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

[Torblimey]

2 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.

I thought so too, perhaps mine is a one off 🥴

At least I realised it before cruising off into the sunset keeping my bedroom warm and running out of diesel because of it.

Maybe I'll get it changed at some point.

2 minutes ago, Tony Brooks said:

 

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.

That I quite agree with, having stripped and serviced everything this year, not finding a fuel filter and considering everything is around 23yrs old at this point there maybe a few changes coming for 2026 😉

[cuthound]

58 minutes ago, Wafi said:

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

 

Definitely. Below the waterline the temperature will not go below 4°C, above it will.be whatever the ambient temperature is.

[Llamedos]

Just looking for water detection paste on Amazon and came across this...

 

Tank Dryer

 

It's marketed mainly for heating oil tanks and anyway sucking out the water is probably far better for a boat. Nevertheless has anyone ever used a tank dryer like this and did it work?

[Tony Brooks]

It will need pushing to the lowest point in the tank if you want to remove all the water, and it will only absorb about a pint and a half before you need a new inner. I have doubts about how it will deal with water-fuel emulsion, let alone signs of bug. I think it is far more effective to pump/suck the bottom of the tank yourself so you can see what you have pulled out. Especially if you put it into plastic milk "bottles" so you can allow it to settle and this get a far better idea of what was in the tank.

[blackrose]

11 hours 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.

 

While the breather is fairly small compared to the size of the tank it's permanently open so water vapour in the air has an infinite amount of time to enter the tank. My understanding is that is exacerbated by fluctuations in temperature, particularly the differential temperatures of the steel tank and air in the headspace of the tank, and also differential temperatures between the air in the headspace and the diesel causing condensation to form with temperature fluctuations. So if you fill the headspace with diesel you solve those issues.

Edited October 20, 2025 by blackrose

[Lily Rose]

14 hours ago, Llamedos said:

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?

I have been thinking about this recently too.

 

Over the last 10 years I have topped the tank up to full every autumn including the addition of Marine 16 (every time I buy diesel).

 

This year I wondered if I should do it differently having read articles, usually by RCR I think, about sticky fuel and about their recommended practice now being to empty the tank for winter.

 

So 3 options:

 

1 Empty the tank but then what do I put more than half a tank of diesel into and then, presumably, dispose of it. Wasted fuel!

2 Leave the tank as it is and risk condensation and maybe rust.

3 Carry on with my usual practice of a top up.

 

I decided to stick with option 3 and topped it up a few weeks ago. This is what I’ve done for 10 years without any problems. If it ain’t broke etc

 

If I do start to have problems then I’ll have to reconsider but for now I’ll carry on with the practice that hasn’t yet caused me any problem.

[Bee]

I think it is time that the practice of just walling off a bit of the counter and bunging a few pipes in was rethought by builders. I think it is a requirement of the RCD (or whatever it is called now) that there should be an inspection lid fitted. I stand to be corrected on that. Tanks really should be accessible, inspectable and ideally have a sloping bottom to collect water. It should be an easy job to remove the inspection plate, have a look inside and if necessary do something about it. One tiny drain point is better than nothing in a flat bottom tank but sludge and muck builds up all over the bottom and that stuff needs scrubbing out.  Guess how I know! Just as integral water tanks are not the best way to store water nor are most diesel tanks on boats.

[nealeST]

Just slightly aside with this topic but I’m wondering if you keep adding diesel to the existing quantity in the tank are you adding to the ‘shelf life’ of the whole amount? I understand it deteriorates to the point of being useless over a period of time. My fuel tank is approximately 90 gallons/400 litres..its brim full. I’ve used about 70 litres since August. If I keep adding what I’ve used do I prolong the life of my fuel?

Edited October 21, 2025 by nealeST

[jonesthenuke]

29 minutes ago, nealeST said:

Just slightly aside with this topic but I’m wondering if you keep adding diesel to the existing quantity in the tank are you adding to the ‘shelf life’ of the whole amount? I understand it deteriorates to the point of being useless over a period of time. My fuel tank is approximately 90 gallons/400 litres..its brim full. I’ve used about 70 litres since August. If I keep adding what I’ve used do I prolong the life of my fuel?

Non bio diesel in a sealed can would last indefinitely (IMHO, others may argue this point). The problem is water entrainment through the mechanisms described above, exacerbated by the addition of bio-fuel which is both hygroscopic and more likely to support growth of bacteria etc.

 

Simple addition of more fuel would not increase the life of the fuel in the tank other than reducing the water ingress through reduction in the air space above as the new fuel has nothing in it to either remove water or halt biological growth. However, if you added a biocide each time fuel is added then it would be an improvement.

[Tony Brooks]

43 minutes ago, nealeST said:

Just slightly aside with this topic but I’m wondering if you keep adding diesel to the existing quantity in the tank are you adding to the ‘shelf life’ of the whole amount? I understand it deteriorates to the point of being useless over a period of time.

 

See below.

 

8 minutes ago, jonesthenuke said:

Non bio diesel in a sealed can would last indefinitely (IMHO, others may argue this point). The problem is water entrainment through the mechanisms described above, exacerbated by the addition of bio-fuel which is both hygroscopic and more likely to support growth of bacteria etc.

 

 

Totally agree, but I think that plain diesel can also support bug growth to a degree, especially when water is present, the bio-content just makes that worse and more likely to be a problem. I understand that "bug" is present in the crude oil as it comes out of the ground, and refining does not totally remove it. Some oil fields are worse than others, I think. Certainly, jet fuel suffered with bug growth before there was any addition of biofuel. I suspect the potentially dramatic effects of bad fuel on an aircraft in flight meant far more attention was paid to it than to road fuel.

[nealeST]

Ok, so the conclusion I draw is once I start using my boat again in April I should use all the fuel I’ve got and not add any new until it’s all but gone. 

[Tony Brooks]

32 minutes ago, nealeST said:

Ok, so the conclusion I draw is once I start using my boat again in April I should use all the fuel I’ve got and not add any new until it’s all but gone. 

 

I don't think so. Drain/clean the bottom of the tank as described earlier to get rid of and accumulated water, water/fuel emulsion, debris and bug, then just carry on as usual. If you have a flush fuel filler, then ideally you would have changed the O ring by now and dressed it with rubber or silicon grease to enhance the seal. You would, in my view, have already added some Marine 16 complete or similar on a "just in case" basis.

 

If the bio-content is worrying you, then you should have used as much up as possible by now, but I suspect you are overthinking the POTENTIAL problems, because others have told you they don't have a problem and just do as they always do. No one can guarantee you won't hit a bug problem because a lot of that is down to where you buy your fuel from and if anyone has used a dirty syphon tube to steal fuel. Just concentrate on minimising the water build up in the tank and getting rid of it.

[Alan de Enfield]

2 hours ago, Bee said:

I think it is a requirement of the RCD (or whatever it is called now)

 

That is open to interpretation, 

If you build the boat to the relevant ISO specifications then the builder has an automaticic assumption of RCD/RCR compliance and needs no further evidence.

 

It the builder does not build to the ISO specs then he is required to show that his methods of build meet of all the 'essential requirements' by independant testing / proof that his methods comply.

 

The ISO spec for fuel tanks states :

 

 

Harmonised standard: BS EN ISO 10088:2013 Small craft - Permanently installed fuel systems BS EN ISO 21487:2012 Small craft - Permanently installed petrol and diesel fuel tanks

 

The requirements for installation of a fuel system on a boat with fixed fuel tanks are given in the harmonised standard BS EN ISO 10088 Permanently installed fuel systems and BS EN ISO 21487:2012 Small craft - Permanently installed petrol and diesel fuel tanks.

The requirement for petrol fuel tanks to be ‘insulated from the engine and all other sources of ignition’ is deemed to be complied with if

a) the clearance between the petrol tank and the engine is greater than 100 mm and

b) all electrical parts on the engine which could create a spark, and any other electrical components in the engine/fuel compartment, are ignition protected. To ensure that these components are ignition protected the boat builder should use a petrol engine that complies with BS EN ISO 15584 Inboard petrol engines - fuel and electrical system components (the engine manufacture should provide this confirmation) and for other parts, e.g. blower fan or electric bilge pump, use only components that have been CE marked in accordance with Annex II 1. The clearance between a petrol tank and any dry exhaust components must be greater than 250 mm, unless an equivalent thermal barrier is provided. For diesel engine installations, the engines used should comply with BS EN ISO 16147

 

Inboard diesel engines – Engine-mounted fuel and electrical components to ensure that the fuel components fitted on the engine by the engine manufacturer are safe. The engine manufacture should provide confirmation that the engine complies with this standard.

Fuel hose used in the system must be fire resistant if used in the engine compartment and Where fuel hose is used the standard requires that only fire-resistant hose to BS EN ISO 7840 may be used in the engine compartment. Such hose should be stamped to indicate compliance

 

It is also a requirement in BS EN ISO 21487:2012 that

 

"If there is a drain in a diesel oil tank, it shall be fitted with a shut-off valve having a plug that can be removed only with tools. Each tank shall have an inspection hatch of at least 150 mm diameter. The inspection hatch shall, as a rule, be located on top of the tank, but for diesel oil tanks it may also be on the tank side. There shall be access to the inspection hatch when the tank is in position."

 

Edited October 21, 2025 by Alan de Enfield

[cuthound]

It can be surprising how much condensation can collect in an uninsulated area of a steel narrowboat.

i used to get 10 or more litres of condensation 

Prior to learning to leave my bow thruster compartment open to allow better ventilation when winterising my boat, I used to get over 10 litres of condesation collecting in there over winter, as detailed in my post below.

 

 

[IanD]

7 minutes ago, Alan de Enfield said:

 

That is open to interpretation, 

If you build the boat to the relevant ISO specifications then the builder has an automaticic assumption of RCD/RCR compliance and needs no further evidence.

 

It the builder does not build to the ISO specs then he is required to show that his methods of build meet of all the 'essential requirements' by independant testing / proof that his methods comply.

 

The ISO spec for fuel tanks states :

 

 

Harmonised standard: BS EN ISO 10088:2013 Small craft - Permanently installed fuel systems BS EN ISO 21487:2012 Small craft - Permanently installed petrol and diesel fuel tanks

 

The requirements for installation of a fuel system on a boat with fixed fuel tanks are given in the harmonised standard BS EN ISO 10088 Permanently installed fuel systems and BS EN ISO 21487:2012 Small craft - Permanently installed petrol and diesel fuel tanks.

The requirement for petrol fuel tanks to be ‘insulated from the engine and all other sources of ignition’ is deemed to be complied with if

a) the clearance between the petrol tank and the engine is greater than 100 mm and

b) all electrical parts on the engine which could create a spark, and any other electrical components in the engine/fuel compartment, are ignition protected. To ensure that these components are ignition protected the boat builder should use a petrol engine that complies with BS EN ISO 15584 Inboard petrol engines - fuel and electrical system components (the engine manufacture should provide this confirmation) and for other parts, e.g. blower fan or electric bilge pump, use only components that have been CE marked in accordance with Annex II 1. The clearance between a petrol tank and any dry exhaust components must be greater than 250 mm, unless an equivalent thermal barrier is provided. For diesel engine installations, the engines used should comply with BS EN ISO 16147

 

Inboard diesel engines – Engine-mounted fuel and electrical components to ensure that the fuel components fitted on the engine by the engine manufacturer are safe. The engine manufacture should provide confirmation that the engine complies with this standard.

Fuel hose used in the system must be fire resistant if used in the engine compartment and Where fuel hose is used the standard requires that only fire-resistant hose to BS EN ISO 7840 may be used in the engine compartment. Such hose should be stamped to indicate compliance

 

It is also a requirement in BS EN ISO 21487:2012 that

 

"If there is a drain in a diesel oil tank, it shall be fitted with a shut-off valve having a plug that can be removed only with tools. Each tank shall have an inspection hatch of at least 150 mm diameter. The inspection hatch shall, as a rule, be located on top of the tank, but for diesel oil tanks it may also be on the tank side. There shall be access to the inspection hatch when the tank is in position."

 

 

So -- regardless of what the ISO spec says -- how many narrowboats actually have an inspection hatch in their inbuilt diesel tank? I don't think I've ever seen one... 😉 

[Onewheeler]

15 hours ago, blackrose said:

 

While the breather is fairly small compared to the size of the tank it's permanently open so water vapour in the air has an infinite amount of time to enter the tank. My understanding is that is exacerbated by fluctuations in temperature, particularly the differential temperatures of the steel tank and air in the headspace of the tank, and also differential temperatures between the air in the headspace and the diesel causing condensation to form with temperature fluctuations. So if you fill the headspace with diesel you solve those issues.

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.

[IanD]

2 minutes ago, 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.

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... 😉 

[Tony Brooks]

15 minutes ago, IanD said:

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... 😉 

 

Or, as I found, when I started bi-annual draining it was a build up over many years. After a couple of drains, I never found much water. I agree flush fillers are also a great issue.

[Naughty Cal]

We used our boat year round so the fuel tank was almost never completely full. We never had any issues with fuel contamination or fuel bug.

 

The trick is to use your fuel rather than storing it. Having a larger turn over of fuel just doesn't give the fuel time to develop any problems.

[blackrose]

13 minutes ago, Naughty Cal said:

We used our boat year round so the fuel tank was almost never completely full. We never had any issues with fuel contamination or fuel bug.

 

The trick is to use your fuel rather than storing it. Having a larger turn over of fuel just doesn't give the fuel time to develop any problems.

 

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?

[IanD]

31 minutes 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?

Having plugged some numbers in (see above, came out at 1cc per month from heating/cooling/weather), it doesn't seem to me that condensation is capable of producing anywhere near enough water even over months or years to cause the problems people have reported. This is backed up by the fact that many boaters who don't worry about keeping their tank full overwinter (including me) have said that they've never had such a problem -- and if condensation was the cause it would have affected them the same as everyone else with a vented tank, which is all boats.

 

Which rather suggests that rainwater getting in round the filler (or in through the vent) is by far the most likely cause, which would also explain why some people have problems but many don't -- including all those whose fillers/vents don't leak... 😉 

Edited October 21, 2025 by IanD

[nealeST]

3 hours ago, Tony Brooks said:

Just concentrate on minimising the water build up in the tank and getting rid of it

The diesel I have has been treated regularly with Marine 16. Good point about the O ring. I’ll get a new one and some silicone grease when I call into Midland Chandlers next week. 
I have no idea how, or if it’s even possible, to inspect or clean out my tank.  I have had part of the floor up to measure it, that’s how I know the capacity. If anyone else has a Hudson boat let me know about how to clean out the tank? All of the boatman’s cabin is sitting on top of it! I don’t think I’ll be attempting to empty 400 litres of diesel to store whilst I dry the tank anytime soon. How many boaters are actually doing this??

 

I think I need to use that supply up ASAP. Roughly 400 litres gives me 400 engine hours. At 2.5 mph…well let’s say it’s easily 800 miles? Now I wished I had diesel heating instead of an Alde gas boiler….

 

[Llamedos]

1 hour ago, IanD said:

Having plugged some numbers in (see above, came out at 1cc per month from heating/cooling/weather), it doesn't seem to me that condensation is capable of producing anywhere near enough water even over months or years to cause the problems people have reported. This is backed up by the fact that many boaters who don't worry about keeping their tank full overwinter (including me) have said that they've never had such a problem -- and if condensation was the cause it would have affected them the same as everyone else with a vented tank, which is all boats.

 

Which rather suggests that rainwater getting in round the filler (or in through the vent) is by far the most likely cause, which would also explain why some people have problems but many don't -- including all those whose fillers/vents don't leak... 😉 

Your calculations back up my original point that condensation probably isn’t the big problem people have made it out to be over the years. I agree with you that leaky filling caps are the more likely to be the culprit. Luckily my boat has a raised filler cap so I can be pretty sure it’s watertight.
 

Reading everyone’s comments, I think I’m going to leave filling the tank right up this year. Give the existing contents a further dose of Marine 16 complete before I close up for the winter. Then suck out any water before the boat moves in the spring. 

[Tony Brooks]

42 minutes ago, nealeST said:

The diesel I have has been treated regularly with Marine 16. Good point about the O ring. I’ll get a new one and some silicone grease when I call into Midland Chandlers next week. 
I have no idea how, or if it’s even possible, to inspect or clean out my tank.  I have had part of the floor up to measure it, that’s how I know the capacity. If anyone else has a Hudson boat let me know about how to clean out the tank? All of the boatman’s cabin is sitting on top of it! I don’t think I’ll be attempting to empty 400 litres of diesel to store whilst I dry the tank anytime soon. How many boaters are actually doing this??

 

I think I need to use that supply up ASAP. Roughly 400 litres gives me 400 engine hours. At 2.5 mph…well let’s say it’s easily 800 miles? Now I wished I had diesel heating instead of an Alde gas boiler….

 

 

 

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.

[blackrose]

2 hours ago, IanD said:

Having plugged some numbers in (see above, came out at 1cc per month from heating/cooling/weather), it doesn't seem to me that condensation is capable of producing anywhere near enough water even over months or years to cause the problems people have reported. This is backed up by the fact that many boaters who don't worry about keeping their tank full overwinter (including me) have said that they've never had such a problem -- and if condensation was the cause it would have affected them the same as everyone else with a vented tank, which is all boats.

 

Which rather suggests that rainwater getting in round the filler (or in through the vent) is by far the most likely cause, which would also explain why some people have problems but many don't -- including all those whose fillers/vents don't leak... 😉 

 

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.