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LOSS PREVENTION - The Shipowners’ Club

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Towards Effective Navigation
© British Crown Copyright and/or database rights. Reproduced by permission of the Controller of Her Majesty’s Stationery Office and the UK Hydrographic Office (www.gov.uk/government/organisations/uk-hydrographic-office).
The information and recommendations in this booklet are given in good faith and are meant to highlight best practices, good seamanship and common sense to reduce incidents that result in related claims. However, Members must take into consideration the guidance and regulatory requirements given by Flag states and other governing authorities when formulating policy in line with the contents of this publication.
NOT TO BE USED FOR NAVIGATION.
The Shipowners’ Club provides P&I insurance for small and specialised vessels and, as such, the majority of our vessels trade in coastal waters. The art of navigation is second nature to deep-sea mariners but it can be something of a mystery to those crews who do not need to use this particular skill very often.
Our Condition Survey Programme often highlights the fact that crews have a limited understanding of the rudiments of basic navigation and we also see claims arising where poor navigation is a main contributing factor. A better understanding of the subject will hopefully go some way in reducing the incidence of those claims.
The purpose of this booklet is to open the window on navigation in a very basic way. It is primarily aimed at those mariners who are not fully trained in the art of navigation.
This booklet is one of three publications in a series and it is envisaged that together they will help eliminate some of the difficulties that befall those not so well versed in marine navigation.
We remain forever indebted to Captain H. Subramaniam for compiling this booklet series for us. Captain Subramaniam was a distinguished member of the nautical fraternity in a career spanning over 6 decades, including over 30 years of teaching experience. Apart from this series, he also authored eight textbooks on the operation of merchant ships which continue to be used by seafarers across the globe. It was his ability to put a subject across in a nutshell that made all his books easy to understand and helpful to those these are intended for.
Foreword
Chapter 1 Principles of navigation 2
Positional references 2
The three-figure system 11
The gyro compass 12
The magnetic compass 16
Mercator charts 37
Types of navigational charts 39
Characteristics of navigational charts 40
Titles of charts 41
Chart symbols 46
Chapter 4 An Introduction to ECDIS 54
Chapter 5 Nautical publications 57
Chapter 6 Notices to Mariners and chart correction 68
Contents
POSITIONAL REFERENCES
Figure 1
Axis of rotation The Earth rotates around an imaginary line called its axis of rotation once a day from west to east as shown in Figure .
Chapter 1
Loss Prevention: Towards Effective Navigation I 3
The North and South Poles The points where the axis of rotation cuts the Earth’s surface are called the poles. The one on top is called the North Pole and the one at the bottom is called the South Pole as shown in Figure 2.
Figure 2
Equator The Equator is an imaginary line on the Earth’s surface, midway between the two poles. The Equator divides the Earth into two halves, called the Northern hemisphere and the Southern hemisphere as shown in Figure2.
4 I Loss Prevention: Towards Effective Navigation
Parallels of latitude Parallels of latitude are imaginary lines on the Earth’s surface that are parallel to the Equator. They are named North or South according to the hemisphere in which they lie as shown in Figure 3.
Figure 3
The Earth’s surface from the Equator to each pole is divided into 90 equal parts. Each part is called a degree of latitude. The Equator is 0°. The North Pole is 90°N. The South Pole is 90°S. The minimum value of latitude is 0°. The maximum value of latitude is 90°N or 90°S.
Loss Prevention: Towards Effective Navigation I 5
Parallels of latitude Parallels of latitude are imaginary lines on the Earth’s surface that are parallel to the Equator. They are named North or South according to the hemisphere in which they lie as shown in Figure 3.
Figure 3
The Earth’s surface from the Equator to each pole is divided into 90 equal parts. Each part is called a degree of latitude. The Equator is 0°. The North Pole is 90°N. The South Pole is 90°S. The minimum value of latitude is 0°. The maximum value of latitude is 90°N or 90°S.
Meridians of longitude Meridians are imaginary lines on the surface of the Earth that go from one pole to the other by the shortest path. Meridians cross the Equator and all parallels of latitude at 90° as shown in figure 4.
Figure 4
If you stand on any meridian, the North Pole will be exactly north of you and the South Pole will be exactly south of you. When you face north, the areas on your right will be to the east of you. The areas to your left will be west of you.
6 I Loss Prevention: Towards Effective Navigation
The Prime Meridian The Prime Meridian is the meridian that passes through the Greenwich Observatory in London. The Prime Meridian divides the Earth into two hemispheres – the Eastern hemisphere and the Western hemisphere (Figures 5 and 6).
Figure 5
Loss Prevention: Towards Effective Navigation I 7
Longitude Look at the Earth from above the North Pole as shown in Figure 6. The circle you see is the Equator. P is the North Pole. The centre of the Earth is directly below the P. The line P–G is the Prime Meridian, dividing the Earth into the Eastern and Western hemispheres.
Figure 6
Starting from G, divide the Equator on the Western hemisphere into 180 parts and call each a degree of west longitude.
8 I Loss Prevention: Towards Effective Navigation
In Figure 7, the line P–G is the Prime Merdian. Starting from G, divide the Equator on the Eastern hemisphere into 180 parts and call each a degree of east longitude.
As shown in Figure 7:
Longitude of A = 045°E Longitude of B = 090°E
Longitude of C = 135°E Longitude of D = 045°W
Longitude of E = 090°W Longitude of F = 135°W.
Figure 7
Notice that the line P–H is both 180°E and 180°W. Hence the meridian PH is called 180° without the E or W attached to it. The minimum value of longitude is 0°. The maximum value of longitude is 180°.
Loss Prevention: Towards Effective Navigation I 9
Position of a place Each degree of latitude and longitude is divided into 60 parts, each called a minute of arc (not to be mixed up with minute of time).
The position of a place or a ship on the Earth is indicated by its latitude and longitude in degrees and minutes.
The following is an extract from Norie’s Nautical Tables :
Latitude Longitude
Keppel Harbour (Singapore) 01° 16’N 103° 50’E
Distance Distances at sea are expressed in nautical miles. Abbreviation for nautical miles is NM and for metres is m.
1 NM = 1,852 m.
10 I Loss Prevention: Towards Effective Navigation
It is interesting to know how the value of 1 NM was arrived at. Figure 8 shows the Earth, with O at the centre and O–B as a radial line. The O–B line rotates by 360° and covers an approximate distance on the Equator of 40,000 km.
Hence 360° = 40,000 km on the Equator.
Or, 1° = 40,000 km
360 x 60
Hence 1 NM = 1 minute of arc = 1.852 km = 1,852 m.
It has been internationally agreed that 1 NM = 1,852 m.
Figure 8
Also = 1.852 km = 6,076 feet = 1.15 land miles. Also 1 km = 0.54 NM.
Cable 0.10 NM (1/10 of a nautical mile) = 1 cable or 10 cables = 1 NM.
Speed A speed of one nautical mile per hour is called a knot. It is wrong to say ‘knots per hour’.
Loss Prevention: Towards Effective Navigation I 11
THE THREE-FIGURE SYSTEM
Directions are normally indicated in degrees clockwise from north. So, 000° is north, 090° is east, 180° is south, 270° is west and 360° (i.e. 0°) is north again as shown in Figure 9. Your ship’s bridge is indicated by 0.
000°
045°
12 I Loss Prevention: Towards Effective Navigation
There are two sources for direction on board a ship: the gyro compass; and the magnetic compass.
THE GYRO COMPASS
The gyro compass is an electrically-driven mechanical instrument that works on the directional property of a wheel spinning at a very high rate.
Its principle is similar to that of a spinning top that children play with. So long as its rotational speed is above a critical level, it will maintain its direction in space (Figure 10).
Figure 10
So long as the power supply is continuous and does not fluctuate, the gyro compass will be reliable.
Loss Prevention: Towards Effective Navigation I 13
In case of power failure, an alternate power supply should normally take over automatically and ensure uninterrupted power supply for several hours.
Steering repeater
Steering flat repeater
Figure 11
Gyro is not affected by external magnetic influences. When in proper running order, the gyro compass points constantly to true north. The main unit is called the master gyro and other units that get input from it are called repeaters, as depicted in Figure 11.
14 I Loss Prevention: Towards Effective Navigation
Gyro error The gyro compass may have a small error which is usually between 0.0° and 0.5° but may sometimes be as much as 2.0°. This varies according to changes in latitude, course and speed of the ship.
When the gyro reading is higher than the true value, the error is termed high or H. Hence, to all readings of the gyro compass, the error is subtracted to get true values.
Example 1: If gyro error is 1° (H):
Gyro bearing of lighthouse
=
=
=
When the gyro reading is less than the true value, the error is termed low or L . Hence, to all readings of the gyro compass, the error is added to get true values.
Example 2: If gyro error is 1° (L):
True course to steer
Loss Prevention: Towards Effective Navigation I 15
Test yourself Gyro error You are given the true (T) and gyro (G) readings. Fill in the gyro compass error and its name:
1 2 3 4 5
Reading (T) 275° 126° 341° 192.5° 044°
Reading (G) 276° 124° 342.5° 191.5° 044°
Error
You are given the true reading and the gyro error. Fill in the gyro reading:
6 7 8 9 10
Reading (T) 144° 186° 337° 000° 359°
Error H 1° L 2° H 0.5° L 1.5° H 1°
Reading (G)
You are given the gyro reading and the gyro error. Fill in the true reading:
11 12 13 14 15
Reading (G) 129° 221° 343° 180° 119°
Error H 2° L 1° L 0.5° H 1° H 1°
Reading (T)
Answers:
Error: 1. 1°(H) 2. 2°(L) 3. 1.5°(H) 4. 1°(L) 5. 0°
6. 145° 7. 184° 8. 337.5° 9. 358.5° 10. 000°
11. 127° 12. 222° 13. 343.5° 14. 179° 15. 118°
16 I Loss Prevention: Towards Effective Navigation
THE MAGNETIC COMPASS
A magnetic compass rarely points to true north. Are you surprised? Normally, the error is predictable and can be corrected with reasonable accuracy. The magnetic compass is very reliable and serves as a check on the gyro compass.
Compass error Compass error (CE) is the angle of the compass between true north and compass north expressed in degrees.
Clarification When you stand on any meridian, the direction of the North Pole is true north. The direction indicated by the north point of the compass is called compass north (CN).
CE is named east (E) if the compass north lies to the right of true north (Figure 12A).
CE is named west (W) if the compass north lies to the left of true north (Figure 12B).
True N 345° (C)
True N 010° (C)
Colour code for arrows: Comp bearing True bearing Comp error
CN 000° (C)
BA Terrestrial object
Comp error = 15 E
Comp error = 10 W
Loss Prevention: Towards Effective Navigation I 17
Figures 12A and 12B are purely theoretical. At sea, how would you know where true north is? If you can answer that question, there is no need for a magnetic compass!
At sea you can always get the compass reading at a glance and apply CE to it to obtain the true direction. This is why a magnetic compass is needed on board.
Conversion of bearings Conversion of compass bearings to true bearings, and vice versa, becomes very easy if a rule of thumb is applied as follows:
Error east, compass least
Error west, compass best
Example 1: If the bearing is 035°(C) and CE is 15° E, find the true bearing.
Using the rule of thumb, ‘error east, compass least ’, you work it out as follows:
Bearing 035°(C)
CE 15° E
Bearing 050°(T)
You can verify the answer by checking Figure 12A.
Example 2: If the bearing is 330°(T) and CE is 10°W, find the compass bearing.
Using the rule of thumb, ‘error west, compass best ’, you work it out as follows:
Bearing 330°(T)
CE 10°W
Bearing 340°(C)
18 I Loss Prevention: Towards Effective Navigation
Test yourself Compass error You are given the true and compass bearings. Fill in the CE:
1 2 3 4 5
Bearing (T) 275° 126° 001° 192.5° 044°
Bearing (C) 276° 124° 350° 201.5° 044°
CE
You are given the true bearing and the CE. Fill in the compass bearing – Bearing (C):
6 7 8 9 10
Bearing (T) 265° 136° 001° 182.5° 034°
CE 16°E 12°W 2.5°E 19.5°E 4°W
Bearing (C)
You are given the compass bearing and the CE. Fill in the true bearing – Bearing (T):
11 12 13 14 15
Bearing (C) 127° 350° 159° 261° 089°
CE 6°W 14°E 17°W 11.5°E 14°E
Bearing (T)
Answers:
CE: 1. 1°W 2. 2°E 3. 11°E 4. 9°W 5. 0°
(C): 6. 249° 7. 148° 8. 358.5° 9. 163° 10. 038°
(T): 11. 121° 12. 004° 13. 142° 14. 272.5° 15. 103°
Loss Prevention: Towards Effective Navigation I 19
Components of compass error The error of the magnetic compass is made up of two variables – variation and deviation.
Geographic North Pole
Magnetic red pole
Figure 13
Causes of variation The core of the Earth is like a bar magnet. Its magnetic axis is about 11.3° to the axis of rotation (Figure 13). The positions of the magnetic poles are not constant. They change erratically by about 8 nautical miles a year. Navigators call the magnetic poles blue and red, as shown in Figure 13.
20 I Loss Prevention: Towards Effective Navigation
Magnetic variations The angular difference of the ship between true north and magnetic north expressed in degrees.
Geographic North Pole
Figure 14
The value of variation depends on the position of the ship with respect to the geographic pole and the magnetic pole. Figure 14 shows the variation at positions A and B.
Value of variation This is indicated by compass roses at various locations on navigational charts. A compass rose shows true north and degrees of directions clockwise from 0° to 360° and the magnetic north. It also shows the variation at that location, the year it was measured and the annual change in that value (Figure 15).
Loss Prevention: Towards Effective Navigation I 21
Annual change of variation As explained earlier, the magnetic poles shift erratically by about eight nautical miles a year. This causes a slight change in the value of variation each year. This annual change is indicated near the value of variation. The change has to be calculated for the current year and applied to the value indicated for the year when the variation was measured (Figure 15).
Figure 15
Compass Rose
22 I Loss Prevention: Towards Effective Navigation
Sample calculation At the location of the centre of the compass rose, the variation was 6° 40’W in 1992. The annual change is 8’E. Find the variation in 2012.
Variation in 1992
Variation in 2012
6° 40’W
4° 00’W
=
=
=
Conversion of magnetic bearings Conversion of magnetic bearings (M Brg) to true bearings (T Brg) and vice versa becomes very easy if a rule of thumb is applied as follows:
Error east, magnetic least.
Error west, magnetic best.
Loss Prevention: Towards Effective Navigation I 23
Example 1: If the bearing is 047°(M) and variation is 8°E, find the true bearing.
Using the rule of thumb, ‘error east, magnetic least ’, we work as follows:
Bearing 047°(M)
Variation 8° E
Bearing 055°(T)
True north
Terrestrial object
24 I Loss Prevention: Towards Effective Navigation
Example 2: If the bearing is 052°(T) and variation is 6°W, find the magnetic bearing:
Using the rule of thumb, ‘error west, magnetic best ’, we work as follows:
Bearing 052°(T)
Variation 6°W
Bearing 058°(M)
True north
Terrestrial object
Loss Prevention: Towards Effective Navigation I 25
Test yourself Variation You are given the true and magnetic bearings. Fill in the variation:
1 2 3 4 5
Bearing (T) 245° 116° 002° 182.5° 065°
Bearing (M) 246° 114° 350° 195.5° 065°
Variation
You are given the true bearing and the deviation. Fill in the magnetic bearing – Bearing (M):
6 7 8 9 10
Bearing (T) 275° 147° 004° 142.5° 039°
Variation 15°E 12°W 6.5°E 15.5°E 05°W
Bearing (M)
You are given the magnetic bearing and the deviation. Fill in the compass bearing - Bearing (C):
11 12 13 14 15
Bearing (M) 120° 352° 169° 260° 099°
Variation 8°W 15°E 17°W 11.5°E 15°E
Bearing (T)
Answers:
Var: 1. 1°W 2. 2°E 3. 12°E 4. 13°W 5. 0°
(M): 6. 260° 7. 159° 8. 357.5° 9. 127° 10. 044°
(T): 11. 112° 12. 007° 13. 152° 14. 271.5° 15. 114°
26 I Loss Prevention: Towards Effective Navigation
Deviation for the ship’s head Deviation is the angle of the compass between the magnetic north and the compass north. It is expressed in degrees and minutes of arc.
Deviation is named east if the compass north lies to the right of the magnetic north (Figure 19). Deviation is named west if the compass north lies to the left of magnetic north (Figure 20). Deviation is caused by the magnetic influence of the ship’s steel and iron structure.
The value of deviation and its name – east or west – depends on the compass course of the ship.
Deviation of the compass is caused by the magnetic properties of the ship’s structure. The compass would point in a direction slightly away from the magnetic north.
If we were on a wooden boat with no iron or steel structure, there would be no deviation. The compass would point to the magnetic north.
The value of deviation depends on the ship’s head. The value and name of deviation can be obtained at a glance from a document called the deviation card illustrated in Figure 18. Note: To avoid clutter, many horizontal lines have not been shown.
Compass course
West East
000º (c)
Loss Prevention: Towards Effective Navigation I 27
If the bearing of an object is 135°(C) while steering 200°(C), the deviation should be taken out from the card for 200°(C) not 135°(C). The value of deviation depends on the ship’s head not on the compass bearing. From the deviation card shown in Figure 18, deviation = 1°W and not 3°E.
Conversion of bearings Conversion…

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