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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?

grouped propeller blades

Therac
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flawr
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    Even though this question is about a boat and not really an airplane, I still hope it is ok to post here! – flawr Aug 22 '18 at 18:41
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    Related if not duplicate: https://aviation.stackexchange.com/questions/43279/what-are-the-benefits-of-a-non-orthogonal-scissor-tail-rotor – Sanchises Aug 22 '18 at 21:30
  • 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? – Vikki Jul 21 '21 at 16:48

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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.

John K
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