2 Stroke RPM

[agaricus bisporus]

I'm planning a project to fit a motorcycle alternator in place of the engine on a small outboard leg, ie in the 2 -3hp class to act as a water turbine. (for a sailing boat)

Q: Anyone done this before?

Q: I'm looking for ballpark figures for engine and prop RPM so I can work out if the speeds I need are within the bearings/gearing capabilities. Are these things geared 1:1 or is there a reduction? If so what sort of ratio are we looking at? I'd probably prefer a slower "prop" and higher ratio to speed up the alternator. Would a 4 stroke leg be different?

Q: Will the prop thrust bearing stand being "pulled" instead of "pushed" because with an impellor (as opposed to a propellor) fitted the thrust on the bearing is reversed. An alternative might be to turn the leg to face the water-flow, but would that give other problems, eg drag or lube-water flow?

Q: Any good ideas welcome.

[boats.net]

Most of small hp models have a 2:1 gear ratio in gearcase, never seen it done not sure what problems might run into.

[99yam40]

I would not think the prop would have enough surface area to turn the shaft fast enough and with enough torque to get anything out of it

[BaileysBoat]

Agree. You would only get about 1900 rpm at 10 mph.

Here is a prop calculator link.

Propeller Calculator / Prop Calculator

[agaricus bisporus]

But I haven't said what prop I'm planning to use! Not that a conventional prop would work in the first place.