in darkness, a randomly selected reduction factor
Update time and
between 0.5 and 1.0 is applied to the ship's speed.
distance traveled (step 6)
This speed reduction factor only applies if the ship
Once ship speed has been established for the
is not under escort, and it is the largest of all those
segment at hand, the length of the segment in time
affecting visibility. If it isn't dark, then the slowing
and distance can be calculated. Segment length is
factor is set to 1.0.
determined by two parameters in addition to ship
speed: a) time in decimal hours remaining until
Adjust speed for
the next sunrise (during darkness) or sunset (dur-
ocean currents (step 5)
ing daylight), and b) distance in nautical miles from
Two types of currents contribute to ship speed:
the ship's position at the beginning of the segment
a) currents induced by wind in either the marine
to the next data or decision node.
surface layer or, in the case of ice-covered seas, the
The algorithm first checks to see if the time re-
pack ice cover itself, and b) permanent marine cur-
maining to the next sunrise or sunset is less than 8
rents that arise from local- and basin-scale pat-
hours. If it is, then the time-length of the segment
terns of ocean circulation. Currents, in effect, rep-
is set to the time remaining to the next sunrise/
resent a bias that is present in the medium through
sunset; otherwise time length is set to 8 hours.
which the ship moves. That is to say that under-
Next the distance traveled is calculated by multi-
lying currents move the water and ice through
plying ship speed by the segment time that has
which the ship sails regardless of ice characteris-
just been established. This distance is compared
tics and visibility conditions. Effects of currents
with the distance from the ship position at the
are superimposed on and largely independent of
start of the segment to the next data point or deci-
all other factors that influence speed. The contri-
sion node. If distance falls short of the next point,
bution they make to ship speed therefore is added
then the calculated time and distance are accepted.
at the end of the algorithm after all other speed-
If the ship overshoots the point or node (that is, if
related factors are accounted for.
the calculated distance traveled is greater than the
Wind-induced currents. Wind-induced current is
distance to the next point), then the distance is
calculated in subroutine ADDWCUR by multiply-
reset to the distance to the next node, time is ad-
ing wind speed by a factor of 0.025. Winds used
justed downward, and calculations for the seg-
are those selected in Step 2 using the MC algo-
ment are complete.
rithm in subroutine WINDS. The magnitude of
the wind-induced current is assumed to be inde-
Save segment statistics (step 7)
pendent of ice conditions. Under ice-free condi-
With all parameters set, they are saved to a data
tions, winds induce a current that moves parallel
structure such that summary statistics can be cal-
to the wind direction in the surface mixed layer of
culated at the end of each voyage and when the
the open ocean. For ice-covered seas, the wind is
entire transit simulation is complete. If detailed
assumed to push the pack in the direction the wind
output of segment parameters is requested, val-
is blowing. Ship speed is adjusted for wind-in-
ues assigned to virtually all variables are written
duced current by calculating the component of the
to the print file at this point in the algorithm. Fi-
wind vector that acts in the direction of ship mo-
nally, a check is made to determine whether the
tion and adding it to the ship speed (or subtract-
transit end point has been reached. If not, the logic
ing it in cases of head winds).
checks to see if a data node has been reached, and
Permanent currents. Permanent ocean currents
control in the loop ultimately transfers back to step
are assumed to remain constant with regard to
1 and the procedure repeats.
both speed and direction throughout the year.
Fixed values assigned to each data and node point
Running the model
are read from the PCRRNT.DAT file (Table B.8) in
The program is invoked by entering the name
subroutine ADDCCUR. Ship speed is adjusted for
of the executable program file (NSR) followed by
permanent current by calculating the component
a series of command line arguments at the DOS
of the current vector that acts in the direction of
prompt. Individual command line arguments are
ship motion and adding it to the ship speed. This
separated by spaces and include variables that
vector is recalculated at each data node and each
specify the month in which simulated transits are
time the ship direction changes.
to occur, the type of ship making the voyage, the
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