This article about fossil fuels on boats was first published in the June 2025 edition of Boat Chat newsletter. Read the full newsletter and all back issues for free at Boatchat.uk
Background
Recent events have made me pause and think about fossil fuels on boats and if they are a recipe for disaster. In particular about petrol systems but also the wider implications of using fossil fuels in boats. A potential survey client asked recently if I could survey a 3.5 litre petrol engined ski tow boat that had been converted to run on LPG (liquefied petroleum gas). Not a common fitment on UK boats and not one I have seen before. Understandably I directed him to his underwriter to give him the go ahead that the boat was going to be insurable before we proceeded with survey.
‘Thankfully’ diesel remains the most popular fossil fuel for small craft in the UK, with petrol a fairly distant second and electric gaining ground but still in third. Solar and hydrogen both remain quite niche applications (for now) and it is unlikely we will ever see nuclear power in anything other than large vessels. Each fuel brings with it opportunities for performance and cost but also challenges to make the system effective, efficient and safe. As the old marine adage goes:
“You can have performance, cost or safety, but not all three”
Boat Explosion in Turkey
This was brought home in a recent and rather alarming boat explosion on an Axopar 45 XC Cross Cabin. Designed by Finnish company Axopar, built in Poland and fitted with three Mercury Verado 300 V8 petrol outboards.
The blast lead to serious but not life-threatening injuries for the three people on board. Thankfully no fatalities reported at the time of writing.
The incident was reported to have occurred following refuelling as the vessel made to depart the fuel pontoon and the incident was caught on the marina CCTV.
The explosion appears to initiate starboard side forward and the pressure increase lifted the deck moulding away from the hull and moved it to port and aft. Without knowing the vessel layout in detail, most likely line of investigation to start with would be an electrical spark igniting the petrol fuel vapour still present following refuelling. Given the vessels movement immediately prior to the explosion, the bow thruster would be a good place to start.
The Axopar 45 range is fitted with triple Mercury Verado 300 4.6 litre V8 petrol outboards. The design shows the boat to be fitted with a 1300 litre fuel tank located beneath the main cabin floor. A full tank would weigh 1000 kgs.
Refuelling Procedures
Refuelling always demands care and appropriate procedures. That is doubly important when refuelling with gasoline / petrol / 87-octane fuel which these engines use. The fuel vapour present may have been caused by a spillage, blocked tank vents, loose fill hose, leaking tank or a design defect. Petrol vapour is significantly more volatile and flammable than diesel vapour and ignites quite easily.
Refuelling a motor boat with two hoses!
For boat owners, it is essential to always allow time for fuel vapours to disperse. Petrol vapours are heavier than air so will naturally sink into the bilge spaces on any boat. That is why it is critical to use your blowers (if your boat has them fitted) to get the fuel vapours out and fresh air in to the bilges and fuel tank / engine space depending on your boat’s layout.
The fuel system layout and specification on many small craft is often less than ideal. Every small craft design is a compromise in some way and often the fuel system is hidden away, out of sight and out of mind. Fuel vapour detectors are a very rare sight during my surveys of small craft but this is in part due the majority of boats having diesel fuel systems. Gas alarms are a lot more common.
The design rules for small craft are usually less exacting than those for small commercial vessels or larger ships. It’s a bit of a stretch though to consider anything 45 feet long with 900hp on the transom to be a ‘small craft’ in the true meaning of the word, especially with a tonne of petrol on board.
Ventilation Is Essential
When using fossil fuels on boats, ventilation is essential in any vessel with an enclosed cabin. It helps with controlling humidity, managing temperature and allowing flammable and other vapours to disperse. Ventilation can be either forced or natural and the design will vary according to the design.
For natural ventilation, ideally all vessels would have a mixture of high and low level vents both fore and aft to allow for vertical and longitudinal mixing. This is a relatively easy thing to set up on a narrowboat or motorboat but trickier on a sailing yacht due to the hull’s heeling when under sail.
Fixed ventilation in a narrowboat
Most yachts have small vents built into foredeck hatches or fitted on the saloon roof as well as in the washboards to allow some air to pass through, but this requires a breeze for them to work. Solar powered vents can create small amount of forced ventilation on still sunny days.
Two foredeck hatches with vents fitted on a yacht foredeck
Forced ventilation can be either positive (blowing air into the space) or negative (sucking it out). Engine room blowers on most small craft tend to be negative pressure systems. These suck air from low level where fuel and gas vapours will collect to then expel overboard and have static high level vents to allow fresh air back in. This way any flammable vapours get removed quickly from the bilge. It is essential that any electric blowers are ‘ignition protected’ to avoid igniting any vapours.
12V Rule 3″ Inline Engine Space Blower
In larger ships it is more common to have a positive pressure system that produces a modest over-pressure in the engine room to provide adequate volumes of fresh air for engine combustion and helps with cooling of the space.
Fuel Quality
Fossil fuel quality and shelf life standards are both reported to have steadily declined as fuel has been legislated to include more bio-diesel. Many of the additives found in premium forecourt fuel are absent from cheaper supermarket fuel and possibly Marina supplies as well.
This has had the unintended consequence of shortened fuel shelf life once it is in the tank. Reports of ‘sticky’ diesel are common on the inland waterways and clogged filters and pipes at the start of the season are the typical result.
Sticky Diesel Residue In Narrowboat Fuel Tank
No easy options to manage declining fuel quality other than buy the best fuel you can get, check where your Marina sources their fuel from and use fuel additives. Diesel Fuel Complete by Marine 16 has been my go to additive for several years.
The old advice was to always fill your tanks before winter storage to avoid water condensation and the dreaded diesel bug. Now it’s preferable to leave them as empty as you can over winter and fill with fresh at the start of the season to minimise the quantity of fuel that goes ‘off’.
Managing Fuel Spills
An interesting observation I picked up on at Crick Boat Show this year was padlock fittings on narrowboat fuel filler caps. They have become an increasingly common sight during surveys and a sign of both the high price of fuel and a reported rise in fuel theft. The lock is a sensible measure to protect the owner’s fuel. However the fuel diverter fitted around the fuel filling point forces any spilt fuel into the canal, rather than into the vessel.
Neat Fuel Filler On Narrowboat
It is also common to see yachts and motor boats where the fuel filling cap is located to allow any spilt fuel to run overboard into the river / sea. Perhaps this design philosophy should be reversed so that any spilt fuel went into the boat and forced the Owner to clear up. Perhaps then people would take a little more care refuelling and protect the environment a little better.
Conclusion
All of which brings us back to the Axopar 45 explosion in Bodrum. The intensity and force of the explosion strongly suggests the ignition of fuel vapour within the hull. It doesn’t take much volume of liquid petrol to produce a large enough volume of petrol vapour to be an explosive quantity. Detecting the presence of excess vapour and having the means to disperse it quickly and safely are essential in any petrol engined craft.
The challenge going forward in using fossil fuels on boats is that they must be safe and easy to operate. Especially in the hands of inexperienced and ham-fisted Owners and sloppy fuel pontoon operators. All whilst being increasingly mindful of the vessel’s environmental impact when things aren’t perfect and don’t go to plan.
For more Top Tips on keeping your boat safe from fire, check out my YouTube video Fire Safety On Boats – 5 Essential Tips
This article was first published in the June 2025 edition of Boat Chat newsletter. Read the full newsletter and all back issues for free at Boatchat.uk
© David Pestridge 2025