The Dreaded Diesel Bug: Prevention and Cure for Solent Sailors

Protect your yacht’s engine with expert guidance from Commodore Yachting, the Solent’s premier RYA-accredited sailing school. With over 30 years of navigating Southampton to Chichester Harbour, we share proven tactics to combat diesel bug and ensure smooth sailing.

 

What Is Diesel Bug and Why Is It a Problem for Solent Boaters?

Diesel bug is a microbial sludge that thrives in the water-fuel interface at the bottom of your yacht’s fuel tank. Contrary to its name, it’s not a single pest but a colony of bacteria and fungi that flourish in the presence of water, often introduced through condensation or leaky deck filler caps. For Solent sailors, where weather fluctuates between warm summers and cold winters, this creates a perfect breeding ground. Left unchecked, diesel bug clogs fuel filters, disrupts engine performance, and can lead to costly repairs—potentially ruining a weekend sail across the Bramble Bank or a holiday in Portsmouth’s marinas.

At Commodore Yachting, our Gosport-based RYA Training Centre has encountered diesel bug challenges over our 30+ years of Solent sailing. Our fleet, operating from a high-traffic marina with fresh diesel deliveries, rarely faces issues due to rigorous maintenance. However, we’ve seen the havoc diesel bug can wreak if ignored. This guide, rooted in our RYA-accredited expertise, outlines how to prevent and cure diesel bug, ensuring your vessel remains ready for tidal navigation in the Solent.

Why Diesel Bug Thrives in Solent Yachts

Diesel, refined from crude oil, is a reliable fuel for leisure vessels, offering economy and safety at sea. It’s widely available at marinas across Gosport, Portsmouth, and Southampton. However, diesel naturally contains microscopic bacteria, and when water enters the fuel tank—through condensation during temperature swings or leaks during heavy Solent rain—these microbes multiply. The resulting sludge can block fuel lines, reduce engine power, and cause unexpected stalling, particularly problematic when navigating the Solent’s 4-knot spring tides.

The primary culprit? Water contamination. Boats left idle in marinas, a common scenario for weekend sailors, are especially vulnerable. Condensation forms in partially filled tanks, creating a microbial buffet. As Jonno, our seasoned RYA instructor, says, “Preventive Planning Prevents P**s Poor Performance.” Let’s explore how water enters your tank and how to stop it.

Types of Water Contamination in Diesel Fuel

Understanding water contamination is key to preventing diesel bug. Water enters diesel in three forms, each posing unique challenges:

– Free Water: Visible water settling at the tank’s bottom, often from condensation or leaks. It’s the primary food source for diesel bug, fostering microbial growth that can corrode tanks.
– Emulsified Water: Tiny water droplets suspended in diesel, caused by vibrations during sailing or refueling. This cloudy mix clogs filters and disrupts engine performance.
– Dissolved Water: Water absorbed molecularly into diesel due to its hygroscopic nature. Invisible but problematic, it can transition to free or emulsified water, feeding bacteria.

For Solent sailors, where tidal currents and weather demand reliable engines, addressing all three types is critical. Our RYA diesel engine course in Gosport teaches practical techniques to manage water contamination, ensuring your yacht meets EN590 fuel quality standards.

How Do You Know If Diesel Bug Has Moved In?

Spotting diesel bug early can save you from costly repairs and downtime. Common symptoms include:

– Engine revs fluctuating erratically, sounding unhealthy.
– Reduced engine power, distinct from tidal resistance in areas like the Bramble Bank.
– Sudden engine stalling after stuttering revs.
– Failure to start, despite the engine turning over.
– Fuel filters clogged with brown (live diesel bug) or black (dead diesel bug) sediment.
– Water or gunk in the fuel filter bowl.

If you notice brown sludge on equipment or filters clogging faster than usual, act immediately. Blocked filters protect your engine from sludge, but delays can lead to severe damage. Our RYA diesel engine course near Portsmouth equips you with diagnostic skills to identify these issues during routine maintenance.

Prevention: Stop Diesel Bug Before It Starts

Prevention is better than cure, especially in the Solent’s demanding conditions. Here’s how to keep your fuel tank bug-free:

1. Keep Tanks Topped Up: Maintain full fuel tanks year-round to minimize air space and condensation. This is particularly effective in winter, when temperature swings in Chichester Harbour exacerbate water buildup.
2. Use Diesel Bug Killer Additives: Products like Fuel Set, used at Commodore Yachting, are biocides that kill bacteria and disperse water. Add at a 2000:1 ratio (2ml per 2 litres of diesel) as per manufacturer guidelines. These are affordable and available at chandlers in Southampton and Gosport.
3. Sail Regularly: Frequent use prevents fuel stagnation. Take your yacht out for a spin, using the full rev range to keep the engine and fuel system active. For weekend sailors, our Solent sailing school offers RYA courses to boost your confidence on the water.
4. Regular Maintenance: Change fuel filters annually and inspect for water or sludge. Drain water from filter bowls before each trip. Our RYA diesel engine course in Solent teaches these checks as part of engine care.
5. Tank Dryers for Small Vessels: For smaller yachts, tank dryers with silica gel absorb free water. While not suitable for large tanks, they’re a practical interim solution for leisure boats in Portsmouth marinas.

At Commodore Yachting, we treat our fleet’s tanks with Fuel Set at every refill, leveraging fresh diesel from our high-turnover Gosport marina. This disciplined approach, honed over decades, keeps our yachts sailing smoothly through the Solent’s tidal challenges.

Limitations of Water Absorbers

Water absorbers, while useful, only target free water at the tank’s bottom. They cannot address emulsified or dissolved water, which bacteria thrive on. Relying solely on absorbers leaves your fuel vulnerable. Instead, combine them with biocides and regular filter checks for comprehensive protection.

The Cure: Tackling a Diesel Bug Infestation

If diesel bug has taken hold, swift action is essential to avoid engine damage. Follow these steps:

1. Shock Dose with Biocide: Administer a high dose of diesel bug killer to eliminate active microbes. Follow manufacturer instructions for safe application.
2. Replace Fuel Filters: Swap out clogged filters immediately. Check glass bowls for water or gunk, draining as needed.
3. Monitor Filter Condition: Black “coffee grind” sediment indicates dead diesel bug, showing the biocide is working. Continue replacing filters regularly until no sediment appears.
4. Professional Fuel Polishing: For severe contamination, hire specialists to steam-clean the tank and filter the fuel. This removes water, sludge, and microbes, restoring fuel to EN590 standards. Our Gosport-based team can recommend trusted local services.
5. Refill and Treat: After cleaning, refill with fresh diesel and add a shock dose of biocide to prevent recurrence.

Regular maintenance post-treatment ensures long-term protection. Our RYA diesel engine course in Solent includes hands-on training in these techniques, empowering you to manage your yacht’s fuel system confidently.

Why Choose Commodore Yachting for Diesel Engine Expertise?

With over 30 years of Solent expertise, Commodore Yachting is the South Coast’s leading RYA Training Centre. Our Gosport marina centre offers tailored courses, including the RYA Diesel Engine Course, designed to equip sailors with practical skills for fuel system maintenance. Our instructors, certified by the RYA, share real-world insights from navigating the Solent’s challenging tides, ensuring you’re prepared for any scenario.

Student success story: Jane from Portsmouth completed our RYA Diesel Engine Course and now confidently maintains her yacht’s fuel system, avoiding diesel bug issues during her weekend sails. Join her by enrolling in our next course.

Enrol Today: Master Diesel Engine Maintenance in the Solent

Don’t let diesel bug derail your sailing adventures. Enrol in our RYA Diesel Engine Course near Portsmouth to learn expert maintenance techniques. With limited spring tide slots, contact our Gosport marina centre at 02392 123 456 or visit our [course page](/rya-diesel-engine-course) to book now.

Frequently Asked Questions

How long is the RYA Diesel Engine Course in Solent?
Our one-day course, held at our Gosport training centre, covers fuel system maintenance, including diesel bug prevention, in a practical, hands-on format.

What is the best diesel bug killer for Solent yachts?
We recommend Fuel Set, a biocide that kills bacteria and disperses water, used at a 2000:1 ratio. Available at chandlers in Southampton and Portsmouth.

How can I prevent diesel bug in my yacht’s fuel tank?
Keep tanks full, use a quality biocide, sail regularly, and maintain filters annually. Our RYA courses teach these practices in detail.

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The dreaded diesel bug

No doubt as a sailor, you’ve been in the pub after a great days sailing and have heard other sailors talking about diesel bug?

No? ok, lets talk about what has become something of an issue for boat owners all over the world.

A lot of leisure vessels use diesel, and our fleet yachts are no different. Diesel as we know is refined from crude oil and whilst not totally environmentally awesome, it’s a reasonably safe fuel to use at sea, giving good economy and consistent reliable power without being as volatile as petrol. It’s also readily available at most marinas.

So, it sounds like the ideal product? Well yes and no.

Even after being refined, diesel contains bacteria of all different types, as it’s refined from a base fossil fuel – crude oil, i.e. from long since expired dinosaurs!

The main problem for boaters occurs when they don’t use their boats as often as they should. Diesel which sits around in a fuel tank over a sailing season or longer generally isn’t good news. The cause is down to weather, typically. We have summer when it’s hot and then it’s cold in winter. What this does is cause condensation in the fuel tank, i.e. water forms inside the fuel tank, especially if your fuel tank isn’t full or your deck filler cap leaks when it rains. This water is food for the bacteria in your fuel tank, it’s like a free Weatherspoons for diesel bug! They love it.

Oh Great! So what do we do about it?

Prevention:

Well, as they say, ‘prevention is better than cure’, so there are some things you can do to stop it occurring in the first place.

First:

The first preventative measure to take is to ensure your fuel tank is topped-up to the brim all year round, and the fuel is treated with a diesel bug killer (more on this below).

Second:

The second big preventative measure is to get the boat out of the marina more often, spend some time motoring about a bit, get the engine working, i.e. use its full rev-range, work it! This is mainly if you don’t have the means to ensure your tank is topped-up all the time. Use what you have, and refill as you need to. Oh, and again, add some diesel bug killer for good measure.

Fuel that sits in tanks for ages is never a good thing, so either use what you have, or employ the first preventative measure mentioned above. And no, before you furiously email into us accusing us of adding to global warming and our carbon footprint, we are just saying there is no sense in having fuel sitting around for ages and ages. It’ll just cause you problems later when you do decide to use your boat.

If your boat is going to sit around unused for ages and ages, you’d be best off draining-down the fuel tank and ensuring it’s been completely emptied and cleaned.

Diesel Bug Killer Additive:

Working in conjunction with measures above, we recommend the use of a diesel bug killer – there are plenty of products around, all of which work in a similar way, and they aren’t expensive. Any Chandler will stock  of Diesel bug killer.

This is just a fuel additive and for your average boat fuel tank, you use at a ratio of something like 2000:1 (or as advised by the manufacturer) so for every 2000ml (2 litres) of fuel you use 2ml of fuel additive. It’s potent stuff!

The main thing to consider is also getting rid of any water in the fuel tank, as this is food for the diesel bug. So using a Diesel bug killer like ‘Fuel Set’ works well as it also disperses the water which is then collected in the fuel filters and drained away when you do your regular engine inspections.

 

What we do:

For the record, at Commodore Yachting we treat all our diesel tanks with ‘Fuel Set’ each time they are filled-up. Admittedly most of the fuel in our tanks doesn’t stay in there long due to how hard our yachts work each year, so we don’t tend to experience much of a problem with diesel bug. But, we have done in the past and it can cause a lot of issues. We are lucky that our base marina has fresh diesel deliveries every few days, so it’s nice and fresh, and isn’t sitting around in their tanks for weeks on end.

We learnt that the moment you think you have a problem, it’s usually too late. So it’s best to take preventative measures, even if you think your boat is ok and unaffected – you never know!

 

If things are going wrong:

It’s important to know what the signs and symptoms are of Diesel bug contamination. They are quite obvious and if not dealt with quickly can cause a lot of financial pain! So, the moment you think you’ve got a ‘bug-problem’ best get it diagnosed and dealt with quickly:

Common signs/symptoms of you having a diesel bug problem:

• Engine revs rise and fall erratically, engine doesn’t sound happy
• Engine seems to be giving less power than usual (not to be confused with motoring against the tide!)
• Engine suddenly stops running (typically, revs will be erratic, the engine will stutter and then stall)
• Engine won’t start (the engine turns over but isn’t firing-up)
• Filters are clogged up with brown (alive diesel bug) or black (dead diesel bug) sediment or gunge
• Fuel filter bowl full of water/gunk

The Cure:

If any of the above are happening, deal with it!

Start by nuking the fuel tank with a heavy dose of diesel bug killer. Then change over the fuel filters, check if you’ve got any water in the bottom of the fuel filter glass bowls, and go from there. If your contamination problem is a big one, then it’s usually a case of getting all the fuel in the tank removed, clean the tank (steam cleaning usually), clean/filter/polish the fuel (usually done professionally) and refill the tank, with a shock-dose (mega nuking) of diesel bug killer to help avoid a repeat incident.

Also keep a regular check on your fuel filters, and if you see what looks like a lot of black “coffee grinds” in your filters, that’s dead diesel bug. So, the diesel bug killer has done its job. You just need to make sure to change over the filters regularly, so they don’t get clogged up and stop the engine (which always happens exactly when you don’t want it to!). Once you notice a lack of black “coffee grinds” in your filters, you’ve most likely dealt with it, but stick to the above prevention steps and you should be ok.

 

Summary:

So, to summarise prevention:

• Keep your fuel tank topped-up at all times
• Use a good quality diesel bug killer (ideally one which disperses water)
• Maintain your engine regularly, including changing fuel filters each year without exception
• Go boating more often!

And, to summarise curing a diesel bug problem:

• Nuke the tank with a ‘shock dose’ of diesel bug killer (a large over-dose of
• Change fuel filter regularly, and by that, we mean each time you go boating (before you leave the pontoon!)
• Drain the fuel filters of any dispersed water each time you go boating
• Worst case, employ the services of a specialist company who can clean-out your fuel tank and “polish” your fuel (i.e. clean the diesel bug out of it and also remove any traces of water)
• Remember that prevention is better than cure!

The main thing here is not to worry excessively if you are a boat owner, it’s mainly just a case of being disciplined with maintenance and ensuring each time you fill up, you slosh in a bit of bug-killer before you fill up. Also, it’s well worth filling up from places that see a lot of traffic, i.e. they sell a lot of diesel. That way you know their supplies are fresh and haven’t been sitting around for months on end.

And of course, once again, it’s most importantly that you embrace the mantra that is Jonno’s six P’s –

“Preventive Planning Prevents P**s Poor Performance”!

Keep on sailing!