ComNav Commander P2 & P2VS Installation & Operation How Autopilots Work
Document PN 29010074 V4.1 - 22 -
Wind and Current Effects
If your boat encounters a crosswind, and/or a tidal current, at some angle to the desired
Heading, it will be pushed away – in the direction of the wind or current. And so, the autopilot
will be constantly making Heading corrections with the rudder, as it tries to maintain your
desired Heading.
As that condition persists, the SPU will gradually begin to maintain the rudder a few degrees
to Port or Starboard of the centred position, to counter the effect of wind and/or current. This
feature, known as
Automatic Rudder Trim
, will keep your boat pointed in the right direction,
despite the wind or current.
However, one side-effect of Auto-Trim is that your boat may not follow the
Track
you want – it
will maintain your desired Heading, but the Track will “slip” sideways in the direction of the
crosswind and/or current. This potential problem can be solved by using another mode that
ComNav Autopilots provide:
NAV
mode, described next.
Following a Track: NAV Mode
So far, we have seen how an autopilot operates in
AUTO
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 or when you know the direct course to your objective.
But for longer trips, which often involve a number of “legs” at different headings, it would be
useful to be able to have your autopilot steer your boat along all the legs, in sequence.
ComNav autopilots allow you to do just that, with
NAV
mode.
The autopilot must be connected to an external source of navigation information (commands
& data) – such as a chart plotter, or a Navigation program running on a PC. A source of
position data is required, too; it may be built into the external Navigation System, or it may be
another device or system: a GPS receiver (such as a ComNav Vector GPS Compass), a
LORAN C receiver, etc.
3
ComNav Autopilots implement the industry-standard NMEA 0183 protocol for reception &
transmission of navigation information. Almost all modern Navigation System equipment
complies with NMEA 0183, so interfacing to a ComNav autopilot is relatively simple.
A long passage will consist of a series of
waypoints
, which are specific locations on the
Earth’s surface (hopefully on the water!) defined by their Latitude & Longitude. These
waypoints define the
route
which you want the boat to travel on, from your staring position to
your destination. You enter them into the external Navigation System when creating the
route.
That system determines the current location of the boat (from the source of position data),
and then calculates the course that must be steered in order to reach the next waypoint.
When the Autopilot is switched into
NAV
mode, it begins to look for some specific navigation
information from the Navigation System: the course to steer to the next waypoint, and the
Cross-Track Error (which is the distance from the boat to the desired Track from the last
waypoint to the next one). Using these two pieces of information, the autopilot steers the
boat on the shortest possible Track to the next waypoint.
Crosswinds and/or currents are compensated for automatically each time the Navigation
System updates the Cross-Track Error. This is why
NAV
mode is the answer to the “track
slip” problem that can occur in
AUTO
mode, when a crosswind/current exists.
When a waypoint is reached, the Navigation System sends the SPU the next set of
navigation information – for the next waypoint – and the SPU then turns the boat to head to
that next waypoint.
3
… all such external devices are in general referred to in this manual using the generic term Navigation System.
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