The Sailings comprise the various methods of calculating (rather than measuring, on a chart the course and distance between two points on the Earth’s surface. When the distance to be sailed by a vessel is not too great (up to about 600 miles), it is usual, whenever practicable, to sail along the Rhumb-Line track between the departure point and the destination point. For greater distances it may be considered an advantage to sail along the Great Circle track in order to reduce the distance sailed. This may not always be beneficial because, the Great Circle track takes the vessel into higher latitudes where the advantage gained in distance may be offset by the greater chance of adverse weather conditions.
In the previous chapter we said that the most convenient path for any vessel to sail was the Rhumb Line that connects the points of departure and destination because the course of the vessel remains constant throughout the passage. We defined a Rhumb Line as an imaginary line on the Earth’s surface that crosses every meridian at the same constant angle. In coastal navigation we are constantly laying off Rhumb-Line tracks on large scale charts between two points, measuring the requited course angle with a parallel ruler or equivalent instrument and measuring the distance between them with a pair of dividers, using the Lat. scale for distance reference. For long distance navigation, such as across the North Sea, Bay of Biscay, or across the Mediterranean, it is not usually possible to measure courses and distances along Rhumb-Line tracks on the chart with any degree of accuracy. This is because of the small scale of the charts available covering these sea areas, so this must be done by calculation rather than by chart measurement.
There are four methods of Rhumb-Line Sailing, each one a development of the previous one as the science of Navigation progressed from early days: Parallel Sailing, Plane Sailing, Mid. Lat. Sailing and Mercator Sailing. The first three of these are described in this chapter together with the use of the Traverse Table, (T.T.) that most useful of all tables for solving navigational triangles. The most modern of the Rhumb-Line Sailing, Mercator Sailing is described in the next Navigation Lesson.
In using all the Rhumb-Line Sailings, the same principles apply as are used in chart plotting. A D.R. position is one that has been worked up from the last position obtained from observations of celestial or terrestrial objects, and makes no allowance for tidal stream, current or leeway. When a navigator desires to know his vessel’s position, he applies the last known observed position courses and distances sailed through the water, and then to this D.R. position he applies an estimated allowance for leeway, tidal streams or current in order to obtain his E.P.. The (E.P.) is the most reliable position obtainable without observations.
When it is necessary to sail into the open sea it is essential, before the land is lost to view, that the position of the is found by terrestrial observation, so obtaining a reliable position from which the calculated course may be set. Such a position is called the Departure Position (not to be confused with departure to be described later in this Lesson). The point desired to reach may not necessarily be the actual final destination of the vessel but merely a position off a distant headland from which a further course or courses will have to be set, but is nevertheless called the Destination Position.
Parallel Sailing. When a vessel sails on any course other than due N. or due S., she moves some distance easterly or westerly. The value of this movement in an E.-W. direction is called Departure (usually abbreviated to dep.) and it may be represented by the arc of a parallel of Lat. cut off between the meridians of the points of the initial and final positions on the Rhumb-Line, measured in nautical miles.
If a vessel sails along the equator between two places then the departure in Long., (d.long.), between the two places in minutes of arc, will be numerically equal to the departure between the two places in nautical miles, because at the equator, minutes of Longitude are equal to minutes of Lat. or nautical miles. But in any other Lat. when a vessel sails along a parallel of Lat. (i.e. when her course is either due E. or due W.). The departure measured in nautical miles will always be numerically less than the d.long. in minutes of arc between the initial and final positions, because of the convergence of the meridians towards the poles.
In fig. 61-1/2 XY is the arc of a parallel of Lat. and is thus the distance along the parallel between the meridians through X and Y. AB is the distance along the equator between the same meridians. XY is therefore the departure between X and Y and AB the d.long. between X and Y. It will be apparent that the nearer the parallel is to the pole (in other words, the higher the Lat.) the shorter XY, becomes, but the d.long. does not alter, XY must therefore bear to AB some relation depending on the Lat.. To find this relation, consider the sections of the sphere DXY and CAB. They are parallel and equiangular, therefore: -
XY = DX
AB = CA
But in triangle DCX, ∠DXC = ∠XCA = Lat. of X, and as triangle DCX is a right-angled triangle, from our knowledge of trigonometrical ratios: - Cosine ∠DXC = DX
Therefore DX = CX x Cosine ∠DXC
But ∠DXC = ∠XCA (Lat.) and CX = CA (as each is a radius of the Earth)
Therefore DX = CA x Cosine Lat.
But XY = Departure, and AB = d.long., so we get the basic Parallel Sailing formula which is:
Departure = cos.lat. --------------------①
From which, by transposing and inversion, we can also get: -
departure. = d.long. x cos.lat. ---------------------②
d.long. = sec.lat. ---------------------③
d.long. = Departure x sec.lat. ---------------------④.
These four formulae are known collectively as the Parallel Sailing Formulae. Parallel Sailing originated in the very early days of Navigation when there was no accurate means of determining Longitude. A vessel sailed due N. or due S. in a known Longitude until she reached the Lat. of her destination, then she sailed due E. or due W. along the parallel of this Lat. to her destination. Parallel Sailing is important to us today in expressing the relationship between departure, d.long. and Lat., all of which are used in more modern forms of Rhumb-line Sailings.
This plane right-angled, triangle is called, the Plane Sailing triangle, but it should be realised that it is not a triangle on the Earth’s surface; it is simply an artifice which shows the relation between a Rhumb--line course and distance, d.lat., and departure. The course angle in Plane Sailing triangles, and indeed in all Rhumb-line sailing triangles, being right-angled triangles, must be an acute angle (i.e., less than 90º). For this reason, the Course steered on the compass, usually expressed in the 360º notation, must be converted to a quadrantal one to get the correct course angle to use in the Sailings formulae.
In other words, the course angle must be expressed from either N. or S. towards E. or W. A quick sketch will usually solve this course angle problem. For instance, a course steered of 158º is obviously in a SSE’ly direction, and the triangle completed by drawing a vertical line for the d.lat. and a horizontal dep. one for the departure. The course angle will then be seen to be 180º-158º = 22º (or S22ºE in the old quadrantal notation).
A course of 210º is SW’ly, and completing the triangle shows the course angle to be 210º-180º = 30º. Similarly, a course of 320º is NW’ly and the angle in this case is 360º-320º = 40º.
Examples of finding the correct course angle are illustrated in fig. 61-4.
sine X = ZY
XY Therefore ZY = sine X x XY
But in the Plane-Sailing triangle: ZY Departure, XY = Distance sailed, X = Course
Therefore departure = sine Course x Distance ______①
In ∠XYZ, cosine X = ZX, therefore ZX = cosine X x XY
Therefore in the Plane-Sailing Triangle:
d.lat. = Cosine Course x Distance ______②
In ∠XYZ, Tangent X = ZX , so in the Plane Sailing triangle:
tan. Course = Departure ______③
In ∠XYZ, Secant X = XY, therefore XY = secant X x ZX
Therefore in the Plane-Sailing Triangle:
distance. = secant Course x d.lat. ______④
These four formulae constitute the Plane-Sailing relationships, and with them the navigator can find the course and distance along a Rhumb-line track between two positions, or, having steered a certain course for a certain distance, he can find the departure he has made good and his final Lat.. It should be noted, however, that it is not possible to find the final Longitude by Plane-Sailing. When sailing obliquely across the meridians, the Lat. is changing constantly and difficulty arises in determining the correct Lat. to use in order to convert departure into d.long. To find the position of a vessel after she has sailed a given distance such that her Lat. and Longitude change, the navigator must use Mid. Lat. Sailing
In fig. 61-5, XY represents a Rhumb Line connecting two places in the Northern Hemisphere, X being on the parallel of Lat. AB and Y being on the parallel of Lat. AY. CD is a parallel of Lat. the exact arithmetical mean between XB and AY; in other words, if XB is Lat. 20º N. and AY is Lat. 60º N. CD would be in Lat. 40º N.
If a vessel sailed from A to Y her departure would be the distance along the arc AY, and if she sailed from X to B, her departure would be the distance along the arc XB, but if she sailed along the Rhumb-line from X to Y, her departure would obviously be greater than AY but less than XB. The actual departure between X and Y must be the distance along some parallel between AY and XB, and this parallel is not, as might be expected, the mean parallel CD between AY and LB, but another parallel, MN1 which is called the Mid. Lat..
If the Earth were perfectly spherical, the Mid. Lat. would be greater than the mean, or average Lat. of two places, but as the Earth is not a perfect sphere, in some cases the Mid. Lat. can be less than the mean Lat. as the correction tables will show. The difference between the mean Lat. and the Mid. Lat. depends on the d.lat. and the mean Lat. of the two places, and is given in a small table Mean Lat. to Mid. Lat. in all nautical tables. The correction taken from the table is applied to the mean Lat. to give the Mid. Lat.. For example, suppose a vessel sailed along a Rhumb-line track between Lat. 17º N. and Lat. 21º N. The mean Lat. would be 19º N, but from the correction tables for a mean Lat. of 19º, and a d.lat. of 4º we find the correction to apply to the mean Lat. is -60', or –1º, so the Mid. Lat. in this case is 19º-1º-18º N.
Having found the Mid. Lat. in this way, the problem of Mid. Lat. Sailing is merely a matter of combining the formulae used in Parallel and Plane Sailing, substituting in the case of the basic Parallel Sailing formula the more accurate Mid. Lat. Sailing formula:
The normal navigational problems, namely: (a) finding the Rhumb-line track course and distance from one position to another, and (b) finding the final position given the course and distance sailed along a Rhumb-line track can be solved by means of Mid. Lat. Sailing as follows.
Course and Distance from one position to another: since the d.long. is known and the Mid. Lat. can be found: departure = d.long. x cosine mid.lat.
But Tangent Course = departure
So Tangent Course = d.long. x cosine mid.lat. ____________①
And distance = d.lat. x secant Course. ____________②
The Position from Course and Distance Sailed: since the course and distance are known and the Mid. Lat. can be found:
d.lat. = distance. x cosine Course. ____________③
departure = distance. x sine Course. - But d.long. = dep. x secant mid.lat.
So d.long. = dist. x sine Co. x sec.mid.lat. ____________④
Students will have noticed that the formulae for Rhumb Line sailing so far discussed do no more than solve an ordinary right-angled triangle, the hypotenuse of which is the distance, the vertical side the d.lat. and the horizontal one the departure. To enable the navigator to solve this triangle quickly, all volumes of nautical tables provide a Traverse Table.
The Traverse Table is an orderly collection of solutions to right-angled triangles, which may be used for solving any plane right-angled triangle, but its main uses in practical navigation are: -
to find the course and distance between two places, and
to find the position after sailing a given course and distance.
As these are the two principal functions of the Rhumb-line Sailings, it will be appreciated that the Traverse Table is, without any doubt, the most useful in any volume of nautical tables.
The Traverse Table tabulates d.lat. and departure for any distance up to 600 miles and for any course angle up to 90º. The table appears to extend only to 45º, but the angles between 45º and 90º are printed at the bottom of the pages and the columns headed d.lat. and dep. at the tops of the pages are labelled, respectively, dep. and d.lat. at the bottom of the pages. The reasons for this will become clear by studying the two triangles in fig. 61-10.
In any right-angled triangle, the two non-90º angles are complementary angles (i.e., their sum equals 90º), because the sum of the angles in any plane triangle is 180º. To solve a Sailing Triangle where the course angle is 20º and the distance sailed is, say, 100 miles, the Traverse Table (fig. 61-12) is entered at angle 20º, and abreast of 100 in the distance column will be found the values 94.0 for d.lat. and 34.2 for departure.
To solve a Sailing Triangle where the course angle is 70º and the distance 100 miles, the Traverse Table is entered at the same page, i.e., at the page for angle 20º (which is the complement of 70º- 90º minus 20º equals 70º). It will be seen that 70º is printed at the bottom of this pages and reading the columns from the bottom of the pages opposite the distance of 100 will be found the values 34.2 for d.lat. and 94.0 for departure. Comparison of these two triangles in (fig. 61-10) shows that in fact they are one and the same triangles that the triangle for course 70º is merely the triangle for course 20º turned round. The angles at the top and bottom of each page of the Traverse Table are therefore complementary and the table need only extend to 45º because the triangles for angles between 45º and 90º are the same as those for angles between Oº and 45º, only turned round so that, in this case, where the d.lat. for a course of 20º is 94.0, for a course of 70º, 94.0 is the departure.
A navigator following a course between 000º and 090º T. is moving both N. and E.; his d.lat. is thus N. and his departure E.. Following a course between 090º and 180º T., his d.lat. is S. and his departure again E.. These facts indicate that, before using the Traverse Table the course angle must be expressed in relation to the cardinal points of the compass as described in section for finding the course angle to use in Plane Sailing triangles. Since the Traverse Table does no more than multiply the distance by the sine or cosine of an angle, as in the above example:
100 X SIN. 20º = 34.2 - 100 X COS. 20º = 94.0
it can be used for converting departure into d.long. or d.long. into departure. for in the Mid. Lat. Sailing formula, dep. = d.long. x cos. mid. lat. Using the Traverse Table for this operation, it is therefore sufficient to treat the d.long. as distance and the Mid. Lat. as the course, reading off the departure in the d.lat. column. The appropriate columns in the Traverse Table are bracketed d.long. and Dep. accordingly.
dep. d.long. dist. dep.d.long.
The Traverse Table is fundamentally simple, but facility in its use can only be acquired with practice, and students should take every opportunity to become thoroughly familiar with this, the most useful table in all navigation.
To convert d.long. into dep. at mid.(mean) lat. 43º 15.0, find the dep. for a d.long. of 160 at 43º and 44º and interpolate.
Enter the Traverse Table at angle 43º, and using the bracketed d.long. and dep. columns, opposite a d.long. of 160 will be found dep. 117.0.
Enter the T.T. at angle 44º, and opposite a d.long. of' 160 will be found a dep. of 115.1.
The departure for a d.long. of 160 at mean lat. 43º 15.0 is therefore 116.5. To find the course and distance, search through the T.T. until a d.lat. of 50 corresponding to a dep. of 116.5 is found.
At angle 66º a d.lat. of 50.0 corresponds to a dep. of 112.4 at Distance 123.
At angle 67º a d.lat. of 50.0 corresponds to a dep. of 112.4 at Distance 128.
The difference between the two departures for 50.0 d.lat. = 117.8 - 112.4 = 5.4.
To convert d.long. into dep. at mid.lat. 56º 17.0 N., find the dep. for a d.long. of 624 at angles 56º and 57º and interpolate for the 17.
Enter the T.T. at angle 56º, and using the bracketed dep./d.long. columns, search for a d.long. of 624 . It will be found that the d.long. column only extends to 600, and when this occurs the procedure is to divide by 2 (or any other convenient figure) and multiply the extracted figure by the same amount. In this case, if we divide by 2, d.long. 312 in the table gives us dep. 174.5 which, multiplied by 2, gives us dep. 349.0 for the required d.long. of 624.
At angle 56º, d.long- 312 gives dep. 174.5 x 2 dep. 349.0) diff. 9.2
At angle 57º 169.9 x 2 339.8)
For d.long. 624 at mid.lat-56º 17.0, dep. = 17 x 9.2 from 349.0 towards 339.8
To find the course and distance, the T.T. must be searched until a d.lat. of 328 corresponds to a dep. of 346.3. It will be found that:
At angle 46º, d.lat. 327.9 corresponds to a dep. of 339.5 at Distance 472.0
d.lat. 328.0 339.6 472.2
At angle 47º d.lat. 328.0 351.8 481.0
We require d.lat. 328 to correspond to a dep. of 346.3, so -
The dif. between the two departures. corresponding to d.lat. 328, =351.8 - 339.6 = 12.2
The dif. between the req. dep. and the dep. for 328 at 46º = 346.3 - 339.6 = 6.7
The required Course angle is 6.7 of the way between 46º and 47º = N 46º 33.0 E. 12.2
The required Distance is 6.7 of the way between 472.2 and 481.0 = 477.0
From River Humber to Oslo Fjord, Course = 046½º T., Distance = 477.0
(which compares with calculated course and distance on page 5).
Enter the T.T. at angle 38º, and reading the columns bracketed Dep./D.long, look for a dep. of 492.4 in the dep. column. It will be found that this column only extends to a dep. of 472.81. so divide the required dep. by 2 and multiply the extracted d.long. by 2.
Final position of yacht: Lat. 38º 00.2 N., Long. 10º 16.2 E.
(which compares with calculated position in Ex. 4)
In general, for distance up to about 600 miles (the limit of the Traverse Table), the Traverse Table may be used to solve all Sailing triangles and the use of the arithmetical mean Lat. without correction to Mid. Lat., will usually suffice in these circumstances. It is, however, advisable to check with the Mean to Mid. Lat. Correction table to ascertain whether the correction is appreciable, if it is, the Mid. Lat. should be used for greater accuracy. When the distance involved is greater than 600 miles, the Sailing should be calculated by Mid. Lat. Sailing or Mercator Sailing (described in the next Navigation Lesson).
In cases where the conversion between departure and d.long. or vice versa is awkward, involving elaborate interpolation, a combination of the use of the Traverse Table and the calculation methods may be used if preferred, extracting the d.lat. and dep. from the Traverse Table and then converting the dep. into d.long. by formula.
It should be appreciated that courses found by the Sailings, whether calculated or extracted from the Traverse Table, are true courses; similarly, final positions found by the Sailings are D.R. positions, as in neither case has any allowance been made for current, tidal stream, or leeway.
The Sailings, can, however, be used to find the Course to Steer between two places, or an E.P., if due allowance is made for current, tidal stream and leeway when appropriate, as shown in the following examples.
In (fig. 61-13), N represents Newhaven and C Cherbourg. The course required to make good is the Rhumb-Line NC. But from our knowledge of chart-work we know that in order to counteract the tidal stream we must lay-off the estimated set and drift D.LAT. from the departure position, (and this is represented by the line HP) and from position P lay-off the course to steer to the destination C. The ∆ NOP can be solved quickly with the Traverse Table to find position P, since the set (∆-ONP) and the drift (side NP) are both known, and these may be considered the course and distance.
From the T.T., angle 70º Dist. 121 gives d.lat. 4.1, dep. 11.3.
Convert dep. 11.3 into d.long. at mean lat. 50¼º = d.long. 17.9
In 4 hours, motor cruiser sails 4 x 7 = 28 miles on course 155º T. (S. 25º E.)
Tidal stream drifts 4 x 1.5 = 6 miles, setting 245º T. (S. 65º W.)
This problem could be worked by solving first triangle NCD (fig. 61-13), to find the D.R. position D, and then solving triangle DEF to find the E.P. position E, but it is more convenient to combine the working to find the total d.lat. and the total dep., and thus solve the triangle NGE.
In (fig. 61-14), N is the initial position off Newhaven, and triangle NCD represents the Sailing triangle for her course and distance in 4 hrs., D being the D.R. position after this time. The line DE represents the set and drift of the current in the same time (i.e., 245º T. x 6 miles), and the triangle DEF (in which DF is the d.lat. and FE the dep.) when solved will give the E.P. position E. By using the combined d.lat's. and deps. of these two triangles (NCD and DEF) however, we can not only find position E but also the direction and length of line NE which represents the course and distance made good in the four hours, because in the triangle NGE, GNE is the course made good, and NE is the distance made good. The figure shows that the d.lat's. from the first two triangles NCD and DEF, NC+DF = NC+CG = NG in triangle NGE, and the deps. from the first two triangles, CD - FE = GF - FE = GE in triangle NGE. We now have the d.lat.(NG) and the dep.(GE) of the triangle NGE, from which we can find course angle GNE and distance NE.
From the T.T. course 25º x distance 281 = d.lat. 25.4 S. dep. 11.8 E.
course 65º x distance 6 = 02.5 S. 05.4 W.
Total d.lat. 27.9 S. Total 06.4 E.
Initial lat. 50º 47.1 N.
d.lat. 27.9 S.
Final lat. 50º 19.1 N.
Initial lat. 50º 47.0 N.
2) 101º 06.1
Mean lat. 50º 33.1 N.
Convert dep. 6.4 into d.long. at mean lat. 50º 33.1. By T.T.:
At angle 50º dep. 6.4 d.long. 10.0 Initial long. 00º 04.0 E.
At angle 51º 10.2 d.long. 10.1 E.
At angle 51½ 10.1 Final long. 00º 14.1 E.
To find the course and distance made good (∠GNE and dist. NE), find a course angle and distance where a dep. of 6.4 E. corresponds with a d.lat. of 27.9 S.
At angle 12º , dep. 6.4 corresponds with d.lat. 30.13 at distance 30.80
At angle 13º , 27.72 28.45
Course = 12º + (2.23 x 60) = S. 12º 56.0 E. (167º T.)
Distance = 30.8 – (2.23 x 2.35) = 28.6 miles.
E.P. after 4 hours = Lat.50º 19.1 N., Long. 00º 14.1 E.
Course and Distance made good. = Course 167º T. Distance 28.6