http://www.westmarine.com/buy/torqeedo--cruise-4-0t-outboard-engine-long-shaft--13146659
Not 15hp, but it says equivalent to 8hp. A bit pricey too. 
Just checked out that one fontona, thing looks cool, but wow is it pricey.
check out the elco electric outboards
I checked out these also greensider, look cool too, but even more higher priced. They need more manufacturers competing in the market to drive the price down.
The propane Lehr I saw at golehr.Com are:
2.5 hp= $ 1050
5hp=$1700
9.9hp=$2400
15hp=$3280
25hp=$3,950
These aren't cheap either, but not that far away from the price of gas outboards.
I figured someone on this site must own one seeing there are so many trout and salmon fisherman/women. I'm not to familiar with those species, but I thought I read somewhere that if you have gasoline on your hands you are not going to catch a trout. So I figured this may be the perfect fit for them. I usually fish for bass. I watched a Doug Hannon video where he takes off his gas cap, dips his 10" worm in the gas, casts and catches a nice bass. So it really wouldn't benefit a bass guy that much I guess, and still wouldn't work for electric only reservoirs.
