==================================================I did a search for posts and general web sources for PWC engines in airplanes. There is nothing here that I could find and precious little on the web. That probably means it doesn't work, but let's take a look at this option.
As I continue to research the engine options for a small plane build, I keep circling around the usual suspects and am routinely shut down by the usual ridiculous prices. I can buy a 350hp LS1 Corvette crate motor for less than a 50hp Hirth. It's ridiculous how much the word 'aviation' costs.
This frustration leads me to the larger world of engines outside those marketed for aviation. And while I'm not stoked about 2-strokes, I dislike the power/weight ratio of half VWs and the like. But, I trust the name Yamaha, having owned and ridden many of their motorcycles and PWCs over the years. As one of the most prolific engine manufactures on Earth, their merit is proven.
Enter the Yamaha 701 2cyl/2stroke
Stock horsepower is 63 to 73 depending on year of manufacture
with popular mods the hp is up to 80-85
Max HP achieved at around 6200RPM
Weight is 36kg/80lbs
Even the older but very similar 650 gets 50hp stock.
The newer 790 gets 90hp stock.
These of course, are liquid cooled/open loop systems, so we would need to convert to a closed loop and add the weight of a radiator and coolant. The advantage may be far better cooling than high output, air cooled 2-strokes are known for. I'd be happy with a little better.
We would also need a PRSU which adds some weight - common in 2-strokes.
To my mind, we have an engine with some of the same considerations as a Rotax 582, but we can get it rebuilt for $595 (plus core)..., a tenth what a rebuilt 582 costs.
I'm certain I am wrong and that plenty of dialog exists to prove it, so please educate me so I can move past this 'too good to be true' flight of fancy. Thanks!
Andy the reason you don't see Jet Ski Engines being used, is there is many Good Used Rotax UL Engines Available, 277UL, 377UL, 447UL, 503UL, 532UL, 582UL, 618UL, and Rotax Rick offers the 670, all fairly Cheap, and also their Counterpart Snowmobile Engines even cheaper. You also have many of the Kawasaki 340/440's that were popular also cheap.
Can you adapt a Jet Ski Engine, Yes. Can all Jet Ski Engines be used, probably not. I don't know about all Jet Ski Engines but the Kawasaki 440/550 used a Threaded PTO. They have a Big Boss around PTO just as some Kawasaki 340/440 Snowmobile Engine had, but not All. The Sled version used an Adapter Plate 3/4" Thick with a Rotax A Gear Drive. Never seen a B Drive Conversion on a Kawasaki. I have seen a Provision 8 Adapter Plate for the C & E Drive. Most Jet Ski Engines don't have Water Pumps, so would have to figure out a Solution for that. Will Fan Cooled/Free Air 440 Cylinders work on a 440 Jet Ski Case, maybe? You could run a Non-Provision Belt Drive like this, Small Pulley could be Threaded for PTO. The Arctic Cat Suzuki 340/440 were copies of the Kawasaki 340/440, so they are a good option.