"Dave Butler" wrote

If the counterweights are involved, and the the torsional vibration issue
is a
propeller/cranskshaft phenomenon, do you change anything in the crankshaft
counterweights when you change the propeller?

Nope, you still don't quite have it.

Did you know that if you have two tuning forks than vibrate (resonate) the
same note, and you hit one to start it resonating, and you put the second
one up to it, the second one will start vibrating? Well, they do.

Same idea with the crank and the prop. The crank vibrates at one frequency,
and at a certain RPM, that is the frequency that it wants to vibrate at. If
the crank has no other things vibrating at the same frequency touching it,
it is stiff enough to not be a problem.

Now you add a prop that *does* vibrate at the same frequency as the crank,
and run it at that critical RPM, the prop starts its vibration, and the
crank is doing the same thing. Think back to the tuning forks, and now the
one excites the other, and it keeps on exciting each other, getting louder
(think more movement) and louder, until something breaks.

So if you put a different prop on, (3 blade) that does not vibrate like a
tuning fork at the same critical frequency as the crank, the crank still
vibrates at its' frequency, but the prop does not, so it does not help the
crank vibrate bigger. (louder) No problem. The restriction for the
combination is removed.
--
Jim in NC