How does an autopilot work

An autopilot is maybe one of the most advanced and technically sophisticated instruments you can have in your boat. It does have all the intelligence needed to automatically steer your boat ones you have told it the direction to steer or where you want to go. Ones installed it’s quiete easy to use it but it can be good to understand how it’s functioning and why it behaves in the way it does if the situation becomes difficult. Knowing your equipment is important for safety at sea. All autopilot on the market works in much the same way so what’s explained below is more or less the same for all brands of autopilots.

When the boat turns off course or the reference course is changed, the autopilot should apply helm in a way which brings the boat back to course fast, but without overshooting the reference course. The correct rudder angle depends on the amount of the error, the speed of the boat, its size and the effectiveness of its rudder. In order to customize the autopilot and adopt it to the surroundings there are control settings needed to be tuned. Normally you don’t need to modify these settings, the factory settings is usually good for most types of boats and weather conditions, but if you need it’s good to know what you are doing.
The most important of these settings are Sensitivity and Rudder factor. They are explained below:

Sensitivity (Yaw, Weather etc)

The main task of an autopilot is to lock the heading of the boat to a predefined reference course. This is the course the boat has when you press the auto button. In the auto mode the autopilot constantly compares the boats true heading given by the compass with the wished reference course in the same way that any control system does. If there is deviation, the autopilot applies power to the steering system and the rudder turns the boat and get it back on track. The rudder angle is feed back to the system. There is a trade off between the accuracy of holding the reference course and the activity of the rudder movements. The more accurate you want to hold the course the more the drive unit has to work. Changing the Sensitivity parameter lets the user calibrate and fine tune the autopilot to a balance between these two factors and adapt to changing circumstances during cruising such as changing weather conditions, heavy weight etc.

The sensitivity control governs the sensitivity and is used to determine the amount of “wander” in the boat steering. A
high value for the sensitivity will allow the vessel to drift off course before correcting. A low value for sensitivity will attempt to keep the vessel more precisely on course. If you want to change the factory setting of this parameter set the lowest sensitivity value that you can, without having the steering motor continuously “hunting” from side to side. This minimum setting depends upon the amount of slack in your steering, vibration around the rudder feedback mounting position, characteristics of the steering drive system and the weather conditions. The sensitivity setting is remembered when the unit is turned off and on. Lower numbers are most sensitive and will give a straighter course steered. Do not set this so low that your steering equipment is continually working or “hunting”, as this will cause premature wear on your system and, in severe cases, may cause malfunction. In heavy weather you can increase the value to let the boat follow the waves without correcting the course all the time.

Rudder Factor

The Rudder Factor is a tool to adapt the autopilot to the steering system of the boat. Depending on the rudder size etc. the amount of force needed to turn the boat can be adjusted using this parameter if you like. This function makes it possible to tune the amplifier to the rudder angle and how many degrees of helm that are applied for a given course error. In large or slow boats it would be more and in light, fast boats it may be less. Setting the rudder factor too high causes over steering. Too low a setting causes under-steer and a slow response.

In the picture to the right it’s shown what happens when the rudder factor is to high (at the top) and when it’s to low (at the bottom). The movements of the boat is of course exaggerated for illustration purpose only. In real life and when correctly tuned the boat should move in a straight line. The adjustment of this parameter should be done at calm sea.

Modes of Operation

An autopilot can work in one of several modes dependent on what you want to do with it. At start up it will always go in to manual or stand-by mode. The different modes are explained below:

Manual Mode, the autopilot is standby and the boat is under manual steering. The display shows the actual heading
course.

Auto Mode, the autopilot is activated and steers the boat to the reference course which is then shown on the display. The refence course will be the course the boat has when auto mode is selected. You can change this desired reference course by pressing the arrow buttons on AP47 and AP55 or by rotating the course knob on AP4 and AP9.

GPS Mode. In this mode the autopilot gets steering information from a separate GPS unit. All Autopilots with GPS connected for navigation can follow a track to a distant bearing and can follow a route of multiple way points and automatically turn to a new reference course when switching to next way point. In the picture at right you can see how it looks on a GPS plotter. Note that the boat will not cross the waypoint, when it’s close enough to the waypoint the autopilot will automatically turn to the next waypoint.

The GPS Navigator must be compatible with standard NMEA 0183 and set to automatically send BOD (Bearing Origin to Destination) to the autopilot. This is not a message of its own but included in the APB message. The bearing is the course from the previous waypoint (n) to the next waypoint (n +1). When the boat is approaching a waypoint the GPS device will send the new bearing to the next waypoint in the route. The autopilot turns the boat asymptotically towards the new destination, and will then try to hold the course to the new waypoint. A GPS that handles a route plan will send information on the current deviation in meters from the desired track to the next waypoint. This data is called the cross track error XTE. The autopilot that receives this information is constantly working to minimize this deviation from the track.

Pictured above right you can see what it looks like on a GPS plotter. Note that the boat will not cross the waypoint, when the target is close enough the GPS will send the new destination BOD to the autopilot.

Many have misunderstood how the autopilot works when connected to a GPS. It is believed that the autopilot will take all information from the GPS unit and no longer needs the compass. This is not true. As described above, the GPS unit sends two messages to the autopilot, BOD and XTE. The current course the autopilot will take from the compass. It is possible to get the course information from the GPS device as well but it is far too slow. It just sends out the course at the most once per second in comparison with the autopilot compass that sends course information 10 times per second.

Power Steering Mode. In this mode the boats helm system is controlled by the helmsman with buttons on the
Display unit. Since the autopilot is in command of the helm system through its drive unit, this is a convenient way
to steer the boat and to get power steering. This can also be done by a remote device on a cable or a second
steering station fixed mounted while away from the main steering wheel. Since the autopilot controls a power
steering system, options are available to use this to steer the vessel by hand while away from the main wheel. This
can be done by a hand-held device on a cable or a permanently mounted second steering station.

Wind Mode. Some autopilots can operate with a wind sensor as input. The behaviour of the autopilot is somewhat different in this case. Instead of locking to a certain reference course it will in this mode lock to certain angle to the wind. When the boat tacks, the new heading will be the same relative wind angle on the opposite side of the boat. The Wind mode is primarily intended for long sailing cruises at open sea when it is comfortable to steer after the wind. When the wind changes the autopilot adapts to a new course where the relative angle to the wind is maintained. Requires a wind sensor that can send information using NMEA messages.