The engine is a Yamaha TZR unit from around 1988. This 2 stroke liquid cooled twin develops around 45HP and revs to 10500 without any trouble. Converting to hovercraft use seems a little daunting but can be done in 2 stages if needed and is not so bad when you get down to it.
The first stage involves removing the contents of the gearbox casing, but leaving all of the existing engine mounts. For the second, you cut the gearbox casing off and bolt mounting bars underneath. I went for option 1.
The first thing to do is to strip down the engine totally and take out the gearbox internals. Now you need to take a drive for the fan and pully straight from the crank shaft. This saves weight and reduces losses but means loosing the oil pump and water pump both of which are driven from the gearbox.
Living without the oil pump it not a problem as this is a 2 stroke and it’s only used to add oil to the fuel, which in turn oils the crank. Just increasing the jets in the carb a little and adding oil to the fuel sorts that one out.
The water pump is slightly more tricky. Several types have been tried in the past including pumps driven by the fan belt but most people have settled on an electric pump. The trick here is to find one that’s rated for around three gallons per minute and can stand nearly boiling water. Don’t be mean on the water pump of you’ll end up paying in engine rebuilds. Also, remember to get the direction of flow through the engine right. In through the bottom, out through the top.
One thing to keep in mind is the problem of vibration. These engines are revving to around 10500 RPM and really do vibrate. If you bolted everything together and to the hull, something would break. So somewhere, there needs to be some rubber mounts. If you go for option one on the engine, as I did, you get the two original rubber engine mounts. With a flexible coupling on the drives shaft, the problem is solved. With option two, cutting off the gearbox casing and bolting mounting bars to the base of the engine, most people bolt this down to the fan/engine frame and then rubber mount that to the hull.