Biggest issue would be where to put tanks
I'm not an expert on alternative fuels. But, I do think that this could be a big concern.
Propane is less dense than gasoline and you must carry more of it to go the same distance. On a per unit of weight basis, liquid propane contains about five percent more energy than gasoline; however, the density is 30% less. The net result is that a gallon of propane contains about 28% less energy than a gallon of gasoline. In addition, Propane tanks can only be filled to 80 pecent of their capacity due to expansion.
I don't know what the European standard is. This comparison is based on the commercial grade of propane for automotive use known as HD-5 in North America. “HD-5” stands for Heavy Duty (propane) containing a maximum of 5% propylene (also called propene) and a maximum of 2.5% butanes and heavier hydrocarbons (also shown as C4+). This grade was developed back in the 1970's to establish a grade of propane, based on composition, that would be suitable for automotive use. That is, if the composition of propane met the HD-5 requirement, then the fuel would be suitable for automotive engines both for stability and for octane quality, without having to actually test every batch for octane or stability.
Propane with a higher butane content should yield better results.
Most automobiles are designed to run about 300 miles on a tank of gasoline regardless of their fuel efficiency (low MPG=big tank, high MPG=small tank). I'm not sure who decided this, but I guess the reason is if your in the middle of nowhere you have 300 miles to find another gas station. Anyway, you would need a large amount of propane to travel 300 miles in an average car. This is one reason why you don't see many LPG vehicles. The LPG delivery trucks are commonly LPG powered because they are carrying lots of it with them. Also, many city buses use it because they are not expected to travel far between fill ups.
I don't know how many gallons a recreation boater would burn in an hour but you would definitely need large tanks.