Direct and Indirect Boat Engine Cooling With the Use of a Raw Water Pump

Anyone that’s new to boating could easily be forgiven for failing to understand how our boat’s engine is cooled. Unless your engine is air or keel cooled (more on this later) its cooling system is very likely to involve a ‘raw water pump’.

Consider this. If your boat’s engine is in an engine room or buried under your decks, how can it be cooled? Your car will have a radiator and a fan. The radiator is a heat exchanger that relies on cold air passing through it to cool the water inside it. The engine hidden away inside your boat does not have this luxury, so we have to cool the engine another way.

A simple way to achieve this is to take a standard water cooled engine and install a pump that takes water from the sea, lake or river and pushes that water straight through the engine and out again. This is basic ‘direct raw water cooling’. To add to the efficiency of this method some bright spark invented the water jacketed exhaust manifold. Instead of the used water being ejected immediately overboard, it is diverted through the exhausts water jacket and out through the exhaust. This not only massively improves the engine cooling but also removes the fire risk associated from an otherwise very hot exhaust. The big drawback to this method, particularly in sea water, is the build up of dried solids from the salt water. This can get so bad as to require an engine change. A further problem is the absence of antifreeze in winter.

A more elegant solution to cooling the engine is ‘indirect raw water cooling’. This method has a sealed water / antifreeze system, much like a car, but instead of a radiator it has a purpose made heat exchanger. This is a vessel with two chambers that are separated from each other by a suitably heat conductive material. Water from the engine is moved by the engines circulatory pump through one side of the heat exchanger, while a raw water pump pumps seawater through the other side of the heat exchanger. This removes heat from the freshwater circuit. The raw water then passes through the exhaust water jacket where it further cools the engine. The build up of solids can affect the heat exchanger over time, but many can be dismantled and cleaned, this is a much cheaper alternative to a possible engine replacement.

This method, used with a thermostat installed, gives us a sealed cooling system full of hot water that we can divert via a ‘calorifier’ to give us a free tank of hot water. A calorifier is simply a tank with metal coils inside that transports the hot engine water through the tank transferring heat on its way back to the engine. The engine cooling water never mixes directly with the water we are going to shower in!

The modern raw water pump is only a fraction of the size of the old inefficient pumps that used to be used, but they are very powerful. They contain a flexible impeller that ideally should be removed according to manufacturers recommended intervals, for careful checking and possible replacement.

Keel cooling, incidentally, is where the engine is cooled by a sealed system, but instead of using a raw water pump the hot engine water is diverted through metal pipes outside the boat where it is cooled and returned to the engine. This system can be affected badly if the boat grounds and the pipes are crushed or breached.