The execution of any passage plan is the formulation of the tactics which are intended to carry the plan through. Considera- tion should therefore be given to the following specific topics: The reliability of ships equipment, specifically the navigation equipment. Its condition and limitations together with its degree of accuracy. Account should also be given to the level of expertise of ships officers and whether they are familiar with that ships type of equipment.
The projection of towards critical points to allow a
more detailed assessment of tide heights and flow. Underkeel clearance (UKC) being a main consideration for the plans execution. By advancing the ETA, while on passage the possibility of anticipating difficulties can often resolve problems before they arrive.
Meteorological conditions will be continually changing while the vessel is on passage. In order to maintain optimum passage
PASSAGE PLANNING time heavy seas and areas of reduced visibility need to be avoided, if at all possible. Historically and at certain seasons, specific areas are prone to or conditions. If transit of these areas can be avoided or co-ordinated to coincide with daylight or similar suitable time, the overall safety aspects of the passage can be raised.
Day-time or night-time passage, especially when negotiating dangers or narrows can often be achieved at a favourable time by early realisation and making an appropriate speed adjust- ment. Speed adjustments can of course be an increase in speed as well as a decrease in speed. However, if an increase in speed is employed, then the conditions should be appropriate and the contents of Rule 6, of the Regulations for the Prevention of Collision at Sea, noted.
It should also be borne in mind that position fixing methods during the day and during the night may well differ, e.g. the use of unlit headlands for visual bearings is not possible at night.
Traffic conditions, notably at navigational focal points like traffic separation schemes, or prominent geographic points should also be considered in light of the projected ETA of the vessel. Speed adjustment again can be a prudent action to arrive at focal points at an appropriate time.
Summary
It has already been stated that no plan is rigid and by its nature, it must be flexible to suit changing conditions. The inclusion of contingency alternatives in many cases will prove to be that item which is not used. However, the plan that doesn't contain the contingency options is very often the one that turns out to need it most. In anticipation of navigational problems where additional personnel may be required to back up routine watch keeping duties. Masters should have suitable manpower routines available to handle all emergencies.
NAVIGATION FOR MASTERS
Passage Plan — Monitoring
The construction of the finished passage plan and the instiga- tion of the plan in the execution phase are commendable in their own right. However, the Master of any vessel is posed
with the question, does he know that the plan is being
complied with, The answer to the question is revealed
by the progress of the vessel being monitored and visual con- firmation that the plan is being drawn to a conclusion.
The monitoring of the vessels movements must therefore be
and If and when problems are foreseen,
or anticipated the Master of the vessel should be informed to allow flexibility in the plan to accommodate possible devia- tions safely. Monitoring of shipboard equipment is common to monitoring the safe movement of the vessel and therefore to ensure continuity of safe navigational practice, recommended checks on navigation equipment should be made at the following times:
Prior to sailing and departure from the berth.
2. Prior to entering known hazardous areas or areas of specific dangers.
3. At regular and frequent intervals during passage time. Reference is made to navigators and watch keepers to consult
the Procedures
Position fixing: — All the navigational equipment of a vessel is at the disposal of watch keepers and should be used to maximum advantage whenever possible. However, the principles of efficient watch keeping should not be lost in the hi-tech world of satellite systems. Visual bearings are still considered the most accurate and reliable means of fixing the ships posi- tion, provided fixes are based on three position lines. Bearing in mind that the use of Decca Navigator, Radar, Omega, Loran or other instrument systems are liable to instrument error, or operator error. This is not to say that they should not be used. On the contrary, instruments may be the only method of position fixing available, as with a vessel in poor visibility.
PASSAGE PLANNING Navigators should use alternate position fixing methods to avoid
a possible continuous error. Full use should
also be made of the echo sounder when practical, to provide corresponding data checks on obtained fixes.
The frequency of fixing the vessels position will depend on the geography and the circumstances prevailing. Obviously certain areas of navigation for the vessel will require more position checks than others and the frequency of charting fixes will be dictated by the prevailing conditions.
Buoys should not be used for fixing the vessels position but may be found to be useful as checks when fixed objects are not available. Transits and clearing bearings can also be gainfully employed in providing margins of safety for the vessel. The use of parallel indexing has grown over the years and has proved itself to be a reliable and effective method of monitoring the ships progress.
Summary
To complete the principles of passage planning, monitoring is that essential action which illustrates the safe progress of the vessel. Regular alternative position fixing methods must be the order of the day.
NAVIGATION FOR MASTERS
Errors in Position
Example 1 Fixed error on compass or regular observer error:
Actual True
Fix no two position lines are correct.Resultant where
Example 2 Variable errors in —
The chance of the true position being within the cocked hat are
1 in 4. practice the position should be taken as it is nearest the danger
NB: If the variable error on bearing passed through 'Z'
then an incorrect but perfect plot is obtained. Random/variable errors because of:
Observational error.
(2) Changes in compass error for different bearings (i.e. compass card not steady).
PASSAGE PLANNING Example 3 Error in distance by vertical angle
Error in
Inaccuracy in this method of fixing is due to: Errors in the measured angle 6.
2. Errors in the height above sea level being employed. Plotting and computing the error.
Example figures illustrate error in range. (Use of distance by vertical angle tables employed, found in Nome's Nautical Tables).
Assume a vessel observes a lighthouse 13 metres high and ob- tains a vertical angle of 0° when at the time of high water. If the range of tide is 10 metres and the same vertical angle is observed at low water time, what would be the two ranges?
(Assumed Object 13.0m. Vert. Angle Range by table 1.0 10m) Object 23.0m
Vert. Sex't Angle Range by table 1.75
If the state of the tide is not taken into account the error in
this example would therefore fall anywhere between nautical
mile and 1.75 nautical miles. Range of error 0.75 n.mile.
Height above sea level, caused by draught or trim of the vessel could also effect the result in a similar manner.
NAVIGATION FOR MASTERS
Example 4 Errors in the use of transferred position lines. Errors arising from an incorrect course being used. Incorrect
course used because of wind effects or unknown currents, compass fault etc.
2nd P/L
Transferred P/L incorrectly drawn through position If correct course is used, transferred P/L should pass through 'Y'. Error in the fix is where position 'Q' is correct fix.
2. Errors arising from incorrect distance being employed between lst/2nd
2nd Observation P/L
PASSAGE PLANNING
The Use of Horizontal Sextant Angles
The impossible fix — when the three objects and the ship, all lie on the same position circle, (con-cyclic). Both constructed position circles would be coincident.
Ships position
Notable errors when employing horizontal sextant angles: Errors in angular measurement from instrument. 2. Plotting errors, (especially when angles exceed 90°). 3. Errors due to the three objects not being in the same
plane.
Unsatisfactory fixes:
1. When the distance to the middle object is large, (from the ship).
2. When the angle of cut of the position circles is small. 3. Using the compass for obtaining difference in bearings when
NAVIGATION FOR MASTERS
Example 6 Errors in astronomical position lines 1. An error in GMT at time of observation:
An error in the GMT will cause an error in the GHA value. e.g. A 4 second error in GMT causes 1' error in GHA, therefore in Longitude,
Longitude = LHA ~ GHA
This would result in an error of 1 mile in the intercept, when
(a) The observed body is on the 'prime vertical' and the position line is N/S.
(b) When the ship is on the equator and D. Long = departure.
If the vessel is not on the Equator but the position line is still north/south the intercept error can be expected to reduce in the same way as departure is reduced for a given D. Long.
Error in intercept
P/L \
Ships position
Incorrect observed longitude
For the same error in GMT the intercept error would be further reduced when the position line is not north/south.
2.
PASSAGE PLANNING In working a sight the chronometer error was taken as 20 seconds slow instead of 20 seconds fast, when in DR position latitude 35° 00' N longitude 70° 00' W. The bear- ing of the body was 140°T and the obtained intercept was 1.8' away. What was the correct intercept?
Scale 1 cm = 1'
P/L obtained from sight
Correct P/L if the only error is GMT.
Long (W) = GHA - LHA Error is 40 seconds fast Therefore GHA is 10' to large.
Therefore observed longitude is 10' too large.
By traverse tables or right angled trigonometry in lat. 35 °N: D. Long. 10'; Departure = 8.2' .
In Triangle ABD, Angle 'D' is a right angle. Angle A = Course 40°, DB = 1 .8 ', AB = 2.8' Therefore BC = 5.4'
Correct intercept = 3.47' towards. Position fixing — Errors and reliability
In all passage plans the action of monitoring the plan is es- sential and the use of navigational instruments plays a major part. It is especially so when visual fixes are not possible either because of poor visibility or extreme range of targets. The use
NAVIGATION FOR MASTERS
of instruments as an aid has become normal practice but navi- gators should beware of human error and the overall standards of accuracy when they are employed.
Many instruments operate in conjunction with the ships speed and this must be an accepted variable depending on the con- ditions. Higher standards of accuracy are also desired at different stages of the voyage:
i.e. Harbour entrances and approaches as to open deep waters. If mid-ocean navigation is compared to the ships navigation when making a landfall, then the availability of any system together with its accuracy should be considered.
Example 7 — Astronomical navigation compared to Loran and satellite navigation in mid ocean.
Astronomical Navigation Restricted and will vary with weather conditions. Clarity of the horizon and the availability of the celestial body will determine the possibility of a fix. Cloud cover and density could well obscure the sun, planets and stars when most needed.
Loran Not available in large areas of ocean.
Satellite Navigational Position fixing may not always be
System available due to satellite or system
faults. Otherwise availability of GPS is considered good for all areas. The accuracy of these systems will vary but in general the following figures may be considered reasonable: —
Astronomical navigation, in good conditions, should deliver
a fix within 5 nautical miles. Considered adequate for
mid-ocean passage but cannot be relied upon for landfall posi- tion fixing, at a specific time.
Satellite navigation system should provide fixes to 100
metres. This accuracy is obtainable in all areas irrespective of position.
PASSAGE PLANNING Loran 'C' is not a world wide operation but where it is active good fixes can be obtained up to 1200 miles (ground wave) from station. Sky wave fixes are also possible. A degree of skill is required by the observer to gain improved accuracy. Accuracy of - 1 nautical mile is usually achievable with groundwave reception; this accuracy falls dramatically when only skywave transmissions are received.
Example 8 Use of Decca Navigator.
The Decca Navigation System operates on medium frequency continuous wave (CW) transmissions. The navigator requires a Decca receiver on board together with the relevant Decca Lattice charts. The apparatus being installed and maintained by the manufacturers.
The system is extremely popular and widely used at sea, even though the range is limited to about 250 nautical miles from the master transmitting stations. The Decca Fix is highly accu- rate and is well used as either the primary or secondary fixing method aboard the vessel. The theoretical accuracy is given to approximately 1/100 of a Decca-Lane width. (About 7 metres when on the base line, lane widths also vary depending on area).
When in operation navigators are advised that both and errors exist and corrections need to be applied from
information gained from the Data Any dis-
turbance in the power supply can also cause malfunction by way of lane-slip. Other causes of this may be in the form of irregular transmissions, interference from other Decca Stations, strong atmospherics, damage to aerial or simultaneous reception of ground and sky waves.
Example 9 — Omega
The Omega system operates with eight world wide transmitting stations, of pair may be used to generate a of Transmission frequency for basic operation is 10.2 kHz which provides a lane width of 8 nautical miles.
NAVIGATION FOR MASTERS
Additional frequencies of 11.3 and 13.6 KHz are also employed and are used for lane identification purposes.
The system requires the operator to insert an
and this requires an accuracy of up to 4 The possibility of lane slip may exist due to any interruption of the trans- mission but some receivers are fitted with chart recorders which provide indication of broken transmissions.
Additional errors may be caused by
i.e. solar activity causing X-Ray emissions may cause a shift of phase at the Omega Receiver. Error values could range from 0.8 to 1.6 miles. Another known error is cap
caused by the magnetic poles attracting charged particles as- sociated with flares'. Range of errors could be from 0.8 to 4.0 Heavy rain showers can also affect transmissions in both Omega and Decca systems which may result in
occurring.
Accuracy is expected at about 1 to 2 miles but in practice it is realistically about 2 to 5 miles with an acceptable probability of error.
Omega requires its own charts and has world wide coverage when stations are operating at full power. Coastal monitoring stations advise vessels in their areas of known Omega errors. Operating recommendations are such that stations closer than 650 miles should not be used as transmitted signals in close to the station may be confused.