If you are the captain of a supertanker sailing down the English Channel, any decision
you take now about changing the direction of the ship will not happen for about 15
minutes, because it is so heavy. Thus the information relevant to you is that required
for forecasting and where you have been, and where you are now, are only important
in knowing where you will be.
In practice, since the ship is so heavy it is not easily manoeuvrable and the decision
on the course will be have to be made many hours in advance and the necessary information
is required then. You will agree this with the Navigating Officer and he/she can
give the helmsman the necessary commands. Since the decision on the course is made
well before a change in direction, there is no need to haul the Navigating Officer
out of bed to make it. A few hours will make no difference – assuming you have recognised
the need to plan!
Contrast this with the situation that a ship has altered course and could collide
with you in 10 minutes. You would wish to know of this risk immediately in order
to take appropriate action, such as telling the other ship to change course.
In both the above situations it is the helmsman who needs the information promptly!
However, you don’t wish to know the position of all ships in the area, that job is
delegated to the Navigating Officer. The information relevant to you is about those
ships which could be a risk and where you have to make a decision.
If the Navigating Officer told you not to worry, as the ship on a collision course
would miss your ship by 38.236m, you might be a bit suspicious. How could he possibly
calculate the figure that accurately, given the uncertainties in the speed and direction
of the other ship? You would be right in suspecting that he really didn’t understand
the figure he was providing and the uncertainties inherent in the calculation.
However, you do want to know about the risk of collision. What’s the most accurate
information? Your own course and speed. Knowing this, and the position of the other
ship, which can be determined with reasonable accuracy from radar, you can determine
the range of speeds and directions for which the ships could collide. If the course
and speed of the other ship lies in that range, you had better give the captain of
that ship a call.