my boat has a vibration at all rpm. i took it to a boatyard which had an instrument which could measure the frequency of the vibration during sea trials. the measurements indicated the vibrations were at propeller shaft rate for all boat speeds. the yard recommended that the propeller shafts be removed, straightened or replaced. are there other causes for shaft rate vibrations?
Yes there are; there are at least five possible causes of shaft rate vibrations. As the yard recommended, a common cause is a bent propeller shaft with the repair being straighten or replace. Following is a list summarizes possible causes of shaft rate vibrations.
1. Bent shaft: Straighten or replace
2. One bent propeller blade: Straighten blade and rebalance or replace propeller
3. Misalignment of shaft: Realign bearings and rotating components
4. One propeller blade has a different section shape: Reshape blades to identical section geometry by grinding or replace the propeller
5. Propeller shaft coupling for gear connection is bored off center: Replace coupling with one machined correctly
