Filling the fuel tank over winter - yes or no?

I keep my tanks full over the winter, but we can experience wide variations of temperatures even in the summer. So if it's going to happen in the winter, it'll happen in the summer as well.
The best form of defence against water and dirt, is to have tanks with drainable sumps. Most fuel tanks used to be made that way, but modern design "skimming dish" boats can't accommodate them, and suffer the consequences. With a sump, any water or dirt, naturally gravitates into the sump, so the fuel/water interface, where bug trouble begins, is tiny.
 
I keep my tanks full over the winter, but we can experience wide variations of temperatures even in the summer. So if it's going to happen in the winter, it'll happen in the summer as well.
The best form of defence against water and dirt, is to have tanks with drainable sumps. Most fuel tanks used to be made that way, but modern design "skimming dish" boats can't accommodate them, and suffer the consequences. With a sump, any water or dirt, naturally gravitates into the sump, so the fuel/water interface, where bug trouble begins, is tiny.
 
>modern diesel fuel deteriates with age

Petrol goes off after a time but I have used two year old petrol in a chainsaw. Diesel doesn't go off, so I would fill the tank. The only issue with diesel is if it originates from Russia where they put in a winter additive.
 
My boat has plastic tanks which were the principal cause of my running problems, my boat is 30 years old the tanks were only four, I made the mistake in assuming the previous owner had used a preventative in the diesel so after changing all the injectors, new filters, glow plugs etc I eventually took out the plastic diesel tanks and there was a lot of black organic life in the bottom. Although these Vetus diesel tanks are well made the disadvantage is that the narrow diesel supply pipe sucks up the diesel from the sender unit at the top and the blobs of jellyfish looking organic matter were getting stuck in the pipe. As it is so important to have clean diesel rather than fill the tanks and leave them over winter. And as it perhaps takes an hour per tank to remove them, in the spring I will take them out, clean them and then fill with fresh April diesel.
And where you are going to fit a Screw on Primary filter, I did consider this because my 30 years old CAV filter head and glass bowl when fitted with a new filter with a modern imitation rubber sealing ring (which is also smaller) still seems to leak, so when I costed up a screw on on filter kit I thought that the filters were quite expensive, so on looking round I found a supplier (will put the name later) where I can buy a "Double" unit, 2 filters, 1 with a glass bowl underneath for around £30. the main advantage is that they use the cheap CAV type filters which you can buy "everywhere" rather than the screw on ones which could be difficult to source.
 
This layup I'm thinking of not filling the fuel tank as I've done in previous years.
The reasons being:-
newish plastic tank therefore no condensation issues;
tank only 1/4 full so when refilled next year will have fresh diesel in it(white road diesel not red);
fuel regularly dosed with MR 16 additive so no bugs;
modern diesel fuel deteriates with age so fresh diesel in the spring will mix with old;
am fitting a screw-on primary fuel filter to replace the c*** CAV filter which is a pita to change.
Is this ok or am I kidding myself?

Plastic tank doesn't prevent condensation.
More air in the tank (less fuel) means increased condensation not reduced.
Diesel doesn't deteriorate with age to any significant level that 6 months over winter would reveal.
Fuel filters and water separators are a good idea, but they don't prevent the problem, only deal with the consequences.

I ensure that our boat has a full tank at season's end, and there's minimal problem with water.
 
pvb said:
But the important question is "how much air is there in the tank?" And, "how much water can that amount of air realistically hold?" The answer is not very much. And there's almost no flow of air in and out of the tank, so the risk of condensation is minimal. In more years of boating than I care to admit, I've never found water in my diesel filter/separator.
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Suppose the tank contains some volatile substance (say like diesel). Then the cyclical changes in temperature are likely to cause a corresponding cyclical drawing in of damp air from the outside - so a tank with diesel/ petrol in will behave differently to an empty tank.
 
nedmin said:
If there,s no air flow why does my swing bin bag vary in its inflation?
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The changing volume of air is tiny, probably less than the volume of the breather pipe, so the idea that it "sucks damp air into the tank" is rather fanciful. And even if some damp air is sucked into the tank, how much water is there realistically in a few cc of air?
 
pvb said:
If you work it out, you'll find the resulting airflow is infinitesimal.
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Atmospheric pressure variations are large enough to expand and contract the bellows in an barometer, which is a very small area compared to a fuel tank. Add to that the variations in temperature.

Addition - If you've done any flying, you'll remember that aircraft tanks should be filled before parking and one of the DI checks is to sample fuel for water content.
 
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bedouin said:
Suppose the tank contains some volatile substance (say like diesel). Then the cyclical changes in temperature are likely to cause a corresponding cyclical drawing in of damp air from the outside - so a tank with diesel/ petrol in will behave differently to an empty tank.
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In winter temperatures, the volatility of diesel fuel is minimal.
 
I have never filled my tank for the winter, my last boat had twin tanks. My current boat has a 30 gallon tank, spent last winter 2/3rds empty and is currently half empty. I also generally leave whatever petrol there is in the tender outboard tank too and have had no problems from either irresponsible actions...
 
Thanks for all your replies. There are certainly different ideas/theories on my query. So I will fit the new filter housing this weekend, run the engine and probably fill the tank afterwards, or not! The filter I'm going to fit is identical to the CAV Head and the filter is screw on with a drain for water, filter no. is delphi 496A which is a common filter. Should just be able to replace the existing head as the threads are 1/2" UNF.
 
PetiteFleur said:
This layup I'm thinking of not filling the fuel tank as I've done in previous years.
The reasons being:-
newish plastic tank therefore no condensation issues;
tank only 1/4 full so when refilled next year will have fresh diesel in it(white road diesel not red);
fuel regularly dosed with MR 16 additive so no bugs;
modern diesel fuel deteriates with age so fresh diesel in the spring will mix with old;
am fitting a screw-on primary fuel filter to replace the c*** CAV filter which is a pita to change.
Is this ok or am I kidding myself?
Click to expand...

Just to address some of your points directly, and more food for thought to be considered:-

White road diesel is more likely to have more bio-element content than red diesel. Therefore more moisture contained within the fuel. The dissolved element is probably where most of the moisture comes from, not the air (although the above debate has made interesting reading)
Diesel prices are currently at very low levels. Do you think they will still be this low next spring when you decide to fill up?
Diesel DOES NOT deteriorate with age
You dose your tanks with M16 so bug not likely, or not likely to survive whether tanks are full or empty
 
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