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Chuck Husick:
Techno-Talk, March 2006 from BoatUS Magazine -
Engine Oil
The “lube” oil in any internal combustion engine is
all that stands between normal operation and certain, sudden, and
gruesome mechanical death. However the oil you use in your gasoline
fueled car engine may not be the right oil for the gas engine in
your boat.
The most significant difference between the engine in your car and
the one in your boat is how hard it must work, usually expressed
as how much of its maximum power it is required to deliver for sustained
periods of time. In many boats the engine is constantly delivering
60-80% of its maximum power rating, compared with the 20% of maximum
power demanded from a car engine. The more highly stressed marine
engine needs superior lubricating oil if it is to operate reliably.
Marine diesel engines also require special oils since most diesel
engine applications involve high continuous loads. Just about any
of the “C” diesel oils will work well. (The first “C”
in the specification identifies the oil as being suitable for a
compression ignition engine).
The lubricating oil identification you see on a container of oil
is created under the auspices of the American Petroleum Institute,
the “API”. The specification that defines the “weight”
of the oil, for example 10W-50, is set by the Society of Automotive
Engineers, the “SAE”. However, in recognition of the
special and often severe stress placed on oil by gasoline fueled
marine engines, the National Marine Manufacturer’s Association
(NMMA), has created a series of specifications for two- and four-stroke
marine engine oils. NMMA Specification FC–W™ defines
the oil requirements for four-stroke engines, NMMA TC–W3®
specification applies to two-stroke engines. The oils that meet
these specifications are specially formulated to provide the special
protection required by our hard working marine engines.
All engine oils serve multiple purposes. Controlling friction between
opposing surfaces is only one of those jobs. One of the oil’s
most important tasks is to transfer heat from parts of the engine
that are best cooled by being sprayed with oil, for example, undersides
of the pistons. Oil provides the critical seal between the inner
surfaces of the cylinder bores and the piston rings, keeping the
high pressure gases created by combustion of fuel from leaking past
the piston rings. This ensures that the expanding combustion gases
are used to provide useful energy while reducing contamination of
the oil in the crankcase.
Additionally, oil provides a vital cushion between surfaces that
impact on one another, for example the tops of the valve stems and
the tappets and some types of gear teeth. Its lubricating qualities
prevent adjacent moving surfaces from contacting one another and
welding themselves into a solid mass from the heat of friction.
Oil also fulfills a particularly critical need in marine engines,
corrosion protection. Recreational vessel engines spend most of
their lives just sitting in what is often a high humidity, salt
air environment. The saying that our engines usually rust out before
they wear out is unfortunately true.
NMMA specification oils are usually identified as synthetic or part
synthetic. The use of the word synthetic does not necessarily mean
that it was created directly from hydrogen and carbon atoms in a
laboratory. Synthetic oils are often made from a mineral oil base
stock which may be partially disassembled and reconstructed to yield
a set of specific performance requirements. Oil constructed entirely
from the chemists’ stockroom tends to be more costly and is
used primarily for special applications such as aircraft jet engines.
All of today’s engine oils contain chemical additives, each
designed to fulfill a specific purpose. These include anti-foaming
agents, corrosion inhibitors, acid neutralizers, viscosity and pour-point
stabilizers and detergents that help keep the inside of the engine
clean and hold the contaminants in suspension so that they can be
removed when the oil is changed.
Although claims that the “oil” in engine oil does not
wear out are true to a degree, the chemical additives do have a
finite service life. Properly conducted oil changes are primary
in ensuring long and trouble free engine life. Change oil only after
the engine has reached full operating temperature. Use the correct
oil, one that matches the engine manufacturer’s specifications.
In most cases, change the filter when you change the oil.
By Chuck Husick
Chuck Husick is a pilot, engineer, sailor and former president of
Chris Craft Boats.
© Copyright BoatUS Magazine 2006
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