Winterising - keeping Diesel tank completely full over the winter - good practice or urban myth?

[Llamedos]

Can I seek the wisdom of the group on this old chestnut? 

 

Winter is approaching and like many non liveaboards I'm starting to think ahead to closing up and winterising my boat. When we bought our boat around five years ago I was told it's good practice to keep the Diesel tank full over winter to prevent condensation forming inside the tank and so contaminating the contents with water. I've always dutifully done this, but at the same time always questioned the science behind it. My scepticism being that the headspace in the tank is going to be full of fuel vapour not water vapour so how can condensation form? Surely, the only way for the outside air to get in will be via the tanks breather which is pretty small in the scheme of things and so unlikely to have that much influence.

 

I should add that my tank is currently between half and three quarters full. It now also occurs to me that it has already been like that since July because we've been trapped on our Marina and unable to cruise due to low water levels on the GU and Oxford. I've been starting her regularly and lately running the Webasto too - all seems good. The fuel is treated with Marine 16.

 

Is there really any advantage in adding yet more fuel to the tank at this point in the year just so it can sit in the tank all winter?

 

 

[Jen-in-Wellies]

Diesel has a very low vapour pressure, compared with say petrol. It is why it is quite hard to set a puddle of diesel on fire. The vapour pressure drops even further as the temperature falls. I would therefore doubt the assertion that the space above the liquid will be mostly diesel vapour, rather than air. 

Your other point that that air and moisture trsnsfer through the vent gauze will be low does seem more reasonable. 

The biggest single source of water in tanks seems to be standing water getting round flush mounted filler caps. Rain and snow through winter makes this more likely. If your boat has one, then treating the cap to a new o-ring now, with maybe a smear of vaseline to help it seal would be a good idea. 

If your boat is likely to get plenty of use next summer to use the fuel, I don't see any harm in having a full tank standing for the winter months, even if boater lore about condensation is actually hogwash. 

[jonathanA]

It does seem that at least canal boat magazine, which seems to have become a mouth piece for RCR is advocating leaving diesel tanks empty during winter. 

 

I think the reasoning is that modern diesel is more likely to degrade during storage due to its bio content, they are suggesting storing no more than 6 to 12 months.

I hope not or the two 210l drums I have stored which will last me a while will be a problem....

 

[Lady M]

Until  fairly recently, there was still a lot of traditional gas oil being supplied as red diesel.  While RCR may be dealing with more fuel problems these days, I think there would be many many more if it was essential to empty the diesel tank over winter.  I wonder how many of the boats RCR helped with fuel problems had treated their diesel.

[Momac]

For the last  several winters (at least a decade) I have not topped up fuel for the winter but have simply left the fuel wherever it happened to be . This year its on about half tank but in past years it has been higher or lower.

In the spring I then consume as much of the old fuel as reasonably possible before adding new fuel but I rarely would let the fuel go below a quarter of a tank. I change the fuel filters after the first one or two weekend runs out (on the river in my case).

I use a fuel additive but it is a fuel system cleaner and not a diesel bug treatment .

I have not had any fuel issues .

 

As said Diesel has a shelf life due to the biodiesel content. Therefore best to not have diesel in storage that could be more than a year old by the time it is consumed.

However that doesn't necessarily mean that one or two  year old diesel will cause any issues. 

 

 

[Tony Brooks]

My view is that adding a biocide at the last refill, as a belt and braces thing, would not go amiss, but I would NOT use an emulsifier, see below. If I had to choose, I would probably use Marine 16 complete, but only because I know that one. There are probably others just as good.

 

Of far more importance, again in my view, is that the bottom of the fuel tank is drained in some way to monitor and get rid of any water and water-fuel emulsion. I did this early in each spring (and again around August if I had time/opportunity). On a typical narrowboat you can use a bent length of copper pipe and some clear plastic tube to "vacuum" the bottom of the tank, right into the corners. You can use an oil extractor, syphon, syphon pump, or electric fuel transfer pump. 

 

 

[Bee]

I don't think it makes a lot (if any) difference. I think diesel simply doesn't store very well.  At the start of every season I take the lid off of Bee's tank and shine a torch in it to see what horrors there are in it. The bottom of the tank slopes so water accumulates in the deepest part and for the last few years it really is only a tiny amount, it is possible to see the difference between fuel and water. Bee's tank is a great big thing that has never been more than half full in the 20 years or so that we've had it. We did have a big problem a few years ago and I don't really know why but it has never returned. Edited to add - I don't use any treatment but as Tony says I would certainly not use an emulsifier, I think this helped cause the past problem.

Edited October 20, 2025 by Bee

[David Mack]

45 minutes ago, Llamedos said:

My scepticism being that the headspace in the tank is going to be full of fuel vapour not water vapour so how can condensation form? Surely, the only way for the outside air to get in will be via the tanks breather which is pretty small in the scheme of things and so unlikely to have that much influence.

There's a pumping mechanism in play here. During the first half of the day the air in the headspace will heat up a little and expand, meaning there is a net flow out of air. In the afternoon and evening, as temperatures cool, the air in the headspace will contract drawing in air from outside. And with the lower air temperatures of winter the outside air will have a high relative humidity I.e. later in the day damp air will be drawn into the headspace. If the temperature drops enough the relative humidity will rise to 100%, meaning a little of the water will precipitate out, and run down to the bottom of the tank below the diesel where it is unable to evaporate again. If there is dew on the outside of the tank in the morning, it is likely there will be on the inside too.

And the process repeats daily.

I don't know how much water comes in this way, but it will be more the bigger the headspace volume.

 

 

[JoeC]

A link to an article about fuel. sticky fuel update - CanalsOnline Magazine

[jonesthenuke]

The temperature driven "pumping mechanism" described above is similarly the cause of moisture ingress and water accumulation in electrical junction boxes/cubicles, a common known problem. As a consequence some enclosures have a silica gel breather so the air getting in is dried.

 

On our boat the diesel tank breather is external, fitted to one of the dollies. Any rain or condensation on the vent gauze can be sucked in when the tank below cools overnight. Consequently I cover the vent over the winter with an empty can or part of a plastic bottle (or I will now that I have remembered!)

 

I keep the tank full over the winter, treat with a biocide when filling up, check for water in the bottom of the tank every year. So far no problems. Boat is approx 20 years old.

 

At home we have a 2500 litre tank for heating oil. The tank is 25 years old. No breather drier. No biocide. The tank is too large for our usage as we use about 1000 litres per year, I generally keep it  between 30% and 90% full as I buy oil whenever I think its at a low price (and this mitigates price hikes like the at the start of the Ukraine war). The oil thus has a long residence time in the tank. I check it for water in the base occasionally.  The fuel is always bright and clear when I pump some out for other uses. So far no problems, though it would seem to be at significant risk after this time.

 

I would be interested to know if boats that have had bad cases of diesel bug also had significant water accumulation in the tank and what the cause of that water ingress was (e.g failed seals on the filler etc). I doubt water accumulation comes form the suppliers as many occurrences would occur if that were the case.

[Tony Brooks]

6 minutes ago, jonesthenuke said:

At home we have a 2500 litre tank for heating oil. The tank is 25 years old. No breather drier. No biocide. The tank is too large for our usage as we use about 1000 litres per year, I generally keep it  between 30% and 90% full as I buy oil whenever I think its at a low price (and this mitigates price hikes like the at the start of the Ukraine war). The oil thus has a long residence time in the tank. I check it for water in the base occasionally.  The fuel is always bright and clear when I pump some out for other uses. So far no problems, though it would seem to be at significant risk after this time.

 

I wonder if heating fuel has the same bio-content that red diesel does. I don't think a high cetane rating is very important in domestic boilers where once ignited the flame is present all the time and there is no practical limit on how long the ignition device can run (glow plug), or its power.

[Llamedos]

Well this is turning into a bigger subject than I first imagined. Thanks for all the comments so far. Some good advice there.

[GUMPY]

Does diesel have much bio content?

It says up to x% is bio, has anybody tested/measured it?

When @nicknorman tested petrol he found the levels almost non existent.

I wonder if like so much today the fuel tank levels is bollox propagated via the interweb.

 

 

 I never bothered with either empty or full just left it as it was and filled when needed

[Paringa]

This from the Marine16 website

Winter Lay up, to fuel or not to fuel?

[Tony Brooks]

5 minutes ago, Paringa said:

This from the Marine16 website

Winter Lay up, to fuel or not to fuel?

 

I am tempted to comment, well they would say that, wouldn't they. I suspect there is more than a grain of truth in it, though.

[jonesthenuke]

26 minutes ago, Tony Brooks said:

 

I wonder if heating fuel has the same bio-content that red diesel does. I don't think a high cetane rating is very important in domestic boilers where once ignited the flame is present all the time and there is no practical limit on how long the ignition device can run (glow plug), or its power.

Kerosene has a lower cetane than diesel (around 40 rather than in the 50s), as you say, it's not important for boilers or jet engines. I have looked at several specifications for UK kerosene and so far I have found no mention of bio fuel content, so it may be just dino fuel.

[WhiteSuit]

I always fell into the camp of filling overwinter with marine 16 and never had any problems. I always did Tony's suggestion in the spring with the bottom of the tank appearing bright and clean. All previous tanks were integral in the stern with a raised filler.

Current boat has a level filler on the gunnel to a separate tank in the engine room so temperature fluctuation will be less but I take on board the o-seal and expansion vent which is on the roof. 

I currently want to empty the tank to attempt to clean it as I had filter problems earlier this year but the closures  mean it's still half full and will remain that way.

 

Edited October 20, 2025 by WhiteSuit
dyslexic fingers

[cuthound]

When I worked on the tools for BT many moons ago, it was an annual routine to check bulk diesel fuel tanks for water ingress using a "water finding paste" on the dipstick. It turned red in the presence of water. Any water found was drained off or if not drain was fitted, pumped out.

 

We rarely found water ingress in the below ground and internal tanks, but often found it in the external above ground tanks, suggesting that condensation was a major factor.

[Wafi]

1 hour ago, cuthound said:

We rarely found water ingress in the below ground and internal tanks, but often found it in the external above ground tanks, suggesting that condensation was a major factor.

I wonder whether it makes a difference whether your fuel tank is above or below the waterline.

[Torblimey]

Last winter I decided to change the way I heated my NB and rely more on the webasto than the morso squirrel.

With a decent thermostat connected I kept the internals at a minimum 18'c when onboard, winding it up to 22'c when I felt like it.

This steadily emptied the tank over winter and with the draw pipe close to the bottom of the tank internally I purposely let the webasto empty it completely (it took until end of march to do this).

Thanks to the webasto any oily water type deposits were sucked up and burnt off (with a bit of misfiring) and I then refilled the tank ready for spring.

Worked a treat as far as I can tell with no show of water deposits left in the tank when I checked in April.

Quick splash of marine 16 before I refilled just in case.

[Tony Brooks]

4 minutes ago, Torblimey said:

This steadily emptied the tank over winter and with the draw pipe close to the bottom of the tank internally I purposely let the webasto empty it completely (it took until end of march to do this).

Thanks to the webasto any oily water type deposits were sucked up and burnt off (with a bit of misfiring) and I then refilled the tank ready for spring.

 

I am not sure that is very sensible, because if you do suffer bug or dirty/water laden fuel, it is likely to block the Webasto filter. The idea of the dip pipe say 3" proud of the bottom of the tank is to give you a settlement volume where the water, bug and much can settle out, ready for you to remove manually.

[Torblimey]

1 minute ago, Tony Brooks said:

 

I am not sure that is very sensible, because if you do suffer bug or dirty/water laden fuel, it is likely to block the Webasto filter. The idea of the dip pipe say 3" proud of the bottom of the tank is to give you a settlement volume where the water, bug and much can settle out, ready for you to remove manually.

The webasto was supposedly fitted by a professional long before I bought the boat (during it's fitout in 2003).

I've yet to find a fuel filter on the webasto fuel line along with it's first major service this year, all I found in the combustion chamber was the melted deposits of the metal mesh from the burner plate.

It had not been changed since 2003 according to the service records for the boat.

It worked for me however may not be suitable for anyone else.

[Momac]

I thought the heater draw pipe should go less deep into the tank compared to the engine fuel draw pipe. 

This is intentionally to ensure the heater cannot leave the engine with no fuel.

Not applicable of course if the heater has its own fuel tank.

[ditchcrawler]

3 minutes ago, Momac said:

I thought the heater draw pipe should go less deep into the tank compared to the engine fuel draw pipe. 

This is intentionally to ensure the heater cannot leave the engine with no fuel.

Not applicable of course if the heater has its own fuel tank.

Our share boat was like that, thankfully

[Tony Brooks]

2 minutes ago, Torblimey said:

The webasto was supposedly fitted by a professional long before I bought the boat (during it's fitout in 2003).

I've yet to find a fuel filter on the webasto fuel line along with it's first major service this year, all I found in the combustion chamber was the melted deposits of the metal mesh from the burner plate.

It had not been changed since 2003 according to the service records for the boat.

It worked for me however may not be suitable for anyone else.

 

I did not mention that the Webasto fuel pump is a dosing pump that measures the fuel, so if any debris get caught under a valve it is likely to produce some odd symptoms. 

 

As long as you are happy, that is all that matters, but I could never suggest it is a good idea for others.