Is propane (LP) or compressed natural gas (CNG) dangerous to have on a boat?

When installed and used properly, it’s a great fuel for heat and cooking.  There are many
CFRs and recommendations written by the ABYC and NFPA pertinent to installation and use
of propane and CNG, and it is something I am very aware of while surveying a vessel.  If it
isn’t right, these problems are written as “Type A” infractions that have to be corrected before
the vessel can next leave the dock.  LP is heavier than air, settles in bilges and should never
be used on a vessel without “sniffer” alarms! The owner of the 40' Irwin shown below ignored
the smell of gas in the cabin and turned on his laptop computer. He later succumbed to his
injuries.
From the Surveyor's Notebook