Not with gears! But a belt can provide any ratio. Of course any
irrational number can be approximated arbitrarily well by a rational
number... but the idea is just to choose a number that can't be
approximated well by a ratio of small numbers, not one that's genuinely
mathematically irrational. (Indeed, the latter concept is almost
meaningless in engineering, which is why I felt free to use "irrational"
as shorthand for the former concept.)

The idea isn't complete nonsense. If you had two identical assemblies,
linked by a belt drive, it'd be exactly the thing to do. You wouldn't
pick a 2:3 ratio, for instance, since that would mean the second harmonic
of one would resonate with the third harmonic of the other.

I struggle with understanding how you will throw a dangerous harmonic down a
viscoelastic belt. I guess it could be done but it doesn't seem as if it
could be done easily. Metal gears or similarly high modulus materials will
have an extremely low tangent delta and therefore have good
transmissibility. Vibration along a broad spectrum of frequencies should be
efficiently transmitted in such materials. Rubber doesn't transmit
frequency very efficiently unless its a glass; then it doesn't bend either
(unless you bend it very, very, very slowly). More likely you select the
ratio that gets you your desired prop RPM at the desired engine RPM. The
rubber belt should be an excellent (not perfect) vibration decoupler.

Charlie Smith
KIS Cruiser 4021