Diesel fuel tanks for Winter - Full or Empty


Active member
Sep 23, 2002
RO Number
I have always been a fan of keeping my tanks full for winter when I had a gas boat.

For some reason, I feel that the diesels would be better served empty.

I say this because no matter what your going to get some condensate. I feel the filters will do their jobs and take this out. Since diesel fuel gets other issues when it sits, like Algae or seperation, seems to me having less in there is better, then add fresh diesel in spring. In other words, more bad can come from having lots of fuel, than less fuel.

Just curious what other opinions are?

Empty, full, or whereever it is when you haul?
Because of ethanol in gas, it is now considered wiser to leave gas tanks closer to empty. Diesel, on the other hand, is better to be topped off at the end of the season (along with a good stabilizer and an anti-bacterial). Most diesel tanks are aluminum. The less air space, the less water will condense. Unlike gas, it is bad to emulsify water into diesel fuel. Good diesel additives demulsify water (that is, break water out of suspension in the fuel so it is trapped in the separators). The fuel injectors in common rail diesels do not respond well to emulsified water in the fuel (water blows the tips off of the injectors because of the extremely high fuel pressure). Also, the water tends to oxidize (rust) the fuel tanks forming pin holes at the water/fuel barrier. Last good reason - Volvo Penta recommends full fuel tanks in their manuals.
I don't fill [don't winterize either, but that's a different topic] for the diesel reasons CurrentSea mentions.
Add a biocide and stabilizer and put it away with whatever it has in it. New fuel filters in spring and once mid-season. Never had an issue on the Viking.
Full tanks/less air space allow less surface area for water to condense on.

Alcohol has always been a fuel additive to 'remove' water in fuel. Thing is, it doesn't remove water. Water readily mixes into alcohol and alcohol readily mixes with gas and probably diesel, too, so it burns off easily in the combustion chamber.

The mixing allows fuel/alcohol/water mixture to pass thru the filters and float bowls(if you have them).

The other things is, water will mix with alcohol...up to a certain percentage before it 'falls out' of suspension and you have water contamination again.
Wouldn't it be better to talk about religion or politics?

Curious, what's your reasoning for empty tanks.
Why empty? Because I hold 600 gallons and in most years I would not come close to using that much fuel in a timely manner.

Old fuel is going to run the risk of problems, more than other factors. I can do more to manage old fuel by volume than any other method.

I know everyone keeps repeating worries of condensation, but living in wet/rainy Seattle and never having once cleaned water from my filter bowls, or dealt with "algae". I'll humbly suggest that while issues with algae/condensation are theoretically possible and some have dealt with them, the significance of these issues is massively over diagnosed as causal factors. I've noted before, not so humbly, that the laws of physics don't work differently on my boat than the one five foot away from mine. I'm constantly frustrated by the simpletons on my dock who complain about condensation issues in roughly same size/location/insulated/etc.. Boats who apparently have "condensation" issues that cause "algae" that they treat constantly and I never have the same issues. Yet we still run screaming up the docks waving the latest magazine articles claiming we should store our tanks full and treat treat treat.

It's a system with more than one variable. Those who take an understanding of the principles and variables and act accordingly, rarely have problems.

Those who will be using their boat, a lot, In 6 months time or less, should probably fill their tanks and stabilize to mitigate moisture and fuel aging.

Most others could benefit from re-thinking whether the typical advice to fill full adequately deals with issues that come from old fuel, more so than moisture. Remember that there isn't really products on the market that ever fix bad fuel, and ask yourself what proportion of advice you are getting is ultimately based on marketing. Subconciously it's easy to overlook the source of "common" knowledge. Also remember that magazine writers are experts in writing first and boating second, or not at all.
What ghost said. IMO most water comes with fuel or bad orings in fill pipes
I've thought about it on the trip to work. Now, let me be the first to emphatically disagree with myself. I said to store "Empty". That's wrong. It's just as wrong as somebody saying that you should store them full. It's just as wrong as somebody asking what they should do without detailing all the competing variables of the equation.

But yes, as Bruce said, if your filter bowl is full of water, look elsewhere for the solution than just condensation or storing your tanks full. I'm not saying that condensation can't be a factor to consider in this debate. I am stating rather clearly that the guy down at the end of the dock changing multiple filters to get rid of water is very likely not having a condensation issue, even though everyone around him is telling him otherwise. Likewise, the guy next to him having an issue with "Algae" does not need a biocide. Yes condensation exists. Yes, organisms in fuel exist. No, that's not likely the MAIN source of the problem. AN issue is not always THE issue.
It sounds logical to manage 100-200 gal at a time and fill to what you need for each trip. Everyone has to deal with their own issues with their particular boat.

I've never had a (gas) fuel problem down here out of SF Bay, but I never use marina gas for my 100 gal main tank and average between 30-50 gal per salmon trip though tuna trips can use 100 gal. I have a pretty rapid fuel turn-over.

Fuel storage at your local marina is probably the bigger issue and how it's treated. I think we've had that discussion before.
Filling up on spring you probably get old fuel anyway