There are two main varieties of cooling system for inboard diesel engines - direct and indirect cooling. On both types of engines, the Water Pump Impellor should be inspected and replaced if necessary before carrying out the following steps. Make sure the Coolant used in the following steps contains a Coolant/Anti-Freeze mix (Most modern Coolants do).

In a Direct Cooling system, water is pumped from outside the boat, through the engine cooling passages and out of the exhaust. To winterise this type of engine:

1. Remove the Thermostat. This lets water circulate in the engine without the engine having to reach operating temperature.
2. Check the Sacrificial Anodes. Make sure that these are not depleted. Replace if necessary.
3. Run the engine with the water pump feeding from a Coolant Water mix. This can usually be done by closing the external stopcock and disconnecting the pump's feed hose from the water inlet. This hose can then be put into a bucket filled with Coolant Water mix. Run the engine until this mix can be seen coming out of the exhaust. All the cooling passages should now be filled with this mix. (Collect the coolant from the exhaust in another bucket and dispose of it in an environmentally friendly manner)
4. Reconnect  hoses etc. 

In an InDirect Cooling system, Coolant is held in a closed system in the engine. Heat from this internal system is dissipated in a heat exchanger to external water which is pumped, from outside the boat, through the heat exchanger and out through the exhaust. To winterise this type of engine:

1. Check that the internal Coolant is at the correct density by use of a hydrometer. The manufacturers of the Coolant will recommend how often the coolant should be replaced totally but it is usually every three years.
2. Run the engine with the water pump feeding from a Coolant Water mix in the same manner as in a Direct Cooling system.

As an engine is used, sulphur in the fuel is burned and Sulphuric Acid is formed. This finds its way into the engine oil and is the main reason to change your oil before laying up the boat for the winter.

It is a good idea to warm up the engine before changing the oil. Both the oil and the oil filter should be replaced at the same time. Normal grade oil is fine for laying up the engine for the winter. The waste oil should be disposed of in an environmentally friendly manner. (Most garages will dispose of it for you)

All main electrical connections should be checked for corrosion before laying up for the winter. Once they are corrosion free, they should be sprayed with WD40 or equivalent so that they won't corrode in the damp weather.

Tha Batteries are very susceptible to damage during the winter season and should be cared for in the following manner.

1. They should be disconnected and the terminals should be cleaned.
2. The electolyte level should be checked and topped up if necessary.
3. The Batteries should be fully charged.
4. Ideally the batteries should be removed and kept indoors for the winter as sub-zero temperatures are not good for them.
5. Also ideally they should be re-charged every month during the winter as the charge will slowly drain over a long period of time.