Marine heat exchangers

Marine heat exchangers are the most common way to cool a boat's engine, using the lake, river or ocean water in which the boat floats. Since this water may be corrosive the engine may be cooled by a sealed mixture of distilled water and antifreeze. Heat from the water-antifreeze mixture is then transferred to the ocean (or lake or river) water which flows into a heat exchanger. The water-antifreeze mixture runs through it dumping heat, but remaining separate from corrosive salts and chemicals found in the water the boat is floating in. If the ocean water eventually corrodes and ruins the heat exchanger it can be replaced at a fraction of the cost of replacing the engine.

To protect the marine heat exchanger from corrosive salts, a sacrificial zinc anode is screwed into the heat exchanger. This anode must be periodically replaced as part of regular maintenance.

Because the water the boat floats in may be contaminated with floating particles such as wood or other detrius most boats will have a filter (often stainless steel mesh) to remove these particles before they flow through the heat exchanger. This filter must be periodically cleaned or else the flow of water to the heat exchanger will become obstructed and the engine will overheat.

Ocean water is caused to flow through the heat exchanger by an impeller, often made of rubber and looking like a paddle wheel. Impellers commonly fail by having their paddles fall off and when this happens the engine will overheat unless promptly shut off. Many marine engines using heat exchangers have a grease reservoir for the impeller. In such cases a screw is often made part of the reservoir and is turned a small amount periodically, such as once a month, to apply a bit of grease to the rubber impeller to reduce friction and prolong its life.

A consequence of using a heat exchanger on an in-hull marine engine is that there must be a thru-hull opening below the waterline to admit the ocean water to the marine heat exchanger. Most boats with this arrangement have a valve which closes this hole when the boat is unattended so that a leak in the cooling system will not cause the boat to fill with water and eventually sink.