There are hundreds of individual parts that can fail and leave the boat stranded. The bathtub curve only applies to fairly simple mechanisms. More complicated systems fail randomly. With random failures there are no maintenance procedures or overhaul schedule that will provide known reliability. The best we can do is to predict a failure far enough ahead of time to fix the problem in time to minimize its impact. We can also plan contingencies to minimize impact.
There are several approaches you can take to determine if your boat is reliable enough. The first question you must answer is; How will you use your boat? Other questions you must answer are; What systems you rely on, what are their functions, how do they fail, what are the consequences of failure? Some of the failures you define will probably never happen and some others might happen but will not cause significant impact to your intended use of the boat. Resources need to be applied to failures that are likely to happen or failures that are less likely to happen but will have significant impact if they do happen.
The general approach is to:
Define the systems and subsystems on the boat
Define how they can fail (failure mode)
What are the consequences of failure
How likely are failure modes
Define contingency plans. What to do if the failure mode occurs? How do you fix the problem?
Prioritize contingencies based on likelihood of failure and consequence of failure.
Determine how to best predict a potential failure with a maximum warning time.
Plan for contingencies based on the prioritization scheme. Apply judgment based on experience and knowledge.
The next sections provide some background of Failure mode analysis and Risk management. Following that a combined approach is outlined and an example is provided.
One tool that is used in factories to determine reliability is the Failure Modes and Effects Analysis (FMEA). In fact it is a process that can be used to help the reliability design for any system or set of systems. FMEA was developed by Society of Automobile Engineers (SAE) Reliability engineers. It allows them to analyze the effects caused by system element failures. Some engineers get hung up on the definition of failure vs. fault and call the process Fault Hazard Analysis. Whatever you call it, the process helps determine how vulnerable a system is to the types of failures that might be experienced. FMEA can be used to determine the best area to put extra redundancy and reliability. It can be used in a factory, a space ship, a boat or any other complex set of components that work together as a system.
is not a firm process for FMEA that everyone uses. Instead, different
companies use different process steps to define an FMEA. Here the
definition is a modified version similar to the one used by the Ford
Motor Company FMEA process [TBD].
The steps of an FMEA process are described below.
Identity the Potential Problems
1. List the equipment. This includes all of the equipment and components on your boat. Here you can break the system down into system, subsystems, assemblies, subassemblies… We have provided an analysis and a breakdown in the accompanying spreadsheet. You may adjust it or do your own.
2. Identify the functions served by the identified equipment.
3. Identify potential failure modes of the identified equipment. In other words what can happen to the component that might cause it to fail? It might be old age, poor maintenance or excessive moisture etc.
4. List the potential effects of the failure of the component.
5. Define the severity rating or consequence of a component failure (see Error: Reference source not found).
6. Define potential failure mechanisms. Why did it fail? What is the root cause of the failure?
7. Assign an occurrence rating to each failure mechanism. This is the Mean time between Failures
Identify the Potential Solutions
8. Identify predictive monitoring techniques. Advanced monitoring reduces the impact of a failure by providing a warning in advance of the actual failure. List the various techniques by which each failure might be detected.
9. Assign a detection rating to each failure mode/mechanism. How well can we detect the problem? Good, Fair, Poor.
10. Assign likelihood to each fault then calculate a risk-factor number (RF) and apply thresholds to determine the risk level oaf each fault type. This is used to help determine which faults should be dealt with first.
11. Identify recommended action for each fault type. This is any action that you take to reduce the probability of the failure or reduce the consequence. For example if you have a clogged filter, You might have a probability of 40 percent that you it will happen. However if you carry a spare then you will reduce the likelihood of that happing to <10 percent
12. Determine based on the analysis which of the mechanisms you will want to address.
FMEA is an excellent tool to determine the reliability of a system made up of many components. However, it takes specialized training and hard to find data in order to complete a meaningful analysis. Moreover, the results are not produced in the language of the domain and are hard to understand by the untrained. FMEA is useful for highly trained engineers with a staff of maintenance personnel. For the average boat owner a better way is needed to analyze the reliability of a boat.