of one would resonate with the third harmonic of the other. But a
propeller and an engine are nowhere close to identical; their resonant
frequencies don't have anything to do with each other in the first place,

What resonance characteristics does a propeller have, and why?

so there's no point in trying to do anything to throw them out of match
with each other. (Unless you get unlucky; if resonance problems show up
in testing, then you can try re-machining one of the belt pulleys to a