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NAVIGATION

(PART 2)

DETERMINATION OF SPEED, DEPTH, AND POSITIONS

THE LOG

1. Besides being particular in noting the true direction in which a ship is advancing, it is of equal importance to the navigator to know the distance traversed on a certain course or combination of courses. This is done by measuring the speed of the vessel. To determine the speed and also the distance covered, an instrument called the log is used.

2. Principles of the Log.-The principle upon which the log is founded is this: Some light floating object is thrown overboard; as soon as it strikes the water it ceases to partake of the ship's onward motion and becomes stationary; the distance of this stationary object from the ship is then measured after a certain interval of time has passed, and from this the approximate rate of sailing is ascertained.

3. The log consists of three parts, viz., the log chip, the log line, and the log glass.

The log chip is the floating object thrown overboard; the log line measures the distance; and the log glass defines the interval of time.

4. The log chip is a triangular piece of light wood c, Fig. 1, about 5 inches in diameter, the lower edge of which is rounded and loaded with a strip of lead sufficiently heavy to make it float in an upright position, as shown in the For notice of copyright, see page immediately following the title page

figure. In each corner there is a hole, and the log line is fastened to the one pointing upwards; in the lower holes. is fastened a sling or bridle, at the end of which is a peg that fits snugly into a wooden socket t, commonly called

Log Line

FIG. 1

the toggle. This peg can be released from the toggle by a jerk on the log line, thus allowing the chip to be pulled in with comparatively small resistance. The inboard end of the log line is attached to a reel r, around which it is wound.

5. The Log Line.-The first 15 to 20 fathoms of the log line from the chip is called the stray line, and is usually marked by a small piece of white or red bunting, the purpose of the stray line being to allow the chip to get clear of the vessel's eddy or wake before the measuring commences. The rest of the line is divided into parts of equal length, called knots, by pieces of cords fastened. between the strands of the line; each piece of cord carries the requisite number of knots according to its number from the stray-line mark. The length of each knot is 47 feet 3 inches.

6.

The log glass is a sand glass of the same shape and construction as the old hour glass. A vessel usually carries two log glasses, one of which runs out in 28 seconds and the other in 14 seconds; the latter is used when the ship is going at a high rate of speed, when the number of knots of the line run out is doubled.

7.

Relation Between the Knot and the Nautical Mile. In order to determine the speed of a ship per hour, the length of each knot must bear the same ratio to the

nautical mile (6,080 feet) as the time of the log glass does to the hour. Hence, the following proportion:

As the number of seconds in an hour is to the number of feet in a mile, so is the number of seconds in the glass to the number of feet in the knot;

or,

3,600 seconds: 6,080 feet = 28 seconds:.x. Whence, the length of the knot, represented by

x=

6,080×28
3,600

=

= 47.29 feet, or 47 feet 3 inches.

8. Method of Heaving the Log.-Generally, an officer and two men constitute the logging party, one man holding the reel and the other attending to the glass. When the stray-line mark crosses the rail the officer says "turn," at which the assistant holding the glass turns it instantly, watches the sand running down, and says "stop" when the last grain passes through the opening. The officer then checks the line and reads off the number of knots run out. The chip should be hove out well to the leeward of the stern and the reel should be held in such a position as to allow the line to run out freely.

9. Errors of the Chip Log. In order to guard against error caused by incorrect length of the knot, the log line should be examined quite frequently by comparing each knot with the distance between two nails, which for this purpose are placed on the deck at the proper distance. There may also be errors attached to the log glass; the condition of the sand or the size of the hole through which it runs may be affected by the change of temperature. Hence, it is important to know how to correct for any of these errors. It is evident that the longer time it takes the glass to run out, the more line will pass over the rail, and the longer the knot, the smaller will be the number of knots passing over the rail in a given time. Hence, the speed shown is directly proportional to the length of the glass and inversely proportional to the length of a knot. In other words, when an erroneous glass is used, the distance shown by the log is to the correct distance as the seconds of erroneous glass is

to 28 seconds; when the line is wrong, the distance shown by the log is to the correct distance as 47.3 feet is to the length of the knot used.

10.

Let

Correction of Errors.

x= correct distance;

d = distance shown by log;
gerroneous time of glass;
1 = incorrect length of knot.

We thus have the following proportions:

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If both glass and knot are incorrect, by substituting in formula 1 the value of x from formula 2, we have the general formula,

28 d l x= 47.3g

(3.)

EXAMPLE 1.-The speed of a vessel as indicated by the chip log is 8 knots per hour. The glass, after being compared with a watch, is found to indicate 34 seconds instead of 28. Find the true rate of speed.

SOLUTION.-Insert values of d and g in formula 1, whence the true rate of speed, or

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EXAMPLE 2.-The length of the knot is only 45 feet, and the glass runs out in 26 seconds; the distance run by the log is 265 miles. Find the true distance.

SOLUTION. In this instance both glass and knot are wrong, hence we use formula 3 to find the true distance, and we have

d = 265, 7 = 45, and g = 264.

Inserting these values in formula 3, we get the true distance, or

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EXAMPLE 3.-The distance sailed according to the log is 195 miles, but when measured, the length of the knot is found to be 50 feet. What is the true distance?

SOLUTION.-In this case, the length of the knot being incorrect, we use formula 2 to find the true distance.

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EXAMPLE 4.-The distance sailed according to the log was 304 miles, and the true distance was 295 miles; the glass ran out in 27 seconds. What was the error in the length of the knots on the log line?

SOLUTION.-We have

d = 304, x = 295, and g = 27.

Inserting these values in formula 3, and solving for /, we get

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=

= 44.2 feet.

Hence, the knot was 47.3 - 44.2 3.1 ft. too short. Ans.

EXAMPLES FOR PRACTICE

Solve the following problems:

1. According to the log, the distance sailed was 217 miles; both the glass and length of knot were incorrect, the former being 3 seconds too long and the latter 48 feet. Find the true distance.

Ans. 198.8 mi. 2. The true distance was 160 miles, and the distance according to the log, 150 miles, the length of the knot being 47.3 feet. Find the error of the glass. Ans. 1.8 sec. too short.

3. According to the chip log, a steamer covers a distance of 195 miles. It is found afterwards, by measuring the line, that the knot is 1 foot too long. What is the true distance covered by the steamer?

Ans. 199 mi.

NOTE.-Before a log line is marked off it should be stretched and boiled in very salty water; by this process the line will be less influenced by the constant wetting to which it is subjected.

11. Patent Logs.-Besides the chip log there are at the present time several patent logs in use. Most of them are

R

FIG. 2

based on the indications given by the revolution of a small

fan, or screw, which is towed by the ship.

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