Diesel bug: How to get rid of it and why it’s getting worse

Diesel bug seems to be becoming more common in yachts' diesel tanks. But what can you do to prevent it? Andy du Port tells all

Problems related to ‘diesel bug’ in yachts are becoming increasingly common. This is almost certainly due to the higher percentage of biodiesel in the fuel we now buy – good for the environment but not so good for long-term storage in boats’ tanks.

Biodiesel is hygroscopic, which means it attracts and holds water more than other fuels, thereby encouraging the growth of diesel bug – a horrible black gunge lurking in the bottom of most fuel tanks. If this gunge finds its way into your fuel lines, your filters can become gummed up and, eventually, your engine may end up being starved of fuel.

Since owning my first diesel-engined yacht, back in 1974, I’ve been fully aware of the necessity to use clean fuel and keep out any water contamination. More recently, I’ve paid particular attention to keeping my fuel tank bug-free.

At the start of each sailing season I use an oil extraction pump to suck out a sample of fuel. A small amount of black muck usually comes up, and I continue pumping until the fuel runs clear. Two or three litres is normally enough. I try to get my diesel from suppliers with a good throughput or, even better, from petrol stations ashore.

To minimise condensation, I never allow the fuel level to fall much below 20 litres from full, and I always top up to the brim for the winter. When topping up I always dose the fuel with a biocidal ‘anti-bug’ treatment.

Despite all of this, the dreaded bug attacked last year as my wife and I were motoring in Kudu, our Hallberg Rassy 34 in a flat calm 20 miles north of Alderney. It chose to make its appearance in the middle of the main shipping lanes just east of the Off Casquets TSS (traffic separation scheme). Initially the revs briefly dropped a couple of times, and a little later the engine stopped altogether.

We were in no immediate danger: the sea was flat, there was no wind, and all the big ships were passing two or three miles to the north or south of us. However, I did start mentally rehearsing an appropriate Sécurité call to warn vessels of our predicament if it became necessary.

Averting disaster underway

The first step required was to check the primary filter. A year earlier I’d replaced the original CAV filter/water separator with a Racor-type filter – which turned out to be a good decision. Removing the filter element on the CAV for inspection had been a tricky, messy job, even when alongside.

Thankfully I never had to do it while underway; it would have been much more difficult. In contrast, the new one just involves unscrewing the top (T-handle, no need for a spanner) and lifting out the filter. In this case the filter appeared to be clean, but there was a lump of black stuff in the bowl beneath it. I opened the drain and caught the bugs, replaced the element, topped up the filter housing with fuel and re-secured the top. The engine started and we were on our way. Job done… or so I thought.

Having experienced one fuel blockage, it was impossible not to keep worrying about when it might possibly happen again. We were lucky to be in the open sea when the bug struck, but if it had happened in the narrow entrance to Portsmouth Harbour with a cross-Channel ferry leaving on a spring ebb at night it would have been a very different kettle of fish. Then, after clocking up another 100-plus engine hours without incident, it happened again.

This time we were under power in the Solent when the engine missed a beat or two (as did our hearts) before recovering itself. The next day – in another flat calm – the engine was clearly not happy. The revs were ‘wandering’ very slightly, and we were on tenterhooks waiting for the engine to stop. And this time we were heading for Portsmouth Harbour! Just in case, I noted the names of nearby yachts who might be able to help, but mercifully we made it back to our berth in Haslar Creek without any further problems.

It was obviously time for further investigation. More pumping revealed more black gunge in the tank and, after taking advice from an engineer, I contacted a tank-cleaning company.

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Help from the experts

There aren’t many people who specialise in cleaning fuel tanks in yachts, but Clean Diesel Company (CDC), based in Port Solent, was recommended by members of our sailing club. CDC is a two-person team: Chris and Sarah Curtis. They proved to be friendly, knowledgeable and efficient, and took great care not to make any mess: plenty of dust sheets on the upper deck, in the cockpit and in the saloon (the tank is located under the port settee).

The job took around five hours, and would have been shorter if I hadn’t asked so many questions. My tank has no access from the top other than from a very small brass cap which can be removed when changing the fuel level sender. It’s also internally divided into three sections by two baffle plates. CDC installed three round plastic hatches (about 15cm diameter), one for each section of the tank. Not only were these necessary for the cleaning process, they now provide very easy access for inspections in the future.

Let the clean-up commence

Although cutting holes in a tank full of diesel is, apparently, perfectly safe, I have to admit to planning a hasty retreat as Chris started up his drill to do the job. He then used an electric pump to transfer the fuel – about 135 litres – out towards Sarah who filled it into plastic containers in the cockpit. Not a drop was spilled. Then the mucky process of cleaning the inside of the tank began.

In CDC’s opinion, plenty of elbow grease and lots of paper roll is the only way to get the best results. I left them to it and retired to the clubhouse for lunch. When I returned to the boat, the cleaning was complete and I was invited to inspect the result before the fuel was filtered and returned back into the tank – which was now spotless inside. I was keen to know what recommendations CDC had for maintaining bug-free fuel.

Their advice was:

• Buy fuel from petrol stations ashore whenever possible. This, of course, is clearly impracticable for larger yachts, but most of us use a relatively small amount of fuel during the sailing season, and I find that it’s perfectly feasible to top up with just a couple of 10-litre cans.
• Try to avoid refuelling from fuel barges or anywhere that does not have a high turnover of fuel.
• Do NOT keep the tank full. It’s much better to let it run down a bit before filling with a good quantity of fresh fuel. It’s also best not to top it up to the brim before you lay the boat up for the winter, but, instead, leave plenty of room for filling it with fresh fuel after launching.
• Always use a fuel treatment. Chris and Sarah’s favourite additive is Fuel Doctor, but I have used Startron and Marine 16 in the past, and mixing them (or overdosing) does no harm.
• Finally, carefully check the seal on the fuel filler cap. If it is starting to show its age, replace it. As we all know, water in the fuel is a recipe for disaster.

I was surprised by the advice not to top up before the winter lay-up, but it certainly makes sense to mix fresh fuel with the old at the start of the season. Some people say all the fuel should be removed, but I don’t know what you’d then do with, say, 150 litres of diesel!

As for costs, CDC currently charges around £480 for my size of tank (150 litres). Fitting hatches is extra, and if any contaminated fuel has to be disposed of there is a small charge per litre. This might seem a lot of money, but the peace of mind of having a clean and bug-free fuel tank is definitely worth it.

Some readers may be thinking that the engine symptoms I have described could be related to air in the fuel system rather than blocked lines or filters. It is certainly possible, but the tank clearly needed to be cleaned anyhow.

Just to be sure, I’ve now gone carefully round the engine checking that all the fuel lines are fully secured and there are no signs of fuel leakages. I have also re-seated the engine fuel filter with a smear of oil on the rubber sealing ring.

What is diesel bug?

Diesel bug is the contamination of diesel fuel by microbes, including more than 100 species of bacteria and fungus. Contrary to the commonly used terminology, these infections do not contain algae. Algae requires light for photosynthesis and there is no light in a diesel tank.

Diesel fuel is actually quite biodegradable, so, given a very small amount of water, microbes will thrive and break the diesel down. Biodiesel is even more biodegradable, and can be host to well over a thousand species of bacteria. With the exposure to bacterial and fungal spores from purchased fuel, or even from the atmosphere, troublesome volumes of diesel bug can grow quickly. This is particularly true in warm weather and with some gentle agitation as your boat moves at the dock or swings on its mooring.

Contamination by water

Water can get into diesel fuel as a result of condensation, rainwater penetration or adsorption from the air (hygroscopy), and biodiesel is especially hygroscopic. A desiccant air filter in the tank breather vent will help minimise humidity in air in the fuel tank and reduce hygroscopy and condensation.

Water reduces the heat of fuel combustion causing smoke, harder starting, and reduced power. It corrodes fuel system components (pumps and fuel lines) and when frozen, ice crystals provide sites for nucleation, accelerating gelling of the fuel, and pitting in pistons. The presence of water also encourages diesel bug growth. This occurs either at the interface between the oil and water or on the tank, fuel pipe, and fuel filter walls, depending on whether or not the ‘bugs’ need oxygen.

What does diesel bug do?

By breaking down the diesel molecules the microbes can cause a change in fuel chemistry, which adversely affects how the fuel is burned in your engine. On a rough day out on the water, clumps of these microbes can break loose and block fuel filters or, worse, the fuel injectors. Microbes in water can also cause the paper-element filters in the system to rot and fail, in turn causing failure of the fuel pump or blockage of the injectors due to ingestion of large particles.

What is biodiesel?

Biodiesel refers to a vegetable oil or animal fat-based diesel fuel consisting of long-chain alkyl (methyl, ethyl, or propyl) esters. It is typically made by chemically reacting lipids such as vegetable oil, soybean oil and animal fat (tallow) with an alcohol (methanol), to produce fatty acid esters.

The latest generation of biodiesel – HVO (Hydrotreated Vegetable Oil) – is gradually replacing the previous biodiesel iteration – FAME (Fatty Acid Methyl Ester). According to industry experts, HVO – with its improved purity, lubricity and cold weather resilience – can reduce diesel emissions by up to 90% while having much improved storage capability.

Biodiesel is fully compatible with most modern materials in your engine, tanks, valves, pipes, and hoses, but not all the old ones.

Here’s how it reacts:

• Plastics: Polyvinyl chloride (PVC) is slowly degraded by biodiesel. High-density polyethylene (HDPE) is compatible with biodiesel.
• Metals: Biodiesel (like methanol) has an effect on copper-based materials (eg brass), and it also affects zinc, tin, lead and cast iron. Copper and zinc also promote oxidation, which can lead to diesel bug. Stainless steel (316 and 304) and aluminium are, thankfully, unaffected by biodiesel.
• Rubber: Biodiesel affects various types of natural rubbers found in some older engine components. Commonly used synthetic rubbers found in modern engines can handle biodiesel in all conditions.

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