Seems like a cool idea. I have a Propane heater (Mr. Heater) that uses disposable green propane bottles. I refill them using an adapter...
The disposable bottles are 3-4 dollars but refilling them costs about 75 cents.
I've had the disposable bottles off-gas a bit out the pressure relief valve as they warmed up after being (over)filled. So be sure to store them in a well ventilated area. Also remember Propane is heavier than air and will sink and pool (as a gas) on the floor or at the bottom of a locker/lazarette if the tank leaks.
Refill these at your own risk as they were not designed for doing so. I've heard the trick to refilling them is to bleed off the gas so the tank fills with liquid propane from an inverted 20lb cylinder...
A bit more info on propane vs gas. This link refers to running a generator but the math is the same for an outboard. Propane has less energy than gasoline so requires more of it to go the same distance...
· Gasoline 1 gallon = 125,000 BTUs
· Natural Gas 1 CCF = 100,015 BTUs (CCF=100 cubic feet)
· Propane 1 gallon = 91,700 BTUs
1 gallon = 91,500 BTU
1 cubic foot = 2,500 BTU
1 pound = 21,500 BTU
4.24 lbs = 1 gallon
36.39 cubic feet = 1 gallon
If we then enter the prices:
· ($2.65) x (10) for gasoline; ($.95) x (12.5) for natural gas; or, ($4.00) x (13.6) for propane
The cost for running the generator 24 hours is:
· $26.50 for gasoline; $11.88 for natural gas; or, $54.40 for propane
His propane cost seems high to me but I didn't research it.
The Relative Efficiency of Gasoline, Natural Gas, and Propane Fuels For Back-Up Generators
Propane is neat and clean and if you already has propane on board it seem like a good idea.