on a few occasions out on the water in the spring, and having to blow out the GIN clear water, I think is proof enough of what happens by not filling your tanks in the fall or keeping them empty. Boats were kept in a dry building. So unless there is a water fairy that fills the tank over the winter, I think condensation is very real. Not myth but reality.

[Sailing Cowboy]

Originally Posted by thinwater

The answer based on lab and field testing is that it requiress VERY specific conditions to accumulate water from the air, but these conditions do occationally occur.

No, an ampty tank is not similar to a 1/2 full tank. In an empty tank, condensation evaporates during the day yielding no net accumulation. In a 1/2 full tank the water drops to the bottom and does not re-evaporate if--and this is the important part--the fuel cannot simply absorb the water. Virgin fuel has a very small but signifigant ability to disolve water.

The thing is it depends on:
* How dry the fuel was.
* How full the tank was.
* Size of the tank (smaller tanks are more vulnerable).
* Temperature swings (Spring and Fall are most troubling, mostly Spring).
* Humidity and water temperatures (50% is low for boat in the water, 100% at night is more common).

In some tanks I could create haze and a few drops in the bottom, and in some not. Never enough to be important and it ALWAYS redisolved when the temperature rose. Thus, you would be very unlikely to ever see it.

However, corrosion rates and bio-growth both increased even without free water, as the disolved water content rose, so keeping out disolved water is also important.

What to do (in order of importance, IMHO).
* Check your filler cap and vent. Most common problem, by far.
* Install a good filter separator.
* Buy from a high volume outlet.
* Burn enough to fuel to change the tank 2x per year. You change your oil, don't you? I understand this is a real challenge for some large tank boats. See my last suggestion. And cruise more.
* Use an additive that prevents corrosion (steel or alluminum tanks).
* Use an additive that prevents bio-growth.
* keep the tank full.
* Consider a fuel tank vent filler. Cheap insurance and they work.

-----

What to do with gasoline tanks is a whole nuther issue. However, I will start with the fact that many yards (the one I use and several other is town) prohibit emptying gasoline tanks in dry storage; the explosion risk is greater and the insurance company says NO.

For gasoline the questions are different, but without getting long winded, empty for very small tanks and full with a vent filter for larger tanks. 1/2 tank is pure trouble.
-----

I'm assuming Cotmar understands that water in the crankcase is from blowby (burning fuel makes CO2 and H2O), not the atmosphere.

Probably the most sensible explanation on here.

[perchance]

If you already have water in your diesel tank (most do to some extent) filling it won't make any difference to the bugs already living there.

[captain58sailin]

While some water can be generated by condensation in the tank, as far as volume goes, far more water comes in through the fill pipe, either in the fuel received or by leakage from the deck fitting. In the north, we do get temp. extremes that can generate a good bit of moisture. It is not uncommon to go from -10* F, to +40* in a single day here, sometimes in a matter of hours. It pays to eliminate as many outside sources of water as possible, if keeping the tanks full can eliminate even a couple of grams of water being generated then what is the hurt? Why not top off the tanks at the end of the season? I don't believe you need scientific proof to know a full tank is better overall for your systems than a half full tank. I have my boat tied up in the harbor while I perform a major refit, I have no water coming into the vessel from shaft logs etc..., however I do have to pump my bilges once or twice a year from condensation. Usually 55 gallons per year. Now I cannot prove that condensation is the only source of water ingress, however if I have a different source I have been unable to find it. The shaft log is inspected frequently for leaks and has never produced any, and if it were, I expect I would have to pump much more frequently than I do.

[Sailing Cowboy]

Originally Posted by perchance

If you already have water in your diesel tank (most do to some extent) filling it won't make any difference to the bugs already living there.

I should clarify that I am referring to gasoline tanks. The tanks were emptied and filled every two days during the season. It was only after winter storage that I had problems and it was water. I use to work at a marina years ago and I remember taking dips on the tanks to check for water. If I remember correctly each tank has about 6 inches of water on the bottom and that was considered normal.

[DeepFrz]

I use to work at a marina years ago and I remember taking dips on the tanks to check for water. If I remember correctly each tank has about 6 inches of water on the bottom and that was considered normal.

Are you talking about the marina's supply tanks from which they pump fuel out to the customer?

[jesperht]

I just had the problem, infected diesel, sorry dont know the english word. Our fuel line was plugged, and we were left without engine power, good practise though!
My tank is stainless steel, and i have allways used diesel additive to prevent infection. I decided to do it proper, an was about to change engine anyway.
I got the tank out, and was shocked to see the amount off infection, it really looked like a cow had taken a dump in the tank, it was that big and rather firm.
Everything is now totally cleaned up, and the tank is modified, so i now got a hatch for inspection, and most important, a small sump with an outlet, so i can drain the water, if the is any.
My plan is to go for empty tank, clean it up in the spring and fill it up. I do not want this experience again....

[Sailing Cowboy]

Originally Posted by DeepFrz

Are you talking about the marina's supply tanks from which they pump fuel out to the customer?

Yes. Cant remember the size of the underground tanks.

[jkindredpdx]

I'm no expert, but the guy who scrubbed my fuel before I took possession of Sedna said to top off the diesel when she was going to sit for awhile. He didn't approve or disapprove of additives, but said since the PO had been using anti-bio, I should continue the process. Best not to change... I guess boats acclimate like people When I'm stuck in town, I like to fire up the engine every couple weeks and, weather permitting, take her for a spin.