HOW TO CHECK YOUR COOLING SYSTEM

Unlike cars, which use air to cool the liquid coolant flowing through the engine and radiator, most boats rely on the water they're floating on to keep their engines cool.

- WORDS: Jon Mendez

Some use this 'raw' water (be it fresh or salt water) to cool the engine directly, continually drawing in water through an impeller pump and circulating it around the engine before expelling it out via the exhaust. This is called 'raw water cooling' and is the simplest, most basic cooling system.

One step up from this is a raw water system with a thermostatic valve. This opens and closes as required to maintain the engine's temperature at or near its most efficient operating range before discharging it. On outboard engines, the hot water is mostly discharged underwater through the propeller hub along with the exhaust gas, although a thin stream of water, known as a telltale jet, is dispersed separately above the waterline so you can see and feel that it is working correctly.

Smaller inboard engines often use a similar direct raw water cooling system, albeit with a single exhaust/cooling water exit above the waterline instead, allowing easy visual inspection to check that everything is working.

Larger inboard engines tend to use a more complex indirect cooling system. This has a closed circuit for the engine coolant - a special mix of fresh water and antifreeze to circulate around the engine - linked to a series of one or more heat exchangers. These heat exchangers have the coolant passing through one side of the system, and 'raw' fresh or salt water from outside the boat passing through the other, so the heat from the sealed side is transferred to the raw water and expelled along with the exhaust as usual.