Proper maintenance of small yacht diesel engines is crucial to ensure their longevity and reliable performance. Here are some daily maintenance tasks that yacht owners should perform to keep their diesel engines in top condition:  

Firstly, check the engine oil level and color daily. The oil should be at the proper level and free of contaminants, such as water or metal shavings. If the oil appears dirty or low, change it immediately.

Secondly, inspect the fuel system for leaks, clogs, or water contamination. Drain any water from the fuel filter daily and replace it regularly as recommended by the engine manufacturer.

Thirdly, check the engine coolant level and make sure it's clean and the sea strainer is free of debris. Overheating can cause serious damage to diesel engines, so monitoring coolant levels is essential. Check the seacock to ensure that cooling water will be available to the heat-exchanger.  Check the date of the last impellor change.

Fourthly, inspect the engine's belts, hoses, and clamps for signs of wear and tear. Replace any damaged or worn-out parts immediately to avoid engine failure. Make sure the vessel carries a spares pack including belts, impellor and oil and fuel filters.

Lastly, run the engine at full throttle for a few minutes daily to burn off any carbon buildup and keep the engine lubricated. This also helps prevent fuel injectors from clogging, which can lead to poor performance.

Performing these daily maintenance tasks will not only extend the life of small yacht diesel engines but also help ensure the safety of passengers and crew. It's also important to follow the manufacturer's recommended maintenance schedule and seek the assistance of a professional marine mechanic for more complex repairs.