Although outboard motors are self draining, there are areas of the cooling system to be mindful of to ensure all water is drained from the system. In addition to water flowing through the block and heads, cooling water in modern outboards is sent to cooling passages in the VST, fuel rails, rectifier/regulators, and additional oil-cooling passages on the block. Usually, the cooling water is routed through small diameter hoses between components.
After running the motor for the last time of the season, and before freezing weather arrives, it is very important to allow enough time for all the water to drain out of all passages. This is particularly important if the boat has been operated in muddy or sandy conditions where mud or sand may have entered the system. The mud and sand can collect in the hose and passages, restricting the flow of water as the system is draining.
To drain the cooling system completely, the motor must be left in a vertical position long enough to allow all water to drain. This is particularly important on models where sections of the cooling system may be lower than the water passages in the block and midsection when the motor is tilted up. Also, if you suspect that ingested mud or sand is restricting water passages it will be necessary to blow compressed air through sections of the cooling system to open restrictions. Tilting up a motor before it is completely drained may allow water to remain in the exhaust passage in the lower unit where it can freeze, cracking the lower unit housing.