Duncan Kent examines the implications of switching from red to white diesel and what you can do to avoid fuel contamination

Is it time to switch to using white diesel?

For years, British sailors have been using marked or red diesel while our continental cousins have been using unmarked white diesel.

Before Brexit, an argument had rumbled on between the EU and the UK about whether leisure sailors should be allowed to use it.

The UK argued that ultimately the colour is unimportant, as long as the correct amount of tax has been paid, while Europe is adamant that the diesel being red (marked to show it can be sold at a reduced tax rate) proves that the correct tax has not been paid.

The ’60/40′ split we’ve all got used to is in fact a fudge while the two sides sort it out.

A yacht moored at Gibbs Island, Strangford Lough, Northern Ireland

From 1 October 2021, cruisers in Northern Ireland will be banned from using red diesel to propel their vessels. Credit: Brian Black

Now that the UK has left the EU, the Government has confirmed that red diesel can still be used by recreational sailors in England, Wales and Scotland to power their yachts.

But boat owners in Northern Ireland will be banned from using red diesel as part of the Northern Ireland Brexit protocol.

The ban was expected to come into effect on 30 June 2021 but has been delayed until 1 October 2021 to allow authorities to examine the lack of marine white diesel pumps in Northern Ireland.

The problem with changing from red to white diesel is twofold.

One is that it is very difficult to clean all traces of red marker dye from your fuel system, and the other is that white diesel can contain a high proportion of biofuel, which can result in severe outbreaks of diesel bug if left in a boat’s tank for long periods.

Red or white diesel – what’s the difference?

Fuel experts state there is little difference between red marine diesel and white road diesel other than the Cetane Number (CN) and sulphur content.

The CN indicates the ‘volatility’ of the fuel, similar to the octane rating of petrol.

The higher the rating, the less time required for the fuel combustion process to be completed.

Most clunky old marine diesel engines are happy running on fuel with a CN of 35-40, but newer, and especially higher-revving, common-rail injection diesels, require fuel with a CN more in the range of 45-55 to run cleanly and efficiently.

Most white road diesel is currently supplied in this category and will do no harm to your older marine engine.

Red diesel

The Cetane Number (CN) and sulphur content is the only difference between red and white diesel

The sulphur content of road fuel is lower than in most generally available marine red diesel.

This helps reduce emissions from road vehicles and has little effect on marine diesels other than to very slightly reduce its self-lubrication properties.

The only reason we should be concerned about using white diesel in our marine engines is the increase in biofuel content.

Biodiesel is a collective name for many forms of non-fossil diesel fuel but is usually created from vegetable oil, animal fats or waste cooking oil.

The ‘bio’ content of all fuels is increasing every few years in the interest of reducing environmental pollution (and to save the producers money).

White diesel is now permitted to contain a maximum of 7% biofuel. This will shortly be increased to 10%, and the rumour is it will go up to 12% in the not-too-distant future.

Changing from red to white diesel

All diesel fuel starts off ‘white’, before chemical dyes are added to indicate their tax status.

Red diesel, therefore, is simply white diesel with red dye added.

Marine diesel engines will run just as happily on white as red.

In the past, problems arose for UK sailors on the Continent with some being fined for having red diesel in their tanks.

Following Brexit, the requirement to use white diesel does not apply to GB vessels, although it remains for Northern Ireland vessels.

According to the Cruising Association (CA), pleasure vessels with GB red diesel in their tanks when visiting states, including the EU 27, are treated as coming from a third country.

Under the Istanbul Convention 1990 (Temporary Admission) the presence of red diesel in tanks both for propulsion and heating ‘shall be admitted without payment of import duties and taxes and without application of import prohibitions or restrictions.’

The CA says this means there is no need for GB vessels to fill up with white diesel

Pumps for white diesel, red diesel and unleaded

UK sailors should keep receipts for diesel bought in the UK and ask retailers to mark them duty paid

Despite this, the RYA has stated that although every individual member of the EU27 should recognise and apply the Istanbul Convention, there is no guarantee that they all will (or will do so consistently).

The Istanbul Convention is similar to an EU Directive – it sets out the rules that the signatory states are supposed to implement but it does not give individuals any right of action should a particular signatory state decline to implement part or all of the Convention – only another signatory state has that capacity.

If you encounter any difficulties when using marked ‘red’ diesel abroad, the extent to which you will be able to rely on the Istanbul Convention will be determined by the national laws of the country in which the ‘offence’ has been committed.

When visiting an EU member state, the RYA recommends that recreational boaters with marked ‘red’ diesel purchased in the UK:

  • Keep receipts for diesel purchased in the UK, to prove that it was bought in the UK, and request that your retailer marks them “duty paid”
  • Log the date of refuelling and engine hours to reinforce these records; and
  • Do not carry marked diesel anywhere other than in their craft’s main fuel storage tanks.

Fuel treatments

There are two primary methods of treating fuel to avoid the risk of diesel bug contaminating your entire fuel system.

One is to use a micro-biological additive that kills the diesel bug bacteria, the other is to employ a ‘fuel polisher’ or centrifugal filtering system.

There are hundreds of different types of additive available today and provided you stick with one of the well-known, proven treatments you should be fine.

Some profess to protect hoses and metalwork as well as killing bacteria, which is a bonus.

Follow the instructions to the letter and make sure you add it to any fuel containers as well as the main fuel tank.

Prevention is better than cure, so using one isn’t going to do any harm – even if your system appears to be clear of bugs.

If you’ve discovered your system is infected and you’ve added a treatment that has successfully killed off the bacteria, you’re still left with the task of cleaning out the remaining residue.

This is where a fuel polishing system comes into its own.

Fuel decontamination

Centrifugal fuel decontaminators/polishers are designed to remove both water and solid particulates from the dirtiest fuel.

Installed between the fuel tank and the primary fuel filter/water separator, they act as the first line of defence by removing water, dirt and sludge down to just 8-10 microns in size (standard filters usually only remove particles down to 25-30 microns).

The centrifugal filter literally throws the contaminants out of the fuel, leaving clean, pure diesel flowing into the engine’s primary fuel filter – which now has far less to do and will consequently last a great deal longer.

These devices can be electrically pumped or powered simply by the action of the fuel being sucked through it.

Black smoke when running an engine is a sign of fuel contamination

Black smoke when running an engine is a sign of fuel contamination. Credit: Graham Snook

You can, though, add an electric pump if you’re using the day tank system mentioned below or you can install an electrically pumped fuel recycling system in which fuel is drawn from the main tank, cleaned via a polisher/filter arrangement and then returned to the tank.

The standard unit simply connects into the existing fuel feed line, between the tank and primary filter.

Some also offer an alarm sensor to warn you when the water/gloop reservoir is full, at which point you simply drain off the contaminates into a container by turning the tap.

The filters on these devices don’t need to be replaced, they just need cleaning occasionally with fresh fuel.

Most cruising yachts have relatively small fuel tanks, so a permanent polishing arrangement might not make as much sense as with a big motorboat.

But if you’re a member of a sailing club you might want to suggest they invest in a portable system to which members can take to their boats to clean their fuel at the start of the season, or just if they suspect it might be contaminated.

You’ll still have to ensure your own tank and system are kept clean, of course.

If you’re planning a long passage and intend to carry several extra fuel containers it might pay to splash out on a simple, 12/24V portable centrifugal transfer pump/filter (about £200).

These not only simplify topping up the main tank and reduce spillage, but they also filter out any bugs and water before the fuel even reaches your tank.

I know all these devices are yet another expense, but when you’re motoring hard against wind and tide, it’s getting dark, the weather’s rapidly worsening and the crew are starting to go green at the gills, I promise you’ll appreciate the investment!

Day tanks

Some larger yachts and motorboats have a secondary ‘day’ tank, from which the engine is fed.

Bulk fuel goes into the main tanks as normal, after which it is cleaned via a centrifugal filtering system before being pumped into the day tank.

Standard filtering is then employed between the day tank and the engine.

I know of several blue-water cruising skippers who have incorporated this into their yachts, especially those that regularly cruise in areas where the quality of fuel supplies is unpredictable.

One skipper I met in Puerto Rico told me that at one time he managed to continue cruising for several months by filling his day tank from a jerry can after his main tank became too infected to use.

Filters and hoses

draining the water trap on the primary filter

Drain the water trap on the primary filter

Marine diesel installations should always have two filters before the fuel pump.

The primary filter should have a clear bowl and tap at its base so you can check to see if there’s any water or dirt collected and drain it off.

The secondary filter is usually a finer one and, in my experience, rarely gets clogged, but the element should still be replaced regularly all the same.

Fuel testing kits

These kits usually comprise a small bottle containing the test chemical within a gel, a specimen bottle and a measuring pipette.

You simply take a small sample of fuel from the bottom of your tank, add some to the gel and shake it vigorously.

Then you keep it somewhere warm for a few days and if your fuel has ‘the bug’ the sample changes colour.


At one time I used to recommend folks topped off their tanks and containers to the brim at the end of the season to eliminate the empty space in which condensation can form.

But nowadays, with increasing levels of biofuel in marine diesel I would suggest completely draining the tank, pipes and filters, plus any fuel containers.

I know it might sound a bit drastic, but it’s actually the only safe way of guaranteeing your fuel system remains uncontaminated with water or diesel bug.

A sailor bleeding air from his engine

Remember to bleed the air out after changing the filter

I would also say the same for tender outboards.

Modern unleaded petrol goes off in a matter of months, so empty the carb and drain the tank when winterising your tender.

If you leave your boat in the water over the winter and like to leave the diesel heater ticking over, I would suggest you fit a separate fuel tank solely for the heater.

Continues below…

At least that way you might only affect the heater, not the engine fuel system, should it get contaminated.

Also, with the EU getting fussier about the use of red diesel in leisure yachts, if you’re a regular Channel crosser you can then convert your main tank to white diesel and just buy red for the heater tank.

Personally, I find good ventilation and one or two simple greenhouse electric bar heaters do the job just as well and they also often prove cheaper to run.

Tips for keeping fuel clean

Clogged engine parts from fuel

Polluted fuel will clog your engine’s arteries

Water can get into your fuel in a number of different ways, but very often it is created by simple condensation – either in half-full fuel containers or tanks – so make sure you keep them as full as possible during the season.

The first sign of a problem is often poor starting, heavy black smoke when running, uneven running, not ticking over or just stopping for no apparent reason.

If you suspect your fuel system is infected stop using the engine immediately and test a fuel sample using a DIY bug test kit available for around £20 from most chandlers.

In summary

  • Keep your fuel tank and containers topped off whenever possible
  • Ensure any containers are spotlessly clean before filling them
  • Add fuel bug killer to main tank and spare fuel
  • Always use a funnel with a water- trap gauze when topping up from a container
  • Ensure you have a water trap fitted to the system and check before every start
  • Clean your filters and trap regularly
  • Ensure deck filler cap seals are in good condition
  • Fit an in-line desiccant filter to the fuel tank breather to prevent moisture being sucked in
  • If your fuel tank allows, drain off any water and sludge at the bottom
  • Buy a manual pump and long rigid tube for taking samples from the bottom of your tank

The dreaded diesel bug

The foremost problem with bio-diesel is that it is hygroscopic, that is, it attracts and absorbs water molecules from natural moisture in the air.

The diesel bug then grows and multiplies on the layer between the fuel and the water in the tank or container.

Diesel bug is a form of bacteria that feeds on water and hydrocarbons from biofuel.

Diagnose fix marine diesel engine problems

A diesel bug contamination can write off the engine

Should you allow it to infect your tanks and filters it can cost a fortune to sort out.

The whole system, from tank to injectors, must be drained and cleaned.

Running your engine with water in the fuel can cause injectors, pumps, valves and pistons to be fatally damaged, sometimes writing off the entire engine.

Another problem with biofuels is ageing – they’re not intended to be kept for longer than six months at the most.

While this is fine with a regularly used road vehicle, most cruising yachtsmen barely use more than a couple of tanks’ worth over a typical season and, if you’re like me, you’ll have at least one 20L jerry can of spare diesel on board that will need disposing of at the end of the season – unless you remember to cycle it whenever your tank is half full.

If you forget, it’s not only nearly £30 worth of fuel to discard, it’s also the tricky problem of finding somewhere to dump it.

This article was updated on 14 July 2021 after Colin Heywood from the Cruising Association’s Regulations and Technical Services (RATS) contacted us to highlight that GB sailors should not be concerned about having red diesel in their tanks when cruising the EU due to the Istanbul Convention 1990 (Temporary Admissions). Thanks to Colin for contacting us. We’ve included the views of the RYA for balance.

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