John,
As another poster has suggested, what you describe is similar to a "Kort Nozzle", so I suggest Googling that for information. Spinning the duct isn't practical due to the tremendous hydrodynamic forces that act on it and the expensive complex bearings it would require. Think about what would happen if you struck ground with it; your boat would be instantly spun around and capsized by the momentum of the heavy duct. The forces against a prop inside a duct are mild, and the duct protects the prop from ground damage. The ducts for a Kort Nozzle are very heavy due to the tremendous forces acting on them, and would make a very heavy assembly that would be very difficult to support.
Kort Nozzles are practical for displacement boats that do lots of heavy pushing at very low speeds, like tug-boats for example. At moderate speeds, the drag caused by the duct reduces the thrust, making them not useful for planing boats at all.
Regards,
Gene
--- In electricboats@yahoogroups.com, John Green <v_2jgree@...> wrote:
>
>
> Hi, Tim,
..stuff snipped...
> I know that this is probably off topic for the group, but I am actually
> currently looking into, and starting discussing off-group with another
> group member, an idea of a propeller that is inverted, that is, the
> blades are inside a tube, and the whole tube rotates. It would look like
> the static blades of a jet engine, that stop the gases rotating with the
> regular blades. My thinking is that the pressure developed immediately
> behind the blades could then not 'spray out' from the sides of the
> prop., but would be forced to release pressure at the rear
> Also, the vacant middle would take in water as the boat moved, which
> might, and I haven't figured this out yet, but might, further feed the
> pressure area at the rear of the prop.
> The snags are difficulty in setting up bearings, and also the
> requirement that the shaft would have to be horizontal, not angled.
> Another snag might be that it just will not work for some reason that I
> have not yet arrived at.
>
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