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Chuck Husick: Techno-Talk, May 2006 from BoatUS Magazine -

information current as of October 2009


If your boat engine is “fresh water” cooled you need to know some things about antifreeze, even if you live as far south as Key West where the lowest temperature ever recorded was 41°F. The first and most critical fact is that if your engine’s cooling system contains just plain water it is highly likely that some part of the system (probably the circulating pump) will fail in a very short time. The system must always be filled with an appropriate antifreeze mixture or an alternative coolant that contains the chemical additives needed to protect its components.

We usually think of water in terms of its taste when we drink it or its temperature when we bathe or swim. We don’t think of water its chemical/physical characteristics, as the Universal Solvent. Owners of boats equipped with water-cooled air conditioning systems may be well acquainted with water’s ability to dissolve things. One day the metal basket in the raw water strainer is there, collecting grass and stuff, the next day most or all of it has disappeared, carried away by the water that flows through it.

Although the 50/50 mixture of antifeeze and water you should have in your engine’s cooling system will protect against freezing down to about -34° F., it’s the ability of the solution to increase the boiling point to about +276° F and the protection provided by the anti-corrosion, anti-scale, anti-foaming and lubricating additives in the antifreeze that are important for your engine. Simply put, plain water is poison in an engine’s cooling system.

If the circulating seawater (rarely above 90° F even in Florida) can dissolve a metal strainer basket consider how efficient the 180° F or hotter water in the engine’s cooling system can be destroying the water seal on the circulating pump in the cooling system, eroding parts of the heat exchanger and in diesel engines built with wet cylinder liners or sleeves (common practice in most large diesels), allowing cavitation bubbles that can form on the exterior of the sleeve to eventually perforate it and ruin the engine.

Choosing the most appropriate antifreeze for your engine is not difficult, although the large number of brands on the market can create confusion. The best authority is likely the engine’s manufacturer. They warrant the engine and have a compelling self-interest in its performance. It’s also a good idea to check the ASTM (American Society for Testing Materials) specification for the antifreeze you are thinking of buying.

Separate standards are published for gasoline and diesel engine use, both for full strength (undiluted) antifreeze and for antifreeze already diluted with pure water to the desired 50/50 mixture. There are two compelling reasons to buy the premixed coolant: you won’t have to obtain the pure (distilled or de-ionized) water needed to dilute the 100% antifreeze and by carrying a spare supply of 50/50 mix on your boat the required antifreeze-to-water ratio will be maintained should you have to top-up the coolant system. The specification numbers for the 50/50 premix are ASTM 4656 for gasoline engines and ASTM 5345 for diesels.

The most common types of antifreeze are either Ethylene Glycol (usually green in color) or Propylene Glycol (usually red or purple colored) based. Either type can be safe and effective in most engines. Regardless of which type of antifreeze you choose the two types must never be mixed. It is best to always use the same type of antifreeze, EG or PG based, that was in the engine when it was delivered from the factory. Making a switch from an EG-based solution to a PG-based product may cause problems unless the system is very thoroughly flushed to remove all traces of the EG-based antifreeze.

Although the antifreeze component in your engine’s cooling system does not degrade over time, the chemical additives that protect the engine have a limited life. You should change the engine coolant at least once every two years, regardless of how many hours the engine has been operated. Long-life coolants are available that can be used for up to five years, however the cost of replacing the coolant every two years is low enough to make use of the long-life products a questionable choice for most recreational boats.

By Chuck Husick

Chuck Husick is a pilot, engineer, sailor and former president of Chris Craft Boats.

© Copyright BoatUS Magazine May 2006

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