The ones in Thailand mount a 4 cyl car engine on a pivot, and run a rigid prop shaft about 8 or 10 feet back, with a prop on the end of it - about 1" bar, no joints. Looks very crude, but they get cheap, efficient engines with as much power as they want. They just run river water through the cooling system, and for brief stops they just tilt the engine to lift the prop out of the water.
I don't think it would comply with Australian safety and water traffic regulations, though.
