From: (Ralph DuBose)

snip

The problem with multi-wing fans is that the blades have a lot of
camber/concavity. THey are better at generating pressure than at
moving air. Serious hovercraft enthusiasts have been to make composite
replacements for better performance. Not very certified.
The new 4 stroke snow-machine motors look great except 1. Still
available only 90 hp and above. 2. I cannot afford one.

http://www89.pair.com/techinfo/MassFlow/ductbook.htm

http://www.cds.caltech.edu/~murray/projects/dfan/

http://www.bridgingworlds.com/DUCKT.HTM


Dan, U.S. Air Force retired