An engine breakdown at sea can be far more than just inconvenient. It can be dangerous. There are a number of common causes for failure, and a bit of planned maintenance and preventative work can avoid those situations.
By far, the most common problems are in the electrical systems. Before setting out, simply checking that there are no loose wires may seem obvious, but it is rarely done. A common cause of electrical problems in some fast, sporting craft is water in the bilge. As the boat accelerates, the bilge water can flow to the back of the boat at splash up onto the flywheel. The spray can then hit the starter motor, stopping you in your tracks. Ensuring that the bilge is empty before setting out, and checking occasionally (and emptying the bilge if water is being taken on) while out can prevent this occurring. Another common problem (on boats with a flybridge) is a failure to start when stopped after a cruise. This can be due to the upper helm controls being not quite disengaged after stopping. These craft have systems in place to prevent starting from the lower helm if the upper help controls are not FULLY disconnected.
Failures in batteries and isolator switches also happen. Smaller boats often experience this particular problem because the parts are often partly exposed to spray. Keeping spare isolator switches on board is a simple solution. Batteries can be low on fluid or have cells drop out, or just be too old to manage any longer. The terminals are also a source of battery failure, often due to the indelicate use of a hammer to get connectors on there! Avoiding these problems is as easy as keeping a (fully charged) spare battery on the boat. There are also products such as portable power-packs available.
Problems with fuel systems are the second most common source of failure. Sadly, this is often due to simply running out of fuel. As basic as it may seem, making sure you have enough fuel for your excursion is vital. Too many boaters rely on their on-board fuel gauge to be accurate. Marine fuel gauges are notoriously inaccurate and cannot be counted on the way a car’s gauge can. Always ensure that you have at least a half a tank when at sea. Dip the tanks to be sure.
An issue that is becoming more common is fouling of the system from the bug that grows in the diesel/water interface. The bug seems to be spreading. There are a number of treatments for it available. Some work well by rendering the dead bugs into a combustible material that just burns up along with the fuel. But some of them just drop the dead lime to the bottom of the tank, and that material clogs the fuel filters. Keeping spare filters on board can save a lot of time and hassles, as long as you have taken the time to learn how to replace them.
Other sources of problems are in the gearboxes, steering apparatus and saildrives. Wear and tear on the clutch will eventually wear the gear out. This is often caused by the operator. Riding the clutch, or allowing it to slip during manoeuvres is often the reason clutches fail. Ensuring that your saildrive propeller is correctly and firmly fitted after the ring anodes are replaced at the beginning of the boating season is obviously critical. But those propellers falling off is one of the notable causes of breakdowns. Hydraulic steering systems also fail due to normal wear and tear. A close visual inspection of cables and fittings, and checking for hydraulic leaks can get those maintenance tasks scheduled before setting off.