Lectures in Navigation Part 20

Compa.s.s Bearing S 42 E.

Variation 4 E.

Required T.C. and Deviation on ship's heading.

3. June 17th, 1919. CT 4h 18m 44s A.M. Longitude 60 14' 59" E.

Lat.i.tude 38 48' 00" S. Ship heading SW x S.

Compa.s.s Bearing 40 Variation 12 W.

Required T.C. and Deviation on ship's heading.

Etc.

WEDNESDAY LECTURE

CORRECTING LONGITUDE BY A FACTOR

We are now almost ready to begin the discussion of a day's work at sea.

The only method we have not taken up is the one which is the subject of today's lecture. It is a method to correct your longitude to correspond with the difference between your lat.i.tude by Dead Reckoning and your lat.i.tude by observation.

Suppose you take a sight in the morning for longitude. The only lat.i.tude you can use is a D. R. lat.i.tude, advanced from your last known position.

Now suppose you run until noon and at that time take a sight for lat.i.tude. In comparing your D. R. lat.i.tude, advanced the true course and distance steamed to noon, and your lat.i.tude by observation taken at noon, suppose there is a difference of several minutes. The question is--How can we correct our longitude to correspond with this error discovered in the lat.i.tude? This is the method which put in your Note-Book:

Find the difference between the lat.i.tude by D. R. and the corresponding lat.i.tude by observation (in most cases secured from a sight at noon or from the Star Polaris). Call this the Error in Lat.i.tude. With the D. R.

Lat.i.tude of the preceding sight and the azimuth or bearing of the preceding sight (always expressed as a bearing of less than 90, old compa.s.s reading) enter Table 47 for the correct Longitude Factor.

Multiply this Factor by the Error in Lat.i.tude. The result is the correction to apply to the Longitude. It is applied East or West according as to whether the Lat.i.tude by Observation is to the East or West of the D. R. Lat.i.tude on the Line of Position (the line at right angles to the Azimuth) of the preceding sight.

Example:

Position about 7:30 A.M. Lat.i.tude by D. R. 25 40' S, Longitude (just secured by observation) 104 05' 38" E. L.A.T. 7h 32m 30s A.M., Declination 4 59' N. Thence ship ran to noon 109, true course, 46 miles, when the lat.i.tude by meridian alt.i.tude of the sun was found to be 25 52' S. Required corrected longitude at noon.

7:30 A.M. D.R. Lat. 25 40' S Lo. 104 05' 38" E 109--46 k. 15 S 48 18 E ---------- -------------- Noon--Lat. by D.R. 25 55' S Lo. 104 53' 56" E Noon--Lat. by obs. 25 52' S ---------- Error in Lat. 3'

Enter Table 47 with azimuth (S 105 E) N 75 E as bearing and Lat.i.tude 25 40' or 26, Factor is found to be .3.

3' (Error in Lat.i.tude) times .3 (Factor) = .9' or 54", Correction in Longitude. Is it East or West? Since azimuth is N 74 E, Line of Position is N 16 W. The D. R. Lat.i.tude and Lat.i.tude by Observation are plotted on this line as follows:

* Lat. by obs. (25 52' S) * Lat. by D.R. (25 55' S)

Lat.i.tude by observation is West of Lat.i.tude by D.R. Hence correction in longitude of 54" is applied West. Position by observation, therefore, is as follows:

Lo. 104 53' 56" E Corr. in Lo. 54 W -------------- Lo. by obs. 104 53' 02" E

Lat. by obs. 25 52' S

Note to Instructor:

a.s.sign the following examples for work in the cla.s.s room:

1. April 20th, 1919 A.M. at the ship. G.M.T. 20d 10h 28m 24s A.M.

(_) 31 55' 40". HE 30 ft. No IE, CC. Lat.i.tude by D. R. 26 30' N.

Longitude 36 55' West.

Ship then sailed a true course of S 36 E--40 knots until noon when observed alt.i.tude (_) 75 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. Lat.i.tude by D. R. 36 20' S. Longitude 96 30' E. CT 1h 37m 16s A.M. CC 1m 30s fast. IE 2' 30" off arc.

HE 36 ft. (_) 7 34' 20". Log registered 114.

True course to noon S 76 E. Log registered 174. Same IE, HE, CC.

Observed alt.i.tude (_) 29 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. Lat.i.tude 38 14' 29" N. Longitude 15 38' 49" W. Observed alt.i.tude (_) 39 05' 40" and bearing by compa.s.s 259. 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 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 alt.i.tude of Star Arcturus 68 30' 40", East of meridian. Same IE, HE, CC.

Changed course to 95 (true). Steamed 60 knots until midnight when ran into heavy fog. Slowed down to 7 knots per hour until 8 A.M. when observed alt.i.tude (_) 48 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 princ.i.p.al 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 cla.s.ses 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 compa.s.s on each 15 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.