63 THE DAILY ROUTINE OF THE NAVIGATOR
247. THE DAY’S RUN.
It is customary in all vessels at sea, to determine the ships position as accurately as possible each day at noon. The distance sailed over the ground between successive noon’s is known as the Day’s Run. This distance divided by the sailing time, gives the average speed of the ship for that day. When making coastal passages, the day’s run is measured direct from the chart, and the difference between this measured distance and the distance recorded by the patent log is usually classified as favourable or adverse current.
When navigating out of sight of land, however, the distance made good must be found by calculation, because the scale of most sea charts is too small for distances to be found accurately by measurement. If a vessel has steered only one course throughout a particular day, the distance made good is calculated by Mercator or Mid. Lat. Sailing. If, however, she has steered several courses, the distance on each leg is estimated as accurately as possible, and the several distances are then summed to give the total run for the day. The direct course and distance made good for the day (i.e., between the two noon positions, ignoring the various legs steered) may then be calculated on sailing vessels, although this has no practical value to the navigator on a power-driven vessel.
The vessel’s noon D.R. Position, reckoned from the Observed Position at the previous noon, is compared with the new Observed Noon Position to give the set and drift of the current or tidal stream experienced during the day (assuming that no other factor has set the vessel off her desired course). Alternatively, if no Observed Position is obtainable, an E.P. can be obtained by applying the estimated set and drift to the D.R. Position.
248. DAYS WORK
Days Work – The business of finding a vessel’s D.R. or E.P., finding her course and distance made good between successive noon positions, finding her average speed, and finding the set and drift of the current experienced during the day, is known as the Day’s Work. It is a nautical form of day bookkeeping dealing with a vessel and her course and speed instead of with cash values.
Before embarking on Day’s Work problems, it is as well to be clear from the outset of certain basic definitions, although these should already be familiar from the practice of Coastal Navigation.
The Dead Reckoning (D.R.) Position (+) is the position deduced from the application of only the course and distance sailed.
The Estimated Position (E.P.) (∆) is the position deduced from taking into consideration the course and distance sailed, the estimated effect of current or tidal stream, and the estimated effect of leeway if applicable.
The Observed Position (Obs.)(⨁) is the position obtained from either visual bearings of shore objects, or from Astronomical sights, or by means of radio aids to navigation.
The procedure in Day’s Work problems is to reduce each course and distance sailed from the Observed position at noon on the previous day into the two components d.lat. and departure, by means of the Traverse Table. The sum of the various d.lat’s. will give the total Northing and Southing, and the sum of the various departures will give the total Easting and Westing.
The difference between the total Northing and the total Southing is the total d.lat. (named the same as greater of these totals W. or S.), and the difference between the total Easting and the total Westing is the total departure (named the same as the greater, E. or W.).
The total departure can be converted into d.long. either by inspection of the Traverse Table or by calculation, and the d.lat. and d.long. used to give the new D.R. Noon Position. If the known or estimated set and drift of the current or tidal stream experienced during the day, and the estimated leeway experienced (if any), have been included in the calculation, their resultant position will be the E.P..
The total d.lat. and the total dep. used in conjunction with the Traverse Table will give the Estimated Course and Distance Made Good for the day’s run. (Note that this is not the actual course and distance made good, as the second position used is only a D.R. or E.P. The actual course and distance made good. can only be calculated if an observed position is obtained at the second noon, and the course and distance obtained between this and the previous observed noon position).
If the first noon position is expressed as a bearing and distance from a known point, the d.lat. and dep. for the reversed bearing and distance is included in the summation of d.lat’s. and dep’s. for the total Northing /Southing and Easting /Westing for the purposes of finding either the D.R. or E.P., but this d.lat. and dep. must be subtracted from the totals again if the estimated course and distance made good is required.
The average speed for the day can be obtained by dividing the total distance sailed by the time sailed. The time sailed is not always necessarily 24 hours as the clocks may have been advanced or retarded during the day to conform with Standard Zone Time if the vessel has changed her Longitude considerably. If only one course has been steered throughout the day the total distance sailed will be the actual distance made good between the observed positions at noon on each day, and the average speed obtained from this will be the true average speed. If no observed position is obtainable at the second noon and an estimated distance made good is used, the resultant average speed will be the estimated average speed.
If several different courses have been steered throughout the day the distances on each leg should be totalled to give the true average speed in a power-driven vessel, or alternatively the actual direct distance made good may be used to give the average speed made good, usually only of value on a sailing vessel.
The true set and drift of the current experienced in the day is found by solution of the triangle between the D.R. Position and the Observed Position at the second noon, either by calculation or by Traverse Table, providing no leeway has been involved. Where leeway has been allowed, the current experienced is between the X position at the end of the C.W.L. (see chapter on Course Plotting).
Ex No. 4: At noon on Jul 20th , a vessel observed Lizard Hd. (Lat. 49º 58 N., Long. 005º 12.W.) brg 030º 15.0′ miles. Log set at zero, course set 230ºT. At 24:00, log 107.0′, course altered 168º T. At 12:00, 21st Jul, log 219.0′. Tidal stream estimated 297º (T) at 2 knots.
Find the E.P., and the Estimated C.M.G. at 12:00 on 21st Jul, and, if the Observed Position at 12:00 on 21st Jul was Lat. 46º 30.N., Long. 07º 36.4 W.,
Find the true set and drift of the current experienced and the Actual Course, Distance and Average Speed Made Good in the day’s run (Sailing time = 24h.00m.)
The first thing to do in problem of this sort is to draw a rough track diagram (fig. 63-1), to sort out just what factors are given and what factors are required. The next step is to draw-up a table (as shown on the next page), in which to sum the total d.lat. and dep. for each course and distance (and bearing and distance where applicable) in the problem.
249. TO FIND THE E.P. (BY TRAVERSE TABLE)
E.P. at NOON 21st Jul = Lat. 47º 08.4 N., Long. 07º 56.2 W.
To Find Estimated Course & Distance made Good (by Traverse Table)
Inspect the Traverse Table (T.T.) to find a course and distance where a d.lat. of 156.6 corresponds with a dep. of 101.5 ( FROM TABLE).
@ 33º, d.lat. 156.3, = dep. 101.5, dist. 186 4′) (Course. = 3/62 x 1 = S. 32º 57.0 W. (213º (T).
@ 32º. d.lat. 162.5, = dep. 101.5, dist. 191.6′) (Dist. = 3/62 x 5.2 = 186.6′ miles.
Estimated Course made Good = 213º T.
Estimated Distance made Good = 186.6′ miles.
If preferred, the above two steps can be calculated instead of using the Traverse Table, as follows: –
250. TO FIND THE E.P. (BY CALCULATION)
251. TO FIND ESTIMATED COURSE & DISTANCE MADE GOOD (BY CALCULATION) EX. A
Estimated Course and Distance Made Good: 213º T. x 186.6′ miles
252. TO FIND THE TRUE SET & DRIFT OF THE CURRENT EXPERIENCED
This is the amount by which the observed position differs from the D.R. Position (not the E.P.), or the bearing and distance of the Observed Position from the D.R. Position. The D.R. Position must therefore be found first, either by the Traverse Table, or by calculation as follows: –
True Set and Drift of Current Experienced = Set 242º T., Drift: 35.2′ miles.
(Rate = 35.2 1.47 knots.)
253. TO FIND THE ACTUAL COURSE & DISTANCE MADE GOOD
Actual Course and Distance Made Good: Course 204½º T., Distance 214.3′ miles.
E.P. at NOON 21st Jul: ——————– Lat. 47º 08.4 N., Long. 07º 56.2 W.
Est.Co. & dist. Made Good: ————— 213º T x 186.6 miles.
D.R. Posn. NOON 21st Jul: ————— Lat. 46º 46.6 N., Long. 06º 51.4 W.
Current Experienced: ——————– Set 242º , Drift 35.2, Rate 1.47 kts.
Actual Co. & dist. Made Good: ———– 204½º T. x 214.3 miles.
Average Speed Made Good: ————- 8.93 kts.
254. DAY’S WORK INVOLVING COURSE CORRECTION & LEEWAY.
In many Day’s Work problems compass courses are given, and these must be corrected to True courses by the application of’ the appropriate Variation and Dev.
Leeway being made, or leeway being applied, must also be allowed for in computing the True course being steered. None of these factors is particularly difficult, but they make Day’s Work calculations somewhat intricate and involved, and extreme care is necessary if errors are to be avoided. Nevertheless, the ability to compute Day’s Work is a vital aspect of a navigator’s training, for it enables him to keep continuous track of his vessel’s probable position, provided he has been recording all the relevant factors in his Log Book.
This will obviously be essential if he is caught out in circumstances, such as poor visibility, when he is unable to obtain an observed position or an accurate fix. Practice in Day’s Work under ideal shore conditions in which there are no elements of stress is therefore highly recommended, so that the navigator will find the process second nature to him when he is at sea, perhaps in adverse weather conditions in which his resources will be fully taxed with many other problems.
Day’s Work problems are an excellent navigational exercise, since they incorporate so many basic navigational principles, including Course Correction, Current and Leeway Application, use of the Traverse Table, knowledge and application of The Sailing formulae, definition of the different types of Position, and Chart work.
Although it has been said that it is customary to compute the Day’s Work at noon each day when making prolonged passages, this does not preclude the use of Day’s Work to calculate a D.R. or E.P. at any time one is required, and indeed every navigator should be able to produce such a position from the information at his disposal at all times of the day or night.
Particular care should be used in the application of leeway. Remember that leeway should be applied ONLY to True courses, never to compass or magnetic courses. Do not be confused by the distinction made between making so-many-degrees leeway and allowing so-many-degrees leeway. In Day’s Work problems the courses given always refer to Courses Steered, so that any allowance for leeway must be made away from the wind to give the Course Being Made Good, regardless of whether it is expressed as making or allowing. (Leeway is only applied towards the wind when finding a Course to Steer; this does not arise in Day’s Work problems).
Example From the following log extract, obtain the E.P. and Est. Cse & Dist. Made Good at 0600hrs., 08th Jun:
As in the previous example, the first thing to do is to draw a rough track diagram (see fig. 63-7) in order to picture the vessel’s movements. In problems of this type, the next thing to do is to correct all the given Compass courses into True courses before constructing the table in which to compute the total d.lat. and total departure.
All Day’s Work problems vary, both in the amount and type of information given, and in what is required to be found, so that no set rules can be formulated in the procedure to be followed in computing them. A sound knowledge of The Sailings and straightforward common sense are the fundamental requirements for calculating Day’s Work.
Lat. of Longships L.Ho. 50º 04.0 W. Convert dep. 27.6 to d.long. @ lat. 50º 40.0 N.
255. TO FIND ESTIMATED COURSE & DISTANCE MADE GOOD (BY CALCULATION) EX. B
Estimated Course and Distance Made Good: Course 341º T. Distance 78.6 miles.