109 deg.--46 k. 15 S 48 18 E
---------- --------------
Noon--Lat. by D.R. 25 deg. 55' S Lo. 104 deg. 53' 56" E
Noon--Lat. by obs. 25 deg. 52' S
----------
Error in Lat. 3'
Enter Table 47 with azimuth (S 105 deg. E) N 75 deg. E as bearing and Latitude
25 deg. 40' or 26 deg., Factor is found to be .3.
3' (Error in Latitude) times .3 (Factor) = .9' or 54", Correction in
Longitude. Is it East or West? Since azimuth is N 74 deg. E, Line of
Position is N 16 deg. W. The D. R. Latitude and Latitude by Observation are
plotted on this line as follows:
\
* Lat. by obs. (25 deg. 52' S)
\
* Lat. by D.R. (25 deg. 55' S)
\
Latitude by observation is West of Latitude by D.R. Hence correction in
longitude of 54" is applied West. Position by observation, therefore, is
as follows:
Lo. 104 deg. 53' 56" E
Corr. in Lo. 54 W
--------------
Lo. by obs. 104 deg. 53' 02" E
Lat. by obs. 25 deg. 52' S
Note to Instructor:
Assign the following examples for work in the class room:
1. April 20th, 1919 A.M. at the ship. G.M.T. 20d 10h 28m 24s A.M.
(_) 31 deg. 55' 40". HE 30 ft. No IE, CC. Latitude by D. R. 26 deg. 30' N.
Longitude 36 deg. 55' West.
Ship then sailed a true course of S 36 deg. E--40 knots until noon when
observed altitude (_) 75 deg. 40' 50" S. What was the position at noon
corrected for Longitude? (Note: Work the A.M. sight by both Time Sight and
Marc St. Hilaire Method.)
2. June 25th, 1919, A.M. Latitude by D. R. 36 deg. 20' S. Longitude
96 deg. 30' E. CT 1h 37m 16s A.M. CC 1m 30s fast. IE 2' 30" off arc.
HE 36 ft. (_) 7 deg. 34' 20". Log registered 114.
True course to noon S 76 deg. E. Log registered 174. Same IE, HE, CC.
Observed altitude (_) 29 deg. 44' 40" N. Required position at noon by Longitude
factor. (Note: Work A.M. Sight by Marc St. Hilaire Method.)
3. At sea, May 30th, 1919. In D. R. Latitude 38 deg. 14' 29" N. Longitude
15 deg. 38' 49" W. Observed altitude (_) 39 deg. 05' 40" and bearing by compass
259 deg.. IE 1' 00" on arc. HE 27 ft. WT 3h 04m 49s. C-W 1h 39m 55s. CC
1m 52s fast.
Changed course to 94 deg. p.s.c. and steamed 75 knots to about 8 o'clock. WT
8h 06m 18s. C-W 1h 39m 58s. At this time observed altitude of Star Arcturus
68 deg. 30' 40", East of meridian. Same IE, HE, CC.
Changed course to 95 deg. (true). Steamed 60 knots until midnight when ran
into heavy fog. Slowed down to 7 knots per hour until 8 A.M. when
observed altitude (_) 48 deg. 45' 10". CT 9h 45m 18s A.M. Same HE, IE, CC.
Required fix at 8 A.M. by Marc St. Hilaire Method, laid down on chart.
Note to Instructor:
Spend rest of period in familiarizing pupils with laying down runs and
intersecting lines of position on Mercator plotting charts.
THURSDAY LECTURE
THE NAVIGATOR'S ROUTINE--A DAY'S WORK AT SEA
You are now familiar with the principal kinds of sights and the methods
used in working them as explained in the foregoing pages. This
information, however, relates only to each individual kind of sight.
Today I will explain briefly how those sights are made use of in your
daily work at sea. Such an explanation necessarily cannot include the
navigator's work under all conditions and on all classes of ships. It
merely gives a brief outline of and a few suggestions relating to
navigating conditions on board a medium-sized transport, in time of war.
I say "in time of war" because navigating then is different, to some
extent, from the ordinary routine in time of peace.
Suppose you are ordered to a ship as navigator. What are your duties (a)
before leaving port, (b) while at sea, and (c) on entering pilot waters?
_(a) Before Leaving Port_
Ascertain the height of the eye of the bridge and any other place on the
ship where you would be likely to take sights.
Have posted in the chart room and on the bridge the deviation of the
compass on each 15 deg. heading, so that it can be easily referred to.
Keep in each chronometer case or in a book nearby the error and daily
rate of all chronometers on board.
Test each sextant for index error and record the result where you can
refer to it easily.
See that all charts of the harbor out of which you are to steam are
corrected to date and are familiar to you, both as to sailing directions
and buoys, and also as to lights and other aids to navigation.
Examine, in detail, the steering engine and steering apparatus. In case
of its disarrangement your intimate knowledge of it may be most
valuable.
See that the patent log and sounding machine are in good order. See that
the lead lines are well soaked in water, stretched, and properly marked.
See that the lighting system in the chart room and the navigator's room
is such that when any door is not tightly closed the lights in the room
are extinguished. Likewise, when the doors are closed, see that the
lights will light and without repeated slamming of the doors.
If possible, provide yourself with a flashlight set back in a metal tube
so that the rays of the light are not diffused but can be focussed only
on one spot at a time.
See that your charts are arranged neatly in the drawers provided for
them in the chart room. If, as is usual, the charts must be folded to
get them in the drawers, mark them legibly on the outside and in the
same place on each chart. Put in the top drawers those charts you know
you will use most frequently. This will save endless time and confusion.
Be sure you have a full complement of necessary instruments, including
sextants, a stadimeter, binoculars, watches, stop watch, dividers,
parallel rulers, pencils, work books; also all necessary books, such as
smooth and deck log books, several volumes of Bowditch, Nautical
Almanacs, Azimuth Tables, Pilot books, Light and Buoy lists, Star
Identification Tables, etc. You will be repaid a thousand times for
whatever effort you expend to have your navigational equipment complete
to the smallest detail. The shortage, for instance, of a pair of
dividers would be an unending annoyance to you. This is also true of
almost any other item mentioned above. Prepare yourself, then, while you
are in port and have plenty of opportunity to secure the equipment you
desire.
_(b) While at Sea_
The least amount of work required of a navigator in time of peace would
include (1) a morning sight for longitude, (2) a noon sight for
latitude, (3) an afternoon sight for longitude, (4) an A.M. azimuth to
check the deviation of the compass, and (5) the dead reckoning for the
day's run from noon to noon.
Navigating in war time requires more work than this. If possible, the
ship's position must be known accurately at any time of day or night
for, in case of an emergency, the lives of all on board may be
imperilled by inaccurate knowledge of your whereabouts. This means that
more sights must be taken and more celestial bodies observed. While
every navigator has his own idea as to the proper amount of work to do
in a day, it would seem as though the following would cover the minimum
amount of work necessary under present conditions:
1. An A.M. sight of the sun for longitude.
2. An azimuth of the sun for checking the deviation of the compass,
taken right after the A.M. sun sight.
3. The watch time of Local Apparent Noon.
4. Ex-meridian and meridian altitudes of the sun for latitude.
5. A P.M. sight of the sun for longitude.
6. An evening twilight sight of three or four stars, preferably one in
each quadrant. If these altitudes are taken correctly your position can
be found to the dot.
7. A morning twilight sight for a fix or, at least, for latitude by
Polaris.
8. The dead reckoning from noon to noon.
9. Distance run during the last 24 hours, from noon to noon
10. Distance to destination.
11. Set and drift of the current.
_1. The A.M. Sun Sight_
In order to make this a valuable sight for longitude it should be taken
when the sun is on or as near as possible to the prime vertical. As the
sun, in North latitudes, passes the prime vertical before sunrise in the
winter, the following remarks do not hold for that season. In winter the
only rule to follow is to observe the sun as soon as it is 10 deg. or more
above the horizon. In summer find out from the Azimuth Tables the local
apparent time when the sun will bear 90 deg.. Estimate, as closely as
possible, the longitude you will be in the next morning when the local
apparent time is as just found in the Azimuth Tables. This can be done
by calculating the dead reckoning from the previous sight, or, what is
even simpler, laying the distance off on the plotting chart. With this
information find the W.T. corresponding to the L.A.T. mentioned above by
some such formula as this: L.A.T. +- Lo. = G.A.T. +- Eq. T. (sign
reversed) = G.M.T. +- C.C.(sign reversed) = C.T. - (C-W) = W.T. This will
not be absolutely accurate, for the longitude you are in is only
approximate, but it will be close enough for good results. This
resulting W.T. will be the time to take the A.M. sight. About fifteen
minutes before that time compare your watch with your chronometer to get
the C-W. Also bring up the C.C. to date and make a note of it so that as
much as possible of this detail work is accomplished before the sight is
taken. Next, take your sextant and test it for index error. This should
be done regularly before each series of sights as it is impossible to
tell what may have happened when the sextant is lying idle, except by
the above test. Now, with your sextant, watch and notebook, go to the
place from which you have decided to take your observations and, at the
proper watch time, start taking your altitudes. It is always advisable
to take a number of sights, closely following each other, so that an
error in one may be corrected somewhat by the others. Take at least
three sights in close succession. At the same time have the log read and
enter it in your notebook. An equally good method in fair weather is to
secure the distance run from the revolutions of the propeller.
Having taken your sights, go to the standard compass and get a bearing
of the sun, at the same time noting in your book the W.T. of the bearing
and the compass heading of the ship. You are now ready to go below into
the chart room and work out your position. What method shall you use?
That depends upon your preference. You have missed the point of the
previous lectures, however, if you forget that the New Navigation is
based upon the Marc St. Hilaire Method, and this is undoubtedly the
method your captain will prefer you to use if he is an Annapolis
graduate. In this connection let me remind you again of the one fact,
the oversight of which discourages so many beginners with the Marc St.
Hilaire Method. The most probable fix, which you get by one sight only,
is not actually a fix at all. Nor does any other method give you an
accurate fix under like conditions. What the most probable fix is, and
all it claims to be, is a point through which the required Sumner line
is to be drawn. If your D.R. position happens to be only one mile away
from the most probable fix, that is no assurance that the most probable
fix is near the actual position of the ship. You may be 25 miles away
from it. But the important information gained is that, though you may be
25 miles away, you know on what line you are, and when this line is
later crossed with another line of position that fix will be accurate.
"Two sights make a fix" is the whole matter in a nutshell.
_2. The Compass Error_
Having secured your morning sight, the next duty is to get the compass
error. From your morning sight computation you know the watch time
corresponding to the L.A.T. of the same sight. Find the difference
between the two and apply this difference to the watch time of the
compass azimuth. That will give you the L.A.T. with which to enter the
Azimuth Tables to get the true bearing corresponding to the compass
bearing recently observed. Apply the variation from the chart to get the
magnetic bearing. The difference between this magnetic bearing and the
compass bearing will be the required deviation, which you should compare
with your Deviation Table. If there is a marked difference, and you are
sure of your figures, use the new deviation in computing courses on this
heading of the ship.
_3. The Watch Time of Local Apparent Noon_
You are now ready to figure the watch time of local apparent noon.
Unless you have a decided preference to the contrary, do this by the
method explained in the Saturday Lecture, Week VI. Do not forget that in
subtracting the L.A.T. of the morning sight from 24 hours to get the
total time to noon, in case the ship were stationary, you do _not_ use
the L.A.T. of the D.R. position, but the L.A.T. found by subtracting
from G.A.T. the longitude of the most probable fix. This will give you
the L.A.T., based on the longitude of the most probable fix, which will
be slightly different from the L.A.T. based on the D.R. longitude. When
you have secured the watch time of local apparent noon, subtract 30
minutes from it and notify the quartermaster that at that time by your
watch the deck clocks are to be set to 11.30 A.M. If this change of time
is very great (providing you are on an almost easterly or westerly
course), it is wise to have the clocks set back in the night watches to
allow for most of the time you figure you will lose. This will not work
such a hardship or such an advantage to the officers and men who have
the forenoon watch and will also be easier for the cooks. The clocks can
then be slightly but accurately changed at 11.30 A.M., as mentioned
above.
_4. Ex-Meridian and Meridian Altitudes_
You know the principles and methods governing sights of this character.
To know your latitude exactly at noon is usually required when you are
steaming in convoy, for at that time your position signals are hoisted,
and it is a matter of pride with the navigator not only to have his
position exact but promptly. If your A.M. sight was taken when the sun
was on or near the prime vertical, a change in latitude at noon will
make no change in longitude. Hence you can figure your longitude at noon
just as soon as you have secured the corrected time from the A.M. sight
to noon (which you have done right after working the A.M. sight). You
will have your longitude, then, before you go on the bridge to observe
for ex-meridian and meridian altitudes.
Sharply at noon you take your meridian altitude and tell a messenger to
notify the captain that it is noon at the ship. The captain then orders
eight bells struck, and you are ready to hand in your noon report,
consisting of latitude and longitude by observation, latitude and
longitude by dead reckoning, deviation of the compass on the ship's head
at 8 A.M., distance made good since the preceding noon, distance to
destination, set and drift of current (Note:--When steaming in convoy
this is unnecessary and usually omitted), and any other pertinent
remarks. If the sun was not taken on or near the prime vertical at the
time of the A.M. sight, take out your longitude factor for the coming
noon position and calculate your D.R. latitude at noon. By correcting
the longitude of the A.M. sight, run to noon, with the difference of
longitude, readily found at noon with the longitude factor and the error
in latitude, you will have the correct noon longitude to hand in, with
only a moment's delay. It will be very hard, however, to get all this
information in on time without the use of latitude constants. There is
no room for a discussion of these constants here, but they are easy to
work and you should learn how to use them. The information is in
Bowditch Art. 325, p. 128, and Art. 405, p. 181.
_5. The P.M. Sun Sight_
This is another longitude sight and so any previous remarks about sights
of this character are applicable here. If the day is fine you need not
work out this sight until after evening twilight, for a fix then by
stars will give both latitude and longitude, whereas your afternoon sun
sight will only give you a longitude. This P.M. sun sight is a good
check sight, to be used or not, according as to whether other earlier or
later sights have been obtained.
_6. The Evening Twilight Sight_
The beauty of using stars is that by almost simultaneous altitudes of
different ones you can ascertain your position, both as to latitude and
longitude. In the North Atlantic during the summer months Vega, Deneb or
Altair in the East, Antares or Deneb Kaitos in the South, Arcturus in
the West, and Polaris, Mizar, or Kochab in the North form an ideal
combination which includes every quadrant of the compass. In the winter
months, Capella, Castor or Pollux in the East, Sirius or any star in
Orion's belt in the South, Deneb in the West, and Polaris in the North
are equally as good.
_7. The Morning Twilight Sight_
In clear weather this should be primarily a sight for latitude, since
the A.M. sun sight for longitude will follow it. A latitude by Polaris,
and at the same time some star in one of the southern quadrants, as a
check, will give admirable results.
_8. The Dead Reckoning from Noon to Noon_
If there is no change of course in the late forenoon, as is usually the
case, the dead reckoning for the day's run can be figured any time
before noon so that it will be all ready to hand in to the captain with
the other noon data. It is much easier to lay this off on the chart than
to go to the trouble of calculating it by Table 2, Bowditch. On the
other hand, such a calculation checks the chart work and should be
worked out if you wish to make "assurance doubly sure."
_9. Distance Run During the Last 24 Hours_
Here, again, an answer by chart and an answer by figures is a good thing
to secure. As you become accustomed to your work you will find the
answer by chart infinitely easier and quicker to get. It is just as
accurate, too, if you lay the distances off carefully with the dividers
and parallel rulers.
_10. Distance to Destination_
The same remarks as are made under (9) hold true here.
_11. Set and Drift of Current_
Find the difference between your D.R. position and your position by
observation at noon, i.e., the course and distance from your D.R.
position to your position by observation. The course is the set of the
current, the distance the amount of drift, all of which is easily
calculated by Table 2, Bowditch. This difference between the two
positions is seldom due to current. It is due to all errors of steering
and the like. But these are all ascribed to current, for the sake of
convenience. This calculation of the current is seldom used now,
particularly when steaming in convoy.
It is obvious that a schedule, such as outlined above, cannot be adhered
to in all kinds of weather or under all conditions. It is merely an
outline of what might properly be included in a 24 hour day, the weather
conditions of which will lend themselves at any time to taking the
observations mentioned. The weather of each succeeding day may force you
to adopt a different routine. Nevertheless, the closer you can keep to
the above schedule the more exact will your various positions be.
_(c) On Entering Pilot Waters_
See that all charts of the locality you intend to enter are corrected to
date.
Study these charts carefully, making notes, in detail, of the aids to
navigation that you intend to pick up.
In noting lights give their distinctive appearance, range of visibility,
approximate time of sighting them, and any other information that you
think you may need. If you have this information with you when on the
bridge it will save much time and trouble that you would otherwise have
to spend, at possibly a critical time, in the chart room.
See that log lines, sounding machine, etc., are in order for instant
use.
Remember that in entering pilot waters the safest landmarks are
permanent ones. Buoys, cans, etc., may drag from their positions or be
lost altogether. This can also happen to lightships.
Become familiar with soundings, rise and fall of the tides, and the
like, in the neighborhood in which you intend to anchor. If possible
choose an anchorage that will enable you to get bearings from two or
three fixed points on shore. As soon as possible after anchoring secure
your bearings by pelorus and have them checked up by the quartermaster
at regular intervals. This will determine how much, if any, dragging has
taken place.
Lastly, always remember that no amount of advice can make up for your
own carelessness. Hold yourself ready for any emergency, keep cool, keep
patient and keep pleasant. Common sense is the best antidote in the
world for strange situations. If you have that, and the knowledge you
should have secured from these lectures, you cannot go far wrong.
While the day's work which follows does not include every sight in
regular sequence as given in the above discussion, it will give a fair
idea of the navigator's work during a day's run. Put it in your
notebook. (Note to Instructor:--Spend the rest of the period in
explaining carefully each step of this example.)
A DAY'S WORK AT SEA
Departure taken from noon position Jan. 25th, 1919, in Latitude 30 deg. 01'
N, Longitude 73 deg. 47' 20" W. Course p.s.c. NE 3/4 N. Deviation 2 deg. E.
Variation 4 deg. W. Log registered 20. Ship continued on this course until
about 6:30 P.M. when log registered 98 and observed altitude of Star
Rigel, East of meridian, 39 deg. 36' 20". WT 6h 33m 19s P.M. C-W 4h 55m 04s.
CC 2m 16s slow. HE 37 ft. IE 0' 20" off arc.
Changed course to 40 deg. (true) and steamed until 3 A.M. when log
registered 198. At this time ship ran into heavy NE gale. Slowed down to
7 knots per hour until about 8:30 A.M. when observed (_) 18 deg. 25' 10"
and bearing by compass S 46 deg. E. Variation 7 deg. W. WT 8h 31m 16s A.M. C-W
4h 55m 04s. IE + 0' 10". Same HE, CC.
Ship then steamed on true course of 39 deg. at 7 knots per hour until noon
when log registered 261 and observed meridian altitude (_) 37 deg. 59' S.
Same IE (+ 0' 10"), HE, CC. Required:--
1. Position by D. R. at noon.
2. Position by observation at noon (corrected for Longitude by a
factor).
3. Deviation of compass at 8:30 A.M.
4. Watch Time of Local Apparent Noon.
(See Next Page)
At Sea, Jan. 25, 1919. Lat. in 30 deg. 01' N Lo. in 73 deg. 47' 20" W
Steamed until 6:30 P.M.
==========================================
Course | Dist. | D. Lat. | Dep. | D. Lo.
------------------------------------------
N 35 deg. E | 78 | 63.9 | 44.7 | 51.5
==========================================
Lat. Left 30 deg. 01' N Lo. Left 73 deg. 47' 20" W
D. Lat. 1 03 54 N D. Lo. 51 30 E
------------ ------------
Lat. in 31 deg. 04' 54" N Lo. in 72 deg. 55' 50" W
Mid Lat. 30 deg. 30' N
----------------------------------------------------------------
At 6:30 P.M. Obs. * Rigel.
Obs. Alt. * 39 deg. 36' 20" W.T. 6h 33m 19s Log hav "t" 8.92502
E + 00 20 C.W. 4 55 04 Log cos Lat. 9.93269
------------- C.C. 2 16 Log cos. Dec. 9.99544
39 deg. 36' 40" ----------- --------
HE - 7 09 G.M.T. 11h 30m 39s Log hav S 8.85315 Nat hav S .07130
------------- (.).R.A. 20 14 58 Nat hav L--D .11349
T.C.A. * 39 deg. 29' 31" (+).C.P. 1 53.3 ------
------------- ----------- Nat hav ZD .18479
Dec. of * 8 deg. 17' 42"S G.S.T. 31h 47m 30.3s 50 deg. 55' 00" ZD
Lat. 31 04 54 N --W.Lo. 4 51 43 -90 00 00
------------- ----------- ------------
L.~D. 39 deg. 22' 36" L.S.T. 2h 55m 47.3s 39 deg. 05' 00" C.Alt.
------------- *R.A. 5 10 41 39 29 31 T.Alt
----------- ------------
"t" 2h 14m 54s 24' 31" Alt. Diff. Toward.
[Illustration: TZ N 135 deg. E, S 45 deg. E]
================================================
Course | Dist | D. Lat. | Dep. | D. Lo.
------------------------------------------------
S45 deg. E | 24.5 | 17.3 | 17.3 | 20
================================================
Lat. Left 31 deg. 04' 54" N Lo. Left 72 deg. 55' 50" W
17 18 S 20 E
------------- -------------
Lat. in 30 deg. 47' 36" N Lo. in 72 deg. 35' 50" W
Changed course to 40 deg. (true)
===============================================
Course | Dist | D. Lat. | Dep. | D. Lo.
-----------------------------------------------
N40 deg. E | 138.5 | 106.1 | 89 | 105
===============================================
Lat. Left 30 deg. 47' 36" N Lo. Left 72 deg. 35' 50" W
1 46 06 N 1 45 E
------------- -------------
Lat. in 32 deg. 33' 42" N Lo. in 70 deg. 50' 50" W
8.30 A.M.