Unit 1
Navigation
1.1. Safety
The safety of passengers and crew, ship and cargo is determined by both internal and
external factors, but most of all by the Human Factor. The internal factors (IF) that
may influence safety are: the ship's dimensions and draft, her rudder, propulsion
system and navigational instruments. The external factors (EF) are formed by
weather conditions, currents and tides, characteristics of fairways and unforeseen
events. In documents the term "Act of God" indicates that the shipowner will not be
held responsible for damage caused by natural disasters. Good seamanship refers to
The Human Factor (HF) and depends on the quality of the seafarers on board. And
because most vessels have been manned with multi-lingual crews, special attention
should be paid to the introduction of the internationally standardised form of maritime
communication on every ship in the world.
1.2 Heading, course, track and drift
By heading is understood the direction in which the vessel is pointing. It is the angle between
the fore-and-aft line of the vessel and True North, expressed in degrees.
Heading constantly changes due to sea and
wind influences and steering errors.
By course is understood the intended sailing
direction of the vessel. In other words: it is the
direction in which the vessel is steered in order
to reach her destination.
It is expressed in degrees.
A track consists of one, or a number o f course
lines along which the navigator intends to
proceed for a safe passage, e.g. when clearing
a danger.
A great circle course will form the shortest
connection between two places on the earth.
A Rhumb Line, or loxodrome, will form a line
whereby all the angles made by the course line
and the meridians are equal. This implies that
in (Mercator) sea charts the earth is not a
sphere, but a square.
Composite sailing combines the advantages
of the great circle and the rhumb line: it will offer the shortest route, while the vessel can keep
constant true directions. Due to the influence of the wind and current the vessel’s destination will
not be reached without any alterations of course.
By course made good is understood the course that the ship will follow after allowing for the
effects caused by wind. Now the effect of current on the vessel’s path must also be corrected,
resulting in a course over ground that will lead to her destination.
Position
The vessel’s position can be determined in a number
of ways, for example by means of pilotage (coastal
navigation), cross bearings, a cocked hat, a running
fix, dead reckoning, astronomical navigation or
satellite.
The vessel’s position may be indicated by a
latitude/longitude – reference, or as a bearing and
distance from a fixed point. In VHF – communication
and written reports the letter position indication must
always be preceded by the word “bearing”.
Positions are expressed in degrees and minutes.
Distances are indicated by nautical miles (1852
meters) and cables (one tenth of a nautical mile –
185.2 meters).
Position indicated as a bearing and distance
from a fixed point, and in Latitude/Longitude.
1.2.2. Pilotage (coastal navigation)
When sailing along the coast, compass bearings of conspicuous objects are taken at regular
intervals.
A conspicuous object (conspic) is an object on land or at sea that is mentioned and described in the
pilot book. Cross bearings form an intersection of two bearing lines that have been taken of two
conspicuous objects.
If a third conspic is available, a third bearing (“check line”) can be taken.
Because the vessel is proceeding, the two bearing lines and the check line will form a triangle,
called cocked hat.
The ship’s position is in or at the cocked hat.
Cross bearings with cocked hat
1.2.3. Running fix
A “running fix” is made when there is only one conspicuous object available.
It is done by taking two bearings of the same conspic at interval.
(The angle between the two bearing lines must be > than 30 degrees).
Example
The first bearing is taken at 16.50 hrs.
Log reading: distance travelled = 380 nautical
miles.
The second bearing is taken at 16.55 hrs.
Log-reading: distance travelled = 381 nautical
miles.
The difference between the first log-reading and
second log-reading is 1 mile.
Now, with the aid of chart dividers, “1” is measured
from the chart scale at the side of the nautical
chart and transferred to the course line. The
position of the ship is at the intersection of the
second bearing line with the line that runs
parallel to the first bearing line.
Running fix
1.2.4. Dead Reckoning
“Dead Reckoning” is determining (reckoning) one’s position by deduction.
In other words: finding one’s position by taking into consideration a number of standard data:
-last known position and time
-course and speed
-sea and weather condition
The expression “dead reckoning” is not spelt correctly.
“Dead” should actually be spelt “ded” as an abbreviation of deduced.
1.2.5. Astronomical Navigation
With astronomical navigation, or celestial navigation, observations are taken of celestial bodies
(the sun, the moon and stars).
With a sextant the angle between a celestial body and the horizon is measured.
Now with the aid of the chronometer and the tables in the nautical almanac the ship’s position can
be determined.
(This way of fixing a position is quiet obsolete. However, it comes with good seamanship and
should never be deleted from the maritime studies).
1.2.6. Satellite Navigation and Radio Navigation
In the (Differential) Global Positioning System a signal is transmitted from the satellite. Within the
shortest possible time the ship’s position is determined be data received from the satellites, and
shown on the GPS-display on the bridge.
1.2.7. Depth
The depth of water must always be determined accurately.
In areas where shoaling has been reported, in restricted- and
inshore waterways, in harbours and ports, and in areas of
which the navigator does not have any local knowledge, the
risk of going aground is always imminent for deep draft
vessels and very often shallow draft vessels, too.
Furthermore the navigator should always be aware of the fact
that charted depths may sometimes be unreliable and must
be increased or decreased due to sea states, swells and
winds.
Therefore Notice to Mariners should be studied carefully and
regularly to avoid grounding due to insufficient depths of water or an unexpected tide below the
prediction.
Places where depths are maintained by regular dredging-operations are indicated as areas with
controlling depths. These depths are always very reliable.
The old-fashioned way of sounding (determining depth of water) is by means of the handlead.
Nowadays the most common way to determine depth is with the aid of the echo sounder,
whereby a signal is transmitted to the seabed.
The signal “bounces” back and is received again by the echo sounder.
From the time elapsed between transmission and reception of the signal, the depth of the water
can be determined.
Protruding obstacles on the seabed, such as wrecks and rocks, are not easily detected by signals.
Therefore the depth over a protruding obstacle is obtained by wire sweeping, whereby a cable is
swept over the seabed between two survey vessels. This routine is continued until the wire will
experience no resistance from any obstacle.
Now the depth of the specific position can be determined and recorded.
1.2.8. Directions
The terms that are used to indicate directions of proceeding are: starboard, port, ahead and stern.
These terms are used in helmorders and engineroom orders to indicate in which direction the
vessel must be steered.
Helmorders (“Starboard 5!”, “Port 10!”, “Midships!”, “Steady!” etc.) and engine room orders (“Full
ahead!”, “Dead slow astern!” etc.) are given by the mate to the helmsman and must be repeated
by the latter before they are actually executed.
To indicate directions towards the vessel, e.g. when a pilot, gale or Tsunami is approaching the vessel, the
terms “on”, “before” and “abaft” are used, followed by the appropriate section of the vessel.
Like so:
on the stem/stern, on the starboard bow, on the port quarter, before the starboard beam, abaft
the port beam, etc.
Directions that indicate the position of other vessels, objects or dangers with respect to our own vessel are
indicated by the words starboard, port, ahead and astern.
(“I will overtake you on your starboard/port side” / “wreck ahead of you”/ etc.)
