The Offshore Voyaging Reference Site

The Bug Is Taking Over

Following our arrival in the Canary Islands last autumn we watched an endless stream of yachts line up at the pontoon alongside the boatyard, all of them with fuel contamination problems. In my experience, it’s usual to find that in cases like this the outbreak can be isolated to one or two suppliers who have a problem with storage or maintenance – the word soon gets around. But the engineer, who seemed to spend his entire time doing nothing but pump out and clean tanks said no, there was no common source for the fuel, but it was in his view part of a rapidly increasing problem linked to the increasing use of biodiesel blends, as there had been a major spike in the level of boats affected since its more widespread uptake.

What’s Behind It?

Fatty Acid Methyl Esters (FAME) known as biofuels, are now being combined with conventional diesel on a wider scale, and often in increasing amounts. Automotive diesel specification EN590 allows up to 7% of FAME to be incorporated, even though it is known that levels of above 2% are more susceptible to microbial infestation. The reasons behind this microbial problem are varied, but two stand out. The first is that FAME is hygroscopic, and so absorbs water, the principal culprit behind most bug growth, and the second is that it is an excellent nutrient resource for the microbes to feed upon. The effects this can cause are bad enough on the road, but have the potential to be far more serious for those of us who go to sea.

The problems that FAME diesel can cause are well known, and include blocked filters and damage to rubber and plastic components in fuel systems, especially in older engines. Equally, there are cures available in the form of biocides that block the bugs’ ability to feed, or dispersants that remove their critical habitat, water, by lifting it into suspension. Both types can be effective, although it seems that the biocides may have an edge in terms of a short term cure, whilst the dispersants may be more effective as preventative treatments. Some are more quick acting, whilst others can leave residues that may affect your filters, so always keep a couple handy after any treatment other than a simple preventative one.

Prevention Is Better Than Cure

Obviously there are ways to try and avoid this issue in the first place, but depending on where you are it may not be that simple. In the UK, for example, it has been estimated that 75% of all diesel fuel supplied to the marine market will be low sulphur and FAME-free, but that still means that 25% will potentially include FAME. So it will obviously make sense to check on what the local market situation is, and if in any doubt, ask the supplier before putting the nozzle anywhere near the tank. Avoid buying spare fuel in cans from petrol stations, where the likelihood of buying FAME-blended diesel will be far higher. And, just as before, keep your tanks topped up whenever you leave the boat for any length of time to reduce the chance of condensation forming on the tank walls, and so creating the best possible opportunity for the bugs to thrive. And consider using a preventative treatment of biocide or dispersant at every fuel fill up – the cost compared to the cure is small.

Another step is to regularly inspect the contents of your tank, whether or not you’ve had a problem with filters. The best way to do this is via a drain tap mounted as low as possible in the tank(s) so that a sample can be drawn from the very area where problems are going to be the most obvious. Failing that, drawing off a sample via a tube inserted through the top of the tank into the bottom corner can be equally effective. If there’s any sign of contamination, water or sticky deposits, then it’s time to drain off or pump out the bottom section (at least) of the tank until no more contamination is present, then treat with a biocide or dispersant, and change the filters. In really bad cases it will mean pumping out all of the affected fuel for disposal. The tank should then be cleaned  mechanically and with a biocidal system cleaner, as in extreme cases there will be deposits on the tank walls that can cause serious microbial corrosion that will need to be removed – if you’ve got tanks welded into your steel or aluminium hull, think very carefully about this!

But Can You Get At It?

Sadly, all of this is dependent on the builder of your boat having installed at least one of the following: (a) a correctly sited drain plug, (b) a sampling tube, (c) an aperture, (d) a removable inspection plate big enough to allow access to all corners of the tank, or (e) a tank that can be removed without demolishing the boat. Having been on a boat affected by ‘the bug’ which only had (c), I can tell you that cleaning out the tank properly is an impossibility. So with FAME-blends becoming more prevalent, if your boat hasn’t got any of the above then maybe it’s time to think about installing whichever option is the most practical and cost effective in your case. Otherwise you may find yourself joining the queue at the boatyard!

And if you have any experience of diesel contamination problems linked to biofuels, do leave a comment.

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We had a problem with blocked fuel filters after one year using 5% biodiesel.

15 years ago I had the same problem with adding STP Diesel Fuel Injector Treatment to the tanks, the result was almost the same, blocked fuel filters, just a few miles out of the coast Rio de Janeiro, no wind no engine and battery empty, and no response on the marine radio.


Hi Carlo

that’s an ugly image, and shows just what you can be up against with biodiesel blends. And it’s not just ‘the bug’ that can block filters. Biodiesel can act as a solvent, cleaning residues off tanks and pipes leading to the blocking of filters with the detritus that (of course) makes its way to the filters.

But in out of the way places where you have to take whatever fuel is on offer, compounded with language difficulties, how do you now what you’re taking on board?

Thanks for the evidence!



Sorry, I’ve no experience or advice to add, more thinking aloud and hoping it comes across as helpful and stimulates some discussion.

I wonder what the effect of putting a fuel polishing pump/filter system in would have?

The polishing dip/extract pipe could go right to the deepest part of the tank (I believe engine dip pipes are an inch or so short of the bottom) and the return could go to the opposite side of the tank and create a nice circulation of fuel. Normal fuel filters are not designed to kill the bugs, just filter crud and hence their byproduct out. I suppose they would at least remove the water from the tank and make it harder for them to grow.

I wonder about water filters/purifiers for backcountry hiking. Some bugs are big enough to be mechanically filtered out (giadia, crypto), others purify the water by bringing the bugs into contact with poisonous chemicals as they pass through a membrane (fixed iodine crystals), others use a UV light to make the bugs infertile by disrupting their RNA so they cant breed.

A fuel polishing system equipped with a filter/purifier able to tackle biodiesel bugs might be an answer? A water remover combined with fixed pesticide or UV exposure?

Regards, Dave

John Harries

Hi Colin and Dave,

We have a fuel polish system on “Morgan’s Cloud” that I built myself some years ago out of an electric fuel pump and a Raquor 500 series filter. It can also be used to transfer fuel between tanks. It’s on a simple electrical timer switch and I run it for a couple of hours, at least, every time we motor. Works great.

Having said that (do I use that phrase too often?) we have not sailed in Europe since bio-diesel became widely used, so I don’t know if our polish system would solve that problem.


Hi Dave and John

good points, and it would be interesting to hear from anyone out there whether John’s fuel polishing system might be an answer.

And for anyone Like John who likes cold places, there’s another reason (!) not to want to take on biodiesel, as it can affect the Cold Filter Plugging Point (CFPP) of the blend, making it gel out at higher temperatures.

Best wishes



this brings to mind the recent flap with the fcc giving preliminary go-ahead approval to lightsquared co. for them to operate their stronger subscriber transmissions on frequencies dangerously near to those set aside for much weaker gps operation…first of all how could they (and lightsquared too) be so ignorant as to do that ? secondly how can the fcc be so arrogant to do that without at least publicizing it significantly in advance of doing it to discover possible problems ? it is as if they said: well, we’ll do this and then sit back to see what the fallout might be…if there isn’t much then we’ll issue the final go-ahead…ha-ha

well, there is plenty of immenently predictable fallout from us peons…hopefully enough so the fcc might be more circumspect with future such deliberations…my understanding at this point is that while lightsquared’s request is shelved, it is amazingly not yet totally dead…as my grandaughter would say: go figure…richard in tampa bay (m/v cavu’s skipper, formerly s/v sidra’s skipper)

Chauncey M Freeman

Good reading and a much needed warning. Please do notify me with any followup comments/advise. It is so sad that most of the managing positions where it counts are filled with monkey’s , another description of Lawyers, Politicians, and Lobbyists.

Chauncey M Freeman
Senior Chief,USN,RETIRED


Hi Richard and Chauncey

It often seems that our interests don’t get noticed, despite the fact that many organisations (like our own RYA) put in sterling work to get our voice heard.

Decisions at this level can have knock on effects, too. It’s getting harder to source fuel in harbours around the UK, as the cost of maintaining what are in effect small supplies becomes no longer cost effective due to maintenance and Health and Safety costs etc. One marina operator told me that supplying fuel was a loss maker as far as he was concerned, and more money could be made by converting the fuel dock into berths. They simply maintained the fuel dock as a service – and he wasn’t sure how long that would continue.

If the same firm had a problem due to an inadvertent supply of a biodiesel blend that contaminated their tanks and caused problems for their customers (with an inevitable loss of reputation), then maybe that could be the clinching point where they might decide to give up the supply of diesel altogether.

And then there are less supplies available, and before too long you’ll maybe have to take what you get…..

Best wishes


Viv and Mireille

This is why this site is so useful! Thanks for the article on Bio Diesel problems. I have forwarded this to sailing friends.



Hi Viv

Well, we try! Hopefully it may prove useful to them, as I know to my own cost what a pain in the neck it can be (at least) and what potential hazards ensue can from a failed engine – not recommened.

Best wishes


Peter Hollings

I’m a wanna-be sailor with some experience with Diesel fuel contamination in my 1977 Mercedes 300D (360,000 miles). When I bought the car about 6 years ago the tank was grossly contaminated and my fuel pre-filter would become clogged every few hundred miles. I turned down the option of having the tank removed for steam cleaning because of the high cost. Instead, I regularly use the Startron dispersant product for Diesel which is available at WestMarine. It is an enzyme which breaks down the Asphaltene into fine enough particles such that they pass the filter and are combusted. I also use a biocide and a lubricity enhancer/cetane boost since my older injection pump was designed for high sulfur fuel which had inherent lubrication properties. As a result of this strategy I had to change my pre-filter every few thousand miles for a couple of years, but this requirement tapered off to the point that I cannot recall when I last did it. I have not inspected my tank interior, but I believe that it would be essentially clean.

Here are some additional points/suggestions:

1. A loss of engine power (falling RPMs under load) is a tip off that a filter is likely clogged.
2. Asphaltene forms in all Diesel fuels, not just with those containing FAME, and the cause may not be solely biological.
3. Pre-filters can be recycled many times by back flushing them. I do this by vigorously shaking the filter (with the fuel still in it) so the fuel flows out of the inlet.
4. Not all of the contamination is in sediment. I once had the screen (which is above the tank low-point) on my tank outlet clogged by a floating mass that was described to me by my mechanic as like a floating Kleenex.
5. Considering that an engine failure at critical times like entering port can be hazardous, I would consider installing a second pre-filter in parallel with a y connection and valve on a boat with contamination problems.

Peter Hollings


Hi Peter

Lot’s of good advice there, especially for owners of boats with older engines.

And as you point out (and I know to my own cost), ‘the bug’ has always been around, it’s just that with the introduction of FAME blends it’s far more likely to occur.

And like many safety conscious owners we do have a twin pre-filter system, that enables us to switch filters at the first sound of the engine missing a beat – not a long term fix, but in extremis a very good idea indeed.

Best wishes


Hans Jakob Valderhaug

This thread seems a strong argument in favour of coloured diesel for boats, regardless of the taxation issues.

Working on the next edition of The Norwegian Cruising Guide we have recently been in contact with the major oil companies in Norway. We are told that while biofuel is added to all “road-diesel”, none of the suppliers add biofuel to their “marine diesel”. Diesel for boats here is still exempt from road tax hence dyed green. Since all suppliers have a separate production line for this coloured diesel there is never any doubt the diesel you put in your tanks is free from biofuel.

As European readers may be aware the fight for coloured diesel sadly seems a lost battle in the EU.


Hi Hans Jakob

In the UK we still have ‘red’ diesel which has traditionally been for marine use (and other uses such as farm machinery), although as you rightly point out, the position regarding such dyed diesel is becoming far less clear. This may be the worst of all worlds, as has been recognised by the British Marine Federation, who issued a guidance note in 2010, that pointed out that ‘the overriding concern is that some suppliers will provide road diesel with a red marker dye and this supply will contain up to 7% biodiesel or FAME’. In a world where dyed diesel is becoming less available, it would seem to me that this risk will be enhanced – which would be far from welcome.

Best wishes


John Lundin

Interesting, I just learned about the growing bacterial problems last week, apparently from the addition of biofuels. A diesel mechanic I spoke to recommended adding a Fuel Biocide at every re-fueling, such as Biobor. He also recommended running the tank as low as possible between re-fueling to avoid contaminating new fuel with the remains of the old fuel.

Between the tank bug and the dissolving rubber/plastic components of fuel systems, this biofuel issue packs a lot of surprises!


Hi John

The preventative use of biocides seems sensible, although there seems to be some debate over which is the most effective in this role, a biocide or a dispersant.

Good point about running the tanks low before re-fueling, which I suspect would also give advance warning of a problem – as the tank level drops low, you’re getting into the place where any sediment is concentrated, and filters start to ‘nip’. As I know to my cost….

Best wishes



“Listening” to the above dialogue, I would like to add the following, that hopefully may both news and to the benefit of some;

While searching for possible ways to beat the cr… out of the diesel bugs, I by change came across a rigorous test of diesel fuel additives undertaken by the reputable German Segeln Magazine 3/2009 ( As the test is not freely available, I purchased it to gain access to the results. According to the test, most diesel additives cost a lot and do not do the job, i.e. a waste of precious cash. One product, however, stood well up to the test – Grotamar 71.

This is what the test stated:
In den von uns gewählten Dosierungen wirkte das Mittel perfekt, beide Proben waren vollständig keimfrei. Die Dosierungsangaben sowohl für Schock- als auch für die Permanentdosierung sind auf der Flasche zu finden, allerdings sehr weit gestreut (Unterschied mit Faktor 20!). Mit der Frage, welche Konzentration denn jetzt die richtige ist, wird der Nutzer etwas allein gelassen. Wir haben uns deshalb für einen realistischen und (ab-) messbaren Wert von 0,05 % für die Permanentdosierung entschieden. Die Warnhinweise sind vollständig, der Verschluss gesichert

Google translate for what it is worth:
At the doses we selected the remedy worked perfectly, both samples were completely sterile. The dosage information for both shock and for the permanent dosage can be found on the bottle, but very far spread (difference by a factor of 20!). The question of what concentration is it now the right, the user is left alone for a bit. We have therefore opted for a realistic and (ab-) measurable value of 0.05% for the permanent metering. The warnings are fully secured to the closure.


The problem is that all additives you put into diesel… including biodiesel is part of a molecular family called a surfactant. A surfactant increases the ability of the fuel to hold water. ULSD has a low interfacial tension, and it gets lower when you add biodiesel. This means that the water binds with the fuel molecules more tenaciously as the percentage of biodiesel increases.

Unfortunately most traditional filters can’t get rid of the this emulsified water. Most can get rid of the free water and saturated water (makes fuel look cloudy), but the emulsified water is the issue. Untreated it can pass right into your engine.

So, putting additives including biodiesel into fuel raises the ability of the fuel to absorb water which leads to a faster degradation of the fuel… more sludge and fuel issues.

A fuel polishing system is suggested… but one that can work with emulsified water. That means a filter that has passed testing for SAE J1488 ver. 2010on ULSD and ULSD blended with biodiesel.

You can google that for further information and answers.


Hi Peter

Thanks for that valuable explanation.

I’ll follow up on that recommendation and see where it leads in terms of availability – and pass it on.

Kind regards


Peter Hollings

Colin & Everybody —

I neglected to mention that Startron sells two different diesel additives. Both contain enzymes which break up the asphaltene, thus cleaning your system. One product is called “Tank Cleaner”. Don’t make the mistake of thinking that is the stronger one. I was advised by their VP Sales that it was the other one (whose name I have forgotten) that was stronger. West Marine sells it.


Paul Mills

Hi all

Learning a lot from this thread… . Just got my seasons supply of fuel treatment and found that Grotamar 71 has now been superceded by Grotamar 82 which claims to be even better.



Just a brief comment to update this article, that has particular resonance for UK based boat owners.

In recent years the derogation for using dyed red ‘marine’ diesel (which is far more likely to be FAME free) has been eroded as European Union countries oppose the sale of duty-free diesel. This has finally reached the stage that it will no longer be legal to run your engine on red diesel in EU waters. Boat owners found with red diesel in their tanks (or spare cans) can be fined, and apparently this has already occurred in some instances.

As a result, if you want to take your boat across the Channel and don’t want to risk a fine, then you’ll have to changeover to white ‘ automotive’ diesel – which is far more likely to contain FAME, as I outlined in the original post. Not good news…..


Ed Seling

I have used all the standard treatments for contaminated tanks and the only one that has been able to cure stubborn algae outbreaks has been Starbrite’s tank cleaner. One of the few products I recommend by name. And I get no kickback! 🙁

Bill Hoffman

This thread may be dormant, but I’m wondering if anyone has information about the impact of diesel additives on diesel heating systems. We have an Espar diesel heater, which required an expensive repair after several years of excellent performance. We were told that because the diesel heater burns fuel at a lower temperature than the combustion temperature of an engine, additives in the fuel are not completely burned in the heater, causing carbon buildup and ultimately malfunction. Anyone with experience/information about this?

Bill Bowers

This is the closest thread I could find.
I am thinking of relocating our fuel vents from the transom as suggested by Nigel Calder’s Cruising Handbook to avoid JSD seas dosing our fuel.
Is there any reason not to relocated the vents to our adjacent propane locker?

Perhaps with a tube leading down to a spill-catching plastic bottle between the propane tanks.

John Harries

Hi Bill,

I think that will work fine. In fact, that’s exactly how our boat is set up.

Make sure that if you add the tube you mention that it has a small hole in it right at the top where it exits the vent to act a siphon break.

Bill Bowers

Thanks John for the confirmation about venting into the propane locker.

Next question is why do we really need twin vents for the two tanks versus a sturdy bulkhead connection with an adjacent barbed tee?


John Harries

Hi Bill,

Hum, this is one of those situations where I have a gut feeling that it would not be a good plan, but then can’t come up with a solid reason why not. You would want to make sure that the join point was well above a level the fuel could reach when healing.

Anyone else got any thoughts?

Geir ove

What is the status around europa now, do they al ad Biodiesel, 5-10% ?
Norway is, Sweden is, Danmark is not for boats, what about

-The biodiesel is not good for our engines. so i hope as the oil price has gone down, it want pay of to make biodiesel, and they can stop adding it ?


Most countries offers only biodiesel. Greece, Spain, Portugal, Germany. In Italy some filling stations said they did not sell biodiesel. I very much doubt that is true.

Biodiesel is not only absorbing more water it also destroy your engine oil quicker as the boiling point is higher. Diesel that blow by, down in to the engine oil will not boil away. It is recommended to shorten the time between oil changes.

Another disadvantage is that the modern diesel has very low sulphor content. That might be good for the enviroment, but is promoting bacteria growth in the diesel. The sulphor was preventing bacteria growth.

If you are sailing in Europe you can not avoid bio-diesel. Here are some things you can do to limit the problem.

-remove water
-use a biocide regulary
-Do not top up if you have a large tank and are not planning to use it in a month or two. (Shelf life in 30 degrees is 2-3 month. That is from the refinery!)
-Do not winter store your boat whith full tanks. The condensation is not a problem. If you think it is, seal the breather!
-Change oil more frequently than what the manual is recommending.

I´m sure that there is more than one biocide that works well. But I have had very good results whith Grotamar after picking up bad fuel in Ceuta, Spain. It took less After adding the Grotamar it took 5-6 hours until the fuel was clear. After that it was just to pump out the sludge that formed in the bottom of the tank.

After this expearance we have changed our fuel routines. When it is your tanks it is a big mess.

We try to stay one day in the marina. Day one we buy 10 l that sits in the cockpit over night. If the sample look nice next day we take the boat to the fuel dock.

Colin Speedie

Hi Roland
thanks for a really useful contribution on this subject – some great ideas.
You may be interested to learn (if you don’t already know) that Grotamar came out as one of the two best products in a very well conceived trial run by Britain’s Practical Boat Owner magazine some years ago, with a British product (that we use ourselves) called Marine 16. Both products apparently share a very similar formulation, so there’s obviously a link.
Best wishes

Bill Attwood

I have just come across a new fuel from renewable resources with some pretty extravagant claims. Being a cynic, I researched the internet, and it seems as if the claims may be genuine. The fuel is “CARE Diesel”, produced 100% from renewable resources. The manufacturer is Neste Corporation of Finland, formed in 1948, and a major refiner in Finland. They are also (according to the firm themselves) the world´s leading supplier of renewable diesel. The fuel has just gone on sale at our marina (Kröslin on the German Baltic coast) and is priced significantly higher (about 40%) than standard diesel, but the claimed advantages, and its green credentials, may make it attractive. The small sample bottle I have seen and smelt shows it to be water clear and completely odourless. It is also unattractive to the “diesel bug” and doesn´t suffer from long periods of storage. Maybe worth investigating?

Colin Speedie

Hi Bill

that’s a very interesting development. I’ve just read the details on this product and it does seem to answer many questions on conventional diesel let alone biodiesel.
Thanks for bringing it to our attention.
Best wishes


Hi Colin, Condensation : “And, just as before, keep your tanks topped up whenever you leave the boat for any length of time to reduce the chance of condensation forming on the tank walls, and so creating the best possible opportunity for the bugs to thrive.” I live in the tropics, 30 degrees celsius day time, 24 degrees celsius night with a humidity of 95%. I get different opinions from boaters in the marina. Some share the same view as you but there are others (Master Marina, Yacht surveyor, Yacht masters etc) who are of the opinion that condensation doesnt occur. That got me thinking because my boat has a capacity of 1000 litres in 2 tanks and the diesel can stay in there for years as I do mostly coastal sailing where a marina is not more than 1-2 week between trips. Keeping diesel in tanks runs the risks of contamination and I was thinking of keeping only 300 litres of diesel (150 litres in each tank) and refueling frequently. Then I thought I should do my own research into condensation in partially filled diesel tanks. Here is my logic. Water vapour in the air condenses onto colder surface (assuming the tank walls). So, the the tank walls are more or less at the same temperature as the air around it, there will not be condensation. So, the sea must be colder than the diesel in the tanks for that to happen. Ie in cold climates and not really in the tropics. Secondly, in the tanks, wouldnt the air space around it be filled with a great proportion of diesel vapour rather than air? Does that make sense?

John Harries

Hi Rob,

Colin may not respond, see comment guidelines #3.

Although I have always been told to fill my tanks before storage and have always followed that advice, I think your logic makes sense.

The key, I think, is that there is no temperature variance in your scenario.

On the other hand, if storing a boat out of the water, particularly in a place like Nova Scotia with big and fast temperature swings, I would always press the tanks first.

The other thought is to treat your fuel with BioBor JF. I have been using the stuff for years and have stored our boat for as long as 20 months with fuel tanks full and come back at the end of that time to find the fuel sweet and clean.

Marc Dacey

John, a data point here. I filled two 50 gallon keel tanks with diesel (topped right up, but with vents left open, no biocide, but, significantly, I think, we used pure diesel, not ethanol) before a re-engine, repropping, etc. It took years. By the time we launched, the diesel was at least five or six years old.

For my engine testing, I went with a jerry can. Then I installed my fuel polishing setup and decided to see the state of the diesel I had to hand. I inspected the tanks and pulled the pickups and found very little evidence of “goo” or contamination. So I ran it through the Racors and it’s been fine. I’ve used about 40 gallons of that original 100 and have kept both tanks filled for the most part with newer diesel. There have been no hiccups and no evidence that the fuel filter pump has needed to be on. Am I just lucky or was the use of “old-school” unadulterated diesel (not “red”, just gas station grade before ethanol) to account for my good fortune?

That said, I will likely throw in stabilizer at haul out because I’m now unable to easily find pure diesel and have no good idea what I’m being sold in terms of hydrophilic additives.


Hi John, thank you for your reply. I do use Racor Biocide without any problems yet. But I have never run my tanks to 1/4 full. Maybe i should try BioBor JF. BTW, what is a fuel polishing set up? What is it being polish off?

John Harries

Hi Rob,

A fuel polishing system allows one to circulate fuel through a filter to clean it. We have one I built, but you can also buy one complete.

Worth it is you cruise remote places, otherwise probably not worth the complication.


Thanks John.