1

It's imperative to run a blower to clear excess fumes from an inboard boat engine bay before starting.

Many modern cars have full skid plates on the bottom, and a fairly well sealed hood (fumes should rise, right?). So why don't we need to run a blower on a car engine? I can imagine the right set of circumstances there could be reasonable risk.

7

The front of the vehicle is designed, in most cases, to allow air to enter easily for the various radiators (AC, engine etc).

During WWII, the desert drivers would always park nose into wind when stopping to help cool the engine.

Boats don't have "free air access" as designing a big leak is probably not a good marketing point.

Worth noting that petrol fumes do sink so with a small fuel leak the fumes sink. Perhaps only a small risk but...

There are some videos on youtube that show the effects of the fumes spreading across the ground when people pour petrol on a bonfire to light it and wait too long...

See https://www.youtube.com/watch?v=3f4lPzxSm5A

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3

Petrol fumes are heavier than air.

In a boat, the fumes drop to the bottom of the bilge and stay there. A bilge blower is needed to get the fumes out, the inlet is usually at the bottom of the bilge.

A car is usually open under the engine, not airtight for sure. So the fumes drop under the car and are likely to disperse to the surrounding air.

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