I just installed a Kiwi Feathering Prop on Tuesday.
In really light winds Wednesday we gained between 0.4 and 0.9 knots. Wind speeds started at 5 knots and dropped quickly to 2knots. On other nights we would have quit long ago.
At night we were also able to see the phosporescent trail of the rudder and offset prop with the old fixed prop. With the feathering prop we could see the trail disapear as the blades feather.
Real world motoring speed added .4 to .7 over most of the range. Top cruising speed climbed both from a reduction in vibration and increased power (5.4 to 6.7). Reverse is much better.
Prop has un-geared composite blades on bronze hub. Boat is 41 ft fin keel, spade rudder, about 21,000 lbs