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
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