Condition Monitoring – Vibration Analysis

Routine maintenance of yacht and sailboat machinery is mainly based on running hours as recommended by equipment manufacturers, or so it would appear from the many conversations I have had with engineers working on various sizes of motor yacht and sailboat alike. Overhaul times have then been adjusted by experience and the requirements of flag state and classification society surveys. The operating and maintenance experience of engineers provided the foundation for planned maintenance systems. Unfortunately, planned maintenance decrees exact periods between overhauls and tends to remove the flexibility and sensible alteration of running hours, by engineers. The result is that planned maintenance has proved to be not cost effective.

Equipment has been frequently taken out of service for planned maintenance and found to have no faults and a potential for many more hours of operation. The cost of stripping down is paid not only in terms of replacement joints, seals and parts but sometimes in damage inflicted and ultimate breakdown built in, during the procedure of opening the item of equipment. Dismantling machinery and equipment for survey may be costly in the same way.

The alternative to planned maintenance, of repairing after breakdown, can be expensive in terms of the extent of resulting damage. Breakdown maintenance is not the remedy of the prudent engineer.

Condition monitoring, another and more acceptable option, has long been used as part of planned maintenance systems on commercial vessels to determine when machinery should be overhauled, however it is not widely known about or used in the world of yacht engineering.

Classification societies are willing to accept, by mutual agreement, condition monitoring as an alternative to taking machinery apart for survey.

One surveillance method of condition monitoring is vibration analysis. Since the level of vibration changes with wear it is possible to monitor the condition of certain machinery by measuring the vibration characteristics. This method is particularly useful when monitoring rotating machinery. The main parameters measured are the vibration’s acceleration, velocity and displacement. An accelerometer is used to measure the above quantities and by recording successive readings then a trend of the change in the vibration can be obtained. By correct analysis of the readings, from a graph plot, deterioration and condition can be accurately monitored and it is possible to plan bearing replacements, for example, at least six months in advance of their failure.

Vibration readings are taken mainly on the bearing housings of rotating machinery as close as possible to the shaft. The vibration pick-up is placed on each bearing in turn to record in the vertical, horizontal and axial directions. Readings may also be taken on casings, supports and at other relevant points. Readings of vibration magnitude and frequency are recorded manually from basic instruments but some equipment incorporate data collectors from which information is fed to a computer for analysis.

An investigation of excessive vibration relies mainly on vibration frequency readings to identify normal and abnormal vibrations. The readings are obtained with the machinery in a steady running state. Other factors which will contribute, perhaps, in a major way to the identification of problems are also taken into account. Thus details such as speed, load and operating temperatures are noted, together with any history of component failure.

The advantages of this form of condition monitoring are obvious. Genuine maintenance overhauls and part replacements can be planned months in advance, therefore, cost effective and labor efficient. The needless running hour planned maintenance and spare part replacement, plus associated work time, are avoided thereby giving rise to a more cost effective and efficiently run yacht operation.