ComNav 1101 Autopilot2 - 4 HOW AUTOPILOTS WORK
Following a Track With an Autopilot
So far, we have seen how an Autopilot operates in AUTOPILOT mode.
In this mode, a new desired heading must be selected on the Control
Head each time you wish to change direction. This is handy for short
trips, when a boat is working or when you can see your objective. For
longer trips, which might involve a number of legs, it would be useful
to be able to pre-program your Autopilot to follow a given track. All
ComNav Autopilots will allow you to do just that.
Before using this feature, the Autopilot must be connected to an exter-
nal source of navigation data such as a LORAN C receiver, a chart
plotter or a GPS receiver. Fortunately, most of the hundreds of com-
mercially available navigation devices transmit data in the NMEA 0183
format, so interfacing a navigation device to the ComNav Autopilot is
relatively simple.
A long passage will consist of a series of waypoints which are discrete
locations on the water’s surface defined by their latitude and longi-
tude. These waypoints are entered into the navigation device. The
navigation device is able to determine the current location of the boat
and calculate a course to steer to reach the next waypoint.
Now the Autopilot is switched into NAV mode. The Processor reads
the information from the navigation device, which includes a course to
steer to the next waypoint and the crosstrack error, which is the dis-
tance (in nautical miles), from the boat to the desired track. Using
these two pieces of information, the Processor steers the shortest
possible course to the next waypoint. Tides, crosswinds or currents
are compensated for automatically each time the navigation device
updates the crosstrack error.
As the boat nears a waypoint, the Autopilot will alert the user by acti-
vating an alarm. When a waypoint is reached, the Processor will turn
the boat to head to the next waypoint. If the last waypoint is reached,
the boat will circle the waypoint until the Autopilot either is switched
out of NAV mode or given new instructions. Because the boat will turn
automatically and without input from the user, when planning the trip it
is crucial that the user verify that there will be sufficient room to ex-
ecute the turn at each waypoint.
Power Steer
Some Autopilots have a POWER STEER mode which allows Full Follow
Up steering. Control is usually through two buttons on the Autopilot.
One button will move the rudder to starboard, the other button will
move it to port. Using the buttons, the rudder can be placed in any
desired position where it will remain. This feature is useful when dock-
ing or maneuvering because the person steering the vessel can take
their hands off the controls between rudder movements.
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