ComNav Commander P2 & P2VS Installation & Operation Basic Operations
Document PN 29010074 V4.1 - 137 -
An example of NAV Mode in Operation
Here’s an example of
NAV
mode in operation, to help make the real-world effects of all that
math clearer!
Let’s say that your boat is running on a waypoint-to-waypoint leg which has a charted bearing
of 120° True, and the autopilot is using degrees True Heading data from an NMEA source.
You’ve selected XTE Correction in the NAV menu, and set up the Navigation System to
output Bearing Origin To Destination data in degrees True, and XTE data (in NM), with both
in the NMEA APB sentence.
When you departed the first waypoint, you were right on the Track between the waypoints,
and so the data in the APB sentence was BOD = 120° True, & XTE = 0.0 NM. Thus, the
autopilot calculated a Commanded Heading of 120° True, and that’s also the boat’s actual
Heading.
That’s good … you’re going straight to the next waypoint … you can relax!
Ooops … where did that cross-current come from? Seemingly all of a sudden, you’re a tenth
of a mile off-track to Port, although the boat’s actual Heading is still ~120° …
No worries! The autopilot will handle it:
•
BOD is still 120° T, but XTE is now 0.010 NM
… so the Commanded Heading is now 120° + (0.010 NM x 400°/NM) = 124°, and the
actual Heading is something close to 124°
A few minutes later, you’re a little more than half-way back to the Track:
•
BOD is still 120°, XTE is now 0.004 NM, so Commanded Heading is now 121.6° (and
the actual Heading is close to that)
And after a few more minutes, you’re back on-Track, with both Headings holding at ~120°.
Some other points about
NAV
mode to keep in mind:
•
The XTE value the autopilot uses in its correction calculations is always in Nautical
Miles, and has a precision of 3 decimal places (0.001 NM).
When XTE is 0.01 NM or less, the full 3-digit precision is used. But, when XTS is
greater than 0.01 NM, the value is rounded to 2 decimal places (0.01 NM), before the
correction calculations are done. The reason for that is to provide smoother & more
efficient steering when XTE is “large”.
To explain: there’s really no point to making many fine adjustments to Commanded
Heading when the boat is a long way off-track. For example, if XTE above were to
change from 0.024 NM down to 0.015 NM, the Commanded Heading would stay at
128°, rather than going in 0.4° steps from 129.6° down to 126°. But … the total time
to come back on-track would be about the same.
And it’s easier on the rudder & drive mechanism too. A few larger changes cause
less wear & tear than many small ones – and also use less power!
•
Very small changes in, or values of, the XTE value may not always cause a change
of Heading - depending on the Sea State parameter value (see page 126).
Example: if Sea State is 1, a change in XTE of 0.001 NM yields a change of Heading
of 0.4° - but that’s less than the 0.5° of Yaw that is allowable for Sea State 1.
On the other hand, if very small XTE values exist for a time, what will happen instead
is that the autopilot’s Automatic Rudder Trim will gradually build up a “trim” which
does
have the same effect as changing the Commanded Heading.
In the example above, a steady XTE of 0.001 would cause the rudder to wind up a bit
off-centre to Starboard – keeping the boat “crabbing” a bit into that cross-current from
Starboard, so that you stay right on the Track that you want.
To Next Page
Loading...
