Eric,
I guess I should have made myself more clear.
Streams don't flow at anywhere near 6 knots and the impeller I am thinking of is in the stream channel and has no penstock or dam. There are also tidal generators that use screw impellers in a tidal channel and extract energy with good efficiency at much lower current.
(I do read your posts very carefully, I am apologize for making you feel the need to repeat yourself.)
Alan,
That is a very insightful statement, that with a regular prop the Bernoulli force is working against you instead of with you. I had not thought of that. On an autoprop this would not be the case as the blade would be in the reversed position. I suspect your observed issue is a huge contributor, probably the biggest contributor, to the lack of successful regeneration.
RE: reduced drag quoted on the autoprop website
It appears from the design that the autoprop never really feathers. If you want a feathering propeller there are much better solutions, a folding prop would reduce the drag even further than a fully feathered prop. It does appear that it maintains laminar flow in all conditions, and as Alan pointed out also insures that the Bernoulli force and Newtonian forces are always working together rather than against each other (as is the case in reverse or regen in a fixed prop).
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My contention is this:
A stream of water 18" in diameter moving at 4 knots contains a significant amount of energy, doesn't it? If we aren't able to extract that energy we are doing it wrong.
Personal Conjecture:
From a marketing perspective it seems that if, for every five hours of sailing, you could motor for an hour, electric drive would be very attractive to a large number of sailors. A 20% total system efficiency doesn't seem to optimistic. A package that ran on regen would be compact, simple, and mass-marketable. Sailors could get back to sailing, rather than engine management, and still have the ability to maneuver and dock under power.
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