10 PRACTICAL ASTRO-NAVIGATION
32 THE DAILY ROUTINE OF THE ASTRO-NAVIGATOR
123. THE NAVIGATOR AT WORK
The Master or skipper of a vessel carries the burden of responsibility for the safety of the vessel under his command. Closely related to safety is the navigation of the ship. Knowing where the vessel is at any time and, more important, knowing how the vessel is moving over the ground and being able to forecast here likely position at any time in the near future, are the principal problems of the navigator. When the vessel is away from land, in the absence of electronic aids to navigation, the principles and practice of Astro-navigation must be brought to bear in seeking answers to these problems.
On every vessel, large or small, one person is usually allotted the position of navigating officer, or more simply, the navigator, although this does not imply that others on board are not involved in the navigation of the vessel from time to time, and indeed the more people on board who are capable of taking sights and assisting the navigator the better that vessel, will be navigated. On merchant’s vessels, the navigation of the ship is performed collectively by the officers of the watch., although the Second Mate is usually designated the navigating officer. Each officer during his watch sees to it that the ship is never on an unsafe course. He checks the ship’s rate of progress by dead reckoning, using a compass and the log readings, and when possible and practicable by finding her position by observation. Such an ideal state of affairs may not be possible on a small craft; very often the skipper is the only one on board with the necessary navigational training and must act as skipper/navigator, but this is not a recommended practice since on a long passage if the skipper becomes incapacitated through accident or illness, the crew and the vessel are left stranded in mid-ocean without a navigator. At least one other person on board should be capable of navigating the vessel., this person is designated the navigator and thus leaves the skipper free to his overall responsibility for the safety of the vessel.
Some yacht skippers follow the dubious practice of advertising for a cook/navigator. This is highly reprehensible: the navigator will want to be out on deck when meals are being prepared and should not be expected to serve two masters. The cook is an important member of the crew: and so is the navigator. A cook/navigator is likely to come ashore after a voyage very tired, having done more than his (or her) fair share during the voyage, while the crew are likely to have been poorly fed and the vessel indifferently navigated.
The navigator should be responsible to the skipper. not only for the daily navigation of the vessel but also for the care and maintenance of all navigational instruments and equipment including the steering and hand-bearing compasses, sextants, chronometer and/or deck watch and clocks, the marine log, such electrical and electronic devices which may be fitted (such as echo-sounder, radio D/F set, radar etc. ) navigational charts and nautical publications ensuring that all of these are available for instant use when required.
The navigator should see that the charts for use during a voyage are corrected to the date of the latest available Notice to Mariners, and they should be arranged sequentially in order of use in a chartroom drawer. The chart used when the vessel is away from the land is usually a small-scale general chart of the area on or from which rough courses and distances may be plotted or measured. Course lines should be labelled with the true course printed neatly along the line in a three-figure notation. Times should be printed in 24-hour notation with reference to the appropriate Zone-Time (e.g., 16:45 (Zone-1). and all Descriptions should be neat and legible and located so that they least interfere with the navigator when he uses the chart or plotting sheet. Plotting is an important part of the work of an Astro-navigator. The tools of plotting include a plotting chart or sheet (those available were described in § 21-23 of this Astro-navigation Study). a sharp but soft pencil, a pair of dividers, a graduated straightedge and a good protractor. For plotting on a navigational chart, a parallel rule is another necessity. When plotting on a navigational chart or plotting sheet it is important that the chart in use alone occupies the chart table. The practice of using one chart or plotting sheet placed on top of another should be condemned, on account of the possibility of using the of the chart below for marking off distances and positions on the chart on top. Seldom do navigational charts have the same scales.
It is important that the navigator works accurately and methodically when solving his sights. To facilitate accurate working it is advisable to keep to a standardised system of reducing sights. Most navigators use systems which they have evolved according to their varied experiences and sight-working habits.
Familiarity with the layout of a solution to a sight or series of sights assists in checking and finding possible mistakes, and the use of standard sight forms such as those published by the Admiralty under reference N.P. 400A for use with Sight Reduction Table N.P. 401(H.D.605). similar to those supplied with this Course, is strongly recommended. The practical as well as the theoretical aspect of dealing with errors in positions and position lines, and a discussion on the cocked hat and multiple star fixes, will be given later in this chapter.
It will be obvious from the previous study of this study that the publications on board should include a set of Sight Reduction Tables, a copy of any standard set of Nautical Tables (such as Burton’s, Nories, or Inman’s), and of course, the N.A.. It is, however, important to ensure that the volume or volumes of Reduction Tables are adequate for the range of Lat. to be covered during the proposed voyage and also that if the duration of the voyage extends from one year into the next a copy of the next year’s N.A. is carried in addition to the one for the current year.
Other nautical publications required will include Sailing Directions and Light Lists for the areas in which the vessel will be navigated, not forgetting the destination areas at the end of a long sea passage. The Admiralty publication N.P136 Ocean passages for the World is another essential for long-distance passages.
All of these publications should have been studied before the commencement of the voyage, but they should also be available for reference at every stage of the voyage.
The navigator should wind the chronometer and/or deck watch at the same time each morning. He should also check the chronometer error from radio time signals at least once a day when the vessel is at sea, keeping a record, in the Chronometer Rate Book (see § 6-8 of this AN Course), of the daily rate, the accumulated error and the run and minimum temperatures in the chronometer locker. The navigator is also responsible for winding the ship’s clocks (usually once a week if they are of the 8-day variety) and for keeping them on the correct Zone Time by adjusting them one hour when the vessel passes from one Zone to the next.
The compass bearing of the Sun during the daytime., or that of a star or planet at night, should be observed at least twice in every 24 hours and compared. with the body’s true bearing to obtain the compass error and Dev. for the vessel’s head at the time of the observation. Since these are comparatively simple observations, there is no reason why they cannot be delegated to watch-keepers during the navigator’s normal watch below, provided the navigator accepts responsibility for the overall care and checking of the compasses throughout the voyage.
There are five times in a day when the navigator should take the opportunity (when available) of taking celestial observations for position fixing. The frequency of celestial observations and the routine and type of sight taken will depend on circumstances, location, and weather conditions. A point to bear in mind is that the last sight taken may be the last sight the navigator is able to take for perhaps several days. For instance, in mid-ocean if the weather is fair and no sights are seen for a week., then foul weather sets in and sights cannot be taken for a further six days owing to overcast conditions, storms or fog, it would then be nearly a fortnight without knowing the vessel’s position except by D.R. and in this interval the D.R. could be a long way out.
When more than 100 miles from land, the daily routine should be a forenoon Sun. sight, a meridian altitude at (local) midday, an afternoon Sun sight, plus multiple: (simultaneous) sights at morning and evening twilight. The forenoon and afternoon sights should be taken at about the time when the Sun is bearing as nearly as possible due E. or due W. i.e. is on the prime vertical. If it does bear exactly 090º or 270º then the sight will provide the Longitude at the time. When this position line is crossed with a meridian altitude position line at local noon the navigator will have a perfect cut at 90º. However, there are times and localities where the Sun will not cross the prime vertical or will do so only when it is at too low an altitude for a good sight. The Sun does not cross the prime vertical when the Lat. of the position and the Sun’s Dec. are of contrary names. In these circumstances, the best that can be done is to observe the Sun as soon as its altitude has reached about 15º in the morning, or when it has fallen to about this altitude in the afternoon. The position line will then not provide the immediate Longitude but (providing the Sun’s bearing is within say, 30º – 40º of due E or W) will produce a position line which may be transferred to cross the meridian altitude position line to give an acceptable cut.
The value of observing multiple (simultaneous) sights at morning and evening twilight lies not only in the facility of obtaining an instantaneous fix on the vessel’s .position, but also verifying the-accuracy of the sights being taken at other times in the day. If three or more (ideally six) sights are taken simultaneously, and all the position lines cross nearly at a point, or form only a small cocked hat, all must be good sights, correctly worked out and plotted. If only two sights are taken simultaneously, or if one is using only the Sun and a transferred position line, then one could consistently be getting false position lines and thus a false observed positions without being aware of it. This could occur for example, if one regularly mis-placed the Sun on the horizon (e.g. a gap between Sun and horizon, or the Sun’s limb overlapping the horizon) or were using an incorrect H. of. E. or a miscalculated I.E..
At morning and evening twilight the navigator has the choice of observing multiple stars, planets or the Moon as available. He should have no difficulty in recognising stars observed during morning twilight., as he will have had the opportunity of studying them during darkness before the time of sights. Stars observed during evening twilight are not so readily identified at the time of sights but the approximate bearings and altitudes of a suitable selection can be calculated in advance using one of the methods described in § 24-28 of the Astro Navigation study.
During evening twilight the eastern part of the sky becomes darker more quickly than the western part. Conversely, during morning twilight the eastern part of the sky becomes brighter more quickly than the western part. For this reason it is best that bodies in the eastern sky during morning twilight, and those in the western sky during evening twilight should, in general, be observed before those in the opposite half of the sky.
124. KEEPING NAVIGATIONAL RECORDS
Good navigation starts with good records of all navigational matters since written records are clearly necessary for a number of purposes. For navigation, accurate details of courses steered, times, distances sailed, compass error and Dev., variation, leeway, tidal streams or current, wind direction and strength, chronometer error, visual bearings observed, celestial bodies observed position lines and observed positions or fixes obtained are all required. For the historical records, broader details of passages, times of departure and arrival, names and next-of-kin of the crew, and similar details are required. To attempt to incorporate all this in a single book would be cumbersome and confusing. It is better to keep several different record books, each one for a specific purpose.
The records recommended are:-
A Deck Log Book (the vessel’s official log book) records all the vessel’s dimensions, equipment and crew on board in the opening pages, followed by daily pages on which are recorded all the hour-by-hour events, courses and positions together with times (in the appropriate Zone Time) and log readings (in miles and tenths). An example is shown in fig. 32-1.
A Scrap Log Book (usually identical to the Deck Log Book) made of waterproof paper for the helmsman or watch leader to record all events as they occur for later transfer to the Deck Log Book or fair copy.
A Sight Record Book, kept close to the chronometer or deck watch locker in which the altitude and D.W.T. of every celestial observation is recorded as it is observed together with the Zone Time and log reading.
A Chronometer Rate Book (fully described in § 06-08 of this study) for recording checks by Radio Time Signals and ascertaining the daily and accumulated rates of the chronometer or deck watch.
A Navigator’s Sight Form Book, which may be either a printed one such as the standard sight form N.P.400A referred to earlier, or an ordinary notebook ruled up by the navigator himself for working out his sights.
It is important that where an observed position is obtained after either simultaneous or double sights, this position is recorded (after plotting) in the Sight Form Book and in the Deck Log Book together with the Zone Time and log reading. The Sight Form Book is also used for calculating E.P.s, D.R. positions and the Day’s Run (course and distance made good and average speed between successive noons). all of which should also be entered by the navigator in the Deck Log Book.
A Ship’s Log in which, probably the skipper, will keep a narrative record of each passage, listing times of departure and arrival, describing interesting occurrences at sea and ports visited etc.
Of the above records the navigator will be solely responsible for (c), (d) and (e) but will also have to make important entries in (a) from time to time. Whenever the navigator takes a celestial sight the E.P. (E.P.) will be required. Whenever a fix (by landmarks) or an observed position (by celestial sights) is obtained, the fix or OP should be compared with the E.P. The E.P. is then abandoned and the fix or OP, if considered reliable, is then used to plot forward further E.P.s till a new fix or OP is obtained.
In earlier explanations of sights, reference was made to the D.R. position. The strict definition of a position by D.R. (Dead Reckoning) is one arrived at by reference only to the course steered and the distance sailed. The E.P. is therefore the D.R. position corrected for leeway and tidal stream or current. The E.P. should, of course, always be determined and used for working up sights.
When an observed position or a fix is compared with the E.P. for the same time and an appreciable difference between the two is found, it is advisable to investigate further and to endeavour to account for the difference. First, is the OP by sights reliable? If so, then perhaps the log is misreading, or the courses reported in the scrap log were not correctly recorded. Possibly the tidal stream or current, or the leeway allowed was wrongly assessed. Such checks give a backlog of experience on which to judge the reliability of the positions and to assess the validity of assumptions which have been made.