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For instance, if it were required to construct a deviation diagram by the following deviations, the result would be as
represented in Fig. 5. The deviation on any point can now be found by measuring the perpendicular distance between the curve and the desired point on the vertical line.
20. Illustration of Use of Diagram. – To find, for instance, the deviation for ESE JE proceed as follows: From c, Fig. 5, representing the compass point E SEE, draw a line perpendicular to a b; then, with the scale, measure the distance c d, which, in this case, is equivalent to 6. divisions or 6.5°. Since the curve at that point lies to the east, the deviation is easterly; hence, the deviation for E SE ŽE 6.5° east.
21. For larger deviations and greater accuracy, diagram called, after its inventor, Napier's diagram is commonly used. Its construction is somewhat more complicated than the one just shown, but it may be used with advantage in turning compass courses, or bearings, into magnetic courses, or bearings, and vice versa.
Too much reliance, however, should not be placed on tables of deviation or deviation diagrams, since the magnetic condition of the ship, and, consequently, the deviation, may change considerable with the lapse of time. This is especially true in regard to a new iron ship, which, as a rule, loses much of its magnetism in the first two years after being launched.
Proper Placing of the Compass.- Considerable care should be bestowed on the selection of a suitable place for the standard compass on board of iron ships, remembering that no iron or steel of any kind must be placed near or permitted to remain within a certain distance of it. All vertical iron, such as stanchions, davits, etc., should, if possible, be at least 14 feet away from the standard compass. It should be placed in a position selected, not for the convenience of the helmsman or builder, but in a place where the surrounding magnetic forces will least affect it, and where every facility exists for the examination of its error.
23. To Protect Compasses From Electrical Disturbances. – With the introduction of electricity on board ships a new form of compass disturbance was created, inasmuch as the magnetism of large electromagnets used in dynamos and the electric current in general may disturb a compass even at a considerable distance. The committee of Lloyd's Register of British and Foreign Shipping has suggested the following in reference to compasses and electricity on shipboard:
1. That dynamos and electric motors should be placed as far as possible from all compasses, and at a distance of at least 30 feet from the standard compass.
2. That wires conducting electric currents should not come nearer than 16 feet to any compass, whereas wires conducting strong currents should be at a still greater distance.
3. That the compensation of compasses should be done when the dynamos are at rest, while the operation for determining the deviation should be performed when the dynamos are running.
24. Cautionary Remarks. – In connection with compass adjustment and the determination of deviation, it must be borne in mind that these subjects are as equally important to the iron steamer on the Great Lakes as to the oceangoing steamship; and perhaps more so to the former, on account of its sometimes close proximity to more or less iron-bearing mountains. To steamers carrying iron ore, in particular, the subject of deviation is of the utmost importance, inasmuch as the cargo may change the magnetism of the ship materially; the error of compasses on such vessels should therefore be carefully and frequently investigated. Mere mechanical adjustment, however, should be regarded as only a temporary expedient. The careful navigator will therefore avail himself of every opportunity of determining his compass error by astronomical observations, and compåre the result so obtained with the tabulated deviation.
CORRECTION OF COURSES
25. There are
three kinds of courses, viz., compass course, magnetic course, and true course.
26. The compass course is the angle cOS, Fig. 6, that the ship's track OS makes with the direction of the magnetic True
needle. It may be affected by variation and deviation, and, consequently, must be corrected for both in order to obtain the true
27. The magnetic course is the angle bOS that the ship’s track makes with the magnetic meridian. This course is affected only by variation, the application of which converts it into true course. It is evident that
where there is no deviation the compass and magnetic course will be identical.
The true course is the angle a 0 S that the ship’s track makes with the true, or geographical, meridian, indicating true north and south.
It is evident that at places on the earth's surface where neither deviation nor variation exist the true course will be identical with the compass course.
29. Important Remarks. – In dealing with problems of correcting courses, always remember that, since the compass is the representation of the visible horizon, the position of the observer is considered to be at the center of the compass card. Hence, when applying corrections, whether to the right or left, the student must consider himself to be stationed at the center of the card, looking in the direction of the course to be corrected. Thus, one point to the right of ENE is E by N, and one point to the left of E NE is N E by E; similarly, one point to the right of SSW is S W by S, and one point to the left is S by W. A glance at Fig. 10, Lake Navigation, Part 1, will illustrate the above.
30. Leeway. – In order to obtain the true course made good from the compass course, it is sometimes necessary to apply, besides variation and 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. In reference to a steamship making a voyage in calm, fine weather, no correction for leeway need be applied; but, with the wind and sea on the side, or in any direction not parallel with her fore-and-aft line, they have a tendency to affect the ship’s course, and in such cases a correction for leeway is necessary.
Leeway may also be defined as the angle SOS', Fig. 7, that the ship's keel makes with her path through the water. Thus, if NOS is the true course steered and OS is the ship's actual path over ground - the arrow indicating the direction of the wind-the angle SOS is the leeway. From this, it is evident that the correction for leeway must be applied from the wind. Thus, if a ship steers N E and a strong NNW wind is blowing, she will make a more easterly course than is indicated by the compass; hence, if the leeway is one point, the course made good will be N E by E, although the ship is continually heading N E.
31. The amount of leeway varies with the strength of wind, the form of the hull under water, and other matters; therefore, no positive rule can be laid down for estimating its value. Experienced navigators usually estimate the leeway by eye, being guided by the angle between the ship's
wake and her keel or fore-and-aft line. Generally, no allowance is made for leeway in steamers, except in strong winds and when lightly loaded so that a large portion of the hull is exposed to the pressure of the wind.
In lake navigation, owing to the flat surface of the bottom of lake steamers and the absence of external keels, leeway must be and is considered in all steamers, no matter at what speed they may be going when loaded. If light or encumbered with tows, it is of still greater importance.
32. 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 solving of the problems considerably.
33. 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.