Originally Posted by Billy Goat
Why not use a conversion kit? It is not new science, many companies produce the necessary kits to convert most kinds of gas motors to propane. If you are the man with carburetors, you can probably do it in 2 hours or less. It has to be a cheaper route than a new motor. I do believe they are only for 4 stroke motors though.
Yeah, you can't premix oil with propane.
You might be able to use a newer oil-injected 2-stroke, but who wants to smell a 2-stroke on a river trip?!
Newer 4-stroke motors aren't cheap, either, but they burn really clean, and they're becoming common enough that they're coming down in price. You need a propane regulator that turns liquid propane into propane gas, and you need a mixer that takes the place of the carbueretor and mixes air with the propane gas.
Most propane "fuel" tanks have a dip tube and draw liquid propane off the bottom of the tank. If you ran it off a barbecue tank, I'm not sure if you'd pull it so fast that the tank would freeze up. With the relatively small amount of fuel used by a 2-5hp engine, it's possible you might be able to run a 20lb BBQ tank and gaseous propane and skip the regulator. Without the regulator, you'd want some sort of solenoid valve that would shut off the flow of propane if the engine died--both to save fuel and prevent pollution, and to also minimize the risk of a KABOOM!