On Mon, 22 Jun 2009 06:12:55 -0700 (PDT), Loogypicker
wrote:

I know that on a hovercraft, a duct around the thrust fan, in a
certain shape and close tolerances to the prop really helps the
thrust.

All the lifeboats on the cruise ships have had steerable shrouds,
called Kort Nozzles. The tenders, with identical hulls, have a
conventional rudder. They have bigger engines and are faster, and also
have radar on a retractable mast. Nozzles have high drag and cut top
speed, but offer more manuverability at low speeds. They are found on
some tugs.

Casady