Over-heating and Cooling Systems
Fourth place goes to a failure in the cooling systems which lead to the engine over-heating. The most common cause is due to an air-lock in the system which is simple to identify and resolve. To identify if this is the issue, feel the top and bottom of the swim tank, if everything is working fine there should be a difference in temperature, if not then both top and bottom will be hot or cold. To remedy this, locate and unscrew the bolt that sits on top of the 'swim' tank and this will release the air locked in the system. However overheating can be caused by many issues from a coolant hose rupturing, (look for leaks) a water pump failing or a fan belt shredding (which drives the water pump) or the worst case, a head gasket failing.
There are a number of factors that contribute to alternator failure, from poor battery conditions resulting in the alternator working harder to charge the batteries, to battery management systems that over work the alternator to keep batteries continually charged up. Ultimately, one of the biggest issues is that alternators operate in a damp, hot environment, which is not healthy for any electrical product.
Like starters, they're often left for long periods during the year and then used continuously for short periods. During this 'down period', rust can develop and affect operation. There's no way to prevent this occurring other than to correctly winterise your vessel and regularly visit the boat and run the engine. This year's call-out increase could be down to the amount of damp and rain, especially if water has ended up in the bilges. If your bilges are full of oil and water, this will be thrown over the alternators when the engine is running and as they are electrical components, the oil and water will bring about their early failure.