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Suppose the Var. 20° W., then, using Deviation Card, p. 21.
2° W., II
I S. 88° W. | . " N. • , 14 W * * 8: 10 W. But the converse of this rule, when, knowing the True Course you wish to set the corresponding Compass Course, does not exactly hold good.
LEEWAY. When a course is set, the helmsman keeps the ship's head to it by means of the helm, and by watching the coincidence of the given compass-direction with the lubber's line. But the ship does not at all times progress in the direction of her length, for she is under the influence both of the wind on the sails, and of the resistance of the water through which she is moving; and between the two forces she makes a compromise, as it were, progressing nearly in the direction in which her head lies, but more or less to leeward, according to the strength of the wind, the quantity of sail set, and the angle the wind makes with the side of the ship. This deviation of the ship's track from the direction of the fore-and-aft line is called LEEWAY, and it may be expressed as the angle which the ship's keel makes with her actual path through the water.
The amount of leeway, reckoned from a quarter of a point upwards, is either estimated according to the circumstances of the case, or set by compass by comparing the direction of the fore-and-aft line of the ship with that of her wake ; and it is always allowed in a direction from the wind; hence Rule.-Wind on Starboard side, allow LEEWAY to the left.
Wind on Port side, allow LEEWAY to the right. Or you may say Starboard tack for starboard side, and Port tack for port side. Ex.-Ship’s head N.N.W., leeway 11 points, with wind N.E., gives the actual direction
of her course N.W. . N. Ex.-Ship’s head W.S.W., leeway # point, with wind South, gives the actual direction of
her course W.8.W. & W. These examples will make the method of allowing for leeway clear to you; but remember that lreway is not an error of the compass-only a correction to be applied to the course under special circumstances.
CURRENT. A ship's course and distance are affected by what is called the set (direction) and drift (rate) of any current in which she may be sailing. A tidal current, generally called the stream of tide, is only experienced when near land, and in some places runs with great strength ; allowance has to be made for this on the course set, as otherwise she might soon be ashore; I shall have to speak more at length on TIDE8, and therefore at present merely make a note that the stream of tide affects the ship’s course in the same manner as a general current.
You know that in different parts of the ocean there are great oceanic currents setting in various directions, and in some places, as in the heart of the Gulf Stream, and round the Cape of Good Hope, with great velocity; we must understand their effect on the ship’s course..
In a current the whole body of water is moving and drifting a ship with it, without altering in the least her apparent course. When the set of the current is in the same direction as the ship’s, no alteration of the ship's course will ensue, aor, indeed, if the current sets in a direction exactly opposite to this ; in either case, the only effect will be to increase or diminish the rate of the ship's motion by the whole amount of the drift of the current.
Thus, with the ship's course N.E., sailing at the rate of 8 miles an hour, in a current setting N.E. at the rate of 3 miles an hour, her course remains N.E., bat she has changed her position (8 + 8) Il miles.
Also, with the ship's course West, sailing at the rate of 9 miles an hour, in a current setting East, at the rate of 4 miles an hour, her course remains West, but she has changed her position only (
9 4 ) 5 miles. If, however, the set of the current is across the ship’s track, then both the course of the ship and her rate will be affected, as shown in Fig. 4.
Let us suppose the arrow at D to represent the set and drift of a current in which a ship is sailing, -the direction of the arrow is the set, and its length is the drift or rate : tben, let the line from A to B be the course and distance of the ship if unaffected by current : we draw a line B o, parallel with D and of the same length, then the line A C will represent the actual course and distance the ship makes, on account of the current; the actual course is more to westward than the apparent course owing to the westerly set, and the distance run is greater than the rate at which she appeared to be moving, because the current is partly running
in the direction of the ship's motion. · Charts generally show the current by means of an arrow, and if there is a numeral near it, you have also the mean drift; in a Day's Work, allowance must always be made for the effect of current.
Thus far we have spoken of the compass, which, when corrected for its permanent and accident variations, gives us the TRUE DIRECTION in which the ship has sailed. We must now know something of the method adopted to determine the DISTANCE run. The instrument used for this purpose is called,
THE LOG. The Log consists of the LOG-8HIP and LOG-LINE. The log-ship is a quadrantalshaped piece of hard wood, about 5 or 6 inches radius, and a quarter of an inch
thick; the circular part is loaded with lead to make it swim perpendicularly, and just sufficient to immerse it. (See Fig. 5.)
The outer extremity of the logline terminates in two or tbree ends,
for as many holes as are in the
flat of the log-ship, - and which, when fitted, form a sling or bridle; the end of one part of the bling being a leather peg, this is fitted into one of the holes near the circular edge of the log-ship, and draws upon being checked, after the operation of heaving the log
has been effected, and thus the log-ship is more easily hauled in: the other (or inner) end of the log-line is attached to a REEL, around which is wound 120 fathoms of line. The log-ship end is marked off, to the length of 10 fathoms or more, according to the size of the ship, by a bit of rag or bunting; and this length is called stray-line,—its use being to carry the log-ship out of the eddies of the ship's wake before counting commences. The line from the stray part inwards is marked (knotted) off in equal lengths called knots; the distance between each knot is usually subdivided into 8 equal parts called fathoms ; but a better subdivision would be 10 parts, each representing the tenth part of a knot. Each knot is the representative of a nautical mile, and its length is proportioned to the seconds of the log (sand) glass used when heaving the log.
When the log is hove, a seaman holds the reel by the two ends, another seaman takes the log-glass, and an officer of the watch asks, “ All clear ?" On receiving a reply, “ All clear,” he throws the log-ship well out to leeward from the lee quarter; as soon as the whole of the stray line has gone, the officer calls “ turn;" the seaman then turns the sand end of the glass uppermost; the log-ship, being perpendicular in the water, and presenting a face towards the ship, by its resistance to the ship's progress draws the marked part of the line off the reel; when the sand has run out, the seaman calls “stop ;" the officer at that instant clutches tight the line, and the number of knots which have passed out indicates how many nautical miles per hour the ship is moving through the water. · The LOG-GLASSES are common sand-glasses (or filled with metal filings) running to seconds; the long glass runs out in 30 seconds, or in 28 seconds; and the short glass in half the time, viz., in 15 seconds, or in 14 seconds. · When the ship's rate is more than 5 knots, the short glass is used, and the number of knots shown by the log-line is doubled.
As the glasses may be affected by variation in the temperature, and the sand certainly by damp weather, it is necessary to examine their accuracy, from time to time, by comparing them with a seconds' watch; they can be made true, by drying the sand, or changing its quantity, to do wbich it will be necessary to remove the cork at the stoppered end of the glass.
The Nautical Mile.-In Navigation, DISTANCE is invariably measured in NAUTICAL MILES. One such mile is the 21,600th part of the earth's circumference (360° x 60 = 21600'), and, from various admeasurements, it is considered to be about 6087 feet, or 2029 yards.
We now come to the LENGTH OF A KNOT on the log-line, and you can at once understand that the length between two adjacent knots should be the same part of a nautical mile, that the seconds of the glass are to an hour. But before we commence to determine this length, you must, in the first place, know that as it is safest to have the reckoning ahead of the ship (to apprise the seaman the sooner to look out when approaching the land), an allowance in the length of the knots is made such that it shall obviate any causes of error arising when heaving the log: hence, for practical purposes at sea, the length of the nautical mile is taken to be 6000 feet.
For the 30-second glass we readily get the length of the knot; 30 seconds. or half a minute, is the 120th part of an hour, and 6000 divided by 120 gives 50 feet, which is the length generally taken; and it must be clear to you that if the 120th part of a mile (as measured on the log-line) runs out in the 120th part of an hour, the ship would be dragging along at the rate of 1 mile per hour; on the same basis, if eight 120ths passed out, she would be sailing at the rate of 8 miles an hour.
But we must adopt a method of computation equally applicable and simple for all glasses. Now, there are 3600 seconds in an hour, and 6000 feet in a nautical mile; and the ratio of 3600 to 6000 is as 6 to 10, or as 3 to 5; hence, we have
Rule for finding the length of a knot corresponding to a glass running seconds :-Multiply the seconds (of the glass) by 5 and divide the product by 3, and the result will be the length of the knot in feet.
. Thus 28 x 5 = 140; and 140 divided by 3 = 46.6 or 46 ft.
It is no uncommon thing to mark the length of the knot by two copper nails, driven into the deck, at the proper distance apart.
The subdivisions of the knot, already referred to, and erroneously called fathoms (8 to a knot), are really nautical furlongs, after the same manner as 8 statute furlongs make a statute mile.
Note.-The log-line no less than the glass varies in its indications : especially does it contract by wetting. You may consider that,one being correct, and the other faulty-. For error of seconds glass-Glass too short gives distance too short;
Glass too long gives distance too long.
Knot too long gives distance too short. If both are faulty :-Multiply faulty length of knot by erroneous distance, and divide product by faulty time glass runs; three-fifths of the result gives true distance. - Patent Log.-In days gone by there was no other mode of finding the ship's rate of progress through the water except by the use of the log and seconds' glass, but the PATENT LOG has now, to a certain extent, superseded the old log ship, especially in the case of steamers. This instrument, consisting of a rotator and register, is kept towing astern with sufficient length of tow-line to carry it out of the immediate wake of the ship, and then the revolụtions of the rotator indicate on the register the distance run, which can be ascertained from time to time by hauling it in. The mechanism of the patent log is the same in principle as that of the screw propeller; the rotator, of three or four
flanges (B), revolves more or less rapidly according to the rate at which it is drawn through the water, and, by revolving, sets in motion a system of wheelwork which turns the hands of three indices on the register (a). The distance run, according to the number of revolutions of the rotator, is registered first in miles, then up to 10 miles, and lastly up to 100 miles.
It is needless to dwell on the patent log as there are many different kinds by different makers, and "descriptions for use” accompany all of them. Up to å recent date it has been pecessary to haul in the log to ascertaiu' by the register.
the distance run: but the latest improvement in their mechanism is the arrangement by which the rotator alone tows, and the register is connected to it on board, so that without any hauling in the distance run can be known at any moment. The chief makers are Massey, Reynolds, and Walker, and the logs of each bave specialities of their own,-some shipmasters prefering one maker's instrument, and others another maker's.
I have dwelt long on the Compass and Log, by which together the ship’s direction and distance run are ascertained ; I have done so advisedly, because, as if by common consent, both are treated with more indifference than they should be, and hence candidates for Board of Trade certificates have more difficulty with the Day's Work than with any other problem submitted to them. Moreover the Compass and Log are the instruments by which alone the common JOURNAL OF A SHIP's VOYAGE is carried on; hour by hour the rate of sailing, the compass course, the wind, the leeway, with any other particulars of note, as the deviation and variation of the compass, the current, the weather, &c., are entered on the LOG-BOARD, and every day at noon, the course and distance made good from noon to noon, and the Place of the Ship at the given noon by Dead Reckoning is known, and her progress, in the absence of observations, is approximately determined.'
SAILINGS. The computations in what is termed DEAD RECKONING are made by certain methods which, in Navigation, are called SAILINGS, and of these the chief arePlane Sailing, Traversé Sailing (which may be defined as compound plane sailing), Parallel Sailing, Middle Latitude Sailing, Mercator's Sailing, and Globular or Great Circle Sailing; and there are others which, at most, are but modifications of those just mentioned. We will not go into them further than is required to solve the Day's WORK by Inspection;—the method by inspection being a kind of rule of thumb, for which the Traverse Tables and Meridional Parts are all that is necessary: I have, however, thought it well to indicate the “sailings” by name, as I shall have to speak of some of them.
A TRAVERSE AND THE TRAVERSE TABLES... · In addition to what has already been explained to you in respect to Latitude and Longitude, Courses and Distance, there are many other things, connected more or less with them, that you must know before you can work the Day's Work; I shall endeavour to make these clear to you by working what is called a Traverse. · Traverse denotes a thing that goes athwart another; that is, crosses and cuts it at an angle. If the winds and currents were always favourable, a ship, bound from one port to another, would take as direct à route as possible, and her course for days together would tend in the one direction ; but when winds and currents are alike adverse, her course is altered, from time to time, to suit the shiftings of the wind, and thus to keep as near as possible to her proper course. Her oblique (or zigzag) progression against the direction of the wind is called