From DIY Boat Owner Magazine
Your Systems’ Trusted Sentinels
By John Payne
These meters, instruments, and gauges are the windows to the vital signs of your engine and electrical system.
Here’s what you need to know about the operation, maintenance, and troubleshooting of gauges and meters
auges and meters sit unobtrusively on our boat-engine control
panels, switchboards, and navigation stations; they’re windows
into our boat systems and without them, systems and equipment would
operate invisibly and fail without warning.
Such devices are crucial to ensuring that
boat engines run correctly within normal
operating parameters and that we don’t
overload electrical systems.
These days our gauges and meters may
be an array of discrete analog meters, just
a couple of basic meters, or a new generation of integrated digital and visual screen
displays. I’m one who still favors analog
displays and it’s for good reason that aircraft still use them as parts of the latest
high-tech screen displays. A quick glance
is all that’s needed to see that your systems
are all “go.” That is an important point
to remember because, when considering
meters and gauges, it’s not always about
the numbers; it’s also about monitoring
trends, up or down, of the system that the
gauge or meter is monitoring.
We’re all familiar with engine gauges.
While there’s a trend to simply have an
alarm, I prefer the ability to monitor pressures and temperatures on a gauge or
meter. Of course, there are many possible
pressures and temperatures to monitor; it
all depends on what and how much you
want to monitor. The basics are sufficient
for most boaters.
Oil pressure is one of the fundamental parameters that need a gauge. The
oil-pressure sender or transducer unit for
the meter uses variable resistance inside a
housing that changes in proportion to the
pressure applied. Low oil pressure readings
are caused by low lube oil level or a clogged
oil filter or, worst case, by a faulty oil pump.
Always believe the meter indication and
stop the engine. Many people doubt the
meter and learn from the consequences.
Typical multiple gauge connection arrangement.
Temperature is another vital parameter that we always monitor. The proper
monitoring of water temperature is essential to the safe operation of your engine
as temperature extremes can cause serious damage. The meter transducer units
are resistive, and output is a resistance
that’s proportional to the temperature. The
main causes of high temperatures typically
include a faulty freshwater-pump impeller,
low engine-cooling water levels, and fouled
coolers. In addition, your meter might be
telling you of problems with the saltwater
(raw-water) cooling system.
The engine tachometer is another
essential meter and is indispensable for
monitoring engine speed. Observing this
information enables us to make informed
decisions on fuel consumption and boat
performance. There are several tachometer
types, based on the sensing system with
the meter. The generator tachometer inputs
a signal from a mechanically driven gen-
erator unit, which outputs an AC voltage
proportional in amplitude to the speed,
and this is then decoded by the tachom-
eter. Variations in speed give a proportional
change in output voltage, and therefore
a change in meter reading. The inductive
tachometer has an inductive magnetic sen-
sor that detects changes in magnetic flux as
the teeth on a flywheel move past the sen-
sor head. This transmits a series of on/off
pulses to the meter that are counted and
displayed as speed on the tachometer. The
alternator tachometer takes a pulse from
the DC charging alternator AC winding.
This signal is a frequency directly propor-
tional to the engine speed.
Monitoring fuel and water quantities
is essential with a simple electrical gauge.
Most tank sensors operate on the same
principle of varying a resistance proportional to the tank level volume.
The immersion pipe sensor consists of
a damping tube that has an internal float
that moves up and down along two wires.
These units are generally only suitable for