A little info:
1.  A pound propane cylinder is 21,500 btus and a gallon of gasoline
125,000 btus (~6:1 ratio).
2.  Propane cylinder cost ~$2-3.  If you refill from a 20# tank (use a
$20 adapter), it's ~$1.33 if the tank is a typical fill of 15# and costs
$20.  Refilling a cylinder from a tank to full takes care and can stress  the
cylinder.
3.  A gallon of gasoline is ~$4.
4.  Therefore, the cost for propane per btu is ~2 to 4.5X.  Use your
costs for a better comparison.
5.  As a cylinder is less than a quart of gas, runtime is a problem
unless only a 2hp or so.  I know of no 2 stroke propane outboards.  A 9hp
engine would require an external tank - kind of awkward in a  dinghy.
6.  But some like the advantage of no gasoline on board, cleaner
emissions,  easy starting, no carb issues, fuel storage life, fuel compatibility
with a  grill.
7.  I prefer gasoline.
8.  Also - a question to all - I have been told that premium  gasoline
is always ethanol free - True?
Regards, Jim

In a message dated 11/7/2012 1:08:26 P.M. Central Standard Time,
trawlers-request@lists.trawlering.com writes:

Does  anyone know any downside?

I agree with your basic conclusion Jim for gasoline. But a gallon of gas is
about 6 lbs and therefore roughly equivalent in btus to 6 lbs of propane.

Our city for a while ran some of its fleet of municipal vehicles on propane
to be politically correct and save the planet and all that sort of good
stuff.  But one of their mechanics testified in a public hearing that those
engines quickly had significant problems, as I recall for valve seating and
carbon build up. The engines, though converted for propane, were not happy
with the stuff.

David Sorenson
Duluth

Over "here" the newest rage is to have your gasoline powered boat converted to run on either 20/33 kg LPG bottles.
The companies claim that one can run "all day" on two bottles.
Here is what I know.

A liter of LPG at the fuel station is about 2.75 TL/liter.
A liter of gasoline just about 5 TL/liter
One liter of propane weighs about 509 grams (per wiki.answers) ~ 24k btus
one liter of gasoline weighs about 737 grams ~ 33k btus

So, while gasoline is 2x more expensive than LPG, BTU wise, gasoline gives about only 1.375 more Btus per liter and if the analogy holds, all other things being equal, 1 liter of gasoline will allow the car/boat to travel 1.375 times farther than LPG.

A 50 liter gasoline liter fill up will give about 1650k btus of energy for a cost of about 250 TL
to get the same BTUs from LPG, need to fillip with ~69 liters for a cost of about 190 tl or about 24% cost savings.

Of course, the closer the price of gasoline is to LPG is, the less one saves.
But, given the above, it appears that a full day of cruising "here" can be made cheaper by switching to LPG.

Lee
Marmaris, Turkey

On Nov 9, 2012, at 15:51 , JHWardJr@aol.com wrote:

A little info:

1. A pound propane cylinder is 21,500 btus and a gallon of gasoline
   125,000 btus (~6:1 ratio).  ....