Simrad NSS12, Technical Manual

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Autopilot | NSS Installation Manual
Layline steering
Layline steering is useful when navigating. Cross Track Error (XTE) from the navigator will keep
the boat on the track line. If the XTE from the navigator exceeds 0.15 nm, the autopilot will
calculate the layline and track towards the waypoint.
Manually adjusting steering parameters
The autotune function in the autopilot is so refined that the majority of boats will need no
further adjustments of the steering parameters. On some boats however, or in particular
sea conditions, fine tuning of the steering parameters may improve the performance of the
autopilot.
Transition speed
Refer previous description.
Rudder
This parameter determines the ratio between commanded rudder and the heading error. The
higher rudder value the more rudder is applied.
If the value is too small it will take a long time to compensate for a heading error, and the
autopilot will fail to keep a steady course.
If the value is set too high the overshoot will increase and the steering will be unstable.
Counter rudder
Counter rudder is the amount of rudder used to try to prevent the boat from yawing around
the set course. Higher counter rudder settings result in more rudder being applied.
The best way of checking the value of the Counter rudder setting is when making turns.
The figures illustrate the effects of various Counter Rudder settings;
A: Counter rudder too low; overshoot response
B: Counter rudder too high; sluggish and creeping response
C: Correct setting or counter rudder; ideal response
A B C
Auto trim
This parameter defines how fast the autopilot shall correspond after having registered a
heading error.
The standard value is 40 seconds which should work well on most boats. Rule of thumb: Set
to same value (seconds) as the boat’s length in feet. On boats operating on VRF the value
should be set to 20 seconds.
Rate limit
Sets the maximum allowed rate of turn.
The value should be kept at 6.0°/second unless there is a need for more rapid response in
turns.
Minimum rudder
This parameter filters small rudder commands to prevent high rudder activity.
Some boats may have a tendency to not respond to small rudder commands around the
“course keeping” position because of a small rudder, a rudder deadband, whirls/disturbance of
the water-stream passing the rudder or it is a single nozzle water jet boat.