Change the oil at the end of the season and replenish with fresh oil. After use oil's ph balance changes and it becomes slightly acidic; this oil may attack the engine's internals if left in place.
Set up a system so that you can run the engine ashore. Some people create a closed loop using a bucket that can be hung at the level of the exhaust to catch the cooling water. The bucket has a hole in the bottom fitted with a hose that can be jammed into the raw water intake. Others use a similar arrangement but attach the hose to the inlet of the water pump.
Do not be tempted to use pressurised water from the mains as this can force it's way past the impeller and end up in the cylinders.
Some people recommend removing the thermostat to help the antifreeze circulate. Some recommend draining the engine water jacket using the taps on the engine, before adding antifreeze. Note that draining the engine water jacket will not drain the gear box jacket.
Cover the engine mounts with cling film or alternative before starting. spilt oil on the engine mounts destroys the rubber in the mounts.
Run the engine to get the oil to it's working temperature. Once this is achieved pour antifreeze into the cooling system and let it run round the system. A mix of equal parts water and antifreeze is generally used. This prevents water in the engine block and gear box jacket freezing and also helps prevent corrosion in the engines cooling circuit. On a Centaur with raw water cooling, when the engine is hot and with the engine running, remove the plastic inlet tube from the raw water inlet sea cock. Insert the tube in a can of antifreeze and allow the water pump to empty the can. Stop engine as soon as the can is empty. Re-attach the tube to raw water sea cock.
The engine can now be stopped and the oil removed. This can either be done using the sump plug or a suction pump (like the PELA 6000) deployed through the dip stick housing. If using the former method toddlers disposable nappies are a great way of mopping up any oil spills. Accessing the sump plug is impossible on some engines.
Once the new oil is added to the correct level all air intakes should be closed off to prevent moist air entering the engine, condensing and forming pin pricks of corrosion inside the engine. This normally means an oily rag or cling film over the exhaust and air filter.
Lastly, fill the diesel tank as full as possible to prevent condensation and the growth of the "diesel bug" in the fuel tank.