Gear
J
ust about every
engine installed in a
yacht or power boat
has some degree
of instrumentation,
ranging from the basic rev
counter, warning lights and
audible alarm (as in this
project) to a full suite of
dials and displays.
Modern chart plotters and
marine instruments, in addition to
displaying the chart, radar and
many other functions, also offer a
comprehensive set of data pages
to display information for one or
more engines, transmissions and
electrical systems. Here, we take a
Moody 31 and its Volvo Penta 2003
engine and add a host of engine
and electrical data to display via
the NMEA2000 network.
Three stages of conversion
are involved, beginning with the
sensors themselves – referred to
as senders – which will convert the
parameter being measured into an
electrical signal. The second and
third stages, accomplished by the
data converter unit, are to convert
the electrical signal into a digital
number (digitisation) and arrange
it into a format suitable for
transmission onto the NMEA2000
network. Some larger engines
use the automotive J1939
network, which would then
have to be converted into the
NMEA2000 format to display
on the chart plotter.
The most common senders are
resistive, such that their resistance
changes with the parameter
being measured, or pulse-type
senders where the frequency of the
pulses changes with the speed
being monitored. The coolant
temperature, oil pressure and fuel
level senders used here are all
Diesel data on screen
Andy Johnson reveals how you can add engine, transmission and
electrical system data pages via the NMEA2000 network by linking
three different data converters to the chart plotter on his Moody 31
changing the resistance in small
steps as the float moves up and
down. Once the flange is fitted,
the actual sender simply screws
in – providing there is enough
height above the tank to poke
it through (check this first!).
Measuring current presents more
of a challenge. It would be useful
to display total charging current
and current consumption from the
domestic batteries. Current can be
measured by a ‘resistive current
shunt’, which has a very small
fixed value of resistance, creating
a voltage across its terminals
proportional to the current flowing
through them (remember Ohm’s
Law?). Alternatively, a Hall effect
sensor measures the magnetic
field generated by the flow of
current in the cable (passing
through its aperture), producing
a small voltage proportional to it.
This type of sensor also needs a
12V supply. Both are 60A rated,
as they would be damaged if
subjected to prolonged currents
greater than their rating. They
give 1mV/A output, and this is
increased by the use of differential
amplifiers to multiply this verysmall signal by 50,
increasing it to
50mV/A.
Measuring
battery voltage
only requires
a wire from the
battery terminal
to the data
converter
channel input, or is
measured internally
on some units.
To minimise
electrical
interference, a
new 0V bus was installed for
all the instrumentation, connected
directly back to the domestic
battery negative terminal and
screened multi-core cable
was used for the sender
signal connections.
The data converter itself is an
electronic unit with configurable
input channels, taking in the
signals from the resistive and
pulse-type sensors. The resultant
numbers, representing the
values of the parameters being
measured, are then assembled
into the correct format and sent
to the display via the NMEA2000
data network.
Data converters either
piggyback off existing sensors and
instruments, picking up the sensor
outputs at their terminals, or new
sensors are fitted and wired to the
data converter inputs, or a mixture
of both.ABOUT THE AUTHOR
Electronics engineer Andy Johnson has focused in
recent years on teaching RYA courses and own-boat
training in navigation, radar, boat electrics and
electronic systems. He and wife Sue sail their
Moody 31 wherever the weather and time allow.resistive. Measuring current is
achieved using either a resistive
device called a current shunt or a
Hall effect sensor (which measures
current in a different way). The
alternator itself provides the pulse
output for engine revs so no
additional sender is required.Selecting the senders
The temperature and pressure
senders need to fit in the same
locations as the original senders,
so they need the correct thread.
The temperature sender should
measure from about 25°C to above
the alarm point (95°C). The oil
pressure sender must cope with
the highest possible pressure for
a cold engine at start-up (7 Bar,
100psi in this case).
There are many tank level
senders on the market: the one
chosen was a simple tube type,Oil pressure and coolant
temperature senders with
integral alarm switchesCables from the alternator and
other sources of charging are
passed through the Hall effect
sensor aperture