« Reply #2 on: Sep 14, 2021, 4:59 pm »
I did look at those blades types some years ago but concluded they were only quieter in a very specific rev and load range (the main advantage when using the large chord versions was being able to spin them slower to get the same flow/pressure - I suspect that was where most of the noise reduction actually came from).  You'd need to play around with the MultiWing or WingFan fan calculators (free download from their websites) to get a better idea of the expected performance (they'll give indicative noise levels which could be compared with the 4Z).  Without a major transmission re-design the rpm is obviously fixed (attached to the engine) which may limit the selection to some degree.

« Reply #1 on: Sep 14, 2021, 2:08 pm »
I am considering changing my Nine bladed 4ZR lift fan to explore the possibilities of reducing (altering) lift fan noise in my Otter as the thrust fan is much less intrusive and thus is more acceptable within the current noise profile.
To avoid re-inventing does anyone have any experience regarding the new batch of sickle bladed "low noise" profiles that perhaps should be explored?
KingFisher 1 (the one I designed) DID have a separate WINGFAN lift blade-  from memory 5 blade about 850 diameter which indeed WAS very quiet as one could swish along on lift only with the declutched thrust fan stationary. No directional control via rudders obviously, but I could steer via walking about the craft!
Replacing the OTTER lift fan is such an easy procedure and I am going to take Decibel readings before alterations for comparison purposes.
Whilst the Otter clearly already HAS a low noise profile, I feel its sensible to explore further improvements which could then be shared across general hovercraft usage. 
Memories are BETTER than Dreams---"Capn" FLINT