Marine Diesel Fuel Quality And Storage In Australia

Most “engine problems” that turn out not to be engine problems are fuel problems. In a boat, fuel sits still, warm and humid, for months at a time — the worst possible storage conditions.

Why boat fuel goes bad and car fuel doesn’t

A car empties its tank every week or two. A boat’s tank may hold the same diesel for a season, in a hull that heats during the day and cools at night. Each cycle draws humid air in through the tank vent and condenses water on the tank walls.

Water settles under the diesel. The interface between the two is where microbial growth lives — the “diesel bug”. It is not a fuel additive problem or a fuel quality problem in the first instance; it is a water problem.

Australian conditions make this worse in the north: warm water, high humidity and long tropical layups are ideal for growth.

What contamination actually does

• Blocks filters, often suddenly and usually in a seaway when the tank is being stirred
• Wears injection equipment — water is not a lubricant, and modern high-pressure systems are intolerant of it
• Corrodes tanks, especially steel and aluminium, from the bottom up
• Sheds sludge that coats the tank interior and re-suspends when the boat moves

The failure pattern is characteristic: fine in flat water, loses power in a chop.

Practical prevention

Keep the tank full over layup. Less air space means less condensation. This is the single most effective habit.

Drain water traps regularly — the primary filter/water separator bowl is there to be looked at, not just fitted.

Use a primary water-separating filter ahead of the engine’s own filter, with a vacuum gauge if possible so you can see restriction building before it becomes a stall.

Take fuel from busy suppliers. High turnover means fresher fuel.

Watch what happens after fuelling. Problems that start within a few hours of a fill are worth reporting.

Testing and cleaning

Sampling from the bottom of the tank tells you more than any additive marketing. Water and sludge are visible.

Fuel polishing — circulating the tank contents through filtration and water separation — cleans the fuel that is there. It does not clean the tank walls.

Tank cleaning through an inspection hatch is the real fix for a badly contaminated tank, and it is the reason fitting inspection hatches to a tank that lacks them is a worthwhile job.

Biocides kill growth. They also release the dead biomass into the fuel, which then goes to your filters. Dose before a cleaning cycle, not before a passage.

Biodiesel content

Diesel sold at Australian pumps may contain a biodiesel component. Biodiesel holds more water in suspension and has a shorter storage life than mineral diesel, which matters for a fuel that will sit for months.

If you are laying a boat up for a long period, that is worth knowing about the fuel you are laying it up with.

Frequently asked questions

What is diesel bug? Microbial growth that lives at the water/fuel interface in a tank. It produces sludge that blocks filters and acids that corrode tanks.

Should I keep my fuel tank full or empty over winter? Full. Empty tank space breathes humid air and condenses water.

Does fuel polishing fix a contaminated tank? It cleans the fuel, not the tank walls. A badly fouled tank needs opening and cleaning.

Why does my engine only lose power in rough water? Because the motion stirs settled sludge and water off the tank bottom into the pickup. It is one of the clearest signs of tank contamination.

How often should I change fuel filters? At the manufacturer’s interval as a minimum, and immediately whenever restriction or contamination appears. Carry spares aboard and know how to bleed the system.