I have always associated Rotax with Karts and microlights but on looking at the company website of BRP , the Canadian company who own them i discovered they make a huge range of engines now, Karts, Ski mobiles, Ski Doos and planes.
The most powerful is a marine engine with 300 bhp from only 88kg.
That's from only three cylinders. I assume its a bit of a constant revs device as it powers Ski Doo's on water but still impressive.
I wonder what the emissions etc are? it is a 4 stoke but I presume less rules at sea?