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4.2.2.1.4 Plan track
From the data type “Ship data”, it is primarily the dimensions of the ship (length, width, draught) which
are required for track planning. Furthermore, this datum contains information on the sensor technology
available to execute the voyage plan (particularly the accuracy of the sensors used). In general it should
be borne in mind that not all elements of a composite data type are necessarily used by the particular
process.
In the process “Stipulating the track limits” (DFD 1.1.4.2), it is possible to distinguish two sub-areas de
limited in relation to one another. The result of the process “Determine maximum transverse deviations”
(1.1.4.2.1) constitutes the limits which are taken into account within the frame of course control (i.e.
‘Track control”). The maximum longitudinal deviation (1.1.4.2.2) primarily serves as a limit for control
ling speed.
As becomes clear on comparing DFD 1.1.4.3 with DFD 1.1.3, it is also necessary to make a distinction
in the case of the way point list between a “track way point list” and a “route way point list” - in analogous
manner to distinguishing the states “route" and “track” in the voyage plan data. The process 1.1.4.3.1
“Stipulate track way point list" comprises the functions “generate, modify and delete way points of a
track”. The stipulation of the rudder setting points (process 1.1.4.3.3) likewise comprises the determina
tion of early warning positions “Wheel over lines”, “Forwarding distances”) before reaching the rudder
setting point. For the purposes of this, appropriate early warning times and distances must be taken
into account.
When considering DFD 1.1.4.4 it should be borne in mind that the maximum rudder position determined
here actually represents a physical limitation of the maximum rudder angle to the value set here. This
value is generally set in fixed manner in the auto pilot by the user. Only by manual control it is possible
to exceed this value. In contrast to this, the limit value ROT/turning radius does not represent a “true”
physical limitation, as a result of the inertia of the ship system. The controller will instead trim when the
value is exceeded. At present, there is freedom of choice with regard to determination of the transition
behaviour at course changing points. In principle, both control via the turning behaviour (ROT) and also
position-assisted control via the turning radius to be followed are conceivable. As a rule, the user also
has both alternatives available in track planning. Internally, control is generally effected by means of the
ROT, since errors in the position determination processes (eg static fluctuations when using GPS) may
have serious consequences on the behaviour of the ship in the event of changes in course. There are
however highly accurate track control systems with radius control in the course of development. The
current value for ROT considered by the auto pilot is mostly calculated internally - even when it is avail
able in the system.
With regard to the correction information flowing into the planning process, it should be noted that this
must relate to the current voyage plan data. In the event of simultaneous modification of way points and
other voyage plan data (eg manoeuvring limits), care should therefore be taken to ensure consistency.