With boat show season comes lots of sales and new commands. A vibration analysis survey is always a prudent investment. In a transaction, it can be a testament to the condition of the vessel. Whereas the selling price point can be negotiated for the buyer, one of these surveys can protect the seller from faulty claims after the sale.
When a captain or chief engineer take over a new command, a vibration analysis can bring to light issues or service items that need attention. This is extremely important if maintenance records weren’t kept well or if the previous crew left without disclosing information.
Although everyone has heard of vibration analysis, few truly appreciate how valuable it can be as a predictive and preventative maintenance tool.
Everything has vibration. Vibration is a mechanical principle in which oscillations occur around a point of equilibrium. One of the most important things to remember about vibration is that it is never solely a cosmetic issue, especially on a yacht. When vibration is loud enough to be heard or felt, it’s much more than a nuisance. It’s evidence that the vessel is not running at optimal performance, and mechanical components are suffering.
There are several terms that must be understood to learn what vibration isolation theory is. The first is frequency. Frequency is the number of complete cycles of oscillation that occur in a given period of time.
Natural frequency is the number of complete cycles of oscillation a mass will vibrate in a given period of time if a force displaces it from its center, allowing it to vibrate freely.
Disturbing frequency is defined as the frequency of vibration caused by an unbalanced, rotating or reciprocating movement of mass. When the disturbing frequency equals the natural frequency of the isolation system, exaggerated movement occurs. This is called resonance.
In the quest to reduce vibration, the theory of isolation is extremely important to understand, especially when it pertains to engine mounts. All machines in operation emit vibration of some sort that will vary in intensity or amplitude. Since vibration is a force, introducing an opposite force can reduce the transmission. To succeed at “isolating” the vibration, the environment of the installation must be assessed, namely three important factors: the weight of what is supported, the disturbing frequency of the machine, and the rigidity of the structure of the supporting machine and its foundation.
When seeking to reduce the transmission of vibration, a robust material should be introduced. The material must be so resilient that it returns to its original height after the load is removed. Rubber in engine mounts are a perfect example. When engine mounts deflect from the mass of the engine, they establish low natural frequency of the isolation system. When the isolation system is at a lower frequency than that of the machine, vibration is absorbed by the rubber in each phase of its cycle.
The lower the natural frequency and the higher engine RPM, the more efficient the isolation system. Successful isolation systems also aid in noise reduction, as they break the flow of structure-borne noise coming from the machine.
Vibration analysis uses collected data to break down vibration into individual frequency components. AME uses three methods to analyze data: manufacturer’s requirements, comparing it to another similar machine that is known to be operating well, and using published vibration standards.
The frequency, direction and amplitude of the vibration are then examined. Using those factors as a basis, mechanical issues such as deteriorated or incorrectly installed engine mounts, bent shafts, engine misfire, exhaust deficiencies and propeller issues can be detected.
The benefits of vibration analysis go way beyond forecasting mechanical failures. The analysis provides valuable information that is backed up by science. By making educated maintenance decisions that have sound, scientific backing, objectivity is eliminated, making a captain’s or engineer’s approach to maintenance a targeted one, allowing for more efficient and cost-friendly yard periods.