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

[IanD]

16 minutes ago, blackrose said:

 

But I think you're missing the point that water itself isn't necessarily a problem unless it gets to the height of the fuel take off to the engine or unless diesel bug grows. Many boats have some water sitting at the bottom of the tank and it doesn't always cause a problem. However what it does do is create the conditions for a problem to occur in the form of diesel bug which grows on the interface between the water and diesel. Like any bacteria, diesel bug has to come from somewhere - it doesn't just appear by itself, so if it gets into a tank with water it can grow, but if it doesn't then it won't. So whether water in the tank comes about via a leaky filler (which I accept is a major cause) or by condensation, then contrary to what you say it may not affect all boats without a full tank in the same way.

I'm not arguing that if water gets in it's a bad thing, as you say it encourages diesel bug growth -- to get to the takeoff height you'd indeed need *huge* amounts, typically tens of litres. My point is that this is unlikely to come from condensation, otherwise all boats not keeping the tank full to the brim over the winter would have a problem -- and many/most don't.

 

And if your filler doesn't leak the amounts likely to get in from condensation are so tiny (a few cm3) they're unlikely to even cause a diesel bug problem -- or at least, not make it any worse... 😉 

 

40 minutes ago, Tony Brooks said:

 

 

With a tank on the baseplate, you have a problem with cleaning it. All I can suggest is ballasting the boat so it sits with the filler pipe directly above the lowest point and letting the contents settle, then use a pipe or hose to pump from through the filler. The idea is not to drain the tank, but remove the heavier elements that have collected below the fuel. This is water, water/fuel emulsion, bits of rust, bits of rubber hose, and bug debris.

 

If this were my boat and I intended to keep t a long tome, I would definitely be looking at the feasibility of fitting an inspection hatch in the tank.

 

If you have a day tank it might not be so bad because you can probably drain/clean that fairly easily & regularly so  you get a fair idea of what is going on in the main tank.

 

I don't have an inspection hatch, but I do have a drain at the bottom of the tank -- and before @Alan de Enfield pipes up, yes it's fitted with a stop valve and a plug according to ISO specifications... 😉 

 

Edited October 21, 2025 by IanD

[Tony Brooks]

16 minutes ago, IanD said:

I don't have an inspection hatch, but I do have a drain at the bottom of the tank

So did mine, but as the boat rimmed down by the stern, there was still a lot of water and emulsified fuel in there. That may have been caused by an emulsifying additive like Fuelset etc.

[Tacet]

5 hours ago, IanD said:

Some numbers -- if you don't like them, look away now... 😉

 

Changes in atmospheric pressure (typically 980mb to 1030mb or 5%) and tank temperature changes are going to drive air in and out of the tank, but only slowly -- if there's 100l of free airspace then a typical change of 50mb will move 5l of air in or out over usually a few days -- maybe 1l per day typical assuming 10mb pressure change? If the air inside a diesel tank changes temperature at the same rate as my electrical cabinet, this changes about 5C between day and night which would move about 1.5l of air in/out per day -- maybe double this (3l/day?) in a steel tank more exposed to the outside temperature? So at a guess the total air movement due to pressure and temperature changes is maybe 4% of the tank free air space per day, mostly due to temperature changes, and it's not all swapped since it mixes inside, which means it would take more than a month to replace all the air with fresh.

On top of this, the likely amount of water condensing out from warm moist air to cool dry air is perhaps 10g/m3, which is 1cc of water in 100l -- that's 1cc per month, which certainly can't account for the water buildup in diesel tanks that people have reported.

 

So either there's a *lot* more air going in and out of the tanks than these figures -- maybe due to wind gusts, but even this seems unlikely to provide many litres of water which is what people have found -- or the water is coming from leaks round fillers. Which seems much more likely... 😉 

I've never bought in to the keeping-the-tank-full theories.  Much the same venting issues occur for road vehicles and yet problems are few

 

Flush deck fillers are more likely culprits, I reckon

 

[blackrose]

1 hour ago, IanD said:

My point is that this is unlikely to come from condensation, otherwise all boats not keeping the tank full to the brim over the winter would have a problem -- and many/most don't.

 

No, you're still missing the point. All boats not keeping the tank full wouldn't have a problem. Some would and some wouldn't depending on which developed diesel bug.

[PaulJ]

41 minutes ago, Tacet said:

I've never bought in to the keeping-the-tank-full theories.  Much the same venting issues occur for road vehicles and yet problems are few

 

Flush deck fillers are more likely culprits, I reckon

 

Me neither since I commisssioned a never used 15 year old tank on a narrowboat.

It was fitted with a vent but no filler had ever been fitted or cut into it so any water could only come through that vent (it had been pressure tested so that ruled leaks out)

I was convinced it was going to be crusty and full of rusty water and would be in need of the top cutting off and at the very best some major derusting carried out. Cut a hole for the filler first, stuck a camera in and was amazed to see it was like brand new- just some gingering. Unbelievable but true.

 

[IanD]

3 hours ago, blackrose said:

 

No, you're still missing the point. All boats not keeping the tank full wouldn't have a problem. Some would and some wouldn't depending on which developed diesel bug.

<sigh> whatever you say...

 

(in case you didn't get it, the "problem" I was referring to was water in the diesel, which was the subject of discussion -- you're the one with diesel bug in your head...)

Edited October 21, 2025 by IanD

[Naughty Cal]

23 hours ago, blackrose said:

 

Yes, you're right, but that's a different issue. Using your fuel won't prevent condensation build up in the tank which could still lead to issues. My tank is rarely full too and my fuel turnover isn't that great, but I think I've just been fortunate not to have had any problems. Perhaps it's less of an issue in a GRP boat? Is it a steel tank?

It was indeed a stainless steel tank.

[system 4-50]

If you moor your boat online in some places during the winter then there are other issues to be considered when leaving your tank full.  For further details please post where you leave it.  If you also post details of your tank security arrangements then I can advise as to whether these are appropriate as well.  😁

[ditchcrawler]

1 hour ago, system 4-50 said:

If you moor your boat online in some places during the winter then there are other issues to be considered when leaving your tank full.  For further details please post where you leave it.  If you also post details of your tank security arrangements then I can advise as to whether these are appropriate as well.  😁

I lost quite a bit at Napton and ran out half way up Hatton 

[Onewheeler]

On 21/10/2025 at 12:05, Onewheeler said:

I suspect (partly on the basis of calculations I did when a wage slave looking at exchange of breathing vessel contents with the outside) that external pressure fluctuations have at least as large an effect - changes in pressure from weather systems passing, and rapid fluctuations from wind gusts.

My old work was done on a rather large breathing vessel. I've just run some rough numbers for a 100 L headspace and get around 50ml gas flow for a 10 m/s gust. Probably not enough to worry about although frequent pulses in and out will exchange with the tank interior to some extent.

[Llamedos]

On 20/10/2025 at 10:10, JoeC said:

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

I'm revisiting this thread because I've just come across some information others might find useful.

 

The article above posted by @JoeC, written by RCR and mentions not storing fuel treatments for more than a year. It got me thinking about how long all of those bottles of Marine 16 we see in chandlers or order online have been sitting around before we even buy them, so I dropped Marine 16 themselves a line and asked them how I could be sure the product was fresh and therefore still effective. This is their reply...

 

"We have been batch coding product now for about 18 months so there will be an inkjet batch code on the base of the bottle This will detail batch code, manufacture date and expiry date. Expiry is 2 years but you would be safe going a year beyond that as long as it is kept cool and dry."

 

So basically before you buy a bottle look on the base.  If you find a bottle with no dates like these showing you know it's going to be at least 18 months old.

 

[PineappleGuy]

It's a bit late now, but is there such a thing as winter diesel for canal boats or an additive that effectively converts normal diesel into a winter version? I don't see anything in the RCR article about it, or indeed in this thread. Or is it not an issue for canal boats?

[Momac]

16 minutes ago, PineappleGuy said:

It's a bit late now, but is there such a thing as winter diesel for canal boats 

Yes there is winter diesel sold at all road fuel stations in winter and it can be used all year round.

 

 

[ditchcrawler]

17 minutes ago, Momac said:

Yes there is winter diesel sold at all road fuel stations in winter and it can be used all year round.

 

 

When I was offshore we use to run diesel stocks down in autumn to fill with winter diesel. No idea what it was, its was just what was needed for winter

[Tony Brooks]

3 minutes ago, ditchcrawler said:

When I was offshore we use to run diesel stocks down in autumn to fill with winter diesel. No idea what it was, its was just what was needed for winter

 

Almost certainly a lower waxing temperature so it may be a bit more expensive to refine and possibly of a slightly lower calorific value.

[jonesthenuke]

1 hour ago, PineappleGuy said:

It's a bit late now, but is there such a thing as winter diesel for canal boats or an additive that effectively converts normal diesel into a winter version? I don't see anything in the RCR article about it, or indeed in this thread. Or is it not an issue for canal boats?

Winter diesel has a lower waxing point (circa -12C rather than -4C for summer diesel.  As to whether you need this depends on how far North you will be and if you want to cruise in sub zero conditions where the canals are probably frozen over. I doubt many people would be concerned.

 

However, if you lay you boat up over winter but go to check on a boat that has been frozen in, then you may well find fuel filters blocked by wax.

 

When I was working we had large tanks of diesel for emergency use, we aimed to keep these filled with winter diesel all year round The normal consumption was very low, just routine test runs but we had to buy some diesel in summer months. It became increasingly difficult to get assurance that diesel being supplied in warmer months was compliant with winter diesel specifications and resorted to testing every tanker delivery to check the cold filter plugging point. Additional assurance was provided by immersion heaters in the tanks and trace heating of the delivery lines to the engines.

 

PS some benefit may be gained by adding kerosene to the boat's tank, but this like other additives will not fully stop the waxes in the diesel from solidifying in really cold conditions (and the tax man will not like it)

Edited March 23 by jonesthenuke

[Mike Coombes]

12 minutes ago, Tony Brooks said:

 

Almost certainly a lower waxing temperature so it may be a bit more expensive to refine and possibly of a slightly lower calorific value.

 

Experienced HGV lorry drivers - my best mate Dennis for example - would add a gallon of petrol to a full tank during a cold spell. He told me there was no downside and his fuel never waxed. His diesel tanks were usually 40 gallon jobs. Owner driver too, so not likely to risk his expensive engine.

 

As an aside, I saw an assault on the Crowmarsh Gifford side of Wallingford bridge in, IIRC, 1994. A truck had just gently rolled over the bridge, stopping outside Wilders Yard. The driver, no mug, knew the issue so got some newspaper and a few sticks, a rag dipped in diesel and placed it under the twin filters between the tank and the cab. After setting it on fire to warm the waxed fuel in the filters, he moved out of the biting wind while his little fire did its work. A motorist, going the other way, saw the fire, thought he would be a good Samaritan, so jumped out with a CO2 extinguisher and put it out. The truckie gave him a right thumping!

 

Made me chuckle.................................

[PineappleGuy]

So, from the answers so far, I conclude that:

a) winter diesel is not sold canalside,

b) it is not common to add something like STA-BIL Diesel Winter to the tank instead,

c) waxing might be an issue if one's boat is laid up somewhere that experiences sub-zero temperatures.

I'm asking because we've just replaced the filter in the attached photo, the boat having spent the winter in a marina in the Midlands.

Edited March 23 by PineappleGuy

[LadyG]

I think it might be a good idea to put a date sticker on the fuel filter, its not always changed timeously,   ditto the gearbox fluid. Attention to detail varies a lot when it comes to servicing.

Ive decided not to add anything to my diesel, 

[Alan de Enfield]

3 hours ago, PineappleGuy said:

It's a bit late now, but is there such a thing as winter diesel for canal boats or an additive that effectively converts normal diesel into a winter version? I don't see anything in the RCR article about it, or indeed in this thread. Or is it not an issue for canal boats?

 

Yes - it has an anti-waxing agent 'added' into the diesel, normally after the 1st October, never fill your boat up in August / September if you will not use the fuel up before Winter.

 

If you are not going to use the boat at all during /winter than it makes no odds as the fuel will wax at low temperatures, but will go back to normal once the weather gets warmer.

 

I can have problems with my digger and tractors in very cold weather - sometimes it can be weeks before they will start.

 

 

1 hour ago, PineappleGuy said:

winter diesel is not sold canalside,

 

Yes it is - when the Diesel is manufactured at the refinery they do not know which is going to a canal boat and what is going to agricultural use.

Edited March 23 by Alan de Enfield

[PineappleGuy]

1 hour ago, Alan de Enfield said:

 

when the Diesel is manufactured at the refinery they do not know which is going to a canal boat and what is going to agricultural use.

So do the marinas etc know what they're putting into your tank (i.e. normal or winter grade)? It had never occurred to me to ask!

 

1 hour ago, Alan de Enfield said:

 

Yes - it has an anti-waxing agent 'added' into the diesel, normally after the 1st October, never fill your boat up in August / September if you will not use the fuel up before Winter.

That's very interesting, am I the only one who hadn't heard that advice before?

[Alan de Enfield]

3 minutes ago, PineappleGuy said:

So do the marinas etc know what they're putting into your tank (i.e. normal or winter grade)? It had never occurred to me to ask!

 

It is an automatic addition at the refinery, so if their delivery is before early October they will be getting Summer grade, hopefully they plan their stock levels so that they are not left with 1000s of litres of Summer grade as they go into Winter (and presumably reduced sales volumes)

 

 

6 minutes ago, PineappleGuy said:

That's very interesting, am I the only one who hadn't heard that advice before?

 

I doubt it - it generally gets a mention most years

I seem to remember even posting pictures of my 'waxed up' filters last Winter, or maybe the Winter before.

[jonathanA]

I was told that before anti waxing agents were introduced it was normal to mix a small amount of petrol In diesel in winter. Never saw it happen, but can remember the sheets of newspaper being set on fire under truck diesel tanks on the first frosty morning of winter...

 

Incidentally the same problem can happen with bulk calor, there is a summer and winter version...

 

Eta with calor its the tank regulator that ices up not waxing . Just to be clear.

Edited March 24 by jonathanA
Clarify

[Jen-in-Wellies]

27 minutes ago, jonathanA said:

Incidentally the same problem can happen with bulk calor, there is a summer and winter version...

Depends on the country. UK is near 100% propane all year round. Some countries mix in more butane in the summer. Too much butane can interfere with it vaporising in winter. https://www.mylpg.eu/useful/lpg-mixture#mixture

[Alan de Enfield]

54 minutes ago, Jen-in-Wellies said:

Depends on the country. UK is near 100% propane all year round. Some countries mix in more butane in the summer. Too much butane can interfere with it vaporising in winter. https://www.mylpg.eu/useful/lpg-mixture#mixture

 

In the UK, caravans are using predominently Butane (blue cylinders), but boats are virtually 100% Propane (Red cylinders)

 

When camping / hiking the gas cylinders are a 'mix' of typically 70% butane and 30% propane as Butane provides 9% more energy per unit of liquid volume than Propane.