There are several negative issues with non-geared props. As we all know, the first being the inefficienccy in reverse. The second being the habit of one blade not opening properly. But the third is the response to the second. Out of balance issues get transmitted to some very criticle parts
of the drive system. The first being the aft shaft bearing. The second being the shaft seal
. Depending on shaft length and RPM
, the distance between the prop and any other component can be at a "Critical" length. Much like how a guitar string vibrates. The "resonant" frequency of the shaft can be such that vibration can be amplified to enourmouse amounts and cause major damage to bearing and seal components. The prop is one end of that string, the engine
being the other. Every component inbetween and along the shaft can be subject to seriuose stress. Of course, the last part of the shaft in this instance is the engine
and how it is mounted. So even the engine mounts can get a hammering.