Sitex SP38 Autopilot Installation And Operation Manual Download

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How Autopilots Work

Sitex SP38 Installation and Operation

Document PN

29010101

V2r0

Wind and Current Effects

If your vessel encounters a crosswind, and/or a tidal current at any given angle to the
desired Heading, it will be pushed away, in the direction of the wind or current. At this point,
the autopilot will constantly be making Heading corrections with the rudder, as it tries to
maintain your desired Heading.
If that condition persists, the SPU will gradually begin to alter the rudder a few degrees to

Port or Starboard of the centred position, to counter the effect of the 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 may “slip” sideways in the direction of

the crosswind and/or current. This potential problem can be solved by using another mode

that

Sitex

Autopilots provide, which is the NAV

mode.

Following a Track: NAV Mode

So far, we have seen how an autopilot operates in AUTO

mode, whereas a new desired

Heading must be selected on the Control Head each time you wish to change direction.
This is useful for short trips or when you know the direct course to your destination.
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.

Sitex

autopilots allow you to do just that, with NAV

mode.

The autopilot must be connected to an external source of navigation information (commands

and data), such as a chart plotter, or a Navigation program running on a PC. A source of

position data is also required, it may be built into the external Navigation System, or it may be

another device or system, like a GPS receiver (such as a

Sitex

GPS Compass), a LORAN C

receiver, etc. (all such external devices are in general referred to in this manual using the

generic term

Navigation System

Sitex

Autopilots implement the industry-standard NMEA 0183 protocol for reception and

transmission of navigation information. Almost all modern Navigation System
equipment complies with NMEA 0183, so interfacing with a

Sitex

autopilot is

relatively simple.
A long voyage will consist of a series of waypoints, which are specific locations on the
Earth’s surface, as defined by their Latitude and Longitude. These waypoints define the
route 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.
The system determines the current location of the boat (from the position data source),
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.