COXSWAINS NAVIGATION View as a Pdf
(Material courtesy of A.N.T.A. publications, edited extracts Ranger Hope © 2008)

Information on a Chart

Figure 6.1.3  Depths and Heights

Rocks and beaches that cover and uncover with the tide may have a drying height marked on or alongside them.  This drying height is measured (in feet or metres) above chart datum.  Thus a rock with a drying height marked will not always be visible.  You will only be able to see it when the tide has fallen below the height of the rock.

Heights

Heights ashore are measured in feet or metres above mean high water springs, which is the highest normal high water.  This is so that the height shown on the chart will be the minimum height of the object above water level.  When the tide is below high water the actual height of an object above water level will be increased by the amount of fall of tide.

·        Navigational marks

·        Restricted areas

·        Satellite – Derived Positions

·        Tidal streams

·        Magnetic Anomalies etc

For example,

FORMER MINED AREAS Trinity Opening, Papuan, Cruiser and Lark Passages have been swept and are open to surface navigation only. They are not safe for anchoring, trawling or bottoming by submarines owing to mines.

CAUTION – INCOMPLETELY SURVEY Owing to the incomplete nature of surveys in the areas indicated, shoaler water than charted may exist.

Compass Rose

The compass rose indicates direction on the chart, true north, magnetic north and magnetic variation for a given year.

Depth Contours

A depth contour is a line joining soundings of equal depth, ie 10 metres. On the example, Cape Bedford. Look closely and identify the 2, 5, 10, 15 and 20 metre depth contours. Note, on a photocopy how hard it is to distinguish between the shore, 2 and 5 metre depth contours without colour.

Figure 6.1.5  Cape Bedford (Aus 831)

Nature of Bottom

This is the type of bottom, ie mud, sand, shells etc. On a chart mud is abbreviated as M, sand as S and shells as Sh. There are many variations of bottom types and colour. Look at the example, Cape Bedford and identify the bottom type. All of the abbreviations are found in NP 5011.

NOTE:  Longitude is never used to measure distance.

Measurement of direction

True direction is measured from true north. Direction is defined as the point on the horizon towards which a vessel is heading.

Figure 6.2.2  The cardinal points

NOTE:  The direction the vessel is heading, 070^o T (True). All courses and bearings should be given in a three digit format to avoid confusion.

Figure 6.2.3  True course and true bearing

The true course the vessel is steering is the angle between true north and the vessel’s head. The true bearing of any object from the vessel, is the angle between true north and the line joining the vessel to the object. The compass rose is used to measure true courses and bearings on a chart.

Using the Chart

The navigator's instruments

Data for use in coastal navigation is obtained from the compass and electronic aids such as radar, echo sounder and GPS. To work on the chart, the coxswain needs

·        a soft (2B) pencil

·        a soft eraser

·        a pair of dividers

·        a large compass

·        parallel ruler, either roller, Capt Fields type or navigational triangles

What type of instruments you use is entirely a matter of choice.  The only criterion is that you are able to measure, and transfer, distances and directions accurately and correctly from one part of the chart to another.  This course describes the use of parallel rulers and dividers.  If you are using different instruments, you need to perfect a slightly different technique.

Figure 6.3.1

To obtain a fix, we could take a bearing of a second object and obtain another position line.  We have already stated that the vessel must lie on a position line, so if we have two position lines then we must be at their point of intersection.

For better accuracy, it is better to fix your vessel's posi­tion using three position lines if possible. See Fig 6.3.2.

Figure 6.3.2:  Fix by three cross bearings

How far apart should the bearings be? In general, a good angle of cut is between 60° and 120°, with a third midway between the two.

Position circles

Another way of fixing your vessel's position is by position circles.  This is done by obtaining ranges of various landmarks.  These ranges are usually found by radar.

Figure 6.3.4  Fix by three radar ranges

Ranges must be taken off the adjacent latitude scale and the relevant arc plotted on the chart using compasses. Both ends of the arcs should be marked with a single arrow, the point of intersection circled, and the time of the fix written alongside.

Selection of objects for ranges is as important as it is with bearings.

Plotting Position by Latitude and Longitude

We will consider plotting our position on the chart from a given latitude and longitude.  There are two methods of carrying this out.

You will be able to follow the process by looking at Fig 6.3.5.

Place one edge of the parallel ruler along one of the parallels of latitude printed on the side of the chart and walk the ruler until one edge passes through the given latitude.

Practically the shift is to the south west by approximately 200 yards.

Laying off courses on a chart

Use the largest scale chart available and study it carefully.  When laying off a course bear in mind the following:

(a)                               Keep well clear of hazards and dangers near the coast.

(b)                               It is preferable to keep close to the coast by day so that identification of terrestrial objects is facilitated and constant fixing made possible.

(c)                                                               By night, the distance from the coast should be in­creased keeping within visible range of lights

(d)                               If weather and visibility deteriorate, avoid a course that converges with the land.

(f)                                                                 Bear in mind the traffic density.

Figure 6.3.7:  Reading a Bearing or Course from the Compass Rose

Put your parallel rules on the course line diagram (Fig 6.3.7) and then manoeuvre the parallel rules to the nearest compass rose. Put the edge of the parallel rules through the centre of the rose and look at the edge of the compass rose. Where the parallel rules cuts the edge, you can now read off the course to steer. It should

be 065^o T.

Measuring Distance

Take the dividers and open them until the points are on the two places in question. The dividers are moved to the side of the chart adjacent to the middle of the course and the distance is read.

Figure 6.3.8:  Measuring Chart Distance Using Dividers

On most coastal charts the minutes of latitude are subdivided into tenths and it is usual to express distance in miles and decimals of a mile e.g. 5.8 mile.

Speed, Time & Distance

The day is a unit of time of twenty-four hours.  The start of the day is 0001, or midnight.

The first two figures represent the hours and the second two figures represent the minutes of the hour.  Thus, looking at the clock as you know it, we have the following:

Figure 6.5.1  Transit with Radar Range

Figure 6.5.2 Transit with Sounding

Figure 6.5.4  Four Point Bearing

Figure 6.6.1  Spring tides

Figure 6.6.2  Neap tides

Tidal Streams Tidal streams are horizontal movements of water which result from tides (for example flowing in and out of rivers). Tidal stream information is shown on the chart either as a diamond shape or with arrows.   Figure 6.6.4  (AUS 832) In this example (Fig 6.6.4) you must refer to the tides at Cairns for the nearest time of high water (HW) to use the table. The figure in the Dir column indicates the direction the tidal stream is going. The Sp refers to the spring tide and Np refers to the neap tide – notice the different rates.   Figure 6.6.5  Tidal Stream Chart Symbols (Chart 5011 – Symbols) The arrows indicate the direction the tidal stream is going and the rate in knots (kn). The arrow with ‘feather’ indicates the flood stream, ie when the tide is coming in, and the other arrow indicates the ebb stream, ie when the tide is going out.     Currents A current is a non tidal movement of water caused by weather and oceanographic conditions.     Figure 6.6.6  (AUS 831) The arrow indicates the direction the current is going and the rate is indicated, ie 0.5 knots.   Figure 6.6.7  (AUS 832) Current information is also displayed within the title block. Steering by Compass In the section on navigation one of the tasks you performed was to lay off a course between two places on a chart and find the true course. In this section we take the next step and calculate the compass course to steer on the boat to make good the true course laid off on the chart.   Magnetic variation Courses and bearing laid off on a chart are true bearings but we steer and take bearings from a magnetic compass. The magnetic compass follows magnetic lines of force, the magnetic poles of which are in a different place to the true poles. Therefore, in all but a few places around the world the true and magnetic bearings of an object will be different. This difference is called ‘magnetic variation’ and changes from place to place. The value of the magnetic variation is always given in the compass rose on the chart. The North magnetic pole is located north of Canada and wanders about in the general area. For someone on the cast coast of Australia the North magnetic pole is slightly to the east of true North and the magnetic variation is correspondingly named EAST on charts of that area. For much of the west coast of Australia, magnetic north lies to the west of true North and the variation is correspondingly named west.   Calculating the magnetic variation The value of the magnetic variation is given in degrees and minutes on the chart. For practical purposes mariners work in whole degrees and ½ degrees (not minutes). Calculation of the magnetic variation for the current year involves two steps: Step 1:                                   Add or subtract the change in variation between the chart and the current year.  For example, from Fig 6.7.1.       Figure 6.7.1     Number of years:                  1997 – 1974 = 23 years Increase:                                         3´ x 23 years = 69´ = 1o 09´ Magnetic Variation for 1997:  10o 30´ E (1974)                                                                        +  1o 09´                                                                          11o 39´ E (1997)     Step 2:                                   Round off the updated magnetic variation to the nearest ½o                                                From step 1, 11o 39´ E rounds to 11½o E   The rules for rounding to the nearest ½o are straight forward. Use 15´ and 45´ as the cut offs. If the minutes are more than 15 and less than 45 take it the ½o. If they are less than 15 or more than 45 go to the nearest whole degree. eg                             8o 10´ = 8o                                  8o 20´ = 8½o                                  8o 40´ = 8½o                                  8o 50´ = 9o   Application of variation Changing from true to magnetic courses and vice versa requires a simple addition or subtraction of the variation. The trick is knowing when to add and when to subtract. In each of the following figures the:                                       OUTSIDE rose is the TRUE rose and the                                       INSIDE rose is the MAGNETIC rose   Figure 6.7.2 Note Fig 6.7.2. The variation is 10o E. See that in any particular direction the magnetic bearing is always 10o less than the true bearing. In other words when the variation is EAST, the magnetic bearing will always read least (or less than the true).     Figure 6.7.3     Conversely, in Fig 6.7.3 where the variation is 10o W the magnetic bearings are always 10o bigger than the corresponding true bearings. That is when the variation is WEST the magnetic bearing will read best (or bigger than the true). These rules are normally condensed to:                                       if the VAR is EAST, the MAG will read LEAST and                               if the VAR is WEST, the MAG will read BEST   Examples:                                         1                                      2                                        3 True Co 108o True Co 240o True Co 357o Var     4o W Var     8o E Var    6o W Mag Co 112o Mag Co 232o Mag Co 003o                                           4                                      5                                        6 True Co 270o True Co 100o True Co 004o Var   12o E Var   10o W Var    7o E Mag Co 258o Mag Co 110o Mag Co 357o   Transits In the section on navigation we found that a transit was a bearing through any two points that can be identified on a chart, for example, a set of lead lights. For any set of lead lights the true bearing is always given on the chart and this allows us to carry out a simple compass check.   Checking your compass by transit Firstly, the true bearing of the leads is converted to a magnetic bearing by applying the variation. Then, while steaming your vessel along the leads, the compass bearing of the leads is noted (it is your course steered) and compared to the calculated magnetic bearing of the leads. They should both be the same. If your compass bearing does not agree with the magnetic bearing of the leads then your compass is carrying another error called compass deviation. Deviation is investigated and resolved in higher level certificates. For the Coxswain it is sufficient to be able to recognise the presence of deviation from a transit. NOTE:                               A set of leads on a chart are a convenient transit because they are easily seen and the true bearing is given. However, any transit can be used and the true bearing found by laying parallel rules along the transit and reading the bearing from the rose on the chart. Because compass deviation changes as your vessel’s heading changes your compass needs to be checked from time to time, over a number of different transits. Deviation is cause by something within the boat affecting your compass. Therefore, if a deviation of more than about three degrees is discovered when you check your compass, firstly check to make sure there are no steel or magnetic objects placed around the compass. If no cause can be readily found then your second option is to have a licensed compass adjuster ‘swing’ your compass.   Steering a Course In a seaway a small vessel will move about substantially making the compass difficult to read and impossible to hold dead on course. Where possible, use a landmark to steer to, checking your compass from time to time. Where this is not possible and you are forced to steer by compass alone allow the boat to wander or ‘yaw’ to the natural rhythm of the sea (within reason). These random errors should be roughly the same to port and to starboard and the average course should be the required course. Being off course 5o, even up to 10o from time to time, is not dangerous if a frequent check is kept on the average compass course and the initial required course was accurate. If however, there is an error in the required course through incorrect application of magnetic variation or undetected deviation, then the steered course will be biased by that error, be it 5 or 10o, and that is dangerous. Taking Bearings with A Compass On larger vessels provision is made for taking bearings off a main compass (azimuth ring) or by a pelorus and applying it to the main compass. On small vessels there is no such provision and the only way to take accurate bearing with the steering compass is to point the vessel straight at the target and read the bearing from the lubbers line. However, it is much more convenient to use a hand bearing compass (either conventional or electronic). Modern hand bearing compasses have precision sights and easy to read cards graduated in 1o increments. The only correction that can be applied to a hand bearing compass is magnetic variation. They can not be compensated or corrected for compass deviation and are therefore of no value on steel boats.                            Buoyage Description Of Buoyage System “A” Many countries throughout the world have agreed to the use of a uniform coding system of navigational marks. The system, developed with the assistance of the International Association of Lighthouse Authorities, has been in wide use within Australia waters since late 1983. The buoyage system during the day, uses shape, colour and topmarks whilst at night, colour and rhythm to identify the individual mark. Five basic shapes are: cylindrical (can), conical, spherical, pillar and spar. Australia uses IALA Buoyage System A.   Type Of Marks 1.      Lateral            indicates port and starboard hand sides of channels. 2.      Cardinal        indicates that deeper water lies to the direction shown          ie to the north, south, east or west. 3.      Isolated         indicates isolated dangers of limited extent with Danger      navigable waters all round them. 4.      Safe Water   indicates that there is navigable water all round and under the position, eg mid channel buoy. 5.      Special          indicates special feature eg spoil grounds, or             prohibited anchorages.   Lateral Marks They are usually positioned to define well established channels and indicate port and starboard had sides of the navigation route into a port. Where there may be any doubt, the direction of buoyage may be indicated on charts by the symbol. Remember:                                  - Port hand Mark is coloured red and the basic shape is a   can and shows a red light. - Starboard hand Mark is coloured green and the basic                                                       shape is conical and shows a red light at night. When going into port, leave the port hand mark to port. Hence the term, red to red when entering port. When departing it’s the opposite, leave the port mark to starboard.   The Cardinal Marks There are four cardinal marks:- North, South, East and West. A cardinal mark will indicate where the best and safest water may be found. A cardinal mark may indicate – ·        the deepest water in an area; ·        the safe side on which to pass a danger and to draw attention to a feature in a channel such as a bend, junction or an end of a shoal.   Remember:                                The mariner is safe if passing – (a)                                                   North of the north mark (b)                                                   East of the east mark (c)                                                   South of the south mark (d)                                                   West of the west mark.   - both the colour pattern and top mark will indicate which side to pass during the day - at night the cardinal mark exhibits a white light and its quadrant is distinguished by a specific group of quick or very quick flashes - associate the number of flashes of each group with that of a clock face, three o’clock east, six o’clock south, nine o’clock west and twelve o’clock north.               When making up your palm cards note the apex of the topmark always points to where the black is painted on the marker, ie north marker apex up, black on top of the marker. Isolated Danger Marks Indicates an isolated danger of limited extent which has navigable water all round it eg an isolated shoal, rock, reef or wreck – but don’t pass too close. Reme                                            - its colour is black with red horizontal bands with two black spheres.                                                       - at night always a white flashing light showing a group of two flashes.                                                       - the characteristics may be best remembered by association of two white flashes with two spheres as the topmarks. Safe Water Marks Indicates that there is navigable water all around the mark, eg mid channel or land falls buoy. Remember:                                  - always with red and white vertical stripes                                                     - topmark is a single red sphere                                                       - at night a white light, isophase, occulting, a single long flash every 10 seconds, or morse A Special Marks Indicates a special area or feature such as: Traffic separation marks Spoil ground marks Cable or pipe line marks including outfall pipes. Also to define a channel within a channel, eg a channel for deep draught vessels in a wide estuary where the limits of the channel for normal navigation are marked by red and green lateral buoys. Refer to the chart for the exact meaning. Remember:                                - it is always yellow in colour                                                     - it may have a single yellow X topmark.                                                       - at night a yellow light with any rhythm, other than those used for the white lights or cardinal, isolated danger and safe water marks (at night).     Figure 6.8.2  (Qld Tide Tables QT)