To winterize an inboard/outboard boat, change the engine oil, remove the cooling water, add fuel stabilizer, and coat parts of the engine with fogging oil spray to prevent metal corrosion. After that, replace the old gear oil with new oil. Lubricate the grease fittings on the engine, take all the expensive electronics and other items out of the boat, clean the boat, and coat it with wax. Place a protective cover on the boat.
Before changing the oil, start the engine, and allow it to warm up a little. This helps remove more dirty oil. To remove the cooling water, attach a garden hose to the cooling system of the boat, and rinse the system with clean water until the engine reaches normal operating temperature. After that, take out the drain plugs from the engine block and manifold. If there is still water in the system, take out the water pump hose to remove it.
If your boat has an outboard engine, open all the drain holes, run the engine, and rinse the system for several minutes. When adding fuel stabilizer, get rid of old fuel, add high-quality marine fuel stabilizer, and fill the tank entirely with fresh fuel. If there is water coming out of the gear oil unit while you are draining it, or if the oil seems diluted, replace the seals before sailing again.