Check this one out:
http://www89.pair.com/techinfo/MassFlow/ductbook.htm

As this book says, The main advantages of ducted fans are at either low or
high speeds.

My own interest in ducts relates to amphibians. The ides is to get the best
performance from the propulsion system at the point where I need it the
most, getting off the water. After that, a little reduction in performance
might be tolerated.

Besides, It'd look COOL!