Without proper ventilation, the fumes are likely to accumulate in the bilges (the lowest part of a boat).
In the bilge.
bilge
Bilge
Bilge
Bilge
bilge
Bilge
bilge
bilge
They are both heavier than air, and would accumulate in the lower parts of a boat (the bilges).
Gasoline and propane fumes are most likely to accumulate in the bilge of a boat, which is the lowest compartment of the hull where water collects. It is important to regularly inspect and properly ventilate the bilge to prevent the build-up of flammable vapors.