Home / For international owners / Cover in detail
Petrol vapour in the engine bay: how explosions happen, and how to prevent them
Short answer
Petrol vapour is heavier than air and collects in the engine bay.
One spark at start up can set off an explosion.
Ventilate after refuelling, sniff the bay and run the blower,
and you prevent most of these accidents.
Motorboats with a petrol inboard or sterndrive carry a risk diesel owners hardly know: petrol vapour. It comes from refuelling, leaking fuel lines and the tank vent, it is heavier than air and it settles at the lowest point, usually the engine bay and the bilge. A spark from the starter motor, the alternator or a bilge pump switch is enough, and the engine hatch can be blown off. Many of these accidents happen at the fuel berth or just after leaving it.
That is why boats with a petrol inboard have an electric engine bay blower. It draws air from low down in the bay and should run before every start, for at least four minutes, as many owner's manuals and the warning label beside the switch say. When refuelling: engine off, all electrics off, hatches and doors shut so vapour cannot flow below. Afterwards open up, run the blower and smell the engine bay before you turn the key.
At the start of the season, check fuel hoses, hose clips, the tank vent and the filler neck. If you smell petrol, do not start until you have found the cause. A hydrocarbon detector in the engine bay gives a reliable warning of vapour, and a fire extinguisher belongs within reach, not behind the engine. If fire breaks out in the bay, keep the hatch shut, cut the fuel and discharge through the fire port. Fire and explosion are commonly insured perils, but starting despite a strong smell of fuel can be argued as gross negligence and cut a claim.
Worth remembering
Diesel
hard to ignite
Petrol
vapour explodes at a spark
Blower on, nose to the engine bay, then start.
Questions
What owners ask us about this
Why do petrol motorboats explode?
Petrol vapour gathers in the engine bay and a spark at start up ignites it. The most dangerous moment is right after refuelling, before the bay has been ventilated.
How long should the engine bay blower run?
Before every start, for several minutes; many manufacturers say four. Then smell the engine bay before you start the engine, and wait longer if in doubt.
Will hull insurance pay for an engine bay explosion?
Fire and explosion are insured perils in most hull wordings. If the owner acted with gross negligence, for example by starting despite a smell of petrol, the payment can be reduced.
Related
Not sure whether you qualify?
Whether it is “Petrol vapour in the engine bay: how explosions happen, and how to prevent them” or another question: send us your residence and the flag your boat flies. You will get a straight answer in one email; including a no, if that is the answer.
Was this useful?
