Auto Pilot considerations

Most modern modern auto pilots can be operated in  3  different steering modes:

  1. Compass mode (aka auto mode).
  2. GPS mode (aka track mode)
  3. Wind mode (aka vane mode)

Input data necessary for correct operation is received from the following directly connected sensors:

  1. Fluxgate compass
  2. Gyro
  3. Rudder angle sensor

In addition wind angle data from a wind instrument is required for wind (vane) mode. Waypoint  data such as bearing and distance to waypoint and cross-track error is required for track mode. For all steering modes, boat speed or SOG (speed over ground from a GPS) are required for best performance of the auto pilot, as course corrections are speed-dependent.

The simplest and most often used steering mode on most vessels is "Compass" or Auto mode for steering a set course.  In this mode, there is no compensation for drift and current.

In track or GPS mode, the vessel is steered along a pre-determined track to a waypoint. Multiple waypoints may be defined to form a route. This is the preferred steering mode if fixed obstacles have to be avoided. This mode usually requires a chart plotter, computer or GPS with a graphic interface.

If the auto pilot is connected in an instrument network (e.g. Seatalk or Navnet), instrument and GPS data is usually available via the network. If there is also a chart plotter in  the network, the necessary data to steer to a waypoint or along a route in track mode can be generated and is also sent to the auto pilot through the network.

The majority of auto pilots have an NMEA 0183 input port for several reasons:

  1. To make the AP product suitable for installations without the manufacturer specific  proprietry network and allow all instrument and GPS data to be input from NMEA 0183 sources.
  2. To allow input for GPS (track) mode from an alternative source such as computer, in spite of being connected in a network.



If all auto pilot input is through the NMEA port, the data stream has to include NMEA sentences from different  sources. This is no problem if the input is  received from a chart plotter or computer that has all data available, but hard wiring the AP to the chart plotter or computer output is often not possible for the following reasons.  

  1. The chartplotter output baudrate is 38400, due to setting of the input to 38400 bps for support of AIS. The auto pilot requires 4800 bps input. Some chart plotters have more than one output port, but in many cases there is only a single NMEA 0183 port with the same baudrate for in- and output.
  2. The chart plotter or computer is not always switched on. On sailing vessels  it is often required to steer in wind (vane) mode with the chartplotter or computer switched off to save battery power. This means that direct input from the wind instrument is necessary.  In all cases the AP needs speed input from a speed/log instrument and/or SOG from a GPS, also if only Compass (auto) mode is used.

If the auto pilot is connected through a Brookhouse multiplexer model AIS or AISC, solutions are offered in the form of::

  1. Baudrate conversion
  2. Data stream switching

Please follow the links for a detailed explanation.

Auto pilot NMEA 0183 compatibility

When a new chart plotter is installed on a vessel with an existing auto pilot, the problem of NMEA 0183 incompatibility often surfaces. The new chart plotter outputs the latest NMEA version while the auto pilot may only support an older version. NMEA incompatibility would be a bad reason for replacing  the often high quality auto pilot, which can still offer many years of reliable service.

A short script loaded in a Brookhouse multiplexer can convert the chart plotter NMEA output sentences to the older version. This situation is often encountered with B&G auto pilots that support NMEA version 1.5.

Scripts for NMEA sentence conversion are available from Brookhouse free of charge.