Petrol in tanks, whether for your yacht or tender, can go off. Ken Endean suggests an annual precaution

For many years, we have been advised that petrol in storage may ‘become stale’, and that if engines are unused for long periods their tanks should be drained.

One problem affecting modern petrol/ethanol blends is that the ethanol absorbs water from the air and eventually the water/ethanol mix undergoes phase separation.

Recently the introduction of petrol blends with higher concentrations of ethanol has prompted more warnings, including that petrol should not be stored in a container for longer than a month.

I may, inadvertently, have been carrying out a long-term scientific experiment, because my boat’s main petrol tank has not been drained for 35 years and yet the engine runs smoothly.

My Sabre 27 has a 10HP Yamaha 4-stroke outboard, which replaced a 12HP petrol inboard when we bought the boat in 1986.

Petrol tanks are in the port cockpit locker and the engine runs off a normal, plastic, 12-litre outboard motor tank, with reserve fuel in a 50-litre stainless-steel tank.

Separation in fuel visible; petrol above, water/ethanol below

Separation in fuel visible; petrol above, water/ethanol below. Credit: Ken Endean

Fuel is transferred between the tanks by a small hand pump.

At the end of each season, unused fuel is stored in the reserve tank.

Over time the contents of the reserve tank are mixed with new petrol but a proportion will be several years old and some must date back to 1986.

In 2012, alarmed by reports of water in fuel, I used the hand pump to draw liquid from the bottom of the reserve tank, into a clear bottle, where it separated into two layers; I returned the upper layer of petrol to the tank and was left with half a litre of what looked like water.

Since then, I have repeated the exercise at the start of each season and this year the volume of water/ethanol had increased to nearly a litre.

Continues below…

Petrol being poured into a boat engine

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Water in a tank might come from surface condensation but once water sinks to the bottom of the tank it cannot re-evaporate readily.

If only half a litre of water/ethanol accumulated between 1986 and 2012 there is no reason why the rate of such surface condensation should then have increased sharply.

It is more likely that the water arises from absorption by ethanol-blended petrol, because sales of that have increased in the last decade.

Round Britain

Ken Endean is an inshore pilotage enthusiast who has made a close study of coastal sea conditions around the British Isles

Despite reading authoritative reports on other kinds of deterioration in stored petrol, I have not yet experienced any other problems that could be attributed to ‘stale’ fuel.

That’s just as well, because draining the whole tank and disposing of its petrol is not an easy option.

The accumulation of water, however, could obviously cause serious trouble within a few seasons if it is not removed.

I’m not a petroleum chemist, and I’m keeping my fingers crossed.

Although, for petrol-powered boats, it appears that removing some liquid from the bottom of their tanks would be a sensible annual ritual at the start of each sailing season.

Expert tips on caring for fuel

  • Fit a desiccant filter to your tank breather if possible
  • Drain your tank at the end of the season, or at least drain off the bottom portion of your tank
  • Disposal of old petrol is not easy if it can’t be used. Some recycling centres and garages will accept it
  • Check the water separators and filters for signs of water
  • Clean the carburettor if it has been left with fuel in over winter

Enjoyed reading Why you should check for water in your petrol?

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