Most marine inboard engines are fresh water cooled. Much like your car, a captive amount of freshwater (coolant) is continually circulated through a heat exchanger that maintains the engine at a temperature set by the thermostat. Usually this is between 170 and 180 degrees. Unlike your car, a marine engine does not have a radiator since there is no air flow available to cool it. So, what cools the fresh water ? The solution is to utilize the relatively cool seawater, described as raw water. The raw water is pumped from the sea to a heat exchanger tube where coils filled with fresh water (coolant) are surrounded by flowing raw water (sea water). The cold raw water is now cooling the hot fresh water that is keeping the engine from overheating.
Raw water will pass through the heat exchanger only one time before it is expelled back into the sea while the fresh water continues to recirculate. As the raw water exits the heat exchanger, it will mix with the exhaust gases in the muffler and pump out the back of the boat.
The fresh water and raw water systems have dedicated pumps, the raw water utilizing a rubber impeller and the fresh water pump using a metal impeller as not to melt. Because the raw water sometimes carries debris like kelp, eelgrass, plastic bags or even small fish, there is a filter installed between the through hull and the pump, this is known as the sea strainer. The sea strainer is usually made of clear glass or plastic so it can be checked without having to open the filter… and should be checked before starting the engine.
At the very least, upon engine startup, a prudent skipper will check that the boat is “pumping water” by visually inspecting the raw water exiting the back of the boat. It is best to inspect the flow while standing on the dock as it can be hard to see the water flow from the cockpit. Skippers should watch the water flow for at least 2-3 discharges to make sure the system stays primed and continues to pump water. While starting your engine away from the dock you still want to visually inspect the flow but also use your sense of hearing. A dry exhaust (not pumping water) is much louder than a wet exhaust and will be obvious.
Stay cool and always check to make sure you are pumping water when you start the engine and while running the engine!
Great explanation of a critical function for a marine engine. Thanks for posting this. Its great for people to learn the basics for sure.