right of SSW is Sw by S and one point to the left is S by W. A glance at Fig. 10 will clearly illustrate the above. 131. Leeway. In order to obtain the true course made good, it is sometimes necessary to apply, beside the variation and the deviation, a correction for leeway. Leeway is the result of the pressure of the sea or the wind exerted on the hull and the sails of a ship, causing her to drift sidewise. With the wind from aft or on the quarter, there is no leeway worth considering, but when the ship sails as near the wind as she can, which is termed to be close hauled,” or “by-thewind,” or with the wind athwartship, leeway is often quite considerable. Steamers are affected in exactly the same way. 132. The amount of leeway varies with the strength of the wind and depends to a great extent on the area of sails carried, the form of hull under water, and other matters; hence, no positive rule can be laid down for estimating its value. Leeway may also be defined as the angle SOS', Fig. 34, that the ship's keel makes with her actual path through the water. Thus, if NOS is the true course steered by a ship—the arrow indicating the direction of the wind-the course made good will be NOS', while the ship's head is still pointing in a direction parallel to OS. Consequently, the correction for leeway must be applied from the wind. Thus, if a ship steers NE and the wind is NNW it will make a more easterly course than indicated by the compass; that is, if it makes one point leeway the course made good will be NE by E. Experienced navigators usually estimate the leeway by eye, being guided by the angle between the ship's wake and her keel; a good plan, however, is to drop over the stern some floating object and from its bearing, when some few fathoms away, estimate the amount of leeway; still better is to use the log line. 133. Use of Diagrams.-When correcting courses' the beginner should always make use of a diagram-no matter how roughly drawn-representing the cardinal points, and on it lay out his courses and corrections; such a diagram will simplify the solution of problems surprisingly. 134. To find the true course from the compass course. Rule.-Correct for leeway and apply easterly variation and deviation to the right; and westerly variation and deviation to the left. EXAMPLE 1.-A vessel steers NNE I E by compass with wind E by S; the variation is 1 point E, the deviation 2 points W, the leeway point. Find the true course. SOLUTION.- or=N 2 points E Leeway= point N21 points E Variation= 1 point E N31 points E Deviation= 2 points W True course=N 11 points E or=N by E E. FIG. 35 Ans. The dotted line in Fig. 35 will plainly show the application of the various corrections in this example, the arrow indicating the direction of the wind. ENE W E SSE EXAMPLE 2.—The compass course is S W by W, the wind SSE, leeway point, variation i point w, deviation 1 point W. Required the true course. S S5% points W S 44 points W or =SW W. Ans. EXAMPLE 3.-A ship sails -W ESE by the compass, the FIG. 36 wind is south, the leeway point, variation 26° 45' E, deviation 17° 30 W. Find the true course. -MS S 49°11'E -S66415 S 75°56'E HESE FIG. 37 SOLUTION.-See Fig. 37. S 75° 56' E 49° E Deviation= True course=S 66° 41' E. Ans. N by W N45W -570°37'W $56°52'W -5 45°W. EXAMPLE 4.-The compass course is W IN, the wind N by W, the leeway 25°, variation 3 points W, deviation 20° E. Required the true course. SOLUTION.-See Fig. 38. Leeway= 25° 0 N 109° 23' W or, S 70° 37' w Variation= 33° 45' W S 36° 52' W Deviation= 20° 0 E True course=S 56° 52' W. Ans. EXAMPLE 5.-Find the true course having given the compass course NE by E] E, the deviation 4° 10' E, the variation 25° 20' W, the leeway point, FIG. 38 the wind being ESE. SOLUTION.Compass course N E by E } E-N 61° 52' E Leeway= 5° 37' N 56° 15' E N 60° 25' E True course=N 35° 5' E. Ans.' EXAMPLE 6.-Find the true course when the compass course is NW, the wind W SW, the leeway 1 point, the variation 8° 26' E, the deviation 13° W. SOLUTION.- Compass course N W=N 45° 0 W Leeway point= 2° 49' N 42° 11' W N 33° 45' W 135. The student should bear in mind that in all problems of course corrections the deviation applied must be that of the compass course, no matter in what order the corrections are applied. EXAMPLE 1.– The compass course is E SE, the leeway 1 point, the wind south, the variation 5° W, the deviation according to Table V. Find the true course. SOLUTION.- Compass course E S E=S 67° 30 E Leeway * point= 5° 371 S 73° 7' E S 78° 7' E True course=S 69° 7' E. Ans. Deviation= 14° 0 E Variation= 12° 0 E EXAMPLES FOR PRACTICE 1. Compass course is north, wind W NW, leeway 1 point, deviation 221° W, variation 111° E. Ans. North. 2. Compass course is S by E, wind E by S, leeway 13 points, variation 16° W, deviation 9° W. Ans. S 19° 23' E. 3. Compass course is east, wind NNE, leeway 14 points, variation 10° w, deviation 5° W. Ans. N 89° 4' E. 4. Compass course is NW by W, variation 8° E, deviation by Table V. Ans. N 65° 15' W. 5. Compass course is S 45° E, wind s w, leeway 19°, variation 11° W, deviation by Table V. Ans. S 64° E. 6. Compass course is S W W, wind SSE, leeway point, deviation 30° 30W, variation 15° W. Ans. S 10° 45' W. 136. To find the compass course when true course is known. When it is required to find the course to be steered by compass in order to reach a port or position whose true direction is known, the corrections must be applied in exactly the opposite manner to that used in finding the true course from the compass course. |