Chapter 1

Chapter 13,888 wordsPublic domain

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[Transcriber's Note:- Words and phrases surrounded by underscores are italicised. There are four symbols which are denoted by :- (.) Circle surrounding a centred dot - Right Ascension of the Meridian; (+) Circle surrounding a cross - Earth's Central Progress; (_) Circle with centre dot and line under - Observed altitude of sun's lower limb; -(-)- Circle with line through - True Altitude. ]

LECTURES IN NAVIGATION

Prepared for Use as a Text Book

at the

OFFICERS' MATERIAL SCHOOL NAVAL AUXILIARY RESERVE

Lieutenant ERNEST G. DRAPER, U.S.N.R.F.

Head of the Department of Navigation

Officers' Material School, Naval Auxiliary Reserve

FOREWORD

These Lectures have been compiled as speedily as possible to meet the demand for some quick but fairly comprehensive method whereby large bodies of men, divided into small classes, might learn the elements of Navigation and thus assume, without delay, their responsibilities as Junior Officers of the deck, Navigators and Assistant Navigators in the United States Naval Auxiliary Reserve.

I realize that the haste with which the book has been written is apparent in many places, and it is hoped that many evidences of this haste will disappear in case further editions are printed. Besides acknowledging the help and information which was secured from the list of navigational works, mentioned on another page, I wish to mention particularly Prof. Charles Lane Poor's book, entitled "Nautical Science," from which was secured practically all of the information in the Lecture on Planets and Stars (Tuesday--Week V); Commander W. C. P. Muir's book, "Navigation and Compass Deviations," and Lieutenant W. J. Henderson's book, "Elements of Navigation," the text of which was followed closely in discussing Variation and Deviation and Traverse Sailing.

I desire to express my gratitude to Lieutenant Commander R. T. Merrill, 2nd, U. S. N., for suggesting a detailed outline of the whole course; to Lieutenant Commander B. O. Wills, U. S. N., for his valuable criticisms and almost daily help during the preparation of these Lectures; to Lieutenant (j. g.) C. D. Draper, U. S. N. R. F.; Lieutenant (j. g.) R. Brush, U. S. N. R. F., and Lieutenant (j. g.) P. C. McPherson, U. S. N. R. F., for many criticisms and suggestions; and to Captain Huntington, Seamen's Church Institute, for suggesting helpful diagrams, particularly the one on page 44. This opportunity is also taken for thanking the many Instructors in the School for their opinions on various questions that have come up in connection with the course and for assistance in eliminating errors from the text.

E. G. D.

LIST OF BOOKS CONSULTED

AMERICAN PRACTICAL NAVIGATOR, BOWDITCH

NAVIGATION AND COMPASS DEVIATIONS, MUIR

NAUTICAL SCIENCE, POOR

ELEMENTS OF NAVIGATION, HENDERSON

WRINKLES IN PRACTICAL NAVIGATION, LECKY

WHYS AND WHEREFORES OF NAVIGATION, BRADFORD

EPITOME OF NAVIGATION, NORIE

NAVIGATION, HOSMER

FINDING A SHIP'S POSITION AT SEA, SUMNER

GENERAL ASTRONOMY, YOUNG

PREFACE

TO THOSE TAKING THIS COURSE IN NAVIGATION:

These lectures have been written with the idea of explaining, in as simple language as possible, the fundamental elements of Navigation as set forth in Bowditch's American Practical Navigator. They will be given you during the time at the Training School devoted to this subject. At present this time includes two morning periods of one and a half hours each, separated by a recess of fifteen minutes. In general the plan is to devote the first period to the lecture and the second period to practical work.

Not many examples for practical work have been included in this book, but one example, illustrating each new method, has been worked out. If you understand these examples you should be able to understand others similar to them.

Toward the end of the course a portion of each second period will be devoted to handling the sextant, work with charts, taking sights, etc. In short, every effort will be made to duplicate, as nearly as possible, navigating conditions on board a modern merchant ship.

DEPARTMENT OF NAVIGATION,

_Officers' Material School_,

_Naval Auxiliary Reserve_

CONTENTS

WEEK I--PILOTING

_Tuesday_--The Compass 1

_Wednesday_--Pelorus; Parallel Rulers; The Lead, Sounding Machine, Dividers and Log 6

_Thursday_--The Chart 10

_Friday_--The Protractor and Sextant 13

_Saturday_--Fixes, Angles by Bearings and Sextant 16

WEEK II--DEAD RECKONING

_Tuesday_--Latitude and Longitude 20

_Wednesday_--Useful Tables--Plane and Traverse Sailing 23

_Thursday_--Examples on Plane and Traverse Sailing (Continued) 27

_Friday_--Mercator Sailing 28

_Saturday_--Great Circle Sailing--The Chronometer 30

WEEK III--CELESTIAL NAVIGATION

_Tuesday_--Celestial Co-ordinates, Equinoctial System, etc. 34

_Wednesday_--Time by the Sun--Mean Time, Solar Time, Conversion, etc. 36

_Thursday_--Sidereal Time--Right Ascension 43

_Friday_--The Nautical Almanac 47

_Saturday_--Correction of Observed Altitudes 52

WEEK IV--NAVIGATION

_Tuesday_--The Line of Position 55

_Wednesday_--Latitude by Meridian Altitude 58

_Thursday_--Azimuths of the Sun 61

_Friday_---Marc St. Hilaire Method by a Sun Sight 63

_Saturday_--Examples on Marc St. Hilaire Method by a Sun Sight 66

WEEK V--NAVIGATION

_Tuesday_--A Short Talk on the Planets and Stars--Identification of Stars--Time of Meridian Passage of a Star 66

_Wednesday_--Latitude by Meridian Altitude of a Star--Latitude by Polaris 73

_Thursday_--Marc St. Hilaire Method by a Star Sight 74

_Friday_--Examples: Latitude by Meridian Altitude of a Star; Latitude by Polaris; Marc St. Hilaire Method by a Star Sight 75

_Saturday_--Longitude by Chronometer Sight of the Sun 76

WEEK VI--NAVIGATION

_Tuesday_--Longitude by Chronometer Sight of a Star 79

_Wednesday_--Examples on Longitude by Chronometer Sight of a Star 80

_Thursday_--Latitude by Ex-Meridian Altitude of the Sun 81

_Friday_--Examples: Latitude by Ex-Meridian Altitude of the Sun 83

_Saturday_--Finding the Watch Time of Local Apparent Noon 83

WEEK VII--NAVIGATION

_Tuesday_--Compass Error by an Azimuth 88

_Wednesday_--Correcting Longitude by a Factor 89

_Thursday_--The Navigator's Routine--A Day's Work at Sea 91

_Friday_--Day's Work 105

_Saturday_--Day's Work 105

WEEK VIII--NAVIGATION

_Monday to Thursday_--Day's Work 107-108

_Additional Lecture_--Compass Adjustment 109

WEEK I--PILOTING

TUESDAY LECTURE

THE COMPASS

Everyone is supposed to know what a compass looks like. It is marked in two ways--the old way and the new way. Put in your Note-Book this diagram:

The new way marked on the outside of the diagram, starts at North with 0 deg., increases toward the right through East at 90 deg., South at 180 deg., West at 270 deg. and back to North again at 360 deg. or 0 deg..

The old way, marked on the inside of the diagram, starts at North with 0 deg., goes to the right to 90 deg. at East and to the left to 90 deg. at West. It also starts at South with 0 deg., goes to the right to East at 90 deg. and to the left to West at 90 deg..

A Compass Course can be named in degrees, according to either the new or old way. For instance, the new way is just 45 deg.. The old way for the same course is N 45 deg. E. New way--100 deg.. Old way for same course--S 80 deg. E.

There is another way to name a compass course. It is by using the name of the point toward which the ship is heading. On every ship the compass is placed with the lubber line (a vertical black line on the compass bowl) vertical and in the keel line of the ship. The lubber line, therefore, will always represent the bow of the ship, and the point on the compass card nearest the lubber line will be the point toward which the ship is heading.

The compass card of 360 deg. is divided into 32 points. Each point, therefore, represents 11-1/4 deg.. The four principal points are called _cardinal_ points. They are--North, East, South, West. Each cardinal point is 90 deg. from the one immediately adjacent to it. It is also 8 points from the one adjacent to it, as 90 deg. is 8 points, i.e., 11-1/4 deg. (one point) times 8. Midway between the cardinal points are the inter-cardinal points. They are--N E, S E, S W, N W, and are 45 deg. or 4 points from the nearest cardinal point. Midway between each cardinal and inter-cardinal point--at an angular distance of 22-1/2 deg. or 2 points, is a point named by combining a cardinal point with an inter-cardinal point. For instance, NNE, ENE, ESE, SSE, SSW, WSW, WNW, NNW. Midway between the last points named and a cardinal or inter-cardinal point, at an angular distance of 11-1/4 deg., is a point which bears the name of that cardinal or inter-cardinal point joined by the word _by_ to that of the cardinal point nearest to it. As, for instance, N by E, E by N, E by S, S by E, S by W, W by S, W by N, N by W. Also NE x N, NE x E, SE x E, SE x S, SW x S, SW x W, NW x W, NW x N. The angular distance between each and every whole point is divided into 4 parts called half and quarter points and each representing an angular measure of approximately 2 deg. 49'. In mentioning fractional points, the U. S. Navy regulations are to name each point from North and South toward East and West except that divisions adjacent to a cardinal or inter-cardinal point are always referred to that point: For instance, N 1/2 E, N x E 1/2 E, NE 1/2 N, NW 1/2 N, NW 1/4 W, NW 3/4 W, NW 1/4 N.

Boxing the compass is naming each point and quarter-point in rotation, i.e., starting at North and going around to the right back to North again. Every man should be able to identify and name any point or quarter-point on the compass card.

In changing a point course into a degree course, for either new or old compass, a guide is herewith furnished you. This should be pasted into the front of your Bowditch Epitome. It shows, from left to right, the name of the point course, its angular measure in the new compass and its angular measure in the old compass. It also shows at the bottom, the angular measure of each division of one point. In understanding this guide, remember that each course is expressed in degrees or degrees and minutes.

Put in your Note-Book:

In Navigation, each degree is written thus deg.. Each fraction of a degree is expressed in minutes and written thus '. There are 60' in each degree. Each fraction of a minute is expressed in seconds and is written ". There are 60" in each minute. Four degrees, ten minutes and thirty seconds would be written thus: 4 deg. 10' 30".

Although this guide just given you is given as an aid to quickly transfer a point course into a new or old compass course--or vice versa--you should learn to do this yourself, after awhile, without the guide.

Put in your Note-Book:

I will show you just how each one of these courses is secured from the guide just given you.

Note to Instructor: After explaining these courses in detail, assign for reading _in the class room_ the following articles in Bowditch: Arts. 25-26-27-28-29-30-31-32, 74-75-76-77-78-79-80-81-82.

Every compass, if correct, would have its needle point directly to the real or _true_ North. But practically no compass with which you will become familiar will be correct. It will have an error in it due to the magnetism of the earth. This is called Variation. It will also have an error in it due to the magnetism of the iron in the ship. This is called Deviation. You are undoubtedly familiar with the fact that the earth is a huge magnet and that the magnets in a compass are affected thereby. In other words, the North and South magnetic poles, running through the center of the earth, do not point true North and South. They point at an angle either East or West of the North and South. The amount of this angle in any one spot on the earth is the amount of Variation at that spot. In navigating a ship you must take into account the amount of this Variation. The amount of allowance to be made and the direction (i.e. either East or West) in which it is to be applied are usually indicated on the chart. On large charts, such as those of the North Atlantic, will be found irregular lines running over the chart, and having beside them such notations as 10 deg. W, 15 deg. W, etc. Some lines are marked "No Variation." In such cases no allowance need be made. On harbor charts or other small charts, the Variation is shown by the compass-card printed on the chart. The North point of this card will be found slewed around from the point marking True North and in the compass card will be some such inscription as this: "Variation 9 deg. West in 1914. Increasing 6' per year."

Now let us see how we apply this Variation so that although our compass needle does not point to true North, we can make a correction which will give us our true course in spite of the compass reading. Note these diagrams:

The outer circle represents the sea horizon with the long arrow pointing to true North. The inner circle represents the compass card. In the diagram to the left, the compass needle is pointing three whole points to the left or West of True North. In other words, if your compass said you were heading NE x N, you would not actually be heading NE x N. You would be heading true North.

In other words, standing in the center of the compass and looking toward the circumference, you would find that every true course you sailed would be three points to the _left_ of the compass course. That is called Westerly Variation.

Now look at the diagram to the right. The compass needle is pointing three whole points to the right or East of True North. In other words, standing in the center of the compass and looking toward the circumference, you would find that every true course you sailed would be three points to the _right_ of the compass course. That is called Easterly Variation.

Hence we have these rules, which put in your Note-Book:

_To convert a compass course into a true course_

When the Variation is westerly, the true course will be as many points to the left of the compass course as there are points or degrees of Variation. When the Variation is easterly, the true course will be as many points or degrees to the right of the compass course.

_To convert a true course into a compass course_

The converse of the above rule is true. In other words, Variation westerly, compass to the right of true course; variation easterly, compass course to the left.

DEVIATION

As stated before, Deviation causes an error in the Compass due to the magnetism of the iron in the ship. When a ship turns, the compass card does not turn, but the relation of the iron's magnetism to the magnets in the compass is altered. Hence, every change in course causes a new amount of Deviation which must be allowed for in correcting the compass reading. It is customary in merchant vessels to have the compasses adjusted while the ship is in port. The adjuster tries to counteract the Deviation all he can by magnets, and then gives the master of the ship a table of the Deviation errors remaining. These tables are not to be depended upon, as they are only accurate for a short time. Ways will be taught you to find the Deviation yourself, and those ways are the only ones you can depend upon.

Put in your Note-Book:

Westerly Deviation is applied exactly as westerly Variation. Easterly Deviation is applied exactly as easterly Variation.

The amount of Variation plus the amount of Deviation is called the Compass Error. For instance, a Variation of 10 deg. W plus a Deviation of 5 deg. W equals a compass error of 15 deg. W, or a Variation of 10 deg. W plus a Deviation of 5 deg. E leaves a net compass error of 5 deg. W.

LEEWAY

Leeway is not an error of the compass, but it has to be compensated for in steaming any distance. Hence it is mentioned here. A ship steaming with a strong wind or current abeam, will slide off to the leeward more or less. Hence, her course will have to be corrected for Leeway as well as for Variation and Deviation.

Put in your Note-Book:

Leeway on the starboard tack is the same as westerly Variation. Leeway on the port tack is the same as easterly Variation. This is apparent from the following diagram:

As the wind, blowing from the North, hits the left hand ship, for instance, on her starboard side, it shoves the ship to the left of her true course by the number of points or degrees of leeway.

Leave a space and put the following heading in your Note-Book:

I. Complete rule for converting a compass course into a true course: 1. Change the compass course into a new compass reading. 2. Apply Easterly Variation and Deviation +. 3. Apply Westerly Variation and Deviation -. 4. Apply port tack Leeway +. 5. Apply starboard tack Leeway -. II. Complete rule for converting a true course into a compass course: 1. Reverse the above signs in applying each correction.

I will now correct a few courses, and these are to be put into your Note-Book:

+------+-------------+--------+--------+-------+--------- C Cos | Wind | Leeway | Dev. | Var. | New | Old ---------+------+-------------+--------+--------+-------+--------- N x E | NW | 1/2 pt. | 5 deg. E | 10 deg. W | 12 deg. | N 12 deg. E S 67 deg. E | S | 1 pt. | 3 deg. W | 5 deg. E | 104 deg. | S 76 deg. E E x N | SE | 1/2 pt. | 5 deg. W | 10 deg. E | 78 deg. | N 78 deg. E W x N | NW | 1-1/2 pts. | 1 deg. E | 15 deg. E | 280 deg. | N 80 deg. W ---------+------+-------------+--------+--------+-------+---------

Assign for Night Work the following arts. in Bowditch: 36-8-10-13-14-15-16-17-18-19-20-21-22-23-24.

WEDNESDAY LECTURE

PELORUS, PARALLEL RULERS, THE LEAD, SOUNDING MACHINE, DIVIDERS AND LOG

_I. The Pelorus_

This is an instrument for taking bearings of distant objects, and for taking bearings of celestial bodies such as the sun, stars, etc. It consists of a circular, flat metallic ring, mounted on gimbals, upon a vertical standard. The best point to mount it is in the bow or on the bridge of the ship, where a clear view for taking bearings can be had. The center line of the pelorus should also be directly over the keel line of the ship. The inner edge of the metallic ring is engraved in degrees--the 0 deg. or 360 deg. and the 180 deg. marks indicating a fore-and-aft line parallel to the keel of the ship. Within this ring a ground glass dial is pivoted. This ground glass dial has painted upon it a compass card divided into points and sub-divisions and into 360 deg.. This dial is capable of being moved around, but can also be clamped to the outside ring. Pivoted with the glass dial and flat ring is a horizontal bar carrying at both of its extremes a sight vane. This sight vane can be clamped in any position independently of the ground glass dial, which can be moved freely beneath it. An indicator showing the direction the sight vane points can be read upon the compass card on the glass dial. If the glass dial be revolved until the degree of demarcation, which is coincident with the right ahead marking on the flat ring, is the same as that which points to the lubber's line of the ship's compass, then all directions indicated by the glass dial will be parallel to the corresponding directions of the ship's compass, and all bearings taken will be compass bearings, i.e., as though taken from the compass itself. In other words, it is just as though you took the compass out of its place in the pilot house, or wherever it is regularly situated, put it down where the pelorus is, and took a bearing from it of any object desired.

In taking a bearing by pelorus, two facts must be kept in mind. First, that when the bearing is taken, the exact heading, as shown by the ship's compass, is the heading shown by the pelorus. In other words, if the ship is heading NW, the pelorus must be set with the NW point on the lubber line when the bearing is taken of any object. Second, it must be remembered that the bearing of any object obtained from the pelorus is the bearing _by compass_. To get the true bearing of the same object you must make the proper corrections for Variation and Deviation. This can be compensated for by setting the glass dial at a point to the right or left of the compass heading to correspond with the compass error; then the bearing of any object will be the true bearing. But naturally, you will not be able to make compensation for these errors unless you have immediately before found the correct amount of the compass error.

_Parallel Rulers_

The parallel rulers need no explanation except for the way in which they are used on a chart. Supposing, for instance, you wish to steam from Pelham Bay to the red buoy off the westerly end of Great Captain's Island. Take your chart, mark by a pencil point the place left and the place to go to and draw a straight line intersecting these two points. Now place the parallel rulers along that line and slide them over until the nearest edge intersects the center of the compass rose at the bottom or side of the chart. Look along the ruler's edge to find where it cuts the circumference of the compass rose. That point on the compass rose will be the _true_ compass course, and can be expressed in either the new or old compass, as, for instance, 60 deg. or N 60 deg. E. Remember, however, that this is the _true_ course. In order to change it into the compass course of your ship, you must make the proper corrections for the compass error, i.e., Variation and Deviation and for Leeway, if any.

_The Lead and Sounding Machine_

The lead, as you know, is used to ascertain the depth of the water and, when necessary, the character of the bottom. There are two kinds of leads: the hand lead and deep-sea lead. The first weighs from 7 to 14 pounds and has markings to 25 fathoms. The second weighs from 30 to 100 pounds and is used in depths up to and over 100 fathoms. Put in your Note-Book:

Fathoms which correspond with the depths marked are called _marks_. All other depths are called _deeps_. The hand lead is marked as follows: