Page 217 - NAVAL SCIENCE 3 TEXTBOOK

RU LE S OF THE ROA D AN D ,\I AN EUV ER I N G BOA RD 223

""''' MANEU V ER ING BOARD SCALES
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Reference ship's course and speed vector plotted on a maneuvering board for a course of 125°T and a speed of 15 knots.
The CPA Problem To find the CPA of a vessel approaching your own vessel, it is
necessary to construct its relative track on the maneuvering board.
As stated in the beginning of this unit, a major concern of those
To do this, several ranges and bearings to it, usually taken three
in charge of a vessel is to avoid hitting other vessels or things in
minutes apart over a period of several minutes, are plotted on the
the water. To assist in this endeavor, it is very helpful to be able to
maneuvering board and labeled lVI" 1H , and so on (for maneuver-
determine their projected closest points of approach (CPAs) and z
ing vessel). The resulting points. when connected with an extended
their true courses and speeds. Once this information has been
line, define the relative 11IOtiOll lille (RNft) of the approaching ves-
determined, appropriate actions can be taken if required to pre-
sel relative to your own.
vent a hazardous situation from developing or to comply with the
The CPA to your vessel will be the smallest distance from the
rules of the road. The maneuvering board can be used both to find
pole, where your vessel is, to the extended Rl\'lL line. Since by
the other vessels' ePAs and courses and speeds and, if required, to
geometry we know that the smallest distance between a point and
determine the course and speed changes that could be made by
a line is the perpendicular distance between them, it is only nec-
your own vessel to open the ePAs to suitable safe distances.
essary to pick off the perpendicular distance on the plot with the