In a youtube video an airboat is featured with two contra-rotating propellers, each holding nine blades. Interestingly those nine blades are not spaced evenly, but grouped into three groups. I've never seen such an arrangement before, and I expected an evenly spaced configuration to be more efficient. What could be a reason to prefer this grouped configuration?
1$\begingroup$ Even though this question is about a boat and not really an airplane, I still hope it is ok to post here! $\endgroup$– flawrAug 22, 2018 at 18:41
1$\begingroup$ Related if not duplicate: aviation.stackexchange.com/questions/43279/… $\endgroup$– SanchisesAug 22, 2018 at 21:30
$\begingroup$ Comment rather than answer since this is just a guess, but maybe they found it easier to build a propeller with three evenly-spaced groups of three blades each than to build one with nine evenly-spaced blades? $\endgroup$– VikkiJul 21, 2021 at 16:48
If there was a technical reason beyond "it looks cool", I would guess it would be some benefit noise wise, where it may sound more like a 3 blade prop than a 9 blade, and the resulting lower beat frequency of one prop, interacting with its counter-rotating mate, was less objectionable.
I went on a mangrove airboat tour in Forida once in a 454 V8 powered boat and it was earplugs PLUS ear muffs together for me, so I can imagine that anything that helps with noise levels while absorbing all that power would be very sought after.