That number on the prop says 30-12.
A semi educated guess would be 30 inch long, or 30 inch diameter, since they spin in a circle, 12 inch pitch, or it theoretically would move 12 inches per revolution. I agree with the above post about it not having enough power to plane a boat, but a hovercraft doesn't have ANY contact with the ground.
A streamlined bicycle might be scary fast.
Now that we have analyzed the thing, where did you find it? Like, airfield, everglades, body shop?