Vibration Stops and Re-Starts After Engine Alignment???

A customer called us last week with a frustrating and persisting vibration problem with his 50′ Sunseeker.  The customer is a new owner and does not have a history with the vessel.  The vibration was described by the Captain as a “shimmy of the hips”.  We did a quick check in the engine room and found one engine running out 0.014″ behind the coupler – this is a dangerous level of run-out which is vibrating the vessel and could cause damage to the transmission if not repaired.

Here is the strange part of the discussion – the owner had a mechanic perform a simple in-water engine alignment and the vibration went away.  However, a few weeks later it suddenly came back.  This is a very strange symptom that could not be explained until we removed the running gear for a full investigation.

During dis-assembly the couplers were unusually hard to remove.  Once removed the damage was apparent.  Both coupler keys had been damaged.  The coupler on the problematic side was completely sheared and there are clear signs of galling due to the shaft spinning inside the coupler.  This is a catastrophic level of damage.

I don’t have a theory on the key failure (shafts were only slightly bent, struts were in alignment so no apparent signs of a hard grounding).  However, with the visible galling and shaft spinning it is obvious that the coupler was not seated squarely on the shaft and the mechanic aligned the motor to a “cocked” coupler.  Most likely, during a hard run some weeks later, the shaft slipped in the coupler again and caused it to be cocked in another direction – hence the mysterious and sudden return of the vibration.

No big deal in terms of a High Seas repair.  The shafts were straightened, keyways repaired, couplers lapped to remove galling and new keys were made in our machine shop.  The alignment of the struts were checked and an engine alignment performed with our optical scope alignment system.  Once complete the run-out behind the engine was back to an acceptable 0.002″

Another successful sea trial with the Captain; all was very smooth.  Mystery solved and another happy customer added to our reference list.