Seiwa SW AP03 - Manual (Page 8)

The third option may be used on yachts fitted with a compatible wind instrument. In this
case, the reference course adjusts itself to maintain a constant apparent angle to the wind.
1.1.2 STEERING CONTROL
When the vessel swings off course or the reference course is changed, the autopilot should
apply helm in a way which brings the vessel onto course quickly, but without overshooting
the reference course. The correct rudder angle depends on the amount of the error, the
speed of the vessel, its size and the effectiveness of its rudder.
Automatic Tuning
In an auto-tuning autopilot, such as the SW AP system, this choice of the appropriate
rudder correction is made automatically. The autopilot uses data about the type of vessel,
which is entered during the set-up operation. Then, as the vessel travels, the autopilot
continuously monitors the accuracy of course holding and the level of rudder activity. It then
chooses internal settings which achieve the best compromise between these two measures
of performance. These is also an option to operate in a manual tuning mode, in which case,
the following comments apply.
Rudder Factor (Manual)
The sensitivity or RUDDER FACTOR sets how many degrees of helm are applied for a given
course error. A mid-range rudder factor setting applies half a degree of helm for each
degree off course. In large or slow vessels it would be more and in light, fast boats it may be
less.
Setting the rudder factor too high causes oversteering or ‘snaking’ as illustrated in Fig 1.2.
Too low a setting causes understeer and a sluggish response. Fortunately, most vessels
tolerate a range of settings and still steer well.
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