A few years back I bought a decoy that had a moving head and a propeller on the backside. It sucked! The batteries (D sized) lasted like 2 hours! The prop hung in grass and it stopped spinning. It was disappointing. The year after I tried the duck that had the motor underneath the keel (torpedo shaped motor). Again, batteries didn't work well at all.
Since then I have started making my own decoys. I even cut one up, filled it with foam, put some props coming out of the side and threw it in the water. It worked too...
Its a byproduct of dreaming about ducks you know.