On a deisel motor in a boat why does it have coolant and a water pump?

I just got a new boat with a diesel engine and I don't understand why there is a coolant tank as well as a water pump? I think the water pump goes to the heat exchanger but what actually cools the motor - how does this stuff work?On a deisel motor in a boat why does it have coolant and a water pump?this is refered to as a fresh water cooling system. the engine has a coolant tank and a recirc pump that pumps the coolant through a tube and shell heat exchanger,just as a car does through a radiator. instead of air around the radiator, the exchanger has sea(lake)water pumped through it by a engine mounted pump or a pump on a outdrive if so equiped. the engine heat is transmitted from the engine coolant to the exchanger, and out to the sea(lake). this system has many advantages. one of the most important is the fact that salt or brackish water never touches the engine block or components, preventing costly corrosion. secondly it keeps the engine running at a more even or constant temp. this is very important to a diesel as the engine puts out its hp and performance at a set temp. the heat exchanger can have an automatic drain system, draining water out of it and with coolant (antifreeze) in the engine, the boat can be operated in freezing weather without freezing, or draining the block every night. Heat exchangers can be backflushed easier than an engine, and allow the boat to be operated in muddy or contaminated water. (I have seen boats with tandem exchngers in case one plugs up or needs cleaning). gasoline and diesel engines can have exchanger systems, but since diesels inherantly are more expensive and live longer, a heat exhanger system is worth while.On a deisel motor in a boat why does it have coolant and a water pump?if you do indeed have a heat exchangers, you have two loops at work...the coolant circulates in the engine with its own pump; the tank is for topping up just like in a car. The external water pump....the one connected to the hose to the seacock / through hull...draws in cool sea water, passes that through the heat exchanger where it cools the coolant and then the hot sea water goes out the exhaust.



This way you're not circulating hot corrosive sea water through the engine and shortening its life.