35
4.
Set North:
You must do this, if the autopilot is connected to the GPS.
To run a GPS course requires that the autopilot compass is in agreement with the GPS’s course estimate. You need to set
North with the pilot in standby mode. Do not set North except in calm sea’s and un-accelerated conditions. You should
be running at a reasonably high speed, it won’t hurt to be running on plane with your main engine. With your GPS turned
on and setup code 48 selected, press and hold the
GPS
(
Select Load
) button.
If the
Up Arrow
LED illuminates, this means that your GPS connection is not sending course information to the autopilot
correctly. You must point your boat north and then press and release the
Deckmount
switch to set compass north.
If the
Up Arrow
LED does not illuminate, press and release the
Deckmount
switch to set compass north. In this case you
do not need to be pointing north-you only need to be going in a straight line.
Your pilot will power down after north has been set.
Rudder and Counter Rudder Gain.
Let’s assume that you pushed the Auto button when the boat was heading 45 degrees and that after a
few seconds your boat has been turned, by wind, to a heading of 55 degrees. The autopilot (and you)
know that you are trying to steer to 45 degrees and that your current heading is in error by ten degrees to
the starboard. How much port rudder do you suspect is the right amount to steer to bring the boat back
to 45 degrees in a reasonable time? Loaded question! The amount of rudder required depends on many
variables like hull shape, boat loading, how fast the rudder can move, boat speed through the water, and
more. If we had numerical values for all the variables affecting the required rudder deflection, we could
calculate the required rudder gain and apply it with the rule: Rudder is applied in proportion to heading
error and the proportional constant is rudder gain.
Since we can’t make a very good guess at what value Rudder Gain should take, we have to adopt a cut
and try approach to finding the proper value. Fortunately, we can provide some guidelines as to how the
system behaves when the Rudder Gain is too large and when it is too small.
Counter rudder is rudder applied to oppose turning. It is used to “damp” heading changes and is more
responsive to disturbances in heading than the proportional rudder deflections caused by heading error.
Counter Rudder is applied in proportion to the rate of turn, as measured by the gyroscope, and the
proportionality constant is Counter Rudder Gain. We can provide some guidelines as to how the system
behaves when the Counter Rudder Gain is too large and when it is too small.
A good way to hunt for the proper Rudder and Counter Rudder Gains is by commanding the autopilot
to change heading and watch the heading response. The following plot shows how a boat responds in
heading when it is set up real well. At time = 0 the
Straight Right Arrow
button was rapidly pressed
and released five times, the boat heading was initially 0 degrees, in a few seconds the boat heading is
steady on 5 degrees. Tuning should be done with no more than a light breeze and riffles on the water.
Tuning should be done in forward gear.
