longitude, owing to the convergence of the meridians as the pole is approached, are less than 24.8 nautical miles; but, by considering the problem as a case of Mercator's sailing– where the place from is the position by D. R., and the place in the position determ in ed astronomically—the true set and drift of the current is readily found, as * shown in the example. A. 25. Cautionary Remarks. –The dif- || ><-----ference between the B observed position of the ship and the position in dicated by dead reckoning should not always be considered as the effect of a current. It may be due to the leeway made by the ship and insufficiently allowed for in the log, or it may be the result of careless steering, or from inaccuracy of the officer, whose duty it has been to record the course and speed of the vessel. The principal ocean currents are now so well known and understood that no captain or officer ought to accept a current as the explanation until he has satisfied himself that the difference is not due to some other cause. 26. Allowance for Tidal Streams.-When navigating in localities where the direction and velocity of the tidal streams are known, the course to steer must be shaped so as to make allowance for the probable distance over which the vessel will be set by these streams. For instance, if the course across a channel or bay is east, and the distance 70 miles, a vessel sailing or steaming 10 knots will probably experience during the passage both tidal streams. Supposing then that during 6 hours the stream sets north with an average drift of 4 knots per hour, and then south at the rate of 2 knots per hour, the whole drift of the vessel will be about 22 miles to the northward. We will further consider this subject under the heading of Tides and Tidal Currents in Ocean A/eteorology. 27. To Find the Set and Drift of a Current by Bearings.--When sailing or steaming along, and in sight of land, the set and drift of a current can be determined as follows: Let A, Fig. 8, represent the position of a ship determined by cross-bearings, or other method, and A B the course steered and distance run from that point to another point B. At this latter point the ship's position is again determined by reference to some known object or objects on shore; should this indicate the vessel to be at C, it is evident that if B and C are connected by a straight line, the magnitude and direction of B C will represent the set and drift of the current that the ship experienced during her run from A to B. FIG. 8 28. When at anchor the set and drift of the current is found by the log; the set by the direction of the log line, and the drift by the number of knots run out. 29. Charts generally show the direction or set of a current by means of an arrow, and its mean or average hourly drift is usually indicated by a numeral close by. EXAMPLES FOR PRACTICE 1. A ship sails S E by E for a distance of 58 miles and a current sets E N E 40 miles. Find (a) the course and (b) distance made good. a ) S 79° E. Ans. {{}} ... 2. During 24 hours a ship sails as follows: S W 40 miles, W S W 27 miles, and S by E 47 miles, being throughout effected by a current setting S E by S, the drift of which is 1% miles per hour. What is (a) her course and (b) her distance made good 2 Ans (a) S 11° 49' W. \ (6) 117.1 mi. 3. Suppose a port bears east and the set of a current is N N E with a drift of 2 miles per hour, speed of the ship being 9 knots. What course should be steered so as to counteract the effect of the current 2 Ans. S 78° E. 4. At noon the ship's position by astronomical observations is found to be latitude 50° 10' N, longitude 19°28' W.; but, by dead reckoning, its position is indicated to be in latitude 49° 47' N and longitude 19°59’W. Assuming the vessel to have been accurately steered and the distances run exactly determined, find (a) the set and (b) the drift of the current wherein she has sailed. (6) 14 mi, per hr. 5. A port bears N 75° E and a current is running N 40°W 2 miles per hour, the ship's rate of sailing is estimated to be 9 knots. Find what course to steer in order to counteract the current and keep the port on the same bearing. Ans. N 86° 37' E. 6. A ship sailing in a current has by her dead reckoning run S 57° E 48 miles, and by observations is found to have made good 31.6/ southerly difference of latitude, and 44.7’ easterly departure; required (a) the set and (b) the drift of the current. \ (6) 7 mi. THE DAY'S WORK IDETERMINATION OF POSITIONS BY DEAD RECKONING 30. On board of every well-governed ship a careful record is kept of the several courses steered and distances run, also the interval of time occupied on each course; these data are entered up, either on a slate, known as the log slate, or in a book generally termed the rough log, or scrap log book, and from them are computed, at noon of each day, the course and distance made good from the preceding noon, thus obtaining the ship's position by dead reckoning. This operation comprises what is called the day’s work. 31. The data given are the latitude and longitude at the preceding noon (obtained by astronomical observations), the compass courses, the distance run on each course, the variation of the compass, the deviation due to each separate direction of the ship's head, the leeway, the wind, and the set and drift of currents (if known). 32. The data required are the true course and distance made good, the latitude and longitude in, and the compass bearing and distance from the position thus obtained to the port of destination, or that point toward which the ship, by reason of circumstances, is to be directed. 33. Strictly speaking, the day's work includes the determination of the ship's position both by dead reckoning and by astronomical observations; for the present, however, only that by dead reckoning will be considered. 34. Order of Procedure.—The order of procedure in carrying out the day's work is given in the following rule: Rule.—I. Correct each course for variation, deviation, and /ceway, thus finding the true courses, and arrange the same in tabular form, as will be shown. Add together the hour/y disfances for each course and inserf 1/e same in the table of posife the corresponding true course. II. Pond in the traverse fab/es she difference of /a/stude and departure corresponding to each true course and distance, and p/ace them in their respective co/w/m/s, having previously marked (with a horizon/a/ dash) //e spaces not to be fi//ed. III. By adding the quantifies in the four columns and taking the difference befoveon the north and south difference of latitude, and befoween east and west departure, f/he difference of /atitude and departure due to the whose traverse is found, and thence the true course and distance made good according to the proper formulas. I l’ish this true course and distance, the difference of longitude made is found either by para//e/, middle latitude, or Asercator's sai/ing. The position thus found is the position of the ship as defermined by dead reckoning. The following example worked out in detail will illustrate the operations directed in the preceding rules. 35. Illustration.-On July 10, at noon, a ship, by astronomical observations, was in the following position: Latitude 54° 3' N and longitude 42° 18' W.; afterwards she sailed as by the following log-book account. Required the latitude and longitude in on July 11, at noon. |