The growing threat to a yacht’s fuel system

Up to 80% of breakdowns and failures on blue water cruising yachts are due to fuel contamination. With restrictions imposed from the pandemic and sailors unable to visit their yachts for regular maintenance, this is a timely concern.

What is Diesel Bug?

Diesel bug is a general term used to describe microbial bacteria, filamentous moulds and yeasts that inhabit fuel tanks. They thrive at the interface between water and diesel, living off the oxygen in the water and feeding on the fuel.

The microbes have a very short life span, but one microbe can produce more than 7 million new microbes in a 24-hour period! The waste they create falls to the bottom of the tank and with their high reproduction rates, form a large amount of sludge quite quickly. This is then in prime location to get sucked through the fuel lines, blocking filters and eventually causing major damage to the engine. Around 147 species have been identified living in infected non-biodiesel fuel. Biodiesel is much worse with over 1,400 organisms living in the fuel.

How is Diesel Bug formed?

As soon as fuel leaves the refinery, it is exposed to microbes in the air and trace amounts of moisture. Fuel used quickly, by motorists, regular marine traffic or the aviation industry, suffer far less from diesel bug than that used off and on, on sailing yachts, where the fuel is stored for much longer periods. Unfortunately, a half-full tank in a tropical location is the perfect environment for condensation to form on the walls of the tank and trickle down into the fuel. Once there, it descends to the bottom of the tank as it is heavier than diesel. 

As the illustration below shows, diesel bug forms and grows in the layer between water and diesel.

Diesel bug has become a much bigger problem in recent years as biodiesel is added to most petroleum diesel. The average globally is 5-10%, but the really bad news is that biodiesel contains 10-15 times more water than petroleum diesel, according to the American Chemical Society. As a result, it is almost impossible to keep water out of your fuel tank.

Contaminated Fuel

Contaminated fuel is another problem to be aware of. In the remote regions of the world, the quality of the diesel can be questionable and in extreme cases, it’s been well-documented that gas station owners will deliberately add water to the diesel to make supplies go further! In New Zealand, 600 boats suffered severe cases of diesel bug due to a single supplier selling contaminated fuel. The cost to fix the problem was around USD$19,000 per vessel. 

Between 1999 and 2006, the EU and US introduced legislation to reduce the amount of sulphur in diesel in an attempt to reduce harmful emissions. This led to the introduction of ULSD (Ultra Low Sulphur Diesel) fuels. Today, USLD is often combined with Fatty Acid Methyl Esters (FAME), aka biodiesel. Whilst the removal of sulphur is good for the environment, the introduction of biodiesel poses risks to the fuel’s integrity. Research has indicated that sulphur may have a bio-static effect, i.e. suppressing the growth of diesel bug. The inclusion of FAME soyabean oil and other vegetable oils certainly gives the diesel bug more to feast on. In scientific terms, biodiesel has a much higher hydroscopic property compared to regular diesel. Hygroscopy is the phenomenon of attracting and holding water molecules via absorption from the surrounding environment, so an increase in FAME, combined with a lower sulphur content, dramatically increases the potential for fuel contamination.

Biofuels are one of the largest sources of renewable energy in use today. Unlike other forms of converted diesel, biodiesel is compatible with regular engines without the need for additives or modifications. For this reason and the slight reduction in cost, biodiesel is widespread, especially in Europe. 

Effects of water in diesel

It is imperative to test your fuel regularly. A diesel bug infection can bloom quickly and produce some serious side effects. A small infestation can clog filters and injectors, however, a severe infection can cause structural damage to the tanks and injectors.

How to keep your tanks free of diesel bug

Prevention is much better than cure.

Good Husbandry

The best defence of diesel bug is prevention, which means good husbandry. This can be regular visual checks and removal of water if it’s occurring. There will always be low levels of diesel bug present, however, it really becomes a problem at moderate or severe levels of contamination. If possible, top your tanks up to remove the air gap and thus reduce the risk of condensation.

It is a very good plan to regularly use biocides, even at very low levels this will kill it before it starts. 

Regular Testing

This is by far the most cost-effective way to maintain good diesel integrity. Identifying diesel bug early and treating it with a manageable amount of biocide product will keep your tanks free of the damaging diesel bug sludge. 

An immunoassay antibody test is the quickest and easiest way to test your fuel. It is designed to reliably test for the specific microbes that are damaging to fuel compared to other testing methods such as ATP (Adenosine Triphosphate) testing which targets all microbes, even those that are not harmful to fuel integrity. Minimal equipment is needed and can be completed on-site. The results are ready in minutes and are presented in a ‘traffic light’ style system demonstrating severe (red), moderate (amber) or negligible (green) levels of contamination.


There are many options on the market for biocides that heavily vary in cost and effectiveness. In general, there will be different dose types; a maintenance dose for managing clean tanks, or a shock dose if your diesel is compromised. Practical Boat Owner published an article in which they tested many biocide products in-depth and published their results. We would recommend checking out their findings 

Dick Beaumont on White Dragon is currently using Kathon FP 1.5. He has found it to be very effective and it’s available in 5lt and 10lt containers at a relatively low cost per litre compared to other brands. A higher dose of this biocide can be used if testing shows it’s necessary and it will still come out cheaper overall per litre, especially if the alternative is only available in one-litre containers. 

How to combat severe contamination

Unfortunately, severe contamination is expensive to remedy. Recently a yachtsman known to Ocean Sailor, discovered he had a severe case of diesel bug and specialists were called in. The first step was to introduce a shock dose of biocide and top the tanks. This was then left for 24 hours. Upon returning to the boat, the tanks were pumped and the diesel was syphoned off. In moderate cases, this diesel can be used again after it has been polished. however. his diesel was in too bad a state to be used again. The tanks had to be fully steam cleaned, which is no easy job on a small sailing boat with baffles getting in the way. The water created from the steam cleaning had to then be fully removed prior to fresh diesel being added to the tanks. Finally, a maintenance dose of biocide was added to the tanks to maintain healthy fuel.

Kraken’s Answer

Triple fuel tanks, a dedicated running tank and a fuel polishing system. Built into every Kraken as standard.

Fuel polishing and centrifugal separators, while commonplace on superyachts are not included in most cruising boats, even when they are sold as blue water cruisers. Kraken is the exception to the rule, following chairman Dick Beaumont’s experiences picking up dirty or water contaminated fuel in far-flung regions of the world. The unfortunate fact of the matter is that the system is just as important in the developed world where biodiesel is widely used.

The Kraken fuel polishing system consists of three tanks; comprising two main side tanks which are directly filled from the deck fuel access hatch, and a running tank. No fuel can be directly filled into the running tank unless it is transferred from the port and starboard storage tanks via the polishing system. This ensures the fuel is uncontaminated and safe to be used by the engine or generator. Below is a diagram showing the Kraken fuel polishing system.

We have heard some boat builders describe the Kraken triple tank and polishing system as overkill, but you won’t think so if your engine stops as you thread your way through a coral reef or narrow rock strewn channel!

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