LPG outboard on small 30 ft sailboat

[a64pilot]

Originally Posted by Jammer

That's backwards: it's actually 1.35 gallons propane = 1 gallon gasoline. In terms of locker space required it's about 2x because propane tanks are bulkier and can't be filled more than 80% full.



Any gasoline engine can be converted to propane, and I've been around many that have been, from small generators to old farm tractors to 200 horsepower Chevvy Suburbans.


Here's the tradeoff.


Propane engines are easier to start. The fuel lasts "forever" regardless of temperature. The exhaust typically has lower hydrocarbons but somewhat higher NOx: https://www.tandfonline.com/doi/pdf/....1972.10469661


The main problem with propane is fuel handling and cost. It is not practical to pour propane from one container to another, and there are fewer dealers who will fill your cylinder on the spot than was once the case since so many have switched to cylinder exchange programs. These programs usually use short-filled steel cylinders -- you get 15# of propane (sometimes less) in a cylinder designed to hold 20# -- which are unsuitable for marine use.


Usually people use the composite (plastic) or aluminum cylinders, of around 14-20# capacity (3-4 gallons). Gasoline 4-stroke motors typically burn 0.1 gallon per HP per hour at full throttle, so a 9.9 hp propane is going to burn around 1.3 gallons per hour. If you're motoring you'll have to change propane cylinders every 4 hours or so depending on the size of the cylinders and how much you open the throttle.


As with electric alternatives, this works out just fine for motoring in and out of the harbor but tends to fall down when you decide to motor up the river all day since there's no wind.

You know I expected this post to way off. But every bit of it I agree with.
The only thing I would add and this comes from running propane fork lifts, and that’s if you use a good high quality synthetic oil, it would last a very long time, propane burns so clean it doesn’t seem to form much if any carbon, so about all that gets into the oil is wear metals.