Diesel engines have simple tastes, says Graham Keating, but clean fuel is one that can’t be guaranteed in all corners of the world


Give our diesel engines clean fuel to drink, plenty of fresh air to breathe, plus a regular jump-start of amps, and they’ll happily run pretty well forever, given a little routine maintenance of course.

However, guaranteeing that crucial supply of clean fuel is becoming something of a headache for many sailors and it’s certainly challenged us aboard Maunie for the past few years. In 2012 my wife, Dianne, and I set off on a voyage that we’d long been dreaming of and was now, suddenly, a reality.

With our careers firmly paused, our house rented out, the car sold and our furniture in secure storage, we’d moved aboard our 1997 Vancouver 38 Pilot, Maunie of Ardwall, for an adventure that would take us from our home port of Dartmouth down to the Canaries, across the Atlantic to the Caribbean, through the Panama Canal and finally across the vast Pacific.

Our ‘we’ll only be away for about two or three years’ reassurance to our friends and family as we departed turned out to be very optimistic; five years later we finally fulfilled the fantasy of sailing our own boat under the iconic Sydney Harbour Bridge.


Dianne helms Maunie under the Sydney Harbour Bridge

Our unplanned tardiness was nearly all the fault of the Pacific Islands; having reached New Zealand in October 2013, we felt their magnetic pull, so we spent three winter seasons exploring the island nations of Tonga, Fiji and Vanuatu, returning to spend each summer cyclone season in New Zealand, before finally shaking ourselves free to return to ‘first world’ life after over 30,000 miles of sailing.

The remote islands of the Pacific suddenly focussed our attention on the issue of diesel quality in a way that we’d never previously considered. Through the daily cruisers’ SSB radio nets and conversations with other crews in idyllic anchorages, we heard of yachts being affected by contaminated diesel, some with serious consequences.

Even when fuel purchased in the islands appeared to be clean, it was widely rumoured that it was of distinctly inferior quality; certainly, Maunie’s white transom would gain a sooty topcoat after any extended periods of motoring, a problem that magically disappeared after the first tank refill in Australia.


Fuel transport in Vanuatu

Marinas and alongside fuel-berths were a very rare luxury in the islands, of course, so we routinely bought fuel from often scruffy roadside garages, lugging heavy 25lt jerrycans to the dinghy dock and hoisting them aboard.

One example was in Vanuatu, where in 2016, at a ramshackle dinghy dock, we managed to borrow a rusty wheelbarrow from a hardware store to wheel our load of empty fuel cans across the wide and rather chaotic main road.

The proprietor of Luganville’s only fuel station was delighted to see us and our jerrycans, with their combined 150lt capacity. Almost all his other customers were local taxis – mostly tiny, Korean-built Daewoo Matiz cars that showed very few signs of maintenance – and we were amused to see that most of the drivers would put only one or two litres into their tanks.

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It wasn’t uncommon to see the result of driver miscalculations with a taxi being pushed onto the forecourt, having run out of fuel a few metres short of the entrance.

The sight of a couple of British yachties, red-faced from both heat and exertion, filling and heaving jerrycans of diesel was clearly something of a novelty for the good humoured but slightly reserved locals. Financial transactions completed (and the diesel was, thankfully, very cheap there), we were ready to run the gauntlet of that busy road.

Refuelling routine

Having obtained our precious fuel and heaved it aboard the boat from the dinghy, our routine was always the same. Leaving the jerrycans to stand in the cockpit overnight to allow any impurities to settle to the bottom, we’d then peer in through the cap to get an idea of the quality we were dealing with.

We used a syphon, the end of its tube held at least an inch above the base of the can, to transfer the fuel into our main tank, with a Baja filter placed in the deck filler to strain out any remaining impurities or water.


Maunie’s 53hp Yanmar 4JH2E is 25 years old and has 4,300 hours on the clock. Regular maintenance has kept it running smoothly

The last litre or so of fuel remaining in the can often had some ‘interesting’ debris in it, so we’d pour that through a fine gauze-lined funnel into a designated ‘dregs’ jerrycan that would be left to settle overnight again before its contents could be safely syphoned.

It was a very messy and unpleasant job that we didn’t relish but, with 100-hour precautionary changes of its primary fuel filter, our Yanmar engine thankfully never missed a beat.

Diesel bugged

In 2018, the demands of both time and work saw us take the decision to have Maunie shipped back to Southampton from Newcastle in New South Wales and, after that, undertake a major refit to replace her teak decks (a project covered in PBO June 2020).


Loading Maunie aboard a freighter in the east coast port of Newcastle, Australia, for the journey home

Our sailing had become constrained, at least temporarily, to the UK and Ireland. It was therefore ironic that it was here, in home waters, that we suffered our first ever fuel contamination problems as we motored down the Irish Sea last summer.

“What just happened?” Dianne and I looked at each other to confirm we’d both heard it, a momentary change in the engine note as we motored through a windless but rolling sea near the Isle of Man.

Seconds later, the Yanmar’s revs dropped away to a rough idle and, shortly after that, the engine died. This moment of surprised silence was to be our introduction to the dreaded diesel bug.


Maunie has slightly old-fashioned but effective water-separators and primary filters. We fitted the second one when we added an emergency fuel tank, to allow an easy change-over if the main filter became blocked

Since our return to UK waters, we’ve filled our tanks only at reputable marinas and fuel barges and have followed the advice to top-up fuel levels over winter to minimise the risk of condensation forming in the tank. The blockage of our primary filter, replaced only 80 hours previously, came as quite a shock.

Thankfully Maunie has a second emergency tank, complete with its own filter, so a simple switch-over of a couple of valves had the engine restarted and we were on our way again.

Fearing a potential repeat of the problem, though, we did stop the engine an hour later to carry out a slightly nausea-inducing swap of the engine’s primary filter element.


With Maunie ashore for the winter, we decided it was time to devise a way of keeping our diesel clean

Diesel bug has become an increasingly concerning issue, of course, and an excellent article in PBO’s sister-publication Yachting Monthly in February this year explored the issue in some detail.

The problem has been exacerbated by the introduction of FAME (Fatty Acid Methyl Esters), a bio-fuel element made from recycled cooking oil and vegetable oils, that is now being added to all road fuel (currently at an inclusion rate of 7%) and, unfortunately, to many marine fuels.

Adding a bio-fuel element to diesel of course makes complete sense to reduce the CO2 emissions of road transport and, according to the YM article, the addition level is set to rise to 12.4% by 2032.


We fitted this 60lt fuel tank before we sailed to Australia. It fits neatly under the floor of the pilothouse food locker

However, its introduction to marine fuels brings significant risks. The FAME is hygroscopic (clearly not great news in a damp marine environment), forming a viscous layer where it interfaces with any water in the fuel, and it also apparently reacts with minerals in the diesel to form carboxylic acid.

These thick, soapy contaminants can block the primary filter, as it did on Maunie, starving the engine of fuel at possibly a very inconvenient or even hazardous moment. Furthermore, the FAME element, being plant based, degrades over time, shortening the safe shelf-life of diesel to around six months.

The conclusion of the article was that boat owners need to take action to actively remove any water in their fuel, filter out any soapy contaminants before they can reach the engine’s in-line filters, and also to use an antioxidant additive to slow down fuel degradation.


The main 180lt stainless steel fuel tank sits under the cabin sole

The solution

Our own experience added an important job to our winter to-do list. We decided to install a fuel polishing system which would allow us to suck fuel from the bottom of our tanks, filter it to remove any water, debris and diesel bug and then return the clean fuel back to the top of the tank.

In common with many yachts, Maunie’s 180lt main tank and her 60lt emergency tank both sit below the cabin sole in her deep bilges. They are accessible only via bolted-down top plates, each fitted with the various send and return pipes for the engine, generator and Eberspächer heater. Each plate has an additional, blanked-off screw cap.

Shopping list

Our original plan was to plumb in a permanent pump and filter system that we could run as and when required and we initially looked at a commercially available piece of kit called the Diesel Dipper.

Maunie on passage from Falmouth to the west coast of Scotland. Photo: Adam Kerner

However its price (around £900, plus the additional cost of valves and pipes), together with the slightly daunting complexity of plumbing it in, made us think again.

Besides, our experience of using jerrycans prompted us to design a solution that would allow us to simultaneously transfer and filter fuel from them, without the messy use of a syphon tube, as well as allowing us to access the bottom of the boat’s two tanks via their blanked-off caps.

A portable 12V system seemed like a practical solution and we turned to Beccles-based ASAP Supplies for advice on the necessary components.

marine-fuel-polishing-system-Screenshot-2022-06-21-at-14.11.57Having searched through their website, we put together a schematic of what we had in mind and sent it to their technical team. We were very pleased to receive a full and knowledgeable reply, suggesting a few modifications, so we ordered the parts shown below (the ASAP part numbers are shown in the drawing).

In essence, the mechanical side of the system is a length of copper fuel pipe that can be bent gently to allow us to ‘hoover’ around the base of each tank, a Racor filter/water-separator, an electronic 12V fuel pump and some valves and flexible hoses.

An order to RS components completed the component shopping list, this time for cables, a switch and an electronic hour-meter, while an on-line search found CPC, a UK supplier of tough Chinese-made waterproof cases in which we would mount the whole thing.

Heading south past Ardnamurchan Point, near the Isle of Mull, July 2021, blissfully unaware of the diesel bug lurking in the tank

Assembly was pretty straightforward, though the addition of the hour-meter made the electrical connections slightly more complicated and required a junction box in which to mount it.

The 12V fuel pump only draws a couple of amps so a direct connection to a cigarette-lighter plug would have been an easy solution; however, the addition of the switch and timer seemed like a neater fix.

The pump delivers about 120 litres per hour, so the hour-meter allows us to work out how much fuel has been circulated each time we use the polisher, as well as logging the time between filter-element replacements.


The polisher’s feed and return copper tubes fit through a spare access port on the top of the fuel tank

I bolted the pump, filter and switch box onto a plywood board which was cut to fit into the plastic case. When not in use, the fuel hoses coil neatly into the case and there’s room for a spare filter as well.

The Racor filters, incidentally, have an additional water-blocking coating called Aquabloc and come in different gauges; we chose 10-micron as recommended but there is a 2-micron option.

With everything assembled, it was time to test it with a can of diesel and the car’s 12V supply. Once we were satisfied that all the fittings were free of leaks (I used Loctite 577 thread-locker on all the threaded connectors, just to be certain), we decided to conduct a properly challenging test and poured a third of a litre of water, coloured with black ink, into 15lt of clean fuel in the jerrycan.


The fuel polisher in action on board the boat

Pouring water into clean diesel felt very wrong, I can tell you, but we could clearly see it sink to the bottom of the can.

The roving copper pipe sucked it up with ease when we switched on the pump and we were delighted to see the Racor ‘turbine’ unit spin the water out into the bottom of its polycarbonate bowl.

Almost miraculously, after only about fifteen seconds, the jerrycan was completely clear of black water. Once the pump was switched off, the black water settled in the bottom of the bowl and it was then just a case of opening the Racor’s drain cock to remove it.


The finished system in its tough, waterproof case. There’s room for the coiled fuel hoses and a spare filter element. Valves are kept closed when not in use to prevent fuel leakage

For just over £450, plus our own time, we now have a hugely useful and reassuring piece of kit stowed in one of Maunie’s deep bilge lockers.

We could have used a cheaper filter system – a CAV filter unit is less than a third of the price of the Racor, for example – but we had read nothing but excellent reviews, so the Racor it was.

We’ve now tried the system in Maunie’s fuel tank and, after an hour, we were happy to find no water. However, there was some black debris that had dropped into the Racor bowl and the filter element, which is easily removed for inspection, was definitely slightly dirty, so it was a rewarding exercise.

The new fuel polisher allows us to start any major passage knowing that we’ve removed any water, debris or diesel bug from our tanks before it has the chance to block the engine filters.

Moreover, we now have a safe and easy way to transfer fuel from jerrycans to main tanks, we can also move fuel between our emergency and main tanks if required, and we have the ability to assist other boats experiencing fuel issues in remote anchorages. We’re hoping that we aren’t called upon to provide that service too often!

How to build a DIY fuel polisher

Summary of costs, including delivery

Item Supplier Cost* (Nov 2021)
Racor 500FG turbine filter ASAP Supplies £224
Facet 12V Fuel Pump ASAP Supplies £63
Shut off valves x 2 ASAP Supplies £14
Fuel hose (10mm ID), 2x 1m length ASAP Supplies £12
Fittings and jubilee clips ASAP Supplies £52
10mm OD copper tube Local chandler £10
Cable and 12V plug RS Components £13
Switch RS Components £2
Hour meter RS Components £13
Junction box RS Components £4
Waterproof case CPC £50
TOTAL £457

*ASAP costs include a 5% discount for membership of the Cruising Association

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