Subject

You can't get there from here.
To get any decent output, you need at least 2,000 alternator RPM.

If you chose to use a belt reduction, 7:1 is about max.

Need 300 RPM on the prop shaft delevering at least 2-3 HP.

That will slow you down at lest 2-3 knots if you could do it.

Trust me, it's never going to happen.

Lew