[hanamizu]

NG is heavier than air and settles at the lowest point, being the basement.

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Worked on a fishing boat many years ago. The boss was super paranoid about LP. Only turned the bottles on when cooking. Turned them off as soon as we finished. Same reason. LP settles in the bilge. One spark and there goes the bottom of the boat.

[bagman]

It’s basic chemistry. The molecular weight of natural gas (methane) is 16. The molecular weight of air is something like 29 (most of air is nitrogen - molecular weight of 28 and most of the rest is oxygen - molecular weight of 32).

Hydrogen (molecular weight of 2) and helium (atomic weight of 4) are used for lighter than airships because their weights are less than that of air. Hydrogen would be preferred (if it wasn’t so combustible) because it is lighter than helium and thus has more bouyancy. In principle, methane could be used, but it wouldn’t have much lift.

Propane (C3H8) has a molecular weight of 44, so, yes, it is heavier than air and will settle. Butane (C4H10) has a molecular weight of 58, so it will settle.