Dunno why I'd never thought about it before, but an editorial in the latest issue of "Ausmarine" asked the question:
Given that a large percentage of cars and taxis the world over are running on LPG as a relatively non-polluting and inexpensive fuel for internal combustion engines, and that the Norwegians have recently introduced a number of gas-fuelled ferries, given the current price of diesel, why aren't we seeing conversion kits for marine engines and outboards?
I'm guessing that fiddly connections might be an issue for outboards and the increased risk of gas in the bilges might worry some - but most people use gas stoves (and have done for years) and have bilges alarms already.
Apart from needing a bigger gas cylinder - if LPG was readily available, is there any reason not to convert a diesel inboard to gas???
From RFC 1925: "(3) With sufficient thrust, pigs fly just fine. However, this is not necessarily a good idea. It is hard to be sure where they are going to land, and it could be dangerous sitting under them as they fly overhead."