Practical Boat-Sailing Part 6

[Ill.u.s.tration: Fig. 1.]

[Ill.u.s.tration: Fig. 2.]

It will be shown too, by experiment, that considerable variation of the bearings, when the angle is large, may be made, without changing very much the position of the yacht, proving how valuable this process is for practical use, as a considerable error in the bearings will still enable one to know almost exactly the position of the yacht; whilst a good observation will give it exact.

EXAMPLE II. (see diagram Fig. 2).--Placing the compa.s.s in front of the observer, it is found that the lighthouse bears W. by compa.s.s, and that the lightship bears S. W. by S. With these two bearings we consult the chart, and lay off the two lines by means of the parallel-rulers; and, if the chart gives the distance in miles of the lightship from the lighthouse, then, by means of a common rule of equal parts, we shall be able to measure the distance of the yacht from the lighthouse or from the lightship. At the foot of most charts, however, will be found a scale of miles, and, having once ascertained the exact position of the yacht by means of cross-bearings, it will be very easy, with a pair of dividers, to find its distance in miles from any desired object within view, or designed upon the chart, and, by the use of the parallel-rulers, the course, by compa.s.s, that should be sailed to reach any desired point.

It is often useful to know how many geographical or nautical miles, which measure at the equator 6,086.4 feet in length, are contained in a degree of longitude at different lat.i.tudes; that is to say, a degree of longitude east or west of 89 N. lat.i.tude is only 1.05 nautical mile in length; and yet, in another sense, this 1.05 is 60 miles, or one degree in length: hence the following table:--

A TABLE SHOWING, FOR SEVERAL DEGREES OF LAt.i.tUDE, HOW MANY MILES DISTANT THE TWO MERIDIANS ARE WHOSE DIFFERENCE OF LONGITUDE IS ONE DEGREE.

+----+------+----+------+----+------+----+------+ LAT. MILES. LAT. MILES. LAT. MILES. LAT. MILES. +----+------+----+------+----+------+----+------+ 15 57.96 26 53.93 37 47.92 48 40.15 16 57.68 27 53.46 38 47.28 49 39.36 17 57.38 28 52.98 39 46.63 50 38.57 18 57.06 29 52.48 40 45.96 51 37.76 19 56.73 30 51.96 41 45.28 52 36.94 20 56.38 31 51.43 42 44.59 53 36.11 21 56.01 32 50.88 43 43.88 54 35.27 22 55.63 33 50.32 44 43.16 55 34.41 23 55.23 34 49.74 45 42.43 56 33.55 24 54.81 35 49.15 46 41.68 57 32.68 25 54.38 36 48.54 47 40.92 58 31.80 +----+------+----+------+----+------+----+------+

TABLE FOR DETERMINING THE DISTANCE THAT OBJECTS AT SEA CAN BE SEEN IN STATUTE MILES.

NOTE.--Enter the table in the column of height in feet, which represents the height of the observer above the sea; and opposite to it, in the column of miles, will be the result.

[A]: HEIGHT IN FEET.

[B]: MILES.

+---+-----+---+-----+---+-----+---+-----+---+-----+---+-----+ [A] [B] [A] [B] [A] [B] [A] [B] [A] [B] [A] [B] +---+-----+---+-----+---+-----+---+-----+---+-----+---+-----+ 1 1.32 13 4.77 25 6.61 37 8.05 49 9.26 105 13.56 2 1.87 14 4.95 26 6.75 38 8.16 50 9.35 110 13.88 3 2.29 15 5.12 27 6.87 39 8.26 55 9.81 115 14.19 4 2.65 16 5.29 28 7.00 40 8.37 60 10.25 120 14.49 5 2.96 17 5.45 29 7.12 41 8.47 65 10.67 125 14.79 6 3.24 18 5.61 30 7.25 42 8.57 70 11.07 130 15.08 7 3.50 19 5.77 31 7.37 43 8.68 75 11.46 135 15.37 8 3.74 20 5.92 32 7.48 44 8.78 80 11.83 140 15.65 9 3.97 21 6.06 33 7.60 45 8.87 85 12.20 145 15.93 10 4.18 22 6.21 34 7.71 46 8.97 90 12.55 150 16.20 11 4.39 23 6.34 35 7.83 47 9.07 95 12.89 160 16.73 12 4.58 24 6.48 36 7.94 48 9.17 100 13.23 170 17.25 +---+-----+---+-----+---+-----+---+-----+---+-----+---+-----+

EXAMPLE I.--Sailing along in the yacht "Firefly," from the top of the house on which I was standing, which brought my eyes to about 12 feet above the level of the sea, I observed seaward the head of a gaff-topsail that evidently belonged to a yacht of about ten tons, and was therefore estimated to be about 45 feet from the level of the sea. How far were these vessels from each other?

In the table,

Opposite 12 feet stands 4.58 miles Opposite 45 feet stands 8.87 ----- Distance apart 13.45 miles

EXAMPLE II.--Sailing towards the land, I mounted the shrouds of my yacht till my eye was about 16 feet above the level of the ocean, where I sighted the top of a known lighthouse that I was looking for, which the chart informed me was 145 feet above the level of the sea. Required the distance of the lighthouse.

In the table,

Opposite 16 feet stands 5.29 miles Opposite 145 feet stands 15.93 ----- Distance 21.22 miles

_Upon seeing the flash of a gun I counted 30 seconds by a watch before I heard the report. How far was the gun from me, supposing that sound moves at the rate of 1,142 feet per second?_

The velocity of light is so great, that the seeing of any act done, even at the distance of a number of miles, is instantaneous. But by observation it is found that sound moves at the rate of 1,142 feet per second, or about one statute mile in 4.6 seconds: consequently the number of seconds elapsed between seeing the flash and hearing the report being divided by 4.6 will give the distance in statute miles. In the present example the distance was about 6-1/2 miles, because 30 divided by 4.6 gives 6-1/2 nearly.

_To find the difference between the true and apparent direction of the wind._

[Ill.u.s.tration: triangle]

Suppose that a yacht moves in the direction C B from C to B, while the wind moves in its true direction from A to B, the effect on the boat will be the same as if she be at rest, and the wind blow in the direction A C with a velocity represented by A C, the velocity of the yacht being represented by B C. In this case, the angle B A C will represent the difference between the true and apparent direction of the wind, the apparent being more ahead than the true; and, the faster the vessel goes, the more ahead the wind will appear to be. We must, however, except the case where the wind is directly aft, in which case the direction is not altered.

It is owing to the difference between the true and apparent directions of the wind that it appears to shift its direction by tacking ship; and if the difference of the directions be observed when on different boards (the wind on both tacks being supposed to remain constant, and the yacht to have the same velocity, and to sail at the same distance from the wind), the half-difference will be equal to the angle B A C. By knowing this, together with the velocity of the yacht B C, and the angle B C A, we may obtain the true velocity of the wind; or by knowing the velocity of the wind and of the yacht, and the apparent direction of the wind, we may calculate the difference between the true and apparent directions of the wind.

Thus, if the velocity of a yacht, represented by B C, be 7 miles per hour, that of the wind, represented by A B, 27 miles per hour, and the angle with the yacht's course, with the apparent direction of the wind B C A, equal to 7-1/2 points, the difference between the true and apparent directions of the wind will be obtained by drawing the line B C, equal to 7-1/2 points; then with an extent equal to 27 miles, taken from the scale, and with one foot in B, describe an arc, to cut the line A C in A; join A B; then the angle B A C, being measured, will be the required difference between the true and apparent directions of the wind.

_Sailing in my yacht, I saw a lighthouse bearing E. by N., and, after sailing 14 miles N. by W., it bore S. E. Required the distance of the yacht from the lighthouse at both stations._

SOLUTION.--Describe the compa.s.s E. S. W., and let its centre X represent the place of the yacht at the first station; draw the N. by W. line, X Y, equal to 14 miles, and Y will represent the second station.

[Ill.u.s.tration: distance calculations]

Draw the line E. by N., X Z, of an indefinite length, and the line Y Z parallel to the S. E. and N. W. line of the compa.s.s: the point of intersection Z will represent the place of the lighthouse, and the distance Y Z, being measured by the same scale of equal parts with which the 14 miles of course is laid off, will be found to be 16-3/4 miles, and X Z 9-1/4 miles.

USE OF THE CHARTS.

Charts can be purchased, at a very reasonable rate, of all the important harbors and the whole coast-line of the United States. They come nicely backed with cloth, so as to stand considerable hard sea-usage. They should be kept, when not in use, rolled up in a large tin box made for the purpose, or a long, narrow wooden trunk, called a chart-box.

In using charts, _great care should be taken to see whether or not the courses laid down to be sailed are magnetic ones; that is, with the variation of the_ _compa.s.s allowed_. Such is usually the case; but there are charts made where the variation of the compa.s.s must be allowed to make the courses true.

Always carefully read all the notes upon the margins of a chart: one will often run across an item of the greatest interest or importance.

Nearly all charts of harbors and the coast-line will be found with two scales of miles upon them; one being marked _statute miles_, and the other _nautical miles_.

Now, the difference is this, the scale that is marked statute miles means a mile of 320 rods of 16-1/2 feet each, or 1,760 yards of 3 feet each, or 5,280 feet; whilst a nautical mile means the sixtieth part of a degree of the earth's surface measured at the equator, which is about 6,086.4 feet in length.

SOUNDINGS

Are very regular upon the American coast; and if the time of tide be known, and the note concerning soundings, on the margin of the chart, consulted, one can often, when caught in a fog, tell quite correctly the position of the yacht, its general place upon the chart being known.

THE LEAD-LINE.--For the purpose of getting soundings, the lead-line must be used, of which there are two kinds,--the _dipsey lead_, i.e., the deep-sea lead, and the hand lead. The deep-sea lead consists of a lead sinker, usually about twenty-five pounds in weight, the lower part of which is hollowed out, and filled with tallow, when it is said to be _armed_: this is for the purpose of bringing up a specimen of the bottom which it strikes upon in its descent, often thus aiding the navigator in determining his position. The line to which this lead is attached is coiled up in a half-barrel or tub, and is usually a hundred fathoms in length, a fathom being six feet. It is generally as large as one's little finger, and is laid up in what sailors call a "left-handed coil,"

the opposite to most other ropes in common use. It is marked up to twenty fathoms in the same manner as the hand lead-line, and then one knot for every ten fathoms, and a strip of leather for each five fathoms. The manner of casting the deep-sea lead is to bring the yacht to the wind, and as nearly stationary as possible, when the lead is taken by hand outside of all the rigging, forward on the weather-side, the tub remaining aft; the person forward then casts the lead as far as possible ahead of the yacht, singing out at the same moment, "Watch! Oh, watch!" and the person aft at the tub allows the line to be taken out by the lead in its descent as fast as possible; and when it reaches the bottom he hauls it carefully up till his hand hits upon the knots, the number of which determines the depth; the yacht is then kept on her course, and the line hauled in over the stern, and coiled down in the tub, as it comes in, for immediate use again. When the lead arrives on deck, it is unbent from the lead-line, the arming examined and sc.r.a.ped off, ready for a new cast.

HEAVING THE HAND LEAD.--The hand lead is used in a different manner, and is the most perfect instrument yet devised to warn the yachtsman or sailor of unknown dangers and the rapid shoaling of the water, or approach to some unknown or unexpected shoal.

Custom has, from time immemorial, marked the lead-line in a peculiar and, the writer does not hesitate to say, ridiculous manner, which can be understood by the initiated only. But that it may be done according to "Gunter," and in "ship-shape and Bristol fashion," the following explanation is given:--

Heaving the lead is done usually by a person who is placed in the main-chains for that purpose, on the weather-side, or, in smaller craft, on deck, forward, just clear of the shrouds. It is thrown whilst the yacht is under way, and being kept on her course, and the results announced in a singing voice by the one casting; and, when the water becomes too shoal, the yacht is put about, and stands off from the danger which she was approaching. The one casting the lead takes hold of the line at about a fathom from it, and swings it to and fro till enough velocity is gained to swing it over his head; when at the right angle it is released, and flies forward in the air, striking the water far in advance of the yacht and the bottom, before the yacht reaches the place where it struck the water, so that the line may be kept perpendicular for a moment from the yacht to the bottom of the sea, and the distance measured by means of marks upon the lead-line, which are as follows:--

At 1 fathom one knot.

2 " two knots.

3 " three knots.

4 " nothing.

5 " a white rag.

6 " nothing.

7 " a red rag.

8 " nothing.

9 " nothing.

10 " leather with one hole.

11 " one knot.

12 " two knots.

13 " nothing.

14 " nothing.

15 " white rag.