An autopilot is possibly one of the most advanced and technically sophisticated instruments you can have in your boat. It has all the intelligence needed to automatically steer your boat once you have told it the direction to steer or where you want to go. Once installed it’s quite easy to use it, but it is important to understand how it functions and why it behaves in the way it does This ensures you get the maximum benefit from the equipement and knowing your equipment is important for safety at sea. All autopilots on the market work in much the same way so what’s explained below is more or less the same for any brand of autopilots.
When the boat turns off course or the reference course is changed,the autopilot should correct the helm in a way which brings the boat quickly back onto course 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 adapt it to the surroundings there are control settings that need to be tuned. Normally you don’t need to modify these settings, the factory settings are usually good for most types of boats and weather conditions. If you need to optimise the settings it’s important to know what you are doing.
The two most important
settings are Sensitivity and Rudder factor.
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 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 to
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. 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 on a calm sea.
Modes of Operation (with reference to the TMQ range)
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.
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 mounted 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 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.
